Paul Gilster's Blog, page 103

An Unusually Interesting Asteroid

We learned late last week that the near-Earth asteroid 2017 YE5, discovered just last December, is what is described as an ‘equal mass’ binary. This would make it the fourth near-Earth asteroid binary ever detected in which the two objects are nearly identical in size, both about 900 meters. The binary’s closest approach to Earth was on June 21, 2017, when it came to within 6 million kilometers, some 16 times the distance between the Earth and the Moon. It won’t be that close again for at least another 170 years.

Image: Artist’s concept of what binary asteroid 2017 YE5 might look like. The two objects show striking differences in radar reflectivity, which could indicate that they have different surface properties. Credit: NASA/JPL-Caltech.

What you have above is an artist’s impression of how 2017 YE5 appears, but have a look at the radar imagery below. This comes from NASA’s Goldstone Solar System Radar (GSSR, observations conducted on June 23, 2018), and shows the presence of two lobes. We don’t yet see a binary, but these radar images were enough for Goldstone scientists to alert astronomers at Arecibo Observatory, who had already inserted 2017 YE5 into their observing list.

Image: Radar images of the binary asteroid 2017 YE5 from NASA’s Goldstone Solar System Radar (GSSR). The observations, conducted on June 23, 2018, show two lobes, but do not yet show two separate objects. Credit: NASA/JPL-Caltech/GSSR.

Working with researchers at the Green Bank Observatory in West Virginia, the Arecibo scientists linked the two observatories in a bi-static radar configuration, meaning that Arecibo transmits the radar signal while Green Bank receives the return signal. It was the combination of data from the two observatories that allowed 2017 YE5 to be confirmed as two separated objects.

Image: Bi-static radar images of the binary asteroid 2017 YE5 from the Arecibo Observatory and the Green Bank Observatory on June 25. The observations show that the asteroid consists of two separate objects in orbit around each other. Credit: Arecibo/GBO/NSF/NASA/JPL-Caltech.

A surprising number of near-Earth asteroids may be binaries, according to this JPL news release, which tells us that among near-Earth asteroids larger than 200 meters in size, about 15 percent are binaries with one larger object and a much smaller asteroid satellite. While equal-mass binaries are apparently rare, contact binaries (two equally sized objects in contact with each other) make up another 15 percent of the population in this size range.

Thus at 2017 YE5 we have two objects that revolve around each other every 20 to 24 hours, as confirmed through brightness variations at visible light wavelengths at the Center for Solar System Studies in Rancho Cucamonga, California. As to composition, the two components do not reflect as much sunlight as a typical rocky asteroid, making it likely that 2017 YE5 has a surface as dark as charcoal. Differences in reflectivity of the two objects suggest that they have different composition at the surface or perhaps different surface features.

I was startled to learn that more than 50 binary asteroid systems have turned up in radar studies since 2000, with the majority consisting of one large object and a much smaller satellite. The differences in radar reflectivity found at 2017 YE5 have not appeared in this population. That makes this binary a useful system for the study of binary formation. Further study of combined radar and optical observations may allow tighter constraints on the density of the 2017 YE5 objects, which should give us a window into their composition and structure.

Ross 128b: Analyzing a Planet by the Light of its Star

Red dwarfs have a lot of things going for them when it comes to finding possibly habitable planets. A planet of Earth size in the HZ will produce a substantial transit signal because of the small size of the star (‘transit depth’ refers to the amount of the star’s light that is blocked by the planet), and the tight orbit the planet must follow increases the geometric probability of observing a transit. But planets that do not transit are also more readily detected because of the large size of the planet compared to the star, gravitational interactions producing a strong radial velocity signature, which is what we have in the case of Ross 128b.

About 11 light years from Earth, the planet was culled out of more than a decade of radial velocity data in 2017 using the European Southern Observatory’s HARPS spectrograph (High Accuracy Radial velocity Planet Searcher) at the La Silla Observatory in Chile. The location of the planet near the inner edge of its star’s habitable zone excited interest, as did the fact that Ross 128 is much less subject to flares of ultraviolet and X-ray radiation than our nearest neighbor, Proxima Centauri, which also hosts a planet in a potentially habitable orbit.

Image: Artist’s impression of the exoplanet Ross 128b. Credit: ESO.

What we know about Ross 128b is that it orbits 20 times closer to its star than the Earth orbits the Sun, but receives only 1.38 times more irradiation than the Earth, with an equilibrium temperature estimated anywhere between -60 degrees Celsius and 20°C, the host star being small and relatively cool. But bear in mind that what we get from radial velocity is a minimum mass, because we don’t know at what angle this system presents itself in our sky. Now a team led by Diogo Souto (Observatório Nacional, Brazil) is attempting to deduce more about the planet’s composition using an unusual method: Analyzing the composition of the host star.

If we learn the chemical abundances found in the star Ross 128, the thinking goes, we should be able to make reasonable estimates about the composition of any planets that orbit it. Souto and team are presenting new techniques for making these measurements, using data from the Sloan Digital Sky Survey’s APOGEE spectroscope. Measuring the star’s near-infrared light, where Ross 128 shines the brightest, the researchers have been able to derive abundances for carbon, oxygen, magnesium, aluminum, potassium, calcium, titanium and iron.

“The ability of APOGEE to measure near-infrared light, where Ross 128 is brightest, was key for this study,” says co-author Johanna Teske (Carnegie Institution for Science). “It allowed us to address some fundamental questions about Ross 128 b’s `Earth-like-ness.’”

APOGEE is the Apache Point Galactic Evolution Experiment, an investigation using high-resolution spectroscopy to probe the dust that obscures the inner Milky Way. The project surveyed 100,000 red giant stars across the galactic bulge, but also observed M-dwarfs in the neighborhood of the Sun as a secondary study. Tightening up our knowledge of stellar parameters, the paper notes, offers an indirect route to studying exoplanet composition.

The assumption in this work is that the chemistry of a host star influences the contents of the disk from which planets form around it, which in turn affects the interior structure of any planet. Thus we can hope to tell from the amount of magnesium, iron and silicon available something about the exoplanet. This is the first detailed abundance analysis for Ross 128, and it shows that the star has iron levels similar to the Sun. The silicon level could not be measured, but the ratio of iron to magnesium points to a large core for the planet, larger than Earth’s.

Souto and team believe that knowledge of Ross 128b’s minimum mass (from the radial velocity data), coupled with their data on stellar abundances, can provide a broad estimate of the planet’s radius, a key factor because it would allow a calculation of its density. From the paper:

While both mass and radius are not available for Ross 128b, we can estimate its radius given its observed minimum mass and assuming the stellar composition of the host star is a proxy for that of the planet. We calculate the range of radii possible for Ross 128b using the ExoPlex software package (Unterborn et al. 2018) for all masses above the minimum mass of Ross 128b (1.35M⊕; Bonfils et al. 2017). Models were run assuming a two-layer model with a liquid core and silicate mantle (no atmosphere). We increase the input mass until a likely radius of 1.5R⊕ was achieved, roughly the point where planets are not expected be gas-rich mini-Neptunes as opposed to rock and iron-dominated super-Earths…

Measurements of the temperature of Ross 128 coupled with the estimated radius of the exoplanet and its inferred composition allow the team to calculate Ross 128b’s albedo, the amount of light reflecting off its surface. These estimates allow the possibility of a temperate climate, taking into account the insolation flux (energy received from the host star) and equilibrium temperature. “Our results,” the authors write, “support the claim of Bonfils et al. (2017) that Ross 128b is a temperate exoplanet in the inner edge of the habitable zone.”

But the paper urges caution in the interpretation:

However, this is not to say that Ross 128b is a “Exo-Earth.” Geologic factors unexplored in Bonfils et al. (2017) such as the planet’s likelihood to produce continental crust, the weathering rates of key nutrients into ocean basins or the presence of a long-term magnetic field could produce a planet decidedly not at all “Earth-like” or habitable due to differences in its composition and thermal history. Furthermore, other aspects of the M-dwarf’s stellar activity and its effect on the retention of any atmosphere and potential habitability should be studied, although we find no evidence of activity in the Ross 128 spectra.

Indeed. The number of variables affecting ‘habitability’ is striking. So let’s say this: We have a planet for which mass-radius modeling based on the composition of its host star indicates a mixture of rock and iron, the relative amounts of each being set by the ratio between iron and magnesium. The derived values for insolation and equilibrium temperature are not inconsistent with previous studies indicating a temperate planet at the inner edge of its star’s habitable zone.

The work hinges on modeling of an exoplanet based on a deeper analysis of its host star than has previously been available for an M-dwarf. Tuning up such modeling will demand further data, in particular applying these methods to the host stars of transiting worlds (think TRAPPIST-1) to test their accuracy and reliability in characterizing planets we cannot see.

The paper is Souto et al., “Stellar and Planetary Characterization of the Ross 128 Exoplanetary System from APOGEE Spectra,” Astrophysical Journal Letters Vol. 860, No. 1 (13 June 2018). Abstract / preprint.

Pluto Maps Inspire Thoughts of Bradbury

Something happens when we start making maps of hitherto unknown terrain. A sense of familiarity begins to settle in, a pre- and post-visit linearity, even when the landscape is billions of miles away. To put a name on a place and put that name on a map is a focusing that turns a bleary imagined place into a surface of mountains and valleys, a place that from now on will carry a human perspective. It can’t be undone; a kind of wave function has already collapsed.

And what place more remote than Pluto? At the dwarf planet’s Tenzing Montes, we find striking peaks, some of them running up to 6 kilometers in height, and all this on a world that, until 2015, we weren’t sure even had mountains. Certainly we weren’t expecting mountains this tall, or a terrain this rugged. Given how many years may pass before we have another chance to visit Pluto/Charon, these first official validated topographic maps of the dwarf planet and its moon, just released, will carry our science — and our imaginations — for a long time to come.

Image: Perspective view of Pluto’s highest mountains, Tenzing Montes, along the western margins of Sputnik Planitia, which rise 3-6 kilometers above the smooth nitrogen-ice plains in the foreground. The mounded area behind the mountains at upper left is the Wright Mons edifice interpreted to be a volcanic feature composed of ices. Area shown is approximately 500 kilometers across. Image credit: Lunar and Planetary Institute/Paul Schenk.

The maps are the work of New Horizons researchers led by Paul Schenk (Lunar and Planetary Institute). The team examined all the images from New Horizons’ Long Range Reconnaissance Imager (LORRI) and Multispectral Visible Imaging Camera (MVIC) systems as the raw material for their mosaics. They aligned surface images where they overlapped and performed digital analysis of stereo images both cameras acquired, producing topographic maps for each region, then assembling these into integrated topographical charts for both Pluto and Charon.

Pluto’s mountains are likely made of water ice, because ices from volatiles like methane and nitrogen would not be strong enough to support such tall features, and as the images show, the steep peaks along the southwestern edge of Sputnik Planitia, itself a frozen sheet of nitrogen, have slopes pushing 40° or more. The topographical maps put large-scale features into perspective and help us see both Pluto’s and Charon’s surfaces in a broader context.

Sputnik Planitia is a good example. Fully 1000 kilometers wide, it contains an ice sheet that averages 2.5 kilometers below Pluto’s mean elevation, which corresponds to sea level on our own planet. The maps also show us that the outer edges of the sheet are an even deeper 3.5 kilometers below mean elevation. These are the lowest known areas on Pluto, a fact that emerges only through study of the stereo images and subsequent elevation maps they spawned. The deep ridge-and-trough system running north to south near the western edge of Sputnik Planitia is more than 3000 kilometers long, evidence for extensive fracturing, as this LPI news release explains. It is the longest known feature on the dwarf planet.

