Paul Gilster's Blog, page 53

A Useful Nearby Super-Earth

Gliese 486b is, in the words of astronomer Ben Montet, “the type of planet we’ll be studying for the next 20 years.” Montet (University of New South Wales) is excited about this hot super-Earth because it’s the closest such planet we’ve found to our own Solar System, at about 26 light years away. That has implications for studying its atmosphere, if it has one, and by extension sharpening our techniques for atmospheric analysis of other nearby worlds. The goal we’re moving toward is being able to examine smaller rocky planets for biosignatures.

But we’re not there yet, and what we have in Gliese 486b is an exoplanet that has now been identified as a prime target for future space- and ground-based instruments, one that, given its proximity, is an ideal next step to push our methods forward. The paper on this work shows that two techniques can be deployed here, the first being transmission spectroscopy, when this transiting world passes in front of its star and starlight filters through the atmosphere.

So-called emission spectroscopy happens when the planet orbits around to the other side of the star, making parts of the illuminated surface visible (think phases of the Moon as an analogy). Astronomers can deploy spectrographic tools in both methods to work out the chemical composition of the atmosphere, and according to Montet, Gliese 486b is the best single planet yet found for emission spectroscopy out of all the rocky planets we know. Moreover, says the astronomer, it’s the second best for transmission spectroscopy.

I asked Dr. Montet about this, wondering about the absolute best planet for transmission spectroscopy. His reply:

The best planet for transmission spectroscopy is TRAPPIST-1 b. Our new planet is #2, and the third best is L98-59 d, a planet discovered by TESS in 2019. We’re quantifying relative goodness using the Transmission Spectroscopy Metric from Eliza Kempton’s work in 2018.

Image: The graph illustrates the orbit of a transiting rocky exoplanet like Gliese 486b around its host star. During transit, the planet obscures the stellar disk. Simultaneously, a tiny portion of the starlight passes through the planet’s atmospheric layer. While Gliese 486b continues to orbit, parts of the illuminated hemisphere become visible like lunar phases until the planet vanishes behind the star. Credit: © MPIA graphics department.

430 degrees Celsius make Gliese 486b a nightmarish place, perhaps one with rivers of lava and, moreover, gravity that is 70 percent stronger than Earth’s. The planet orbits its star, an M-dwarf, every 36 hours. We can only imagine what an orbit this tight means for flares and coronal mass ejections on the surface, and it’s likely that the atmosphere itself could be threatened. What we find here in future studies will help us calibrate atmospheric survival and composition on planets orbiting red dwarfs.

The work on Gliese 486b comes out of the CARMENES Consortium, which is leading an effort that includes over 200 scientists and engineers from eleven institutions in Spain and Germany who have designed the 3.5 meter telescope at Calar Alto in southern Spain. The purpose is to monitor 350 M-dwarf stars in search of low-mass planets using a spectrograph mounted on the instrument. The word CARMENES is actually an acronym, and one that takes us into epic territory in terms of length: Calar Alto high-Resolution search for M dwarfs with Exoearths with Near-infrared and optical Échelle Spectrographs.

So far, what we know about Gliese 486b — using transit photometry and radial velocity spectroscopic data from a variety of Earth-based instruments as well as space-based TESS — is that it is about 2.8 times as massive as the Earth and about 30 percent larger. Calculating from mass and radius measurements, the astronomers on the team led by Trifon Trifonov (Max Planck Institute for Astronomy) find a mean density that indicates a rocky world with a metallic core, and as mentioned above, a gravitational pull about 70 percent stronger than Earth’s.

But is this planet’s tight orbital distance (2.5 million kilometers) too close for any atmosphere to survive? The paper makes it clear that this is possible but by no means certain:

With a radius of 1.31 RE, Gliese 486b is located well below the radius range of 1.4 to 1.8 RE, under which planets are expected to have lost their primordial hydrogen-helium atmospheres owing to photoevaporation processes. It remains unknown how stellar irradiation and planet surface gravity affect the formation and retention of secondary atmospheres.

Which makes this an interesting test case, because the numbers are provocative:

Planets with Teq > 880 K, such as 55 Cancri e, are expected to have molten (lava) surfaces and no atmospheres, except for vaporized rock. Gliese 486 b is not hot enough to be a lava world, but its temperature of ~700 K makes it suitable for emission spectroscopy and phase curve studies in search of an atmosphere. Our orbital model constrains the secondary eclipse time to within 13 min (at 1σ uncertainty), which is necessary for efficient scheduling of observations. Compared with other known nearby rocky planets around M dwarfs, Gliese 486 b has a shorter orbital period and correspondingly higher equilibrium temperature of ~700 K and orbits a brighter, cooler, and less active stellar host.

Image: The diagram provides an estimate of the interior compositions of selected exoplanets based on their masses and radii in Earth units. The red marker represents Gliese 486b, and orange symbols depict planets around cool stars like Gliese 486. Grey dots show planets hosted by hotter stars. The coloured curves indicate the theoretical mass-radius relationships for pure water at 700 Kelvin (blue), for the mineral enstatite (orange), for the Earth (green), and pure iron (red). For comparison, the diagram also highlights Venus and the Earth. Credit: Trifonov et al./MPIA graphics department.

We’re going to be learning a lot more about Gliese 486b as the effort to investigate it continues. How well rocky planets retain their atmospheres under extreme conditions will help us understand possible atmospheric processes going on in their stars’ presumably more clement habitable zones. Given their ubiquity, red dwarfs could be interesting places to look for life, but as this planet shows us, that investigation is in its early stages. For now, hot super-Earths are the best way to proceed.

The paper is Trifonov et al., “A nearby transiting rocky exoplanet that is suitable for atmospheric investigation,” Science Vol. 371, Issue 6533 (5 March 2021), pp. 1038-1041 (abstract).

Delivery Mechanism? Comet Catalina Shows Abundance of Carbon

Were the rocky worlds of the inner Solar System depleted in carbon as they formed, the so-called ‘carbon deficit problem’? There is evidence for a system-wide carbon gradient in that era, which makes for interesting interactions between our Sun’s habitable zone and the far reaches of the system, for as the planets gradually cooled, the carbon so necessary for life as we know it would have been available only far from the Sun.

How much of a factor were early comets in bringing carbon into the inner system? This question underlies new work by Charles Woodward and colleagues. Woodward (University of Minnesota Twin Cities / Minnesota Institute of Astrophysics) focuses on Comet Catalina, which was discovered in early 2016. He sees carbon in the context of life:

“Carbon is key to learning about the origins of life. We’re still not sure if Earth could have trapped enough carbon on its own during its formation, so carbon-rich comets could have been an important source delivering this essential element that led to life as we know it.”

Image: Illustration of a comet from the Oort Cloud as it passes through the inner Solar System with dust and gas evaporating into its tail. SOFIA’s observations of Comet Catalina reveal that it is carbon-rich, suggesting that comets delivered carbon to the terrestrial planets like Earth and Mars as they formed in the early system. Credit: NASA/SOFIA/ Lynette Cook.

Let’s zoom in on this a little more closely. Volatile ices of water, carbon monoxide and carbon dioxide are found mixing with dust grains in the outer system, an indication that the young Solar System beyond the snowline was, in the authors’ words, “not entirely ‘primordial’ but was ‘polluted’ with the processed materials from the inner disk, the ‘hot nebular product.'” Or to slip the metaphor slightly, we can say that comets were salted with materials that were originally produced at higher temperatures. Comets can offer a window into this process.

The work is anything but straightforward, for although we’ve learned a lot through missions like Giotto, Rosetta/Philae and Deep Impact (including, of course, abundant telescope observations from Earth and a sample return mission called Stardust), the interplanetary dust particles we’ve been able to analyze from comets 81P/Wild 2 and 26P/Grigg-Skjellerup differ considerably. The paper explains:

The former contains material processed at high temperature (Zolensky et al. 2006), while the latter is very “primitive” (Busemann et al. 2009). For these reasons, it is necessary to determine as best as we can the properties of dust grains from a large sample of comets using remote techniques (Cochran et al. 2015). These include observations of both the thermal (spectrophotometric) and scattered light (spectrophotometric and polarimetric). The former technique provides our most direct link to the composition (mineral content) of the grains.

The research team drew on data from the Stratospheric Observatory for Infrared Astronomy (SOFIA), a Boeing 747 aircraft carrying a 2.7-meter reflecting telescope with an effective diameter of 2.5 meters. At altitude (SOFIA generally operates between 38,000 and 45,000 feet), the observatory is above 99 percent of Earth’s atmosphere, which can block infrared wavelengths. SOFIA data show Catalina as a carbon-rich object.

The paper points out that carbon dominates as well in other comets we’ve seen, both those in closer orbits (103P/Hartley 2) and Oort Cloud comets like C/2007 N3 and C/2001 HT50. It also turns out that dusty material from comet 67P/Churyumov–Gerasimenko was rich in carbon, although the authors note that comets can show changes in their silicate-to-carbon ratio, sometimes even during the course of a single night’s observations. The paper adds:

A dark refractory carbonaceous material darkens and reddens the surface of the nucleus of 67P/Churyumov–Gerasimenko. Comet C/2013 US10 (Catalina) is carbon rich. Analysis of comet C/2013 US10 (Catalina)’s grain composition and observed infrared spectral features compared to interplanetary dust particles, chondritic materials, and Stardust samples suggest that the dark carbonaceous material is well represented by the optical properties of amorphous carbon. We argue that this dark material is endemic to comets.

All this suggests that carbon delivered by comets is a part of the evolution of the early Solar System. Each carbon-rich comet we study has implications for how life may have been spurred by impacts, making the investigation of carbon-rich Oort Cloud comets a continuing priority for SOFIA, which can be deployed quickly when comets are found entering the inner system.