And then there’s Charon. Who would have dreamed in 1978, when astronomer James Christy discovered it, that we would ever have the kind of detail that shows below?

Image: Perspective view of mountain ridges and volcanic plains on Pluto’s large moon Charon. The ridges reach heights of 4 to 5 kilometers above the local surface and are formed when the icy outer crust of Charon fractured into large blocks. The smoother plains to the right are resurfaced by icy flows, possibly composed of ammonia-hydrate lavas that were extruded onto the surface when the older block sank into the interior. Area shown is approximately 250 kilometers across. Image credit: Lunar and Planetary Institute/Paul Schenk.

Maps and Sea Change

In my first paragraph today, I summoned quantum mechanics for inspiration, saying that producing maps created the collapse of a kind of psychological wave function. Adam Alter made the same point a few years back in an article in The New Yorker, where he talked about our association of linguistic labels with the things they denote. The effects can be subtle. Northerly movement, for example, is associated in psychological testing with going uphill, apparently a remnant of the decision of ancient Greek mapmakers to put the northern hemisphere above the southern one.

Alter goes on to speak of what he calls a ‘linguistic Heisenberg principle,” meaning that as soon as you label a concept, you change how people perceive it, and I would assume this goes for landscapes as well. So we’d better choose the place names we put on our maps with care, given the freight they carry in our imaginations. Ray Bradbury knew this as well. His ‘The Naming of Names’ takes Earth colonists on Mars to strange places indeed as they begin to name the places they see.

For in the world of The Martian Chronicles, Mars is a place with a long, long history, and pretty soon the new place names the colonists have chosen begin to morph back into their ancient forms, as spoken by the original inhabitants of the planet. Before long not just the names but the people themselves are changing, returning to existences ancient, rich and strange:

The nights were full of wind that blew down the empty moonlit sea-meadows past the little white chess cities lying for their twelve-thousandth year in the shallows. In the Earthmen’s settlement, the Bittering house shook with a feeling of change.

Lying abed, Mr.Bittering felt his bones shifted, shaped, melted like gold. His wife, lying beside him, was dark from many sunny afternoons. Dark she was, and golden, burnt almost black by the sun, sleeping, and the children metallic in their beds, and the wind roaring forlorn and changing through the old peach trees, violet grass, shaking out green rose petals.

It’s a great tale, and one worth re-reading any time mapping new landscapes comes to mind.

The papers are Schenk et al., “Basins, fractures and volcanoes: Global cartography and topography of Pluto from New Horizons,” Icarus Vol. 314 (1 November 2018). abstract; and Schenk et al., “Breaking up is hard to do: Global cartography and topography of Pluto’s mid-sized icy Moon Charon from New Horizons,” Icarus Vol. 315 (15 November 2018). Abstract. Adam Alter’s “The Power of Names” appeared in The New Yorker’s May 29, 2013 issue.

TVIW Symposium on The Power of Synergy

Ever since I started Centauri Dreams in 2004, I’ve been talking about the question of infrastructure within the Solar System. My thinking has always been that while we will doubtless get off interstellar missions beginning with robotics on an ad hoc basis during this century, the prospect of a sustained effort will require a built-out infrastructure that will help us create and test out deep space systems of many kinds, from new propulsion technologies to closed loop life support experiments. One step at a time, but do this right and we may push deep into the Kuiper Belt, then the Oort Cloud and, we can hope, beyond.

That’s a long-term vision and it clashes with what we’ve seen since Apollo, a retreat from lunar exploration by humans that may eventually be reversed as we think about partnerships between commercial aerospace and government space programs. To explore these concepts, an upcoming meeting called the TVIW Symposium on The Power of Synergy is to be held in Oak Ridge, TN from October 23-25, 2018. Participants from NASA, DOE ARPA-E, Oak Ridge National Laboratory, the Y-12 National Security Complex, and several private companies are being tasked with the challenge of evaluating where we stand in just such an infrastructure.

TVIW stands for the Tennessee Valley Interstellar Workshop, which has held symposia for a number of years in Oak Ridge, Huntsville and Chattanooga — I’ve been pleased to attend most of these, and you can find my reports from past meetings in the archives here. The upcoming meeting is a departure, a gathering convened to explore a set of specific technologies in the context of the resources and technologies being readied in these high-tech areas.

Synergy — that unpredictable, frequently rewarding process of getting more out of a partnership than the apparent sum of its parts — is to be the theme throughout. Focusing on how the work of government laboratories can mesh with private industry, the symposium is to look at a set of seven key technologies, the thinking being that many of these are reaching the stage where they can create transformative progress in space within a decade. That’s a bracing thought, but the organizers believe that multi-agency cooperation can accelerate space exploration.

Participants in the symposium will be examining, for example, high-impulse nuclear propulsion, as studied in DARPA’s Timberwind Program. Political issues always swarm around nuclear ideas, but high-performance technologies realized through upper-stage nuclear rockets fired only once they have reached Earth orbit or beyond could allow faster transit times, enough so to make human expeditions to Mars far more practical than currently envisioned. Going nuclear has ramifications as well in space solar power and cislunar operations including manufacturing.

Have a look at the symposium website for more on the ideas to be discussed, which include high-energy lasers of the sort now being considered by Breakthrough Starshot as a way to propel small sailcraft with miniaturized payloads to the Alpha Centauri triple system. Closer to home, power beaming in space can help to build a transportation network in the inner system and incentivize exploratory missions to the outer planets. Likewise transformative are high-temperature superconductors, developed for several decades at Oak Ridge National Laboratory. Magnetically inflated cable (MIC) technologies can help in the construction of large space structures. Large-scale 3D printing, another ORNL specialty, points toward manufacturing capabilities in space that would be a necessary part of a permanent human presence.

Rounding out the list of enabling technologies are self-replicating von Neumann machines, solar power satellites and lightweight large-aperture optics. Can we reach the point where small machines can build larger ones out of abundant space resources found, for example, in nearby asteroids? For that matter, can we consider asteroids themselves, suitably modified by such means, as habitats safe from dangerous radiation from cosmic rays or solar storms?

And on the astronomical front, large-aperture optics offers the prospect of space telescopes that dwarf the scale of today’s efforts, including interferometer arrays for the imaging of exoplanets and advances in our knowledge of cosmology. What the symposium organizers are arguing is that all of these technologies are developing at a pace sufficient to think realistically about fleshing out a near-Earth infrastructure that can swiftly be extended to Mars and beyond.

The speaker list is being fleshed out now, but among those scheduled so far are Michael Raftery (Boeing and Explore Mars, Inc) on the ‘NASA Lunar Gateway Concept;’ Franklin Chang-Diaz (Ad Astra Rocket Company) on ‘Living and Working in Space;’ Phil Lubin (UCSB) on ‘Directed Energy Propulsion – Interplanetary and Interstellar;’ John Mankins (Artemis Innovation Management Solutions) on ‘Space Solar Power Stations;’ Bill Peter (ORNL) on ‘Large 3D Printing;’ Robert Bagdigian (NASA MSFC) on ‘Environmental Control & Life Support;’ and Joel Sercel (Trans Astronautica Corporation) on ‘Capture & Uses of 10 Meter Asteroids.’

The venue in Oak Ridge will be the Y-12 New Hope Visitor Center. Those interested in attending can visit the TVIW Symposium on the Power of Synergy website for more information.

Listening in on Enceladus

When I was a boy, I used to scan shortwave frequencies with an old Lafayette receiver in search of distant stations. When I learned that Jupiter was a radio source, my passion for radio DXing took a new turn, merging with my interest in astronomy. When I tried to log the planet’s violent outbursts, I learned with a little digging in the library that Jupiter could be detected from about 15 MHz up to 40 MHz, with the best window somewhere between 18 MHz and 28 MHz.

Called ‘decametric noise storms,’ the Jovian bursts sometimes sounded like ocean waves hitting a shore, but there were also short bursts that could be confused with local lightning, and to this day I’m not really sure whether I really heard Jupiter or not. When you’re listening for something that sounds like the ocean in the shortwave bands, it’s all too easy to think you’re hearing it in the background noise, and a little imagination makes you think you’ve found your target.

These days we can listen to just about anything on the Internet, so I’ll point you to the Io B storm of November 27, 2001, on a page that offers charts, links and an anecdotal account of a reception. Jupiter seems to have acquired a fan base among amateur radio astronomers.

But enough of Jupiter. This morning we need to talk about Saturn and the plasma waves Cassini detected moving from the planet to its rings and the moon Enceladus. These produce the distinctive sound you can hear in the YouTube video below. (If you get Centauri Dreams through email, the video isn’t going to show, but go to this link to see it). Here the recording time was compressed from 16 minutes to 28.5 seconds.

Image: New research from the up-close Grand Finale orbits of NASA’s Cassini mission shows a surprisingly powerful interaction of plasma waves moving from Saturn to its moon Enceladus. Researchers converted the recording of plasma waves into a “whooshing” audio file that we can hear — in the same way a radio translates electromagnetic waves into music. Much like air or water, plasma (the fourth state of matter) generates waves to carry energy. The recording was captured by the Radio Plasma Wave Science (RPWS) instrument Sept. 2, 2017, two weeks before Cassini was deliberately plunged into the atmosphere of Saturn. Credit: NASA/JPL-Caltech/University of Iowa.

The plasma wave interactions are the subject of a recent paper from lead author Ali Sulaiman (University of Iowa), who is a member of the Radio Plasma Wave Science team, RPWS being the instrument on Cassini that recorded these waves traveling on magnetic field lines.

“Enceladus is this little generator going around Saturn, and we know it is a continuous source of energy,” says Sulaiman. “Now we find that Saturn responds by launching signals in the form of plasma waves, through the circuit of magnetic field lines connecting it to Enceladus hundreds of thousands of miles away.”

Image: NASA’s Cassini spacecraft’s Grand Finale orbits found a powerful interaction of plasma waves moving from Saturn to its rings and its moon Enceladus. Credit: NASA/JPL-Caltech.

Enveloped by Saturn’s magnetic field, Enceladus is a geologically active place, emitting the famous geysers we’ve so often examined in Cassini imagery. The plumes of water vapor from what appears to be an inner ocean become ionized, accounting for the strong interaction between Enceladus and the planet; similar interactions occur between Saturn and its rings.

We get this information thanks to Cassini’s high-inclination Grand Finale orbits, which brought the spacecraft both to its closest approach to the cloud tops and to the inner edge of the D ring. The recording itself was captured on September 2, 2017, just two weeks before Cassini’s final plunge. Measurements of the top of the ionosphere as well as the environment around the rings showed the plasma wave interactions and underline the dynamic nature of the Saturn system.

The paper is Sulaiman et al., “Enceladus auroral hiss emissions during Cassini’s Grand Finale,” published online by Geophysical Research Letters 7 June 2018 (abstract).

The Apkallu Initiative: A Minilithic Artefact for Rebooting Human Civilization in the Event of Global Cataclysm

Kelvin Long is a familiar face on Centauri Dreams, the author of several previous articles here and many publications in the field of interstellar studies. The creator of Project Icarus, the re-design of the Project Daedalus starship of the 1970s, Long was a co-founder of Icarus Interstellar and went on to head the Initiative for Interstellar Studies. He also served as editor of the Journal of the British Interplanetary Society during a critical period in the journal’s history, and authored Deep Space Propulsion: A Roadmap to Interstellar Flight (Springer, 2011). Today he turns his thoughts to catastrophe, and the question of what would happen to human civilization if it were reduced to a small remnant. Could we preserve the most significant treasures of our science, our culture, in the face of a devastated Earth? Exploring these ideas takes us deep into the past before turning toward what Kelvin sees as a possible solution.

by Kelvin F Long

The year is 2050. Earth is a thriving metropolis with a population exceeding 9 billion. Progress has been made in harmonising social-cultural tensions around the world and nation state war is now an infrequent event. A young child of the future steps out into the bright sunshine of a gorgeous new morning. Her day is still ahead of her as she out stretches her arms and smiles at the mellifluous call of the singing birds. But then looking up, she notices something in the distance, a long streak across the sky that is moving rapidly, and seems to be descending towards the ground. It disappears behind the horizon, and shortly later a blinding flash engulfs the world. The girl looks on stunned, eyes struggling against the light, to see the gradual build-up of a mushroom cloud that starts to reach high into the atmosphere. The impact event was hundreds of miles away, yet soon it engulfs the world in a global climate change and sends Tsunamis sweeping over coastal cities destroying all in the path. In response to oceanic earthquakes, the water becomes so big, that it pushes across the flat land masses; unrelenting mega white horses to a trampled poppy field below. One day, this will form into wedge shaped chevron deposits hundreds of feet high, composed of ocean floor micro-fossils. Within days of the event the girl will learn that billions of people are wiped out as the human civilization draws to a rapid stagnation. All infrastructure and governments are gone, and only small pockets of communities around the world survive, numbering thousands at best. She was one of the lucky ones, her small community of one hundred people survived just barely on their high mountain top position. This is fortunate for a girl named Hope.

Introduction

The future is uncertain. Whilst it is important to emphasise the positive reasons for the exploration of Earth and space, it is also important not to be in denial about the risks that really face us; for they are not insignificant. They are many and varied in type. From the potential for nation state warfare, to disease pandemics, to global climate change, to risks from above such as impact events by asteroids or comets or even the possibility alien invasion. The sure way to guarantee our survival is to follow the lead of Elon Musk and to make the human race an interplanetary species; and indeed to go further with an interstellar species. But until we have reached this point we are vulnerable. The proposal made in his article is not an alternative to the current plans for the colonization of space and the continued building up of infrastructure, but it is a complimentary pathway to increase the probability of human survival into the coming centuries. In particular, it should be taken on board that the assumptions of this project is that a possible future exists where rocket technology no longer even exists as a worst case survival scenario.

The Apkallu initiative is a proposed project to help reboot human civilization, on the assumption that some small pockets of human communities survive around the world during a global cataclysm, but all the remnants of our industrialised and developed civilization are destroyed. This includes our cities, our farms, our libraries, our infrastructure, and our transport networks; in essence the human race is thrown back to being a hunter-gatherer species and must begin again. It is named after the Sumerian sages who are said to have helped humankind establish civilization and culture and giving us the gifts of a moral code, mathematics, architecture, agriculture and all ways necessary to teach us how to become civilized. The Sumerian civilization is one of the first to appear in recorded history, which included the invention of its own writing form called Cuneiform. Before we discuss what the Apkallu initiative actually is, it is worth reminding ourselves of some essential context.

Impact Threats and Other Risks to Human Survival

We know that objects have impacted the Earth throughout its history and continue to do so today. Approximately 66 million years ago, it is believed that an impact event resulted in the Cretaceous-Tertiary (K-T) extinction. This led to devastation in the global environment and a prolonged winter which affected the photosynthesis of plants and plankton life. It also resulted in the destruction of a plethora of terrestrial organisms, including mammals, birds, insects and most famously the dinosaurs. The object, an asteroid or comet, was 10-15 km in diameter with a likely impact velocity of around 20 km/s and an associated kinetic energy of impact of around 30,000 – 1000,000 Gtons TNT equivalent, depending on the assumptions. It left an impact crater in the Yucatan Peninsula in Mexico, and likely created 300 feet high Tsunami’s over an impact zone of around 3,000 miles.

Another example is the Arizona Meteor crater, which was the result of a Nickel-Iron object around 50 m in size impacting the Earth 50,000 years ago. With impact velocities ranging from 2.8 – 20 km/s this would have impacted with an associated kinetic energy of 10.7 – 26.2 Mtons TNT equivalent. Today, a crater remains of the impact event, 1.2 km in diameter and over 550 feet deep.

In 1908 a comet is believed to have impacted eastern Siberia, causing a flattening of a forest 2,000 square km in size. Since no impact crater was found, it is believed that the object disintegrated at an altitude of 5 – 10 km above the ground. The estimated energy of the air burst explosion was 10 – 15 Mtons TNT equivalent; depending on the assumptions one makes.

In July 1994 a comet split into 21 fragments ranging in size up to 2 km, and impacted the upper atmosphere of Jupiter with an impact velocity of around 60 km/s. The total energy of these impacts was around 6,000 Gtons TNT equivalent creating dark red spots with some being 12,000 km in size. Had this comet impacted the Earth, it would have posed a major threat to human existence.

During late 2017 we observed the close flyby pass of an asteroid of interstellar origins named ‘Oumuamua. Much of the nature of this objects remains uncharacterised, but some sensible estimates of the maximum potential impact energy suggest 4.2 – 46.9 Gtons TNT equivalent, had it impacted the Earth.

Then in April this year that an object named Asteroid 2018 GE3 passed closed to Earth and was spotted 119,500 miles away, which is closer than the Moon, which orbits at an average distance of 238,900 miles. The object was first observed by the NASA funded Catalina Sky Survey project based at the University of Arizona Lunar and Planetary Laboratory. It was first observed a mere 21 hours before the closest approach to the Earth. The object was estimated to be at least 150 – 360 ft in diameter.

How many more are out there waiting for us? No doubt some will argue that the impact risks are statistically small and we should not be concerned about them. We know there are many asteroids in our own Solar System, varying in size from 1 m up to 1,000 km. Approximately 16,000 objects have been found near Earth, but this is a small fraction of the estimated total that is out there, which varies between 1 – 2 million. Statistically, this presents a threat to human existence and life as we know it. Indeed, it is the belief of this author that impact events which can lead to global devastation of the human population may be as frequent as 1/1,000 – 1/10,000 years.

In addition to impact risks there are many other threats to human existence. This may include the implications of magnetic field reversal. Such an event occurred 41,400 years ago during the last ice age, called the Laschamp event. It caused a magnetic field reversal leading to a drop in its strength. This resulted in more cosmic rays reaching the Earth and an increased production of the isotopes Beryllium 10 and Carbon 14.

There are also the risk of enhanced solar activity such as through large scale solar flares, or the possibility of the Sun entering unstable periods in its evolution for which are current models of stellar-structure are not aware. This could be due to the passage of our Sun through the spiral density arms of the galaxy. There are the risks of nation state war or even global thermonuclear war that could drive us towards extinction, either through direct destruction or through altering the climate. There are the risks of human disease pandemic, which surely must become more probable in an increasing global population. There are the risks of human destruction of elements of the biosphere, such as pollutions of the oceans, soils, deforestation or polluting of the atmosphere. There are the risks that microbes could be introduced into our biosphere from an alien planet that is infectious to our biodiversity.

Then there is the actual risk of alien invasion, from a species set on conquering other lower species or seeking resource acquisition no matter the costs. It may be assessed that some of these are low probability. However, the fact that there are so many risks to the future survival of humankind should be a concern, and it is vital that we take a proactive approach to adaptability and survival, instead of a reactive one when such events occur.

Assumptions of a hypothetical Near-Human Extinction

Imagine a situation where human kind is nearly wiped out by some global cataclysm. This could be an impact event or one of the other risks highlighted earlier. In a worst case scenario, but one where some humans survive, we might make the following assumptions:

1. All infrastructure is destroyed, to include buildings, power utilities, city plumbing, dams, transport networks, agriculture and farming, huge portions of the plant and animal kingdom.
2. All information sources are destroyed, to include all the world libraries, computers and electronic memory. It is possible that some books will be discovered over time as communities explore the rubble remaining from the metropolis. Books would become precious beyond their current value.
3. The global climate is in turmoil and hostile, but with isolated regions of stability such that with determination survival is possible.
4. The geological, climatic, oceanic activity and effects of the cataclysm event, within weeks, months or years will gradually return towards some level of stable Earth.
5. Small pockets of humans survive around the Earth, perhaps 10s to 100s each but with the total not exceeding thousands.

Given this scenario, we can note that the surviving generation will remember the world as it was before. They will use this knowledge to teach their children. At this point knowledge is based upon direct memory. Those children will then grow up, with their parents dying off, and they will remember what their parents taught them and some of those children may even have some memories of the world before. But for the most part we are dealing here with recent history and part mythology. The grandchildren will also be born and grow up, but they will have no direct memory of the world the way it was before. At this point we are dealing with history and mythology. Within the third or fourth generation there is a risk that all knowledge will be lost, and especially if that knowledge is not captured and written down. All received knowledge then becomes both mythology and fantasy.

There are solutions to this practiced by the Native North Americans for example, which is to communicate stories verbally and also use this to impart wisdom, and those stories are accompanied by rituals. However, one cannot believe that such a method of communication does not contain significant information error propagation with each successive generation, compared to the original version.

The History of Humans on Planet Earth

In the event of a global cataclysm, assuming small pockets of human communities survive, but the majority of human civilization and associated technological infrastructure is destroyed, how can we ensure a chance at rebooting human knowledge? Indeed, is it possible that this has in fact occurred in the recent past and this is a part reason for the many Megalithic structures on Earth?

Until recently, Sumer was the earliest known civilization in the historical Mesopotamia, and is located in modern Iraq. It dates back to 3,000 B.C and was likely settled around 4,000-5,500 B.C by proto-Euphrateans or Ubaidians. The people from this era are credited for many great inventions and discoveries which led to the advance of their society. This includes in mathematics, geometry, agriculture, architecture, economics and law to name a few. One of the most famous objects discovered from this period is the Code of Hammurabi, a 2.25 m tall stone wall consisting of 282 laws, such as “an eye for an eye” and is the first legal system from the Old Babylonian period.

The Code of Hammurabi, created 1750 B.C, currently housed at the Louvre, Paris (image credit: K. F. Long)

It is important to note that in the Babylonian creation mythologies, which were written in Cuneiform, there are around a thousand lines of text on seven clay tables. The focus of this text is the creation of humankind for the service of the gods. These texts are called the Enûma Eliš, and arguably they have a clear lineage to the Judeo-Christian Bible. The Cuneiform script was scribed, using a wedge-shaped marker onto a wet clay tablet and also cylinder seals. These are small round objects typically an inch in length engraved with information. Once dried the inscription was permanent. The information preserved on tablets and seals was Cuneiform text but also contained figurative scenes or descriptions of events or objects. Such objects are breathtaking in their clarity, gorgeous in their artistic nature, and contain a wealth of information about the society, its rituals, values, business, science and technology.

Photographs of Sumerian Cylinder Seals from the Private Collection of the Author (image credit: K. F. Long)

The Holy Bible records a flood story that engulfed all of planet Earth. This is recorded in Genesis chapters 6 – 9, and the flood seems to last for around one hundred and fifty days. Other cultures have recorded similar stories. For example the Sumerian tale of Ziusudra and the Atra-Hasis also describes a global flood story that is similar to that told in Genesis. In the Sumerian story the flood lasts for seven days. An account is also told in the Epic of Gilgamesh, which is more similar to the Biblical story. Also, the Hindu mythology tells of a great flood in the Satapatha Brahmana. It is very easy to dismiss the possibility of a global flood as pure mythology, but the occurrence of a similar story in so many cultures around the world is at least suggestive that it may be a memory of an actual event which many today are regarding as mythology. Indeed, science may be catching up with the past.