The paper is Woodward et al, “The Coma Dust of Comet C/2013 US10 (Catalina): A Window into Carbon in the Solar System,” The Planetary Science Journal (2021). Abstract / Full Text.

A Method for Creating Enormous Space Telescopes

As we follow the progress of the James Webb Space Telescope through performance tests in preparation for launch, Robert Zubrin has been thinking of far larger instruments. The president of Pioneer Astronautics and founder of the Mars Society thinks we can create telescopes of extremely large aperture — and sharply lower cost — by using the physics of spinning gossamer membranes, a method suitable for early testing as a CubeSat demonstration mission. In today’s essay, Dr. Zubrin explains the concept and considers how best to deploy next generation space telescopes reaching apertures as large as 1000 meters. We can’t know what new phenomena such an instrument would find, but the Enormous Space Telescope fits the theme of breakthrough discovery outlined in his latest book, The Case for Space: How the Revolution in Spaceflight Opens Up a Future of Limitless Possibility (Prometheus, 2020).

by Robert Zubrin

Abstract

This paper presents a method for creating Enormous Space Telescopes (ESTs). The EST employs a hoop to deploy a slack reflector membrane, such as solar sail material or radio dish. When the EST is simultaneously rotated around its center and accelerated along its axis of rotation, the membrane will assume a parabolic shape, thereby creating a reflector for a very large aperture telescope. The EST reflector can be accelerated along its linear axis by tethering its deployment hoop to a tug spacecraft. The tug can exert force on the hoop several methods, including direct thrust, centrifugal rotation of the tethered tug-reflector assembly, or by lowering the reflector from a high altitude balloon or more massive tug positioned in a higher orbit. A force equivalent to linear acceleration can also be generated to shape an EST without a tug using electrostatic means. ESTs can be used for astronomy across a wide spectrum of frequencies, ranging from the ultraviolet, through optical and infrared, down to radio. A demonstration EST with an aperture larger than the Webb Space Telescope could be flown on a CubeSat mission in low Earth orbit. ESTs with apertures of hundreds of meters could be delivered to heliocentric space in single flights of existing launch vehicles.

Background

There is no better place to do astronomy than space. Therefore, since the dawn of the space age, it has been the ardent ambition of astronomers to place ever more capable telescopes there. The largest such operational instrument, the 2.4 m diameter aperture Hubble Space Telescope, has benefitted from its location above the Earth’s atmosphere to make many great discoveries, and astronomers hold high hopes for more breakthroughs from the long-awaited 6.5 m diameter Webb Space Telescope. As the light gathering power of a telescope increases with the square of their aperture, still larger space telescopes are greatly to be desired. However, as the cost (>$10 billion) and quarter century long development schedule of the Webb telescope have demonstrated, new techniques will be required if construction of much larger observatories is to be made practical. This is the purpose of the Enormous Space Telescope (EST) concept.

The Enormous Space Telescope (EST)

The EST exploits the principle that if a flexible material subject to a linear acceleration is spun, the balance of linear acceleration and centrifugal acceleration forces will shape the material into a parabolic geometry. This technique has been used on Earth to spin cast liquid glass into parabolic dishes for use in telescopes up to several meters in diameter. The EST can employ similar physics with a properly tailored sheet of gossamer material in space to create parabolic reflector dishes with dimensions of hundreds of meters while keeping system masses well within existing launch vehicle limits.

In order to understand how the EST works, let us start by considering it in its smallest and possibly initial form, as a CubeSat demonstration mission. Consider a 13 kg, 12U CubeSat in a circular orbit 400 km above Earth. A one kilometer long tether is extended down from the satellite, and used to suspend a 13 m diameter (twice that of Webb) hoop, whose central axis aligns with the tether. Lines from the circumference of the hoop attach to the tether by a frictionless magnetic bearing, allowing the hoop to rotate freely. The interior of the hoop contains a slack solar sail material, properly tailored to accept a parabolic shape without folds, which is attached to the hoop like the skin of a slack-topped drum. Aluminized balloon film can be used to create solar sail material with a mass density of 6 grams/m2. Taking the hoop mass into account, we will assume 10 gm/m2 as the net mass density for our hoop/film combination, resulting in a mass estimate of 1.3 kg for that subsystem.

At an altitude of 400 km, the CubeSat will be moving with a velocity of 7668.63 m/s, generating a centrifugal acceleration of 8.6762 m/s2, exactly matching the Earth’s gravitational acceleration at that altitude. The reflector, however, hanging 1 km below the CubeSat will only be moving at 7667.50 km/s, generating a centrifugal acceleration of 8.675 m/s2. The Earth’s gravitational acceleration at that altitude will be 8.679 m/s. Thus the hoop will experience a downward acceleration of 0.004 m/s2, or 0.4 milliGees. This will make the sail film in the hoop sag. But if we rotate the hoop with an edge velocity of 0.1 m/s, the film material will also experience an outward acceleration, ranging from 0 at its center to 0.0015 m/s2 at its edge. Taken in combination with the linear acceleration, this will shape the film into a perfect parabola.

Fig. 1 An EST suspended by a tether in LEO. The telescope parabolic dish is spinning around the axis of the tether. Earth (down) is on the right.

This little demonstration EST, with a total mass less than 20 kg, including optics that would be positioned along or suspended from the tether at the parabola focal point, would have four times the light gathering capacity of Webb (about thirty times that of Hubble), while costing on the order of 1/1000th as much.

An even cheaper flight demonstration could be done suspending an EST from a high altitude balloon. Since a balloon moves with the wind, the payload would feel no wind. At 100,000 ft it would be above 99% of Earth’s atmosphere. A triangle of long spars could be employed with a balloon attached to each vertex, to keep the balloons out of the field of view of the telescope.

Such systems would have limitations, since they would be constantly pointing directly away from the center of the Earth. But we can do better.

Let us therefore scale our unit up in diameter by a factor of ten, to a 130 m diameter reflector dish, increasing the mass of the hoop, the optics and spacecraft by a factor of 100. It would still be a quite manageable mass though, about 2000 kg, easily launchable into interplanetary space by a Falcon 9 medium lift booster. In this case. there would be no gravity gradient available to stretch the tether. So we need to use an alternative technique.

One approach might be to spin the hoop around the spacecraft, in the manner of a tethered artificial gravity system, having a second hoop counter-rotating along with the one suspending the dish in order to neutralize gyroscopic effects. But such a system would still need to constantly change its pointing direction, making long duration exposures impossible.

Tugs for ESTs

A more effective approach would be to simply employ a spacecraft as a tug. Sunlight has a pressure of 9 micronewtons per square meter, which would add up to 0.12 Newtons over the whole body of the 130 m diameter sail. If that were the only linear acceleration of the sail, it would shape it into a parabolic reflector with its concave side pointing towards the Sun. As we want to be able to point the telescope the other way, we need to generate more thrust than that. This could be done using either electric propulsion or larger solar sails with a lower mass density then the hoop, or magnetic or electric sails, pulling its tether outward from the Sun.

Let us first consider electric propulsion. If we had an 70% efficient ion engine using argon propellant and a Isp of 7000 s, 50 kWe would be required to produce 1 N of thrust. Assuming a typical solar electric propulsion system mass to power ratio of 20 kg/kWe, that would require 1000 kg. The tug would thus accelerate at a rate of 0.001 m/s2. If the reflector was made of sail material with the minimum mass density of 6 gm/m2, its material would self-accelerate away from the Sun with an acceleration of 9e-6/0.006 =0.0015 m/s2, which is greater than the self-acceleration of the tug, and therefore unsatisfactory. However, the remedy for this is simple: just make the reflector material much thicker. For example, if we tripled its thickness to 18 gm/m2, its self-acceleration would only be 0.0005 m/s2, i.e. half that of the tug. So it would lag behind the tug and the net pull on it of 0.0005 m/s2 would make its center sag back towards the Sun. If we then set it spinning with a velocity of 0.1 m/s at its edge, an edge centrifugal acceleration of 0.00015 m/s2 would be created, shaping it into a 130 m diameter parabolic dish.

Fig. 2 Electric Propulsion tug pulling on an EST.

Operating at 1 N thrust, the thruster would consume 0.014 gm/s of propellant, or about 1.2 kg per day of thrusting (i.e. observing time). Thrust and thus propellant requirements would drop if the telescope were positioned further out in the solar system, since solar light pressure would drop as the inverse square of the telescope’s distance from the Sun. Thus, for example at 3.1 AU, it would only need to use 0.12 kg/day of propellant to generate adequate acceleration.

We could also use solar sails as tugs. In this case no propellant would be needed. Positioning the tug behind the EST would allow it to eclipse solar pressure, as shown if Fig. 3. If the tug is pulling the EST, making tug acceleration greater than reflector material self-acceleration could be assured simply by having the tug sails be larger than the reflector sail, and using a heavy gauge material for the reflector sail.

Fig. 3 Using solar sail tugs to accelerate an EST, by pushing from behind. The EST spins around the central axis. The Sun is on the left. An alternative design would send the mast through the sunward pusher sail, allowing it to deliver its thrust to the base of the mast by a set of shrouds.

A pusher sail telescope would need to point (generally, but not necessarily exactly) outward from the Sun all the time. However if a nuclear electric tug were used, and the telescope were positioned in the shadow of a planet, sunlight impinging on the rear side of the reflector would not be an issue and the telescope could be pointed in any direction.

In the case of radio telescopes, all of this becomes much easier, as there would be no solar light pressure on the rear face of the dish. In that case any kind of tug – solar electric, nuclear electric, or solar sail- could be used, with the EST pointable in any direction simply by maneuvering the tug. The amount of acceleration required from the tug could also be much less.

The Electrostatic EST

An alternative to physical acceleration to impose linear force on the dish is to use electrostatic attraction, In this case the reflector sail would be charged one way, while another sail positioned behind it and held off at a distance by a structural system would be given an opposite charge. The sails would thus attract each other, much as if by gravity, and when the assembly was spun up, both sails would assume parabolic shapes, with their concave sides pointing in opposite directions.