Geologists and climatologists study a period in Earth’s history called the Younger Dryas, which occurred 12,900 to 11,700 years ago and saw a return to glacial conditions which temporarily reversed the gradual climatic warming after the last glacial maximum which began receding around 20,000 years ago. It led to many catastrophic effects including the decline of the Clovis culture in North America and the extinction of many megafauna which included the Mammoths; the last of which survived into the Holocene around 4,500 years ago in Africa, Europe, Asia and North America.

Illustration of the Younger Dryas period

In recent years, evidence is emerging that the Younger Dryas period may have been caused by a cometary impact event on the North American ice sheet, around 12,900 years ago. The evidence for his includes the discovery of a 10 million ton deposit of impact spherules across four continents, and the discovery of a Nano-diamond rich layer. In addition, analysis of underground soils indicates massive wildfire and abrupt ecosystem disruption on California’s Northern Channel Islands. Scientists have also discovered very high temperature impact melt products as evidence for an air burst explosion. All of this is dated to around 12,900 years ago, at the onset of the Younger Dryas. If this is proven to be correct, then a global cataclysm may indeed have occurred in our recent past. Speculating, if any advanced civilizations existed on Earth prior to this date, they may have been wiped out by this cataclysm forcing civilization to start from the beginning again.

At some point in our past we moved from a hunter-gatherer species to an agricultural-farming one, where we embraced the domestication of animals and crops. This is marked by a period called the Neolithic, and occurred around 10,200 years ago. It is considered to be the last period of the stone age and commenced the beginning of the Neolithic revolution. It ended with the emergence of the Copper and Bronze and Iron ages and our new abilities to use metals. It is remarkable that we have apparently exploded technologically and social-culturally over the last 10,000 years or so to the state where we have computers, cars, aeroplanes and communication satellites. What was it that propelled us forward over such a short space of time? Why had we not achieved this level of maturity previously? Was it the formation of a critical population density? Was it global climatic conditions? What is our tribal nature and inability to get organized? What it some other threats to our existence?

Homo sapiens in our modern form may be several hundred thousand years old. Paleolithic cave art certainly goes back to 40,000 years but may be 60,000 years if we include what is currently being claimed to be art from Neanderthal man. Evidence from the out of Africa hypothesis puts homo sapiens at around 130,000 – 180,000 years old. But there are alternative versions which claim populations emerging out of Africa as early as 350,000 years ago. Evidence for older findings includes discoveries of anatomically modern human skull fossils at Jebel Irhour in Morocco (315,000 years) and Middle Awash in Ethiopia (160,000 years). The history of human evolution is far from settled and ‘thinking man’ may be much older than we realised.

Ancient Megaliths

A story from ancient Sumeria is that of an amphibious being called Oannes (also known as Adapa) who apparently taught humankind wisdom. The story was told by Berossus in 290B.C, a Chaldean Priest in Babylon. Berossus described Oannes as having the body of a fish but underneath the figure of a man. He is said to dwell in the Persian Gulf, rising out of the waters in day time and furnishing humankind in the instruction of writing, arts and other subjects. Here are the words of Berossus:

“At first they led a somewhat wretched existence and lived without rule after the manner of beasts. But, in the first year appeared an animal endowed with human reason, named Oannes, who rose from out of the Erythian Sea, at the point where it borders Babylonia. He had the whole body of a fish, but above his fish’s head he had another head which was that of a man, and human feet emerged from beneath his fish’s tail. He had a human voice, and an image of him is preserved unto this day. He passed the day in the midst of men without taking food; he taught them the use of letters, sciences and arts of all kinds. He taught them to construct cities, to found temples, to compile laws, and explained to them the principles of geometrical knowledge. He made them distinguish the seeds of the earth, and showed them how to collect the fruits; in short he instructed them in everything which could tend to soften human manners and humanize their laws. From that time nothing material has been added by way of improvement to his instructions. And when the sun set, this being Oannes, retired again into the sea, for he was amphibious. After this there appeared other animals like Oannes.“

Whether this is pure fiction or has any resemblance to historical events does not matter, but it is this story that has given rise to the idea of building what this author is calling a ‘minilithic artefact’ under the Apkallu Initiative as will be discussed further below. As an aside it is worth noting that in his book Intelligent Life in the Universe, written with L. S. Shklovskii (Pan Books, 1977), the astronomer Carl Sagan opened a discussion on the Sumerian civilization with “I came upon a legend which more nearly fulfils some of our criteria for a genuine contact myth”.

On planet Earth we know that species rise up and fall and suffer extinction. The fossil record has shown this for many a species. There are also arguments that Homo Sapiens are not the only occurrence of intelligence on Planet Earth (see for example the recent book Other Minds by Peter Godfrey-Smith’ on the Octopus, William Collins, 2016). Why then is it not possible, in the last million years, that an earlier species of man, or other life form on Earth, could have evolved to similar levels of intelligence to that which we possess today, to include a technological level similar in extent? Such a people would predate modern recorded history, and it is at least plausible that some memory of them could be preserved in the creation mythologies of our various ancient cultures.

Many ancient Megalithic structures have been found by archaeologists around the world. This includes for example the Great Pyramid and the Great Sphinx in Giza (4,500 years old), Tiwanaku and Pumapunku in West Bolivia (3,500 years old), Stonehenge in England (5,000 years old), Machu Picchu in Peru (550 years old) to name a few. However, recently our linear understanding of human evolution from a hunter-gatherer species to an agricultural-farming one has been placed under scrutiny, by the discovery in 1996 of Gӧbekli Tepe, a site in the South eastern Anatolia region of Turkey, which may date back to 12,000 years old. The site demonstrates a superior knowledge of construction techniques, geometry and other disciplines and to enable its construction would have required a food surplus to exist – before the arrival of the Neolithic revolution. In addition, it is arguable that to get to a point where you can construct something like Gӧbekli Tepe would take thousands of years of advancement of knowledge in itself. This might suggest that the builders were 15,000 – 20,000 years old.

A potentially even older site has also been found in West Java, called Gunung Padang, which was discovered in 1914. It may be the largest megalithic site in South Eastern Asia. Radiocarbon dating puts the site at several different eras spanning 6,500 – 20,000 years ago, although the dating claims are controversial among archaeologist in Indonesia. A large structure has also been discovered beneath the surface some 15 m down and includes large chambers. This discovery, and that of Gӧbekli Tepe, is telling us that our linear understanding of history is in need of revision.

Interglacial Periods in Earth’s History

Given the existence of Gӧbekli Tepe and Gunung Padang, the idea that an earlier intelligent and advanced civilization existing on Earth is not so implausible. However, were there opportunities in Earth’s history for this to occur? An examination of climatic conditions would seem to suggest so.

During the history of Earth there have been five major ice ages, and we are currently in the Quaternary Ice Age at this time, which spans from 2.59 million years ago. Within the ice ages are sub-periods known as glacial and interglacial periods.

Recent measurements of the relative Oxygen isotope ratio in Antarctica and Greenland show the periods of glacial and interglacial periods throughout history over the last few hundred thousand years. This is a measurement of the ratio of the abundance of Oxygen with atomic mass 18 to the abundance of Oxygen with atomic mass 16 present in ice core samples, 18O/16O, where 16O is the most abundant of the naturally occurring isotopes. Ocean water is mostly comprised of H216O, in addition to smaller amounts of HD16O and H218O. The Oxygen isotope ratio is a measure of the degree to which precipitation due to water vapour condensation during warm to cold air transition, removes H218O to leave more H216O rich water vapour. This distillation process leads to any precipitation having a lower 18O/16O ratio during temperature drops. This therefore provides a reliable record of ancient water temperature changes in glacial ice cores, where temperatures much cooler than present corresponds to a period of glaciation and where temperatures much warmer than today represents an interglacial period. The Oxygen isotope ratios are therefore used as a proxy for temperature changes by climate scientists.

The Vienna Standard Mean Ocean Water (SSMOW) has a ratio of 18O/16O = 2005.2×10-6, so any changes in ice core samples will be relative to this number. The quantity that is being measured, δ18O, is a relative ratio calculated as in the units of % parts per thousand or per mil. The change in the oxygen ratio is then attributed to changes in temperature alone, assuming that the effects of salinity and ice volume are negligible. An increase of around 0.22% is then defined to be equivalent to a cooing of 1˚C.

There are differences in the value of δ between the different ocean temperatures where any moisture had evaporated at the final place of precipitation. As a result the value has to be calibrated such that there are differences between say Greenland and Antarctica. This does result in some differences in the proxy temperature data based on ice core analysis, and Greenland seems to stand out, such as indicating a more dramatic Younger Dryas period (11,600 – 12,900) than other data.

An analysis of this data shows that the climate has varied cyclically throughout its history and is manifest of natural climate change. In particular what emerges out of the data are some interesting lessons about the recent history of planet Earth. Data shows the rapid oscillations of the climate temperature from the average temperature of today, indicative of glacial and interglacial periods. In particular, the data shows that during the Holocene period, beginning approximately 11,700 years before present, the temperature varied between 2-4 ˚C.

It is reasonable to assume that human civilizations under development will do better when the climate is kinder. This means that the warmer it is the better civilisations will do, and the colder it is, the harder the struggles. In particular we can expect that during the conditions of a colder climate that agricultural farming will suffer, and so there will be less food to go around, which will affect both lifespan and population expansion. To support this it is worth noting that the current epoch, the last 10,000 years has been one of the longest interglacial period for at least the last quarter of a million years and it is reasonable to therefore assume that this is one of the factors which has allowed human development from the emergence of the Neolithic period coming out of the last ice age.

The data also shows that there was a large global warming period known as the Eemian around 115,000 – 130,000 years ago. The average global temperatures were around 22 – 24 ˚C, compared to today where the average is around 14 ˚C. Forests grew as far north as the Arctic circle at 71˚ latitude and North Cape in Norway Oulu in Finland. For comparison North Cape today is now a tundra, where the physical growth of plants is limited to the low temperatures and small growing seasons. Given that homo sapiens may have been here since around 300,000 years ago, this seems like a major opportunity for the development of human society from a people of hunter gatherers to one of agricultural developers and the development of a civil society.

There have been other interglacial periods that have resulted in global temperatures being either equivalent or above the average today, and the data shows temperature spikes of periods at around 200,000 years, 220,000 years, 240,000 years, 330,000 years and 410,000 years. Each of these interglacial periods will typically last at least 10,000 years.

Temperature Proxy Data Showing Opportunities for the Rise of Advanced Civilization in Recent Prehistory

The Apkallu Initiative

It is fully admitted that much of the above contains speculation, but until we have a firmer grasp of history it would be unwise to rule such possibilities out. We turn our attention then to the future and solving the problem of how to preserve human knowledge in the event of a global cataclysm such that humankind can restart again so that within centuries we mature back to similar levels of today’s technological advancement. Ultimately this is a statistical problem, in that by reducing the time of each cycle for maturing to technological capability, one improves the probability of survival. It is sensible to think of this concept as a civilization accelerator.

The Apkallu Initiative is therefore a proposal to construct a minilithic artefact (analogous to Megalithic artefacts) that can survive for a time duration exceeding 100,000 years. This duration is chosen for three principal reasons:

1. The recent ice core records suggest that within that time period there may be several opportunities (~4) where the climatic conditions are sufficiently supportive for human existence to facilitate growth beyond basic survival.
2. It approximately corresponds to four processional cycles of the Earth around the equinoxes, which typically last 25,920 years. We note that many of the ancient Megaliths seem to have been preoccupied with the measurement of the equinoxes; which may relate to lost memory of previous cataclysms.
3. It is difficult to design for an artefact that can survive longer than this, although desirable.

The artefact would be a form of archaeological-architectural device from the standpoint of future humans who uncover it. The device would be replicated perhaps 1,000 times and distributed around the seven continents of the Earth. Ideally, some could also be placed in space, on the Moon or Mars. The idea is that any future human surviving a global cataclysm that finds this artefact and studies it sufficiently, it will give them the knowledge they need to rapidly advance human civilization at an accelerated rate.

Painting illustrating future man finding the archaeological artefact (credit: K. F. Long)

The artefact would be a form of long distance communication. We have of course attempted message plaques in the past such as the Voyager Golden Record and the Pioneer Plaque. Indeed, the Code of Hammurabi from the Sumerian civilization is a form of minilithic artefact, but just specific to moral and legal codes. Another example would have been the tablets for the Biblical Ten Commandments.

There is a question of what materials to construct the artefact from. Plastics and metals will likely degrade over thousands of years. Electronic memory is not useful if it is subject to flip switching and also requires a computer interface to read it. It therefore seems sensible to construct the artefact out of stone; perhaps in a similar manner to the Sumerian Cuneiform on wet clay tablets. One of the options may be Diorite. It would perhaps be useful to depict both logograms, with syllabic and alphabetic elements, as well as phonetics and even determinatives to create appropriate semantic descriptions.

There is a question of what information should the artefact contain. It should contain the foundation knowledge of human civilization. This is a subjective decision. One example we might take lessons from for example was the Trivium (logic, grammar, rhetoric) and the Quadrivium (arithmetic, geometry, music, astronomy) of the classical world. Both were considered preparation work before delving into the study of philosophy and theology. In addition to these, the artefact might contain many other disciplines of thought, such as human biology, medicine, architecture, chemistry, physics, law, history, music, language, agriculture, botany, ethics and other subjects. Experts in appropriate disciplines would need to be consulted to derive the say 12 base foundation knowledge or tenets that govern a field from which in principle all else can be derived given time.

The goal of the information content imprinted onto the artefact would be as follows:

Goal 1: The continued survival of the human species at peace.
Goal 2: The accelerated technological, social-cultural growth of human civilization from an assumed stagnated level.
Goal 3: The preservation of moral and ethical philosophy

There is also a question of what language. One approach would be to take lessons from historical artefacts which contained several languages to ensure future interpretation. This includes the Rosetta Stone (2,200 years old) which contains ancient Egyptian hieroglyphics, demotic and ancient Greek. Another example is the Fuente Magna of the Americas (5,000 years old), found in Bolivia but contains both ancient Pukara and a proto-Sumerian alphabet. Another example is the Behistun inscription (2,500 years old) found in Iran, which contains three different cuneiform script languages, that of Old Persian, Elamite and Babylonian.

There is also the question of the size and shape of the artefact, and although you want it big enough to find, you also want to manage the construction cost of the project. Something around 6 – 12 inches would seem a good optimum size. The exact shape would have multiple surface areas to facilitate different disciplines of knowledge. One idea is a Dodecahedron, which has 12 faces.

The proposal of the Apkallu Initiative is to form a team which then designs and leads the construction of such an artefact. This can then be reproduced and distributed to different locations around the world. Some would eventually be displayed in art galleries or museums and some will be lost to the land and sea, but the hope is that in the event of the cataclysmic scenario described above that future human will stumble across such an artefact, and after studying it, teach their community everything they need to become a civilized and socially-technologically advanced society. Currently no team has been formed, but this article is an initial invitation of interest and anyone interested can contact the web site: https://www.apkalluinitiative.com/

Our ability to become an interstellar capable species depends in the near term on our ability to survive here on Earth or in near-space. The preservation of the deep knowledge and learning of the human experience is critical to this future, if we are to continue to progress, avoid stagnation and decay or even complete extinction or avoid repeating mistakes of the past.

Finally, such a project has the potential to inspire long-term thinking among differing human societies, and so in itself may be a self-perpetuating mechanism toward social-cultural harmonization and increased global awareness of our fragility in the great Cosmos. In addition, because of its interdisciplinary nature, it has the potential to involve all of humanity on its journey, as we jointly work toward a back-up plan to ensure that humanity can survive in the millennia ahead.

The author dedicates this article to the efforts of Graham Hancock and Randall Carlson, whose significant research inspired this initiative. It was written to garner scrutiny of the idea, before deciding whether to proceed or not. Feedback is invited.

Occator Crater Up Close

It’s startling to think that the Dawn spacecraft, now orbiting Ceres at its lowest altitude ever, may have fired its ion engine for the last time. The event occurred by way of positioning the spacecraft for the best possible track near Cerealia Facula, which is a prominent deposit of sodium carbonate in the center of the crater called Occator. Data from the spacecraft’s visible and infrared imaging spectrometer had been used to identify the bright areas called faculae as calcium carbonate deposits earlier in the mission. Vinalia Faculae is in the same area.

“Acquiring these spectacular pictures has been one of the greatest challenges in Dawn’s extraordinary extraterrestrial expedition, and the results are better than we had ever hoped,” said Dawn’s chief engineer and project manager, Marc Rayman, of NASA’s Jet Propulsion Laboratory, Pasadena, California. “Dawn is like a master artist, adding rich details to the otherworldly beauty in its intimate portrait of Ceres.”

Image: A prominent mound located on the western side of Cerealia Facula, in an image obtained by NASA’s Dawn spacecraft on June 22, 2018 from an altitude of about 34 kilometers. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.

You’ll recall the intriguing bright material that began showing up during Dawn’s approach to Ceres, the investigation of which has been a major theme for mission controllers. Imagine if we had a Pluto orbiter to rival what Dawn is doing at Ceres, with the opportunity to map the entire surface, and to delve deeply into the unusual geology on display there. In the case of Ceres, we can now say that the mound above, located at about 19.5 degrees north latitude and 239.2 degrees east longitude, is similar, in JPL’s words, to ‘a mesa or large butte with a flat top.’

Image: In this image of the northern rim of Occator crater landslides can be seen. The image was obtained by NASA’s Dawn spacecraft on 16 June, 2018 from a distance of approximately 33 kilometers. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.

Researchers have concluded that the dome at the center of Occator is the source of saline solutions that evaporated, leaving the bright deposits identified early in the mission (for further background, see this MPS news release). Here and elsewhere in the crater, we are probably seeing vents that allow a mixture of water and salt to rise from a deeper brine reservoir. The new imagery gives us the best differentiation yet between the bright sodium carbonate and the dark background material, allowing us to probe still further the origin of the faculae.

“The data exceed all our expectations,” says framing camera lead investigator Andreas Nathues (Max Planck Institute for Solar System Research, Germany). “We now hope to understand how the bright deposits outside the crater center came about – and what they tell us about Ceres’ interior.”

Bear in mind as well that new gravity measurements may likewise provide details about the subsurface of the dwarf planet. As with New Horizons, analysis of the data trove will take years as the imagery continues to pour in. The faculae of Ceres are the largest deposit of carbonates ever found outside Earth and possibly Mars, leading to the question of how this material was exposed. This JPL news release notes as major possibilities a shallow, subsurface reservoir of mineral-rich water, or a deeper source of brines percolating upward through fractures.

Image: This close-up image of the Vinalia Faculae in Occator Crater was obtained by NASA’s Dawn spacecraft on June 14, 2018 from an altitude of about 39 kilometers. This image reveals the intricate pattern between bright and dark material across this flow feature. The complex structure of the dark background is reminiscent of lava flows observed on Earth. However, in the case of Ceres, the flow material likely involved a lot of ice. The bright material is mostly composed of sodium carbonate, a salt whose exposure onto the crater floor involved a liquid source. The center of this picture is located at about 21.0 degrees north latitude and 241.3 degrees east longitude. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.

Uranus: Orbital Tilt from a Cataclysmic Collision

Yesterday’s post about exoplanet obliquity inevitably brought our own system to mind, with the stark variations between planets like Earth (23 degrees), Uranus (98 degrees) and Mercury (0.03 degrees) serving as stark examples of how wide the variation can be. Thus seasonality has to be seen in context, and interesting questions arise about the effect of high degrees of obliquity on habitability. While thinking about that I received a new paper on Uranus that has bearing on the matter, with its attempt to quantify the ‘hit’ Uranus must once have taken.

After all, something accounts for the fact that the 7th planet spins on its side, its axis at right angles to those of the other planets, its major moons all orbiting in the same plane. Lead author Jacob Kegerreis (Durham University), working with Luis Teodoro (BAERI/NASA Ames) and colleagues modeled 50 different impact simulations in an attempt to recreate the axial tilt of this world. In play were the planet’s internal structure, rotation rate, atmospheric retention post-impact and the composition of materials injected into orbit by the event. Says Kegerreis:

“Our findings confirm that the most likely outcome was that the young Uranus was involved in a cataclysmic collision with an object twice the mass of Earth, if not larger, knocking it on to its side and setting in process the events that helped create the planet we see today.”

Image: This image shows a crescent Uranus, a view that Earthlings never witnessed until Voyager 2 flew near and then beyond Uranus on January 24, 1986. This planet’s natural blue-green color is due to the absorption of redder wavelengths in the atmosphere by traces of methane gas. Uranus’ diameter is roughly 51,000 kilometers, a little over four times that of Earth. Image credit: NASA/JPL/USGS.

The likely impactor was a young protoplanet, striking Uranus during the early era of Solar System formation some 4 billion years ago. That in itself is not surprising, though it’s good that the idea can be firmed up with high-resolution modeling. But we also learn something else. Debris from the impactor may have formed a thin shell near the top of Uranus’ ice layer. This could have the effect of trapping the heat emanating from the planet’s core, a useful finding because it helps to explain the extreme cold of the upper atmosphere (-216 degrees C).

Those temperatures are a puzzle that is exacerbated by the fact that we know so little about the interior of Uranus. The paper points out that surface emission is in equilibrium with solar insolation, which implies that little heat flows out from the planet, and this is in striking contrast with the other giant planets. Thus the idea of a thin thermal boundary layer between the outer envelope of hydrogen and helium and the inner ice-rich layer. The shell theory fits earlier work on the planet’s evolution, and also gives us some ideas about the type of impact.

Looking at where the mass and energy of the impactor are deposited within the planet, the paper focuses on the top of the ice layer, and suggests the impact was a grazing one:

Higher impact parameters can even lead to a temperature inversion near the top of the ice layer. These more-grazing collisions also leave the impactor ice further out, in a thin shell near the edge of the icy mantle, whereas ∼head-on impacts can implant significant ice up to 0.5 R⊕ further inwards and less-isotropically about the centre. These findings may have important implications for understanding the current heat flow (or rather lack thereof) from Uranus’ interior to its surface.

Thus we have a thermal boundary that can, as the paper argues, suppress convection, a kind of blanket to contain heat welling up from within the planet. We can also dig deeper into the planet’s magnetic field. Unlike the terrestrial planets, the Uranian magnetic field appears offset by approximately 0.3 Uranus radii from the center of the planet and tilted by 60 degrees relative to the rotation axis. Other work in the literature has described models that produce such fields using a layer of convecting electrically conducting ices. The impactor modeled in this work could have created lopsided clumps of rock within the planet that explain the offset and tilt.

An impactor of 2 Earth masses blows rock and ice into orbit, where it is available for the formation of Uranus’ current satellites and ring system, giving us constraints on the angular momentum delivered by the collision. All of which is a reminder of how violent a place a young stellar system can be during the era of planet formation, information which should prove useful as we extend what we are learning about our own ice giants to worlds around other stars.