Let us consider the case of two 50 m radius dishes held 25 m apart by structure, with a potential difference between the two of 10 kV, creating a field of 400 volts/m. From electrostatics we have EA = Q/𝜺, so Q, the charge on each dish will be given by Q=(400)(7854 m2)(8.85e-12) = 2.8e-5 coulombs. The electrostatic force on each sail will be given by F=QE, so the total electrostatic force between the sails will be F=400(2.8e-5) = 0.0112 N. Assuming the sail materials have a mass density of 6 gm/m2, this will result in a self-acceleration each sail towards the other of 0.0112/(0,006)(7854) = 0.00024 m/s2. It may be observed that the field will actually be greater near the center because the dishes would sag towards each other. This, however, could be compensated for by varying the thickness of the sail material, making it thicker towards the center and thinner towards the edge, thereby keeping the linear self-acceleration of the two sails towards each other equal over their entire surfaces.

Fig. 4. An Electrostatic EST. The sails have opposite charges and are held separate from each other by a compressive structure. The mutual attraction of the sails can substitute for linear acceleration of the system

Size Limits of ESTs

There does not seem to be any theoretical limit to the potential size of an EST. However, as we have seen, using current materials, the mass required the create an EST system goes approximately as:

Where M is the EST system mass in kilograms and R is the aperture radius in meters. Thus our 6.5 m radius EST demo unit has an estimated mass, including its associated spacecraft, on the order of 20 kg, while our 65 m radius operational EST would be expected to have a mass on the order of 2000 kg.

Currently the largest operational launch vehicle is the Falcon Heavy, with a capability of about 60,000 kg to low Earth orbit (LEO). More powerful vehicles, including the NASA SLS and the SpaceX Starship system are expected to become operational within the next few years, with capabilities of up to 120,000 kilograms to LEO. Since an EST tug could propel itself out of LEO and into heliocentric space, this may also be taken as the limit of the size of an EST system, deliverable into space with a single launch. If we plus 120,000 kg into equation (1), we find that a practical size limit for relatively near-term EST systems would be an aperture diameter of about 1000 meters. The discoveries that might be enabled by such systems are beyond reckoning.

Conclusion

We find that the EST concept offers a practical path towards creating space telescopes with capabilities dwarfing conventional systems by many orders of magnitude. We also find that ESTs could be used to create space telescopes with comparable capabilities to conventional systems, but with several orders of magnitude lower cost. Furthermore, the EST concept holds these benefits for space astronomy across a wide range of frequencies, from ultraviolet down to radio. We therefore recommend that the concept be studied further, and that a demonstration mission be flown at an early date.

Acknowledgement; The author wishes to acknowledge the assistance of Heather Rose, who provided the illustrations for this paper.

P/2019 LD2: A Brief Interlude Among Jupiter’s Trojans

The orbital interactions between objects in a stellar system result in all kinds of interesting effects, a celestial pinball machine that sometimes flings planets outward to wander alone among the stars. Gas giants can be pulled from more distant orbits into a broiling proximity to their star. But the object known as P/2019 LD2 has a special interest because its interactions are happening in a tight time frame even as we observe them.

We could call P/2019 LD2 a ‘comet-like object,’ because it sometimes acts like an asteroid, sometimes like a comet. It is in fact a Centaur, one of that group of outer system objects that only become active as they move into the inner system. We’re watching a transition from Centaur to Jupiter family comet mediated by the gradually warming environment. This one evidently swung close to Jupiter roughly two years ago, to be flung by the giant planet’s gravity toward the Trojan asteroid group that leads Jupiter in its orbit by some 700 million kilometers.

At least that’s the thinking of lead author Bryce Bolin (Caltech), who used Hubble imagery as a follow-up to Spitzer data by way of identifying comet-like activity on P/2019 LD2. The Hubble work showed a cometary tail some 650,000 kilometers long, and could also resolve features near the nucleus at high resolution. The object’s coma and jets are clearly visible. Bolin believes that the gravitational interactions that put LD2 where it is today are comparatively rare:

“The visitor had to have come into the orbit of Jupiter at just the right trajectory to have this kind of configuration that gives it the appearance of sharing its orbit with the planet. We’re investigating how it was captured by Jupiter and landed among the Trojans. But we think it could be related to the fact that it had a somewhat close encounter with Jupiter.”

Image: NASA’s Hubble Space Telescope snapped this image of the young comet P/2019 LD2 as it orbits near Jupiter’s captured ancient asteroids, which are called Trojans. The Hubble view reveals a 650,000-kilometer-long tail of dust and gas flowing from the wayward comet’s bright solid nucleus. Credit: NASA/ESA/J. Olmsted/STScI.

So this is a new interaction involving an object that was itself discovered only in June of 2019 by the University of Hawaii’s Asteroid Terrestrial-impact Last Alert System (ATLAS) telescopes and then identified in archival data from the Zwicky Transient Facility at Palomar Observatory. Moreover, this particular interaction is fleeting, because Bolin and team used computer simulations to show that another close encounter with Jupiter will occur in two years, one that should push the comet away from the Trojans and fling it into the inner Solar System.

Assuming LD2 has its origin in the Kuiper Belt, it would have been bumped out of its location there by other gravitational interactions with another KBO, warming as it moved closer to the Sun in a process thought to nudge a new short-period comet inward about once every century. LD2 also reminds us how we proceed with the identification of an object as a comet. This one began to show outgassing activity fully 750 million kilometers away from the Sun, which is interesting because at that distance water ice is only beginning to be able to sublimate.

That would imply that outgassing in the form of jets escaping from the nucleus is caused by carbon monoxide and carbon dioxide, which can be converted into gaseous form at lower temperatures. The team’s observations using Spitzer identified gas and dust around the nucleus and motivated the Hubble investigation at visible light wavelengths, as did contact from Japanese amateur astronomer Seiichi Yoshida, who had also seen activity on the object.

Image: The main asteroid belt lies between Mars and Jupiter, whereas Trojan asteroids both lead and follow Jupiter. Credit: NASA/ESA/J. Olmsted/STScI.

Where to from here? Exiting the Trojans and interacting again with Jupiter in two years, the object will likely head for deep space. But that could take some time. Carey Lisse of the Johns Hopkins University Applied Physics Laboratory (APL) describes the possibilities:

“Short-period comets like LD2 meet their fate by being thrown into the Sun and totally disintegrating, hitting a planet, or venturing too close to Jupiter once again and getting thrown out of the solar system, which is the usual fate. Simulations show that in about 500,000 years, there’s a 90% probability that this object will be ejected from the solar system and become an interstellar comet.”

But we should keep in mind what happens with comets as they move into warmer regions — they begin to change. Let me quote the paper on this:

Another consequence of the increased heating from closer proximity of the Sun is that large-scale ablation of the comet’s structure due to thermal stress can occur resulting in it becoming partially or completely disrupted (Fernandez 2009). Since P/2019 LD2 is now in transition between the Centaur and Jupiter Family Comet populations, it seems likely that it has become active for the first time, and as such, its activity will be rapidly evolving in response to the new epoch of increased Solar heating.

It will be fascinating to see that change as LD2 evolves, reminding us of the activity of icy objects nudged out of their distant Kuiper Belt orbits into the giant planet interactions that await them, a process that can take several million years as they move toward the inner system.

The paper is Bolin et al., “Initial Characterization of Active Transitioning Centaur, P/2019 LD2 (ATLAS), Using Hubble, Spitzer, ZTF, Keck, Apache Point Observatory, and GROWTH Visible and Infrared Imaging and Spectroscopy,” Astronomical Journal Vol. 161, No. 3 (11 February 2021). Abstract / Preprint.

Into Titan’s Haze

I can remember when I first read about the experiment that Stanley Miller and Harold Urey performed at the University of Chicago in 1952 to see if organic molecules could be produced under conditions like those of the early Earth. It was a test of abiogenesis, though that wasn’t a word I knew at the time. Somewhere around 5th grade, I was a kid reading a book whose title has long escaped me, but the thought that scientists could re-create the atmosphere the way it was billions of years ago seized my imagination.

Never mind that exactly what was in that atmosphere has been controversial. What thrilled me was the attempt to reproduce something long gone — billions of years gone — and to experiment to find out what it might produce. I just finished Samanth Subramanian’s elegant biography of J. B. S. Haldane, the polymathic geneticist, mathematician, physiologist (and too much more to list here), whose work on the chemical formation of life was strongly supported by the Miller and Urey results, as was that of the Soviet biochemist Alexander Ivanovich Oparin, to whom Haldane always deferred when asked who should be given priority for the idea.

The biography, A Dominant Character (W. W. Norton, 2020) is a gem; I highly recommend it to those interested in these matters. And it was just the thing to be reading when I began to hear about the work of Fabian Schulz and Julien Maillard (IBM Research-Zurich).

Working with colleagues at the University of Paris-Saclay, the University of Rouen at Mont-Saint-Aignan, and the Fritz Haber Institute of the Max Planck Society, the researchers have been experimenting with atmospheres as well, though not of our own world but Titan, a moon frequently described as having analogues to the early Earth. In fact, they’ve re-created its atmosphere in an Earth laboratory, which may eventually tell us much about abiogenesis in both places through the use of atomic-scale microscopy.

Titan continues to fascinate. No other object in the Solar System offers up a nitrogen atmosphere of this density, along with organic processes and interactions between the atmosphere and the surface on a grand and highly visible scale. There is a distinct possibility that Earth’s atmosphere 2.8 billion years ago was close to what we see on Titan today. The timeframe is based on the creation of the first reef systems in the Mesoarchean Era, as cyanobacteria began their photosynthetic work to turn carbon dioxide into oxygen. So this is an obviously fecund arena for researchers to probe.

Image: Although the Huygens probe has now pierced the murky skies of Titan and landed on its surface, much of the moon remains for the Cassini spacecraft to explore. Titan continues to present exciting puzzles. This view of Titan uncovers new territory not previously seen at this resolution by Cassini’s cameras. The view is a composite of four nearly identical wide-angle camera images. Credit: NASA/JPL/Space Science Institute.

We’d like to know a lot more about that frustrating photochemical haze that hid the surface of Titan when Voyager 1 took its jog at Saturn to get a look at the moon. Here we’re seeing nanoparticles made out of organic molecules, with carbon, hydrogen and nitrogen in abundance. All this is the result of radiation from the Sun as it streams into the methane and nitrogen mix making up the bulk of Titan’s atmosphere. Previous lab experiments have focused on the organic molecules called tholins to understand the chemical nature of the molecules from which the haze is ultimately derived.

The term ‘tholin’ was first used in a 1979 Nature paper co-authored by Carl Sagan and Bishun Khare, who would doubtless be thrilled to see how large a role they play in our analysis of material in the outer system. Tholins got a lot of public exposure, for instance, when New Horizons flew past 486958 Arrokoth in the outer Solar System. They’re thought to have accounted for its reddish color, and are in fact common in this distant region as solar UV and cosmic rays interact with organic compounds on icy bodies.

The IBM experiment was structured to allow Schulz and Maillard to observe tholins in the formation process. Co-authors Leo Gross and Nathalie Carrasco explain:

“We flooded a stainless-steel vessel with a mixture of methane and nitrogen and then triggered chemical reactions through an electric discharge, thereby mimicking the conditions in Titan’s atmosphere. We then analyzed over 100 resulting molecules composing Titan’s tholins in our lab at Zurich, obtaining atomic resolution images of around a dozen of them with our home-built low-temperature atomic force microscope.”

Image: Titan’s aerosol analogues as seen by Scanning Electron Microscopy. Credit Nathalie Carrasco.

The work is significant because it is revealing how compounds like those found in Titan’s haze are built by using atomic-scale microscopy. This is a deep look into chemical bonding and structure that goes well beyond previous techniques, and offers what appears to be a new astrobiological tool. The scientists believe their work can be turned toward the analysis of Titan’s methane cycle, which like Earth’s hydrological cycle moves between a gaseous and liquid state, producing the moon’s lakes and seas.

The IBM work confirms that Titan’s orange haze is primarily made up of nitrogen-containing polycyclic aromatic hydrocarbons, with chemical structures that are related to the ‘wettability” of the haze, a factor that determines whether the haze nanoparticles float on the moon’s hydrocarbon lakes. From an IBM research blog:

Finding these new details on the chemical structure of tholins adds to our understanding not only of Titan’s haze but also of the likelihood that aerosols might have favored life on the early Earth in the past.

Did hazes like this at one time protect fragile DNA molecules from the Sun’s radiation? Gross and Carrasco point to the fact that the molecular structures the team has imaged are good absorbers of ultraviolet light. That would be useful information not just about the early Earth but also about the prospects for forms of life emerging on Titan itself. Future missions like Dragonfly should give us much information in this regard.

Meanwhile, I’m most interested in the implications of this work for astrobiology in general. Let me quote from the paper on these laboratory analogues of Titan’s haze:

These molecules are for example good UV absorbers and thus modulate the radiative balance of the atmosphere (Brassé et al. 2015). This chemical structure would also influence the surface energy of the haze particles, controlling their wettability with liquid/solid hydrocarbons and nitriles: it would impact their propensity to trigger methane rains in the troposphere and/or to transiently float at the lake surfaces of Titan (Cordier & Carrasco 2019; Yu et al. 2020).

And keep this in mind for the overall context:

More generally this work showed the potential of AFM technique to reveal the chemical structure of complex organic material of interest for astrochemistry, opening new perspectives in the chemical analysis of rare and complex material such as organic matter contained in meteorites or in the frame of future sample return missions.

The paper is Schulz et al., “Imaging Titan’s Organic Haze at Atomic Scale,” Astrophysical Journal Letters Vol. 908, No. 1 (12 February 2021). Abstract / Full Text.

TOI 451: Three Planets in a Stellar Stream

The planets orbiting the young star TOI 451 should be useful for astronomers working on the evolution of atmospheres on young planets. This is a TESS find, three planets tracked through their transits and backed by observations from the now retired Spitzer Space Telescope, with follow-ups as well from Las Cumbres and the Perth Exoplanet Survey Telescope. TOI 451 (also known as CD-38 1467) is about 400 light years out in Eridanus, a star with 95% of the Sun’s mass, some 12% smaller and rotating every 5.1 days.

That rotation is interesting, as it’s more than five times faster than our Sun rotates, a marker for a young star, and indeed, astronomers have ways of verifying that the star is only about 120 million years old. Here the Pisces-Eridanus stream, only discovered in 2019, becomes a helpful factor. A stream of stars forms out of gravitational interactions between our galaxy and a star cluster or dwarf galaxy, shoe-horning stars out of their original orbit to form an elongated flow.

Named after the two constellations in which the bulk of its stars reside, the Pisces-Eridanus stream is actually some 1,300 light years in length and as seen from Earth extends across fourteen different constellations. And while Stefan Meingast (University of Vienna) and team, who discovered the stream, pegged its age as somewhat older, follow-up work by Jason Curtis at Columbia University (New York) determined that the stream was 120 million years old.

Stars of the same age with a common motion through space occur in several forms. A stellar association is a loose grouping of stars, with a common origin although now gravitationally unbound and moving together (I’m simplifying here, to be sure, because there are a number of sub-classifications of stellar associations). A moving group is still coherent, but now the stars are less obviously associated as the formation ages. The Ursa Major moving group is the closest one of these to Earth. A stellar stream like the Pisces-Eridanus stream has been stretched out by tidal forces, a remnant fragment of a dwarf galaxy now torn apart and gradually dispersing.

Image: The Pisces-Eridanus stream spans 1,300 light-years, sprawling across 14 constellations and one-third of the sky. Yellow dots show the locations of known or suspected members, with TOI 451 circled. TESS observations show that the stream is about 120 million years old, comparable to the famous Pleiades cluster in Taurus (upper left). Credit: NASA GSFC.

As with stellar moving groups we’ve looked at before, the Pisces-Eridanus stream seems to feature many stars that share common traits of age and metallicity. TESS comes into its own when studying a system like TOI 451 because its measurements of stars in the Pisces-Eridanus stream show strong evidence of starspots (rotating in and out of view and thus causing the kind of brightness variation TESS was made to measure). Starspots are prominent in younger stars, as is fast rotation. And all of that helps narrow down the possible age of the TOI 451 system.

The three planets around TESS 451 have a story of their own to tell. With temperatures ranging from 1200° C to 450° C, these are super-Earths, with orbits of 1.9 days, 9.2 days and 16 days. Despite the intense heat of the star, the researchers believe these worlds will have retained their atmospheres, making them laboratories for theories of how atmospheres evolve and what their properties should be. Already we know there is a strong infrared signature between 12 and 24 micrometers, which suggests the likely presence of a debris disk. The paper describes it this way, likening the age of stars in the Pisces-Eridanus stream to that found in the Pleiades:

The frequency of infrared excesses decreases with age, declining from tens of percent at ages less than a few hundred Myr to a few percent in the field (Meyer et al. 2008; Siegler et al. 2007; Carpenter et al. 2009). In the similarly-aged Pleiades cluster, Spitzer 24µm excesses are seen in 10% of FGK stars (Gorlova et al. 2006). This excess emission suggests the presence of a debris disk, in which planetesimals are continuously ground into dust…

And in this case we have a debris disk with a temperature near or somewhat less than 300 K.

Image: This illustration sketches out the main features of TOI 451, a triple-planet system located 400 light-years away in the constellation Eridanus. Credit: NASA’s Goddard Space Flight Center.

A comparatively close system like this one should help us piece together the chemical composition of the planetary atmospheres as well as evidence of clouds and other features, with follow-up studies through instruments like the James Webb Space Telescope using transmission spectroscopy. Adding to the interest of TOI 451 is the fact that there may be a distant companion star, TOI 451 B, identified based on Gaia data on what appears to be a faint star about two pixels away from TOI 451. Or perhaps this is a triple system, as the paper suggests:

We note that Rebull et al. (2016), in their analysis of the Pleiades, detect periods for 92% of the members, and suggest the remaining non-detections are due to non-astrophysical effects. We have suggested TOI 451 B is a binary, which we might expect to manifest as two periodicities in the lightcurve. We only detect one period in our lightcurve; however, a second signal could have been impacted by systematics removal or be present at smaller amplitude than the 1.64 day signal, and so we do not interpret the lack of a second period further.

The difficulty of data collection here is apparent:

TOI 451 and its companion(s) are only separated by 37 arcseconds, or about two TESS pixels, so the images of these two stars overlap substantially on the detector. The light curve of the companion TOI 451 B is clearly contaminated by the 14x brighter primary star.

The non-standard methods used to extract the light curve of the companion star(s) are explained in the paper, and I’ll send you there if interested in the details. Note, too, the useful synergy of the TESS and Gaia datasets, which allowed the age of this system to be constrained and also resulted in the discovery of the three planets. As always, rapid growth in our datasets and cross correlations between them trigger the prospect of continuing discovery.

In connection with this work, I should also mention another finding from THYME, the TESS Hunt for Young and Maturing Exoplanets, out of which grew the TOI 451 work. HD 110082 b is a Neptune-class world of approximately 3.2 Earth radii, assumed to be about 11 times as massive as the Earth in a 250 million year old stellar system, another useful find when it comes to examining planet formation and evolution. The F-class primary is about 343 light years away.