The paper is Kegerreis, “Consequences of Giant Impacts on Early Uranus for Rotation, Internal Structure, Debris, and Atmospheric Erosion,” Astrophysical Journal Vol. 861, No. 1 (2 July 2018). Abstract / preprint.

Probing Exoplanet Obliquity

It’s always a shock for me when the soft air and fecund smells of spring slam into a parched and baked July, but seasonal change is inevitable. At least it is on Earth. We get such seasonal changes because of Earth’s obliquity, the angle of its spin axis relative to the plane of its orbit. For Earth, the angle has stayed pretty close to 23 degrees for a long time, although the tilt’s direction wobbles over cycles of thousands of years. And this very constancy of obliquity turns up in exoplanet discussions at times because it affects conditions on a planetary surface.

Some have argued that without the gravitational effects of the Moon, the tilt of the Earth would be changed by the gravitational pull of the Sun and planets, producing a potentially high degree of obliquity. Contrast our situation with that of Uranus, where we find a 90-degree tilt that leaves one pole in sunlight for half the Uranian year as the other remains in darkness. Without knowing how long the Moon has been able to stabilize Earth’s axial tilt, we can’t say how apparent equatorial ice sheets some 800 million years ago fit into this view of the Moon’s effect.

But obliquity as a factor in habitability continues to energize exoplanetary researchers. At Georgia Tech, a team led by Gongjie Li, working with graduate student Yutong Shan (Harvard-Smithsonian Center for Astrophysics) has developed computer simulations to analyze the spin axis dynamics of two exoplanets, Kepler 186f and Kepler 62f, two planets considered to be in or close to the habitable zone of their stars. The paper argues that without our Moon, Earth’s obliquity variation would range from 0 to 45 degrees over billion-year timescales.

Thus obliquity is an interesting data point. Bear in mind that so far, we have no reliable values for exoplanet obliquity, although ways to infer it from light curves and from high-contrast direct imaging have been proposed in the literature. The authors make the assumption that in both exoplanet systems studied, all planets have been identified. They then go on to study the evolution of the two five-planet systems. The ‘secular analytical framework’ they arrive at allows them to factor in planetary rotation rates, additional planets and satellites, and regions where resonant interactions within the system can produce large obliquity variations. For various realizations of planetary systems, the paper thus describes an ‘obliquity evolution.’

We know that Mars and Earth interact strongly with each other, as do Mercury and Venus; other than Earth, none of these worlds has a large moon. The authors point out that the orientation angle of a planet’s orbit around its host star can be made to oscillate through gravitational interactions. If the orbit oscillates at the same pace as the precession of the planet’s spin axis, large obliquity variations can be induced, the kind of thing our Moon dampens out.

Image: An artist’s depiction of Kepler-62f. Credit: NASA Ames/JPL-Caltech/T.Pyle.

For these two exoplanet systems, we get an interesting result, for even without a stabilizing moon (if none is present), these two planets could be experiencing relatively low changes in their axial tilt:

“It appears that both exoplanets are very different from Mars and the Earth because they have a weaker connection with their sibling planets,” said Li. “We don’t know whether they possess moons, but our calculations show that even without satellites, the spin axes of Kepler-186f and 62f would have remained constant over tens of millions of years. That’s not to say either exoplanet has water, let alone life. But both are relatively good candidates. Our study is among the first to investigate climate stability of exoplanets and adds to the growing understanding of these potentially habitable nearby worlds.”

As Li has just pointed out, we have no knowledge of surface conditions on either of these planets, making the lovely image above nothing more than a guess, and an optimistic one at that. The ‘super Earth’ Kepler 62f, about 40 percent larger and with a mass 2.8 times that of our planet, is in the constellation of Lyra, the outermost of the five planets orbiting a K2-class star some 1200 light years from Earth. Kepler-186f orbits a red dwarf about 550 light years out, part of a five-planet system in the constellation Cygnus. A stable axial tilt would make it likely that both worlds experience regular seasons and thus a stable climate.

But are large obliquity values necessarily inimical to life? Some recent work, considered by the authors, shows that variability in obliquity can keep a planet’s global temperature higher than it would otherwise have been, extending the outer edge of the habitable zone. But it does appear that obliquity variations can produce sharp transitions between climate states. From the paper:

Recently, Kilic et al. (2017) mapped out the various equilibrium climate states reached by an Earth-like planet as a function of stellar irradiance and obliquity. They find that, in this parameter space, the state boundaries (e.g. between cryo- and aqua-planets) are sharp and very sensitive to the climate history of the planet. This suggests that a variable obliquity can easily move the planet across state divisions, as well as alter the boundaries themselves, which would translate into a dramatic impact on instantaneous surface conditions and long-term climate evolution.

Planets with highly irregular seasons aren’t necessarily destined to be lifeless, but if we become capable of determining planetary obliquity, such a value could help us narrow the target list for future space telescopes. The authors also suggest that their framework can provide input parameters for existing global climate models as we analyze habitability in multi-planet systems.

The paper is Shan and Li, “Obliquity Variations of Habitable Zone Planets Kepler-62f and Kepler-186f,” Astronomical Journal Vol. 155, No. 6 (17 May 2018). Abstract / preprint.

The Dipole Drive: A New Concept for Space Propulsion

One reason we look so often at sail technologies in these pages is that they offer us ways of leaving the propellant behind. But even as we enter the early days of solar sail experimentation in space, we look toward ways of improving them by somehow getting around their need for solar photons. Robert Zubrin’s work with Dana Andrews has helped us see how so-called magnetic sails (magsails) could be used to decelerate a craft as it moved into a destination system. Now Zubrin looks at moving beyond both this and solar wind-deflecting electric sails toward an ingenious propellantless solution. Zubrin presented the work at last April’s Breakthrough Discuss meeting, and today he fills us in on its principles and advantages. Read on for a look at a form of enhanced electric sail the author has christened the Dipole Drive.

by Robert Zubrin

Abstract

The dipole drive is a new propulsion system which uses ambient space plasma as propellant, thereby avoiding the need to carry any of its own. The dipole drive remedies two shortcomings of the classic electric sail in that it can generate thrust within planetary magnetospheres and it can generate thrust in any direction in interplanetary space. In contrast to the single positively charged screen employed by the electric sail, the dipole drive is constructed from two parallel screens, one charged positive, the other negative, creating an electric field between them with no significant field outside. Ambient solar wind protons entering the dipole drive field from the negative screen side are reflected out, with the angle of incidence equaling the angle of reflection, thereby providing lift if the screen is placed at an angle to the plasma wind. If the screen is perpendicular to the solar wind, only drag is generated but the amount is double that of electric sail of the same area. To accelerate within a magnetosphere, the positive screen is positioned forward in the direction of orbital motion. Ions entering are then propelled from the positive to the negative screen and then out beyond, while electrons are reflected. There are thus two exhausts, but because the protons are much more massive than the electrons, the thrust of the ion current is more than 42 times greater than the opposing electron thrust, providing net thrust. To deorbit, the negative screen is positioned forward, turning the screen into an ion reflector. The dipole drive can achieve more than 6 mN/kWe in interplanetary space and better than 20 mN/kWe in Earth, Venus, Mars, or Jupiter orbit. In contrast to the electric sail, the ultimate velocity of the dipole drive is not limited by the speed of the solar wind. It therefore offers potential as a means of achieving ultra-high velocities necessary for interstellar flight.

Background

The performance of rockets as propulsion systems is greatly limited by their need to carry onboard propellant, which adds to the mass which must be propelled exponentially as the extent of propulsive maneuvers is increased. For this reason, engineers have long been interested in propulsion systems that require no propellant.

The best known propellantless system is the solar sail, which derives its thrust by reflecting light emitted by the Sun. Solar sails are limited in their performance however, by their dependence upon sunlight, which decreases in strength with the square of the distance, and the laws of reflection, which dictate that the direction of thrust can only lie within 90 degrees of the vector of sunlight. Moreover, because photons move so swiftly, the amount of thrust that can be derived by reflecting light is at best 0.0067 mN/kW (at 100% reflectance, full normal incidence), which means that very large sails, which necessarily must have significant mass and be difficult to deploy, must be used to generate appreciable thrust. As a result, while solar sails have been studied since the time of Tsiolokovsky [1], we are only now beginning to experiment with them in space.

An alternative to the solar sail is the magnetic sail, or magsail, which was first proposed by Zubrin and Andrews in 1988, and subsequently analyzed extensively by them in a variety of further papers [2,3] in the 1990s. The magnetic sail uses a loop of superconducting wire to generate a magnetosphere to deflect the solar wind. Assuming the development of high temperature superconducting wire with the same current density as existing low temperature superconductors, a magsail should be able to generate significantly higher thrust to weight than is possible with solar sails. However such wire has yet to be developed.

Another propellantless propulsion system of interest is the electric sail [4], which like the magsail operates by deflecting the solar wind, in its case by using an electrostatic charge. As a result, like the magsail, the classic electric sail (electric sail) cannot operate inside of a planetary magnetosphere other than as a drag device, has its thrust decrease with distance from the Sun, and is limited in the potential direction of its thrust. Because of the low momentum density of the solar wind, electric sails must be even bigger than solar sails. However, because only sparsely spaced thin wires are needed to create sail area, higher thrust to mass ratios can be achieved than are possible using solar sails which require solid sheets of aluminized plastic.

Electrodynamic tethers [5] have also been proposed, which use the interaction of a current in a tether with the Earth’s geomagnetic field to produce thrust. In addition to facing a variety of engineering and operational issues, however, such systems can only operate in a planetary magnetic field and can only thrust in a direction normal to the field lines, a consideration which limits their applicability.

Finally, we note recent claims for a system called the EM Drive [6], which according to its proponents can generate about 1 mN/kWe, in any direction, without the use of propellant, an external light source or plasma wind, or magnetic field. Such performance would be of considerable interest. However, as it appears to contradict the laws of physics, there is reason to suspect that the measurements supporting it may be erroneous.

As a result, there clearly remains a need for a new type of propellantless propulsion system, which can operate both inside and outside of a planetary magnetosphere, can thrust in a multitude of directions, and which is not dependent upon sunlight or the solar wind as a momentum source. The dipole drive is such a system.

The Dipole Drive

The principle of operation of the dipole drive while accelerating a spacecraft within a planetary magnetosphere is illustrated in Fig. 1 below.

Fig. 1. The Dipole Drive Accelerating within a Magnetosphere.

In Fig. 1 we see two parallel screens, with the one on the left charged positive and the one on the right charged negative. There is thus an electric field between them, and effectively no field outside of them, as on the outside the field of each screen negates the other. There is also a voltage drop between the two, which for purposes of this example we will take to be 64 volts.

Protons entering the field region from the left are accelerated towards the right and then outward through the right-hand screen, after which they escape the field and experience no further force. Protons entering from the right are reflected towards the right, adding their momentum to that generated by the protons accelerated from left to right. There is thus a net proton current from left to right, and a net proton thrust towards the left.

In the case of electrons, the situation is exactly the opposite, with a net electron current from right to left, and a net electron thrust towards the right. Note that while electrons entering from the right will be greatly accelerated by the field, reflected electrons will only be reflected with their initial velocity. There will also be an electron current through the outside plasma to neutralize the net proton flow to the right.

Because space plasmas are electrically neutral, the number density of both electrons and ions (which for the moment we will consider to be protons, but may which – advantageously – be heavier species, as we shall discuss later) will be the same, so the proton and electron electrical currents will be equal, as will the power associated with each of them. However because the mass of a proton is about 1842 times as great as the mass of an electron, the thrust of the proton current will be about 43 times greater than the opposing electron current thrust (because the momentum of particles of equal energy will scale as the square root of their mass, sqrt(1842)=43) and the system will generate a net thrust. The acceleration of the electrons is a form of drag, which is provided for by loss of spacecraft kinetic energy. It therefore could, in principle be used to generate electric power, partially compensating for the power consumed to accelerate the protons. In the following examples, however, we will assume that there is no provision for doing this, i.e. that the efficiency of any such energy recovery is zero.