The paper is Newton et al., “TESS Hunt for Young and Maturing Exoplanets (THYME). IV. Three Small Planets Orbiting a 120 Myr Old Star in the Pisces–Eridanus Stream,” Astronomical Journal Vol. 161, No. 2 (14 January 2021). Abstract / Preprint. The paper on HD 110082 b is Tofflemire et al., “TESS Hunt for Young and Maturing Exoplanets (THYME) V: A Sub-Neptune Transiting a Young Star in a Newly Discovered 250 Myr Association,” accepted at the Astronomical Journal (preprint).

Extraterrestrial: On ‘Oumuamua as Artifact

The reaction to Avi Loeb’s new book Extraterrestrial (Houghton Mifflin Harcourt, 2021) has been quick in coming and dual in nature. I’m seeing a certain animus being directed at the author in social media venues frequented by scientists, not so much for suggesting the possibility that ‘Oumuamua is an extraterrestrial technological artifact, but for triggering a wave of misleading articles in the press. The latter, that second half of the dual reaction, has certainly been widespread and, I have to agree with the critics, often uninformed.

Image credit: Kris Snibbe/Harvard file photo.

But let’s try to untangle this. Because my various software Net-sweepers collect most everything that washes up on ‘Oumuamua, I’m seeing stark headlines such as “Why Are We So Afraid of Extraterrestrials,” or “When Will We Get Serious about ET?” I’m making those particular headlines up, but they catch the gist of many of the stories I’ve seen. I can see why some of the scientists who spend their working days digging into exoplanet research, investigate SETI in various ways or ponder how to build the spacecraft that are helping us understand the Solar System would be nonplussed.

We are, as a matter of fact, taking the hypothesis of extraterrestrial life, even intelligent extraterrestrial life, more seriously now than ever before, and this is true not just among the general public but also within the community of working scientists. But I don’t see Avi Loeb saying anything that discounts that work. What I do see him saying in Extraterrestrial is that in the case of ‘Oumuamua, scientists are reluctant to consider a hypothesis of extraterrestrial technology even though it stands up to scrutiny — as a hypothesis — and offers as good an explanation as others I’ve seen. Well actually, better, because as Loeb says, it checks off more of the needed boxes.

Invariably, critics quote Sagan: “Extraordinary claims require extraordinary evidence.” Loeb is not overly impressed with the formulation, saying “evidence is evidence, no?” And he goes on: “I do believe that extraordinary conservatism keeps us extraordinarily ignorant. Put differently, the field doesn’t need more cautious detectives.” Fighting words, those. A solid rhetorical strategy, perhaps, but then caution is also baked into the scientific method, as well it should be. So let’s talk about caution and ‘Oumuamua.

Loeb grew up on his family’s farm south of Tel Aviv, hoping at an early age to become a philosopher but delayed in the quest by his military service, where he likewise began to turn to physics. An early project was the use of electrical discharges to propel projectiles, a concept that wound up receiving funding from the US Strategic Defense Initiative during the latter era of the Cold War. He proceeded to do postgraduate work at the Institute for Advanced Study in Princeton, mixing with the likes of Freeman Dyson and John Bahcall, and moved on to become a tenured professor at Harvard. Long before ‘Oumuamua, his life had begun to revolve around the story told in data. He seems to have always believed that data would lead him to an audacious conclusion, and perhaps primed by his childhood even to expect such an outcome.

I also detect a trace of the mischief-maker, though a very deliberate one. To mix cultures outrageously, Loeb came out of Beit Hanan with a bit of Loki in him. And he’s shrewd: “You ask nature a series of questions and listen carefully to the answers from experiments,” he writes of that era, a credo which likewise informs his present work. Extraterrestrial is offered as a critique of the way we approach the unknown via our scientific institutions, and the reaction to the extraterrestrial hypothesis is displaying many of the points he’s trying to make.

Can we discuss this alien artifact hypothesis in a rational way? Loeb is not sure we can, at least in some venues, given the assumptions and accumulated inertia he sees plaguing the academic community. He describes pressure on young postdocs to choose career paths that will fit into accepted ideas. He asks whether what we might call the science ‘establishment’ is simply top-heavy, a victim of its own inertia, so that the safer course for new students is not to challenge older models.

These seem like rational questions to me, and Loeb uses ‘Oumuamua as the rhetorical church-key that pops open the bottle. So let’s look at what we know about ‘Oumuamua with that in mind. The things that trigger our interest and raised eyebrows arrive as a set of anomalies. They include the fact that the object’s brightness varied by a factor of ten every eight hours, from which astronomers could deduce an extreme shape, much longer than wide. And despite a trajectory that had taken it near the Sun, ‘Oumuamua did not produce an infrared signature detectable by the Spitzer Space Telescope, leading to the conclusion that it must be small, perhaps 100 yards long, if that.

‘Oumuamua seemed to be cigar-like in shape, or else flat, either of these being shapes that had not been observed at these extremes in naturally occurring objects in space. Loeb also notes that despite its small size and odd shape, the object was ten times more reflective than typical asteroids or comets in our system. Various theories spawned from all this try to explain its origins, but a slight deviation in trajectory as ‘Oumuamua moved away from the Sun stood out in our two weeks of data. That deviation also took it out of the local standard of rest, which in itself was an unusual place for it to have been until its encounter with our Sun caused its motion to deviate.

I don’t want to go over ground we’ve already covered in some detail here in the past — a search for ‘Oumuamua in the archives will turn up numerous articles, of which the most germane to this review is probably ‘Oumuamua, Thin Films and Lightsails. This deals with Loeb’s work with Shmuel Bialy on the non-gravitational acceleration, which occurred despite a lack of evidence for either a cometary tail or gas emission and absorption lines. All this despite an approach to the Sun of a tight 0.25 AU.

The fact that we do not see outgassing that could cause this acceleration is not the problem. According to Loeb’s calculations, such a process would have caused ‘Oumuamua to lose about a tenth of its mass, and he points out that this could have been missed by our telescopes. What is problematic is the fact that the space around the object showed no trace of water, dust or carbon-based gases, which makes the comet hypothesis harder to defend. Moreover, whatever the cause of the acceleration, it did not change the spin rate, as we would expect from asymmetrical, naturally occurring jets of material pushing a comet nucleus in various directions.

Extraterrestrial should be on your shelf for a number of reasons, one of which is that it encapsulates the subsequent explanations scientists have given for ‘Oumuamua’s trajectory, including the possibility that it was made entirely of hydrogen, or the possibility that it began to break up at perihelion, causing its outward path to deviate (again, no evidence for this was evident to our instruments). And, of course, he makes the case for his hypothesis that sunlight bouncing off a thin sail would explain what we see, citing recent work on the likelihood that the object was disk-shaped.

So what do we do with such an object, beyond saying that none of our hypotheses can be validated by future observation since ‘Oumuamua is long gone? Now we’re at the heart of the book, for as we’ve seen, Extraterrestrial is less about ‘Oumuamua itself and more about how we do science, and what the author sees as a too conservative approach that is fed by the demands of making a career. He’s compelled to ask: Shouldn’t the possibility of ‘Oumuamua being an extraterrestrial artifact, a technological object, be a bit less controversial than it appears to be, given the growth in our knowledge in recent decades? Let me quote the book:

Some of the resistance to the search for extraterrestrial intelligence boils down to conservatism, which many scientists adopt in order to minimize the number of mistakes they make during their careers. This is the path of least resistance, and it works; scientists who preserve their images in this way receive more honors, more awards, and more funding. Sadly, this also increases the force of their echo effect, for the funding establishes ever bigger research groups that parrot the same ideas. This can snowball; echo chambers amplify conservatism of thought, wringing the native curiosity out of young researchers, most of whom feel they must fall in line to secure a job. Unchecked, this trend could turn scientific consensus into a self-fulfilling prophecy.

Here I’m at sea. I’ve been writing about interstellar studies for the past twenty years and have made the acquaintance of many scientists both through digital interactions and conversations at conferences. I can’t say I’ve found many who are so conservative in their outlook as to resist the idea of other civilizations in the universe. I see ongoing SETI efforts like the privately funded Breakthrough Listen, which Loeb is connected to peripherally through his work with the Breakthrough Starshot initiative to send a probe to Proxima Centauri or other nearby stars. The book contains the background of Starshot by way of showing the public how sails might make sense as the best way to cross interstellar distances, perhaps like Starshot propelled by beamed energy.

I also see active research on astrobiology, while the entire field of exoplanetary science is frothing with activity. To my eye as a writer who covers these matters rather than a scientist, I see a field that is more willing to accept the possibility of extraterrestrial intelligence than ever before. But I’m not working within the field as Loeb is, so his chastening of tribal-like patterns of behavior reflects, I’m sure, his own experience.

When I wrote the piece mentioned above, ‘Oumuamua, Thin Films and Lightsails, it was by way of presenting Loeb’s work on the deviation of the object’s trajectory as caused by sunlight, which he produced following what he describes in the book as “the same scientific tenet I had always followed — a hypothesis that satisfied all the data ought to be considered.” If nature wasn’t producing objects shaped like that of a lightsail that could apparently accelerate through the pressure of photons from a star, then an extraterrestrial intelligence was the exotic hypothesis that could explain it.

The key statement: “If radiation pressure is the accelerating force, then ‘Oumuamua represents a new class of thin interstellar material, either produced naturally…or is of an artificial origin.”

After this, Loeb goes on to say, “everything blew up.” Which is why on my neighborhood walks various friends popped up in short order asking: “So is it true? Is it ET?” I could only reply that I had no idea, and refer them to the discussion of Loeb’s paper on my site. Various headlines announcing that a Harvard astronomer had decided ‘Oumuamua was an alien craft have been all over the Internet. I can see why many in the field find this a nuisance, as they’re being besieged by people asking the same questions, and they have other work they’d presumably like to get on with.

So there are reasons why Extraterrestrial is, to some scientists, a needling, even cajoling book. I can see why some dislike the fact that it was written. But having to talk about one’s work is part of the job description, isn’t it? It was Ernest Rutherford who said that a good scientist should be able to explain his ideas to a barmaid. In these parlous times, we might change Rutherford’s dismissive ‘barmaid’ to a gender-neutral ‘blog writer’ or some such. But the point seems the same.

Isn’t communicating ideas part of the job description of anyone employed to do scientific research? So much of that research is funded by the public through their tax dollars, after all. If Loeb’s prickly book is forcing some scientists to take the time to explain why they think his hypothesis is unlikely, I cannot see that as a bad thing. Good for Avi Loeb, I’d say.

And whatever ‘Oumuamua is, we may all benefit from the discussion it has created. I enjoyed Loeb’s section on exotic theories within the physics community — he calls these “fashionable thought bubbles that currently hold sway in the field of astrophysics,” and in many quarters they seem comfortably accepted:

Despite the absence of experimental evidence, the mathematical ideas of supersymmetry, extra-spatial dimensions, string theory, Hawking radiation, and the multiverse are considered irrefutable and self-evident by the mainstream of theoretical physics. In the words of a prominent physicist at a conference that I attended: ‘These ideas must be true even without experimental tests to support them, because thousands of physicists believe in them and it is difficult to imagine that such a large community of mathematically gifted scientists could be wrong.”

That almost seems like a straw man argument, except that I don’t doubt someone actually said this — I’ve heard more or less the same sentiment voiced at conferences myself. Even so, I doubt many of the scientists I’ve gotten to know would go that far. But the broader point is sound. Remember, Loeb is all about data, and isn’t it true that multiverse ideas take us well beyond the realm of testable hypotheses? And yet many support them, as witness Leonard Susskind in his book The Black Hole War (2008):

“There is a philosophy that says that if something is unobservable — unobservable in principle — it is not part of science. If there is no way to falsify or confirm a hypothesis, it belongs to the realm of metaphysical speculation, together with astrology and spiritualism. By that standard, most of the universe has no scientific reality — it’s just a figment of our imaginations.”

So Loeb is engaging on this very charged issue that goes to the heart of what we mean by a hypothesis, about the falsifiability of an idea. We know where he stands:

Getting data and comparing it to our theoretical ideas provides a reality check and tells us we are not hallucinating. What is more, it reconfirms what is central to the discipline. Physics is not a recreational activity to make us feel good about ourselves. Physics is a dialogue with nature, not a monologue.

You can see why Extraterrestrial is raising hackles in some quarters, and why Loeb is being attacked for declaring ‘Oumuamua a technology. But of course he hasn’t announced ‘Oumuamua was an alien artifact. He’s said this is a hypothesis, not a statement of fact, and that it fits what we currently know, and that it is a plausible hypothesis and perhaps the most plausible among those that have been offered.

He goes on to call for deepening our commitment to Dysonian SETI, looking for signs of extraterrestrial intelligence through its artifacts, a field becoming known as astro-archaeology. And he considers what openness to the hypothesis could mean in terms of orienting our research and our imagination under the assumption that extraterrestrial intelligence is a likely outcome that should produce observables.

As I said above, Extraterrestrial should be on your shelf because it is above all else germane, with ‘Oumuamua being the tool for unlocking a discussion of how we do research and how we discuss the results. My hope is that it will give new public support to ongoing work that aims to answer the great question of whether we are alone in the universe. A great deal of that work continues even among many who find the ‘Oumuamua as technology hypothesis far-fetched and believe it over-reaches.

Is science too conservative to deal with a potentially alien artifact? I don’t think so, but I admire Avi Loeb for his willingness to shake things up and yank a few chains along the way. The debate makes for compelling drama and widens the sphere of discourse. He may well be right that by taking what he calls ‘’Oumuamua’s Wager” (based on Pascal’s Wager, and advocating for taking the extraterrestrial technology hypothesis seriously) we would open up new research channels or revivify stagnant ones.

Some of those neighbors of mine that I’ve mentioned actually dug ‘Oumuamua material out of arXiv when I told them about that service and how to use it, an outcome Ernest Rutherford would have appreciated. I see Extraterrestrial as written primarily for people like them, but if it does rattle the cages of some in the physics community, I think the field will somehow muddle through. Add in the fact that Loeb is a compelling prose stylist and you’ll find your time reading him well spent.

Crafting the Bussard Ramjet

The Bussard ramjet is an idea whose attractions do not fade, especially given stunning science fiction treatments like Poul Anderson’s novel Tau Zero. Not long ago I heard from Peter Schattschneider, a physicist and writer who has been exploring the Bussard concept in a soon to be published novel. In the article below, Dr. Schattschneider explains the complications involved in designing a realistic ramjet for his novel, with an interesting nod to a follow-up piece I’ll publish as soon as it is available on the work of John Ford Fishback, whose ideas on magnetic field configurations we have discussed in these pages before.

The author is professor emeritus in solid state physics at Technische Universität Wien, but he has also worked for a private engineering company as well as the French CNRS, and has been director of the Vienna University Service Center for Electron Microscopy. With more than 300 research articles in peer-reviewed journals and several monographs on electron-matter interaction, Dr. Schattschneider’s current research focuses on electron vortex beams, which are exotic probes for solid state spectroscopy. He tells me that his interest in physics emerged from an early fascination with science fiction, leading to the publication of several SF novels in German and many short stories in SF anthologies, some of them translated into English and French. As we see below, so-called ‘hard’ science fiction, scrupulously faithful to physics, demands attention to detail while pushing into fruitful speculation about future discovery.

by Peter Schattschneider

When the news about the BLC1 signal from Proxima Centauri came in, I was just finishing a scientific novel about an expedition to our neighbour star. Good news, I thought – the hype would spur interest in space travel. Disappointment set in immediately: Should the signal turn out to be real, this kind of science fiction would land in the dustbin.

Image: Peter Schattschneider. Credit & copyright: Klaus Ranger Fotografie.

The space ship in the novel is a Bussard ramjet. Collecting interstellar hydrogen with some kind of electrostatic or magnetic funnel that would operate like a giant vacuum cleaner is a great idea promoted by Robert W. Bussard in 1960 [1]. Interstellar protons (and some other stuff) enter the funnel at the ship‘s speed without further ado. Fusion to helium will not pose a problem in a century or so (ITER is almost working), conversion of the energy gain into thrust would work as in existing thrusters, and there you go!

Some order-of-magnitude calculations show that it isn‘t as simple as that. But more on that later. Let us first look at the more mundane problems occuring on a journey to our neighbour. The values given below were taken from my upcoming The EXODUS Incident [2], calculated for a ship mass of 1500 tons, an efficiency of 85% of the fusion energy going into thrust, an interstellar medium of density 1 hydrogen atom/cm3, completely ionized by means of electron strippers.

On the Way

Like existing ramjets the Bussard ramjet is an assisted take-off engine. In order to harvest fuel it needs a take-off speed, here 42 km/s, the escape velocity from the solar system. The faster a Bussard ramjet goes, the higher is the thrust, which means that one cannot assume a constant acceleration but must solve the dynamic rocket equation. The following table shows acceleration, speed and duration of the journey for different scoop radii.

At the midway point, the thrust is inverted to slow the ship down for arrival. To achieve an acceleration of the order of 1 g (as for instance in Poul Anderson’s celebrated novel Tau Zero [3]), the fusion drive must produce a thrust of 18 million Newton, about half the thrust of the Saturn-V. That doesn’t seem tremendous, but a short calculation reveals that one needs a scoop radius of about 3500 km to harvest enough fuel because the density of the interstellar medium is so low. Realizing magnetic or electric fields of this dimension is hardly imaginable, even for an advanced technology.

A perhaps more realistic funnel entrance of 200 km results in a time of flight of almost 500 years. Such a scenario would call for a generation starship. I thought that an acceleration of 0.1 g was perhaps a good compromise, avoiding both technical and social fantasizing. It stipulates a scoop radius of 1000 km, still enormous, but let us play the “what-if“ game: The journey would last 17.3 years, quite reasonable with future cryo-hibernation. The acceleration increases slowly, reaching a maximum of 0.1 g after 4 years. Interestingly, after that the acceleration decreases, although the speed and therefore the proton influx increases. This is because the relativistic mass of the ship increases with speed.

Fusion Drive

It has been pointed out by several authors that the “standard“ operation of a fusion reactor, burning Deuterium 2D into Helium 3He cannot work because the amount of 2D in interstellar space is too low. The proton-proton burning that would render p+p → 2D for the 2D → 3He reaction is 24 orders of magnitude (!) slower.

The interstellar ramjet seemed impossible until in 1975 Daniel Whitmire [4] proposed the Bethe-Weizsäcker or CNO cycle that operates in hot stars. Here, carbon, nitrogen and oxygen serve as catalysts. The reaction is fast enough for thrust production. The drawback is that it needs a very high core temperature of the plasma of several hundred million Kelvin. Reaction kinetics, cross sections and other gadgets stipulate a plasma volume of at least 6000 m3 which makes a spherical chamber of 11 m radius (for design aficionados a torus or – who knows? – a linear chamber of the same order of magnitude).

At this point, it should be noted that the results shown above were obtained without taking account of many limiting conditions (radiation losses, efficiency of the fusion process, drag, etc.) The numerical values are at best accurate to the first decimal. They should be understood as optimistic estimates, and not as input for the engineer.