To see what the performance of a dipole drive might be, let us work an example, assuming a 500 W power source to drive the system. The electron current negates about 2% of the thrust (1/43rd) produced by the proton current. The maximum possible jet power is thus about 490 Wj. Assuming additional inefficiencies, we will round this down to 400 Wj, for a total system electrical to jet power efficiency of 0.8.

A Coulomb of protons has a mass of 0.011 milligrams. If the jet power is 400 W, and the potential difference is 64 V, so the proton current will be 6.25 A, and have a mass flow of 0.0652 mg/s.

The relationship of jet power (P) to mass flow (m) and exhaust velocity (c) is given by:

P = mc2/2                                                                         (1)

Taking P = 400 W and m = 0.0652 mg/s, we find that c= 110,780 m/s. Since thrust (T) is given by T=mc, we find:

T = mc = 7.2 mN                                                            (2)

This is a rather striking result. It will be recalled that the electrical power driving this system is 500 W. So what we are seeing here is thrust to power ratio of 14.4 mN/kWe, more than ten times better than that claimed for the EM Drive, but done entirely within the known laws of physics!

If it is desired to deorbit (decelerate) a spacecraft, the direction of the screens would be reversed, with the negative screen leading in the direction of orbital motion. In this case, the screens would become a proton reflector. An electric sail could also be used as a drag device to serve the same purpose. However, because the dipole drive doesn’t merely create drag against passing protons, but reflects them, it would create twice the drag of an electric sail of the same area. If the dipole drive is positioned obliquely to the wind angle, it can reflect protons, with the angle of incidence equaling the angle of reflection. For example, if it is tilted 45 degrees to the wind, a force will be generated perpendicular to the wind, that is “lift” will be created. Such maneuvers could also be done with the dipole drive in acceleration mode, deflecting protons to combine lift with thrust. Using this capability, a dipole drive propelled spacecraft in orbit around a planet could execute inclination changes.

To summarize, in contrast to the electric sail which can only create drag against the wind to lower its orbit, the dipole drive can thrust in any direction, raising or lowering its orbit or changing its orbital inclination. In addition, when used as a drag device, the dipole drive can create twice the drag per unit area as the electric sail.

The Dipole Drive in Planetary Orbit

Let us therefore analyze the system further. The dipole drive exerts no field outside of its screens, so the only plasma it collects is the result of its own motion through the surrounding medium. So how big does its screen need to be?

We consider first the case of the above described dipole drive system operating in LEO at an altitude of 400 km, being used to thrust in the direction of orbital motion. It is moving forward at an orbital velocity of 7760 m/s. The average density of ions at this altitude is about 1,000,000 per cc. Assuming (conservatively) that all the ions are protons, the required ion mass flow of 0.0652 mg/s would be swept up by a screen with a radius of 127 m.

It may be noted however, that at 400 km altitude there are also O+ ions, each with a mass 16 times that of a proton, with a numerical density of about 100,000/cc. These therefore more than double the ion mass density provided by the protons alone. If these are taken into account, the required scoop radius would drop to about 80 m.

Another way to reduce the scoop size would be by going to higher voltage, so that more power can be delivered to a smaller number of ions. If, for example, we quadrupled the voltage to 256 volts, the exhaust velocity would double, to 222 km/s, allowing us to cut the mass flow by a factor of four, and the scoop radius by a factor of two, to just 40 m. The thrust, however, would be cut in half, giving us 3.6 mN/kWe.

As we go up in altitude, the plasma density decreases, as does the orbital velocity, requiring us to go to larger scoops. Examples of 500 W dipole drive systems operating at a variety of altitudes are provided in Table 1. In Table 1, Vo and C are orbital velocity and exhaust velocity, in km/s.

Table 1. Dipole Drive Systems Operating in Earth Orbit (Power=500 W)

It can be seen that the dipole drive is a very attractive system for maneuvering around from LEO to MEO orbits, as the high ion density makes the required scoop size quite modest. It should be emphasized that the above numbers are for a 500 W system. If a 5 W dipole drive thruster were employed by a microsatellite, the required scoop areas would be reduced by a factor of 100, and the radius by a factor of 10.

It may be noted that Mars, Venus and Jupiter all have ion densities in low orbit comparable to those above. For example, Mars has 500,000/cc at 300 km, Venus has 300,000/cc at 150 km, and Jupiter has 100,000/cc at 200 km, making the dipole drive attractive for use around such planets as well. Many of the moons of the outer planets also have ionospheres, and the dipole drive should work very well in such environments.

As one ascends to higher orbits, the density of ions decreases dramatically, while the orbital speed decreases as well. For example, in GEO, the ion density is only about 20/cc, while the orbital velocity is 3 km/s. These two factors combine to make much larger scoops necessary. So, for example, in GEO, a 500 W dipole drive operating at 1024 volts would need a scoop 3.6 km in radius.

Because the effectiveness of the dipole drive decreases at higher altitudes while operating within the magnetosphere, the best way for a dipole drive propelled spacecraft to escape the Earth is not to continually thrust, as this would cause it to spiral out to trans GEO regions where it would become ineffective. Rather, what should be done is to only employ it on thrust arcs of perhaps 30 degrees around its perigee, delivering a series of perigee kicks that would raise its apogee on the other side of its orbit higher and higher until it escaped the magnetosphere and became able to access the solar wind.

The Dipole Drive in Interplanetary Space

The dipole drive can also operate in interplanetary space. Compared to planetary orbit, the ion densities are lower, but this is partially compensated for by much higher spacecraft velocities relative to the plasma wind. As a result, the required scoop sizes are increased compared to planetary orbital applications, but not by as much as considerations of ion density alone might imply.

Let us consider the case of a dipole drive traveling in heliocentric space at 1 AU, positioned at an angle of 45 degrees to the wind, with its negative screen on the sunward side. It would thus reflect solar wind protons 90 degrees, thereby accelerating itself forward in the direction of orbital motion. A diagram showing the dipole drive operating as a sail in interplanetary space is shown in Fig. 2.

Fig. 2 The Dipole Drive Operating as a Sail in Interplanetary Space.

The solar wind has a velocity of 500 km/s, so to insure reflection, we employ a voltage of 2028 volts, sufficient to reverse the motion of a proton moving as fast as 630 km/s. With a density of 6 million protons per cubic meter, the wind has a dynamic pressure of 1.25 nN/m2. As the sail is positioned 45 degrees obliquely to the wind, its effective area will be reduced by a factor of 0.707, with the thrust reduced to 0.9 nN/m2. In this case, virtually all of the protons hitting the sail will be coming from the sunward side, and since they are reflected without adding any kinetic energy, no power is required to drive them. However, we still have an electron current coming from the sunward side being accelerated outward. This requires power. With 500 W, total radial thrust would be 1.27 mN, with 1.27 mN also delivered in the direction of orbital motion, for a L/D ratio of 1. The total effective screen area would therefore need to be 1,414,000 m2, with an actual area of 2,000,000 m2, requiring a radius of 798 m. Total thrust to power would be 3.6 mN/kWe.

If instead we had not concerned ourselves with obtaining complete deflection of each particle, we could have used a lower voltage. This would increase the thrust per unit power, but increase the required sail area for a given amount of thrust. So, for example, if we chose 512 volts, we would have a total thrust of 3.6 mN, for a thrust/power ratio of 7.2mN/kWe, but need a sail radius of 1127 m.

It may be noted that all of these results are for a 500 W dipole drive. A microsatellite might employ a 5 W dipole drive, in which case the required scoop radii would drop by a factor of 10.

The thrust and diameter of a 1 kWe dipole drive system operating as a solar wind sail in interplanetary space at 1 AU is shown in fig. 3.

Fig. 3. Thrust and Diameter of a 1 kWe dipole drive system operating as a solar wind sail in interplanetary space.

Use of the Dipole Drive for Interstellar Flight

In contrast to the electric sail, the dipole drive can be used to accelerate a spacecraft at velocities greater than that of the solar wind. For example, consider a spacecraft moving away from the Sun at a velocity of 1000 km/s. The solar wind is following it at a velocity of 500 km/s, so relative to the spacecraft there is a wind moving inward towards the sun at a velocity of 500 km/s. In this case, to accelerate the spacecraft would direct its positive screen away from the sun. This would cause it to accelerate protons sunward, while reflecting electrons outward, for a net outward thrust. At 500 km/s the protons are approaching the spacecraft with a kinetic energy equal to 1300 volts. It can be shown that employing a screen voltage difference that is about triple the kinetic voltage produces an optimal design for an accelerating system, while one using a voltage difference equal to the kinetic voltage is optimal for deceleration. This is illustrated in figs 4 and 5 which respectively show the kinetic voltage as a function of velocity, and the relative power/ thrust and area/thrust ratios of the spacecraft as a function of the dimensionless parameter Z, where Z=(engine voltage)/(kinetic voltage.)

Fig 4. Kinetic Voltage as a function of spacecraft velocity.

Fig 5. Relative Power/Thrust and Area/Thrust as a function of Z=(engine voltage)/(kinetic voltage.) There is a step factor of 2 increase in thrust during deceleration when Z reaches 1, because protons are reflected. For acceleration, Power/Thrust ~ 1 + sqrt(1+Z), while Area/Thrust ~ 1/(-1 + sqrt(1+Z)).

If we add 3900 volts to the incoming protons, quadrupling their energy, we will double their velocity relative to the spacecraft, thereby providing an effective exhaust velocity of 500 km/s. The solar wind has a density of 6 million protons/m3 at 1 AU, with ambient density decreasing to 1 million/m3 in interstellar space. If we take the former value, we get a thrust of (1.67e-27 kg/proton)(500,000m/s)2(6,000,000/m3) = 2.5 nN/m2. If we take the latter value, it would be 0.42 nN/m2. The proton current at the smaller value would be 80 nA/m2, which at 3900 volts works out to 0.312 mW/m2. The thrust to power ratio would therefore be 1.35 mN/kW. (This ratio would also hold true at the 1 AU value, but the magnitudes of both the thrust and power per unit area would be six times greater.)

If a dipole drive powered spacecraft were receding 500 km/s directly away from the Sun, it would see no relative wind and thus produce no thrust. However, like a modern sailboat that can sail faster crosswind than downwind, because it can generate lift, the dipole drive can get to speeds above 500 km/s by sailing across the wind. As the spacecraft’s crosswind speed increases, it becomes advisable to turn the sail to ever greater angles to the solar wind and increasingly normal to the crosswind. As this occurs, the L/D resulting from solar wind reflection increases while the total solar wind thrust decreases. At the same time, however, thrust resulting from the acceleration through the screens of crosswind protons increases, maintaining total thrust constant at ever higher L/D (relative to the solar wind) levels. Once the crosswind velocity exceeds the solar wind velocity the solar wind becomes increasingly irrelevant and the dipole drive becomes a pure acceleration system, driving the incoming crosswind plasma behind it to produce thrust,

As the speed of the spacecraft increases relative to the wind, it is necessary to increase the voltage in order maintain thrust/power ratio efficiency. For example, let’s say we want to achieve 3000 km/s, or 0.01c. Then the kinetic energy equivalent voltage of the approaching protons would be 47 kV. So, to double this velocity we need to quadruple the total voltage, or add a sail voltage drop of 141 kV. The proton current would have a value of 480 nA/m2, with a power of 68 mW/m2. The thrust would be 15.1 nN/m2, for a thrust to power ratio of 0.22 mN/kW.