Waste Heat

Radioactive high-energy by-products of the fusion process are blocked by a massive wall between the engine and the habitable section, made up of heavy elements. This is not the biggest problem because we already handle it in the experimental ITER design. The main problem is waste heat. The reactor produces 0.3 million GW. Assuming an efficiency of 85% going into thrust, the waste energy is still 47,000 GW in the form of neutrinos, high energy particles and thermal radiation. The habitable section should be at a considerable distance from the engine in order not to roast the crew. An optimistic estimate renders a distance of about 800 m, with several stacks of cooling fins in between. The surface temperature of the sternside hull would be at a comfortable 20-60 degrees Celsius. Without the shields, the hull would receive waste heat at a rate of 6 GW/m2, 5 million times more than the solar constant on earth.

Radiation shielding

An important aspect of the Bussard ramjet design is shielding from cosmic rays. At the maximum speed of 60% of light speed, interstellar hydrogen hits the bow with a kinetic energy of 200 MeV, dangerous for the crew. A.C. Clarke has proposed a protecting ice sheet at the bow of a starship in his novel The Songs of Distant Earth [5]. A similar solution is also known from modern proton cancer therapy. The penetration depth of such protons in tissue (or water, for that matter) is 26 cm. So it suffices to put a 26 cm thick water tank at the bow.

Artificial gravity

It is known that long periods of zero gravity are disastrous to the human body. It is therefore advised to have the ship rotate in order to create artificial gravity. In such an environment there are unusual phenomena, e.g. a different barometric height equation, or atmospheric turbulence caused by the Coriolis forces. Throwing an object in a rotating space ship has surprising consequences, exemplified in Fig. 1. Funny speculations about exquisite sporting activities are allowed.

Fig. 1: Freely falling objects in a rotating cylinder, thrown in different directions with the same starting speed. In this example, drawn from my novel, the cylinder has a radius of 45 m, rotating such that the artificial gravity on the inner hull is 0.3 g. The object is thrown with 40 km/h in different directions. Seen by an observer at rest, the cylinder rotates counterclockwise.

Scooping

The central question for scooping hydrogen is this: Which electric or magnetic field configuration allows us to collect a sufficient amount of interstellar hydrogen? There are solutions for manipulating charged particles: colliders use magnetic quadrupoles to keep the beam on track. The symmetry of the problem stipulates a cylindrical field configuration, such as ring coils or round electrostatic or magnetic lenses which are routinely used in electron microscopy. Such lenses are annular ferromagnetic yokes with a round bore hole of the order of a millimeter. They focus an incoming electron beam from a diameter of some microns to a nanometer spot.

Scaling the numbers up, one could dream of collecting incoming protons over tens of kilometers into a spot of less than 10 meters, good enough as input to a fusion chamber. This task is a formidable technological challenge. Anyway, it is prohibitive by the mere question of mass. Apart from that, one is still far away from the needed scoop radius of 1000 km.

The next best idea relates to the earth’s magnetic dipole field. It is known that charged particles follow the field lines over long distances, for instance causing aurora phenomena close to earth’s magnetic poles. So it seems that a simple ring coil producing a magnetic dipole is a promising device. Let’s have a closer look at the physics. In a magnetic field, charged particles obey the Lorentz force. Calculating the paths of the interstellar protons is then a simple matter of plugging the field into the force equation. The result for a dipole field is shown in Fig. 2.

Fig. 2: Some trajectories of protons starting at z=2R in the magnetic field of a ring coil of radius R that sits at the origin. Magnetic field lines (light blue) converge towards the loop hole. Only a small part of the protons would pass through the ring (red lines), spiralling down according to cyclotron gyration. The rest is deflected (black lines).

An important fact is seen here: the scoop radius is smaller than the coil radius. It turns out that it diminishes further when the starting point of the protons is set at higher z values. This starting point is defined where the coil field is as low as the galactic magnetic field (~1 nT). Taking a maximum field of a few Tesla at the origin and the 1/(z/R)3 decay of the dipole field, where R is the coil radius (10 m in the example), the charged particles begin to sense the scooping field at a distance of 10 km. The scoop radius at this distance is a ridiculously small – 2 cm. All particles outside this radius are deflected, producing drag.

That said, loop coils are hopelessly inefficient for hydrogen scooping, but they are ideal braking devices for future deep space probes, and interestingly they may also serve as protection shields against cosmic radiation. On Proxima b, strong flares of the star create particle showers, largely protons of 10 to 50 MeV energy. A loop coil protects the crew as shown in Fig. 3.

Fig.3: Blue: Magnetic field lines from a horizontal superconducting current loop of radius R=30 cm. Red lines are radial trajectories of stellar flare protons of 10 MeV energy approaching from top. The loop and the mechanical protection plate (a 3 cm thick water reservoir colored in blue) are at z=0. It absorbs the few central impinging particles. The fast cyclotron motion of the protons creates a plasma aureole above the protective plate, drawn as a blue-green ring right above the coil. The field at the coil center is 6 Tesla, and 20 milliTesla at ground level.

After all this paraphernalia the central question remains: Can a sufficient amount of hydrogen be harvested? From the above it seems that magnetic dipole fields, or even a superposition of several dipole fields, cannot do the job. Surprisingly, this is not quite true. For it turns out that an arcane article from 1969 by a certain John Ford Fishback [6] gives us hope, but this is another story and will be narrated at a later time.

References

1. Robert W. Bussard: Galactic Matter and Interstellar Flight. Astronautica Acta 6 (1960), 1-14.

2. P. Schattschneider: The EXODUS Incident – A Scientific Novel. Springer Nature, Science and Fiction Series. May 2021, DOI: 10.1007/978-3-030-70019-5.

3. Poul Anderson: Tau Zero (1970).

4. Daniel P. Whitmire: Relativistic Spaceflight and the Catalytic Nuclear Ramjet. Acta Astronautica 2 (1975), 497-509.

5. Arthur C. Clarke: Songs of distant Earth (1986).

6. John F. Fishback: Relativistic Interstellar Space Flight. Astronautica Acta 15 (1969), 25-35.

Technosignatures: Looking to Planetary Atmospheres

While we often think about so-called Dysonian SETI, which looks for signatures of technology in our astronomical data, as a search for Dyson spheres, the parameter space it defines is getting to be quite wide. A technosignature has to be both observable as well as unique, to distinguish it from natural phenomena. Scientists working this aspect of SETI have considered not just waste heat (a number of searches for distinctive infrared signatures of Dyson spheres have been run), but also artificial illumination, technological features on planetary surfaces, artifacts not associated with a planet, stellar pollution and megastructures.

Thus the classic Dyson sphere, a star enclosed by a swarm or even shell of technologies to take maximum advantage of its output, is only one option for SETI research. As Ravi Kopparapu (NASA GSFC) and colleagues point out in an upcoming paper, we can also cross interestingly from biosignature searches to technosignatures by looking at planetary atmospheres.

Biosignature science is the more developed of the two fields, though we’re seeing a lot of activity in technosignature work, the robust nature of which can be seen in the extensive references the Kopparapu team identifies. As applied to atmospheres, a search for technosignatures can involve looking for various forms of pollution that flag industrial activity.

To my knowledge, most work on atmospheric pollution has targeted chlorofluorocarbons (CFCs), a useful choice because there is no biological source here, although our own use of CFCs occurred in a fairly brief window and for a specific purpose (refrigeration). The NASA work targets the much more ubiquitous nitrogen dioxide (NO2), which can be a by-product of an industrial process and in general is produced by any form of combustion.

As Kopparapu notes:

“In the lower atmosphere (about 10 to 15 kilometers or around 6.2 to 9.3 miles), NO2 from human activities dominate compared to non-human sources. Therefore, observing NO2 on a habitable planet could potentially indicate the presence of an industrialized civilization.”

Adds Giada Arney, a co-author on the paper and a colleague of Kopparapu at GSFC:

“On Earth, about 76 percent of NO2 emissions are due to industrial activity. If we observe NO2 on another planet, we will have to run models to estimate the maximum possible NO2 emissions one could have just from non-industrial sources. If we observe more NO2 than our models suggest is plausible from non-industrial sources, then the rest of the NO2 might be attributed to industrial activity. Yet there is always a possibility of a false positive in the search for life beyond Earth, and future work will be needed to ensure confidence in distinguishing true positives from false positives.”

Image: Artist’s illustration of a technologically advanced exoplanet. The colors are exaggerated to show the industrial pollution, which otherwise is not visible. Credit: NASA/Jay Freidlander.

This is evidently the first time NO2 has been examined in technosignature terms. The scientists deploy a cloud-free 1-dimensional photochemical model that uses the atmospheric temperature profile of today’s Earth to examine possible mixing ratio profiles of nitrogen oxide compounds on a planet orbiting several stellar types, one of them being a G-class star like the Sun, the others being a K6V and two M-dwarfs, one of these being Proxima Centauri. The authors then calculate the observability of these NO2 features, considering observing platforms like the James Webb Space Telescope and the projected Large UV/Optical/IR Surveyor (LUVOIR) instrument.

Usefully, atmospheric NO2 strongly absorbs some wavelengths of visible light, and the authors’ calculations show that an Earth-like planet orbiting a star like the Sun could be studied from as far as 30 light years away and an NO2 signature detected even with a civilization producing the pollutant at roughly the same levels we do today. This would involve observing at visible wavelengths over the course of at least 400 hours, which parallels what the Hubble instrument needed to produce its well-known Deep Field observations.

But adding yet more interest to K-class stars, whose fortunes as future targets for bio- and technosignature observations seem to be rising, is the fact that stars cooler than the Sun should generate a stronger NO2 signal. These stars produce less ultraviolet light that can break down NO2. As to M-dwarfs, we have this:

Further work is needed to explore the detectability of NO2 on Earth-like planets around M-dwarfs in direct imaging observations in the near-IR with ground-based 30 m class telescopes. NO2 concentrations increase on planets around cooler stars due to reduced availability of short-wavelength photons that can photolyze NO2 . Non-detectability at longer observation times could place upper limits on the amount [of] NO2 present on M-dwarf HZ planets like Prox Cen b.

Where work will proceed is in the model used to make these calculations, which will need to be more complex, as the paper acknowledges:

…when we prescribe water-ice and liquid water clouds, there is a moderate decrease in the SNR of the geometric albedo spectrum from LUVOIR-15 m, with present Earth-level NO2 concentration on an Earth-like planet around a Sun-like star at 10 pc. Clouds and aerosols can reduce the detectability and could mimic the NO2 feature, posing a challenge to the unique identification of this signature. This highlights the need for performing these calculations with a 3-D climate model which can simulate variability of the cloud cover and atmospheric dynamics self-consistently.

The authors consider biosignatures and technosignatures to be “two sides of the same coin,” a nod to the fact that we should be able to search for each at the same time with the next generation of observatories. Finding the common ground between biosignature research and SETI seems overdue, for a positive result for either would demonstrate life’s emergence elsewhere in the universe, and that remains question number one.

The paper is Kopparapu et al., “Nitrogen Dioxide Pollution as a Signature of Extraterrestrial Technology,” accepted at the Astrophysical Journal. (Preprint).

Interstellar Travel and Stellar Evolution

The stars move ever on. What seems like a fixed distance due to the limitations of our own longevity morphs over time into an evolving maze of galactic orbits as stars draw closer to and then farther away from each other. If we were truly long-lived, we might ask why anyone would be in such a hurry to mount an expedition to Alpha Centauri. Right now we’d have to travel 4.2 light years to get to Proxima Centauri and its interesting habitable zone planet. But 28,000 years from now, Alpha Centauri — all three stars — will have drawn to within 3.2 light years of us.

But we can do a lot better than that. Gliese 710 is an M-dwarf about 64 light years away in the constellation Serpens Cauda. For the patient among us, it will move in about 1.3 million years to within 14,000 AU, placing it well within the Oort Cloud and making it an obvious candidate for worst cometary orbit disruptor of all time. But read on. Stars have come much closer than this.

In any case, imagine another star being 14,000 AU away, 20 times closer than Proxima Centauri is right now. Suddenly interstellar flight looks a bit more plausible, just as it would if we could, by some miracle, find ourselves in a globular cluster like M80, where stellar distances, at the densest point, can be something on the order of the size of the Solar System.

Image: This stellar swarm is M80 (NGC 6093), one of the densest of the 147 known globular star clusters in the Milky Way galaxy. Located about 28,000 light-years from Earth, M80 contains hundreds of thousands of stars, all held together by their mutual gravitational attraction. Globular clusters are particularly useful for studying stellar evolution, since all of the stars in the cluster have the same age (about 12 billion years), but cover a range of stellar masses. Every star visible in this image is either more highly evolved than, or in a few rare cases more massive than, our own Sun. Especially obvious are the bright red giants, which are stars similar to the Sun in mass that are nearing the ends of their lives. Credit: NASA, The Hubble Heritage Team, STScI, AURA.

These thoughts are triggered by a paper from Bradley Hansen and Ben Zuckerman, both at UCLA, with the interesting title “Minimal Conditions for Survival of Technological Civilizations in the Face of Stellar Evolution.” The authors note the long-haul perspective: The physical barriers we associate with interstellar travel are eased dramatically if species attempt such journeys only in times of close stellar passage. Put another star within 1500 AU, dramatically closer than even Gliese 710 will one day be, and the travel time is reduced perhaps two orders of magnitude compared with the times needed to travel under average stellar separations near the Sun today.

I find this an interesting thought experiment, because it helps me visualize the galaxy in motion and our place within it in the time of our civilization (whether or not our civilization will last is Frank Drake’s L factor in his famous equation, and for today I posit no answer). All depends upon the density of stars in our corner of the Orion Arm and their kinematics, so location in the galaxy is the key. Just how far apart are stars in Sol’s neighborhood right now?

Drawing on research from Gaia data as well as the stellar census of the local 10-parsec volume compiled by the REsearch Consortium On Nearby Stars (RECONS), we find that 81 percent of the main-sequence stars in this volume have masses below half that of the Sun, meaning most of the close passages we would experience will be with M-dwarfs. The average distance between stars in our neck of the woods is 3.85 light years, pretty close to what separates us from Alpha Centauri. RECONS counts 232 single-star systems and 85 multiple in this space.

Hansen and Zuckerman are intrigued. They ask what a truly patient civilization might do to make interstellar travel happen only at times when a star is close by. We can’t know whether a given civilization would necessarily expand to other stars, but the authors think there is one reason that would compel even the most recalcitrant into attempting the journey. That would be the swelling of the parent star to red giant status. Here’s the question:

As mentioned above, this stellar number density yields an average nearest neighbor distance between stars of 3.85 light years. However, such estimates rely on the standard snapshot picture of interstellar migration − that a civilization decides to embark instantaneously (at least, in cosmological terms) and must simply accept the local interstellar geography as is. If one were prepared to wait for the opportune moment, then how much could one reduce the travel distance, and thus the travel time?

Maybe advanced civilizations don’t tend to make interstellar journeys until they have to, meaning when problems arise with their central star. If this is the case, we might expect stars in close proximity at any given era — ruling out close binaries but talking only about stars that are passing and not gravitationally bound — to be those between which we could see signs of activity, perhaps as artifacts in our data implying migration away from a star whose gradual expansion toward future red giant phase is rendering life on its planets more and more unlivable.

Here we might keep in mind that in our part of the galaxy, about 8.5 kiloparsecs out from galactic center, the density of stars is what the authors describe as only ‘modest.’ Higher encounter rates occur depending on how close we want to approach galactic center.

Reading this paper reminds me why I wish I had the talent to be a science fiction writer. Stepping back to take the ‘deep time’ view of galactic evolution fires the imagination as little else can. But I leave fiction to others. What Hansen and Zuckerman point out is that we can look at our own Solar System in these same terms. Their research shows that if we take the encounter rate they derive for our Sun and multiply it by the 4.6 billion year age of our system, we can assume that at some point within that time a star passed within a breathtaking 780 AU.

Image: A passing star could dislodge comets from otherwise stable orbits so that they enter the inner system, with huge implications for habitable worlds. Is this a driver for travel between stars? Credit: NASA/JPL-Caltech).

Now let’s look forward. A gradually brightening Sun eventually pushes us — our descendants, perhaps, or whatever species might be on Earth then — to consider leaving the Solar System. Recent work sees this occurring when the Sun reaches an age of about 5.7 billion years. Thus the estimate for remaining habitability on Earth is about a billion years. The paper’s calculations show that within this timeframe, the median distance of closest stellar approach to the Sun is 1500 AU, with an 81 percent chance that a star will close to within 5000 AU. From the paper:

Thus, an attempt to migrate enough of a terrestrial civilization to ensure longevity can be met within the minimum requirement of travel between 1500 and 5000 AU. This is two orders of magnitude smaller than the current distance to Proxima Cen. The duration of an encounter, with the closest approach at 1500 AU, assuming stellar relative velocities of 50km/s, is 143 years. In the spirit of minimum requirements, we note that our current interstellar travel capabilities are represented by the Voyager missions (Stone et al. 2005); these, which rely on gravity assists off the giant planets, have achieved effective terminal velocities of ∼ 20 km/s. The escape velocity from the surface of Jupiter is ∼ 61 km/s, so it is likely one can increase these speeds by a factor of 2 and achieve rendezvous on timescales of order a century.

My takeaway on this parallels what the authors say: We can conceive of an interstellar journey in this distant era that relies on technologies not terribly advanced beyond where we are today, with travel times on the order of a century. The odds on such a journey being feasible for other civilizations rise as we move closer to galactic center. At 2.2 kiloparsecs from the center, where peak density seems to occur, the characteristic encounter distance is 250 AU over the course of 10 billion years, or an average 800 AU during a single one billion year period.

You might ask, as the authors do, how binary star systems would affect these outcomes, and it’s an interesting point. Perhaps 80 percent of all G-class star binaries will have separations of 1000 AU or less, which the authors consider disruptive to planet formation. Where technological civilizations do arise in binary systems, having a companion star is an obvious driver for interstellar travel. But single stars like ours would demand migration to another system.

We can plug Hansen and Zuckerman’s work into the ongoing discussion of interstellar migration. From the paper:

Our hypothesis bears resemblance to the slow limit in models of interstellar expansion (Wright et al. 2014; Carroll-Nellenback et al. 2019). In a model in which civilizations diffuse away from their original locations with a range of possible speeds, the behavior at low speeds is no longer a diffusion wave but rather a random seeding dominated by the interstellar dispersion. Even in this limit, the large age of the Galaxy allows for widespread colonization unless the migration speeds are sufficiently small. In this sense our treatment converges with prior work, but our focus is very different. We are primarily interested in how a long-lived technological civilization may respond to stellar evolution and not how such civilizations may pursue expansion as a goal in and of itself. Thus our discussion demonstrates the requirements for technological civilizations to survive the evolution of their host star, even in the event that widespread colonization is physically infeasible.

It’s interesting that the close passage of a second star is a way to reduce the search space for SETI purposes if we go looking for the technological signature of a civilization in motion. Separating out stars undergoing close passage from truly bound binaries is another matter, and one that would, the authors suggest, demand a solid program for eliminating false positives.

Ingenious. An imaginative exercise like this, or Greg Laughlin and Fred Adams’ recent work on ‘black cloud’ computing, offers us perspectives on the galactic scale, a good way to stretch mental muscles that can sometimes atrophy when limited to the near-term. Which is one reason I read science fiction and pursue papers from people working the far edge of astrophysics.

The paper is Hansen and Zuckerman, “Minimal conditions for survival of technological civilizations in the face of stellar evolution,” in process at the Astronomical Journal (preprint). Thanks to Antonio Tavani for the pointer on a paper I hadn’t yet discovered.