It may be observed that since the necessary voltage increases as the square of the velocity, with power increasing with voltage but thrust increasing with velocity, the thrust to power ratio of the dipole drive decreases linearly with velocity. This puts limitations on the ultimate velocity achievable. For example, the most optimistic projections for advanced large space nuclear power systems project a mass to power ratio of 1 kg/kW. If we accept this number, then, neglecting the mass of any payload or the dipole drive system itself, then the system described in the previous paragraph performing with a thrust to power ratio of 0.22mN/kilowatt at 3000 km/s would have an acceleration of 0.00022m/s2, or 7 km/s per year. The average acceleration getting up to 3000 km/s would be twice this, so the spacecraft would take 214 years to reach this speed. During this time it would travel 1.07 light years. To reach 6000 km/s (0.02 c) starting from negligible velocity would require 857 years, during which time the spacecraft would travel 8.57 light years. The performance of such a system is shown in Table 2. Note 63,000 AU = 1 light year. The performance shown assumes an advanced 1 kg/kWe power supply. If a more near-term power system with a higher mass/power is assumed, the time to reach any given distance increases as the square root of the mass/power ratio. So for example, if we assume a conservative near-term space nuclear power reactor with a mass/power ratio of 25 kg/kW, the time required to reach any given distance would increase by a factor of 5.

Table 2. Advanced Dipole Drive Performance for Ultra High-Speed Missions (1 kg/kW power)

It can be seen that advanced dipole drive spacecraft could be quite promising as a method of propulsion for missions to near interstellar space, for example voyages to the Sun’s gravitational focus at 550 AU. Unless much lighter power systems can be devised than currently anticipated however, they would still require centuries to reach the nearest stars. Power beaming may provide an answer. However such technologies are outside the scope of this paper.

If a spacecraft has been accelerated to interstellar class velocities, whether by means of the dipole drive or any alternative technology, the dipole drive provides a means of deceleration without power (it could actually generate power) by creating drag against the relative plasma wind. This feat can also be done by a magnetic sail or an electric sail. However because it can also create lift as well as drag, the dipole drive offers much greater maneuverability during deceleration as well as a means to freely maneuver within the destination solar system after arrival.

Dipole Drive Design Issues

Let us consider the case of a 2 kg microsatellite operating in LEO, with 5 W of available power to drive a dipole drive. (Note, a typical CubeSat has a mass of 1.3 kg. At 20 kg/kWe, a 5 W solar array should have a mass of about 0.1 kg.) If we operate it with a voltage of 16 Volts, it will produce 28.8 mN/kWe, or 0.144 mN thrust over all. It would have an acceleration of 0.000072 m/s2. This would allow it to generate a ΔV of 2288 m/s in a year, sufficient to provide extensive station keeping propulsion, substantially change its inclination, or to raise it from a 400 km altitude orbit to a 700 km orbit in 1.6 months. To generate this much thrust at 400 km would require a scoop with a radius of 16 m, while doing so at 700 km would require a scoop with a radius of 58 m. Let us assume that the scoop is made of aluminum wire mesh, using wires 0.1 mm in diameter separated by distances of 2 m. Each square meter of mesh would thus have about 1 m length of wire. This needs to be doubled as there are two meshes, one positive and one negative. Therefore, a scoop with a radius of 16 m would have a mass of 32 grams. If the propulsion system were used simply for station keeping, inclination change, or deorbit functions at the 400 km altitude, that’s all that would be needed. To operate at 700 km, a 116 gram scoop would be required. From these examples we can see that the use of the dipole drive to provide propulsion for microsatellites in LEO could potentially be quite attractive, as the modest scoop sizes required do not pose major deployment challenges.

Now let us consider a 100 kg interplanetary spacecraft in interplanetary space, operating with 500 W at a voltage of 2028 volts. From the discussion above it can be seen that this would generate about 2.54 mN of thrust in the direction of orbital motion. The scoop would need to have a radius of about 800 m. In interplanetary space, the Debye shielding length is ~60 m, and so a screen with a 20 m mesh would suffice. Such a screen would have a mass of about 8.5 kg, which would be well within the spacecraft mass budget. The 2.54 mN thrust would accelerate the spacecraft at 0.000025 m/s2. It could thus impart a V to the spacecraft of about 804 m/s per year. Higher accelerations could be provided by increasing the spacecraft power to mass ratio.

The deployment of large scoops composed of two parallel, oppositely charged meshes poses operational and design issues. Prominent among these is the fact that the two opposite charged screens will attract each other. However the total force involved is not that large. For example, let us consider a configuration consisting to two sails of 500 m radius separated by 500 m with a 2 kV potential difference. Then the electric field between them will be 4 volts/m. The area of each screen will be 785,400 m2. From basic electrostatics we have EA = Q/ε, so Q, the charge of each screen will be given by Q=(4)(785,400)(8.85 e-12) = 0.000028 coulombs. The electrostatic force on each sail is given by F=QE, so the total electrostatic force of each sail will be 0.1 mN. This is about a tenth the thrust force exerted by the screens themselves. Nevertheless, as small as they are, both of these forces will need to be negated. This can be done either with structural supports or by rotating the spacecraft and using artificial gravity to hold the sails out perpendicular to the axis of rotation. An alternative is to use the self-repulsion of the charge of each sail to help hold it out flat. In such a configuration two sails held separate from each other by a boom attached to their centers could be expected to curve towards each other at their edges until the stiffening self-repulsive force on each sail from its own charge balanced the bending forces exerted by the spacecraft’s acceleration, the push of the wind, and the attractive force of the opposite sail.

One way to avoid such issues would be to design the system as a literal dipole, with a rod holding a positive charge at its end to the front of the spacecraft, and a rod holding the negative charge pointing to the rear of the spacecraft. Seen from a distance, such a configuration is electrically neutral and would exert negligible field. However, in the zone between the charges, there is a strong field from one pole to the other. Particles entering this field along the rod center lines would experience the full voltage drop. Particles entering the field at some distance from this central axis would experience a lower voltage drop. The overall functional voltage of such a system, from the point of view of power consumption and exhaust velocity, would be an average over many particles entering the dipole field at all distances from its axis. This is obviously a more complex configuration to analyze than that of the two parallel screens discussed so far, but it may be much simpler to implement in practice on an actual spacecraft.

A critical issue is the material to be used to create the dipole drive. In his original paper on the classic electric sail [4], Pekka Janhunen suggested using copper wires with diameters between 2.5 and 10 microns. This is not an optimal choice, as copper has a much lower strength to mass ratio than aluminum, and such thin strands would be quite delicate. For this reason, in the above examples we specified aluminum wire with 100-micron diameters. A potentially much better option, however, might be to use aluminized Spectra, as spectra has about 10 times the yield strength of aluminum, and roughly 1/3 the density (Aluminum 40,000 psi, 2700 kg/m3, compared to Spectra 400,000 psi, 970 kg/m3.). Spectra strands with 100-micron diameters and a coating of 1 micron of aluminum could thus be a far superior material for dipole drive system, and classic electric sails as well. An issue however is Spectra’s low melting point of 147 C. Kevlar, however, with a yield strength of 200,000 psi, a density of 1230 kg/m3, and a melting point of 500 C could provide a good compromise. Still another promising option might be aluminized strands made of high strength carbon fiber, such as the T1000G (924,000 psi, 1800 kg/m3) produced by Toray Carbon Fibers America.

Some options for dipole drive spacecraft configurations are show in in Fig. 6. As can be seen, small dipole drive systems can be used for spacecraft control, for example as an empennage. Such small dipole drive units could also be used for attitude control on non-dipole drive spacecraft, such as solar sails.

Fig. 6. Options for dipole drive spacecraft configuration. Small dipole drive systems can be used for attitude control.

As with the electric sail, the dipole drive must deal with the issue of sail charge neutralization caused by the attraction of ambient electrons to the sail’s positive screen. In reference 4, P. Janhunen showed that the total such current that an electric sail would need to dispose of would be modest, entailing small power requirements if ejected from the spacecraft by a high voltage electron gun. In the case of the dipole drive, the current would be still smaller because the spacecraft has no net charge. In addition electrons acquired by the positive screen could be disposed of by using the power source to transport them to the negative screen. Alternatively, if an electron gun were used, its required voltage would be less than that needed by an electric sail because external to the screens, the dipole drive’s field is much weaker and falls off much more quickly. For these reasons, the issue of sail charge neutralization on the dipole drive should be quite manageable.

Because the dipole drive does not interact with plasma outside of the zone between its screens, the issue of Debye shielding of its screen system to outside charges is not a concern. Debye shielding of its individual wires within screens can be dealt with by means of adequately tight wire spacing. As shown by Janhunen [4], such spacing may be quite liberal (~60 m in near Earth interplanetary space), enabling sails with very low mass to area ratios. [7]

Conclusion

The dipole drive is a promising new technological concept that offers unique advantages for space propulsion. Requiring no propellant, it can be used to thrust in any direction, and both accelerate and decelerate spacecraft operating within planetary magnetospheres, in interplanetary space, and interstellar space. Unlike magnetic sails and electric sails, it can generate both lift and drag, and its maximum velocity is not limited by the speed of the solar wind. Near-term dipole drives could be used to provide a reliable, low cost, low mass technology to enable propellantless movement of spacecraft from one orbit to another, to provide station keeping propulsion, or to deorbit satellites, as required. Then dipole drive could also be used as a method of capturing interplanetary spacecraft into orbit around destination planets, or of lowering the orbits of spacecraft captured into initial elliptical orbits using high thrust propulsion. The latter application is particularly interesting, because it could enable a small lightweight lunar ascent vehicle to carry astronauts home from the Moon by launching directly from the lunar surface to trans-Earth injection and then subsequently lower itself to LEO to rendezvous with a space station or reentry capsule spacecraft without further use of propellant. Such an approach could potentially reduce the mass of a manned lunar mission to within the launch capacity of a single Falcon Heavy. Because it needs no propellant, the dipole drive offers the unique advantage of being able to provide its propulsion service to any spacecraft indefinitely. While the dipole drive is most attractive in orbital space whether ambient plasma is thickest, it can be used in interplanetary space and even enable interstellar missions as well, becoming more attractive for such applications as ancillary technologies, such as power generation evolve.

There are many technical issues that need to be resolved before practical dipole drive spacecraft can become a reality. However both the theory of dipole drive operation and it potential benefits are clear. Work should therefore begin to advance it to flight status. The stars are worth the effort.

References

1. Jerome Wright (1992), Space Sailing, Gordon and Breach Science Publishers

2. D. G. Andrews and R. Zubrin, “Magnetic Sails and Interstellar Travel”, IAF-88-553, 1988

3. R. Zubrin and D.G Andrews, “Magnetic Sails and Interplanetary Travel,” AIAA-89-2441, AIAA/ASME Joint Propulsion Conference, Monterey, CA July 1989. Published in Journal of Spacecraft and Rockets, April 1991.

4. Pekka Janhunen, “Electric Sail for Spacecraft Propulsion,” J. Propulsion, Vol. 20, No. 4: Technical Notes, pp763-764. 2004.

5. Cosmo, M.L., and Lorenzini, E.C., Tethers in Space Handbook, NASA Marshall Space Flight Center, 1997

6. D. Hambling, “The Impossible EM Drive is Heading to Space,” Popular Mechanics, September 2, 2016.

7. “Debye Length,” Plasma Universe.com, https://www.plasma-universe.com/Debye... accessed Feb 18, 2018.