Paul Gilster's Blog, page 73

Of Sails and Supernovae

When we consider pushing a sail to 20 percent of lightspeed, which is the target velocity for Breakthrough Starshot, it’s interesting to think about how laser propulsion differs from sunlight. After all, while constructing a huge laser array presents numerous challenges on Earth, we already have a star to work with, and sail technology that is beginning to be tested in space. Consider, too, that we have operational spacecraft like the Parker Solar Probe that are exploring regions close to the Sun, helping us learn more about heat shields, even as we plan missions like Solar Cruiser, whose sail would enable interesting non-Keplerian orbits near the Sun.

Wouldn’t it be easier to find ways to use our Sun’s own energies to drive our starship by getting a boost from gravitational effects? A closer look reveals the power of solar sails in nearby space (i.e., within the system), while illuminating the problems at interstellar distances.

For getting a solar sail up to the highest possible speeds, we will probably need an Oberth maneuver, which means falling deep into a gravitational well and applying propulsion at the time when velocity in that well peaks, which squeezes maximum effect out of our boost. With a sail, that would mean a so-called ‘Sundiver’ mission, bringing a furled sail shielded by an occulter (perhaps a small asteroid) breathtakingly close to the Sun and then opening the sail at perihelion for the best possible kick.

Depending on shielding and thus how close we can get to our star, we do get a substantial boost over the velocities of our two interstellar Voyagers, one of which moves at just over 17 kilometers per second. We might squeeze 100 kilometers per second out of the Sundiver maneuver, and perhaps a bit more, with future craft using sails made out of new metamaterials achieving even better. But we’re nowhere near 20 percent of the speed of light.

Solar sails certainly have broad applications as we build a Solar System infrastructure, and we can use them with Sundiver maneuvers to get into nearby interstellar space. But if we find long mission times unacceptable, we need to be thinking of alternatives when we mount a true star mission.

Other civilizations won’t necessarily have to work with the same constraints, depending on the kind of star they orbit. I’m always interested in how intelligence might exploit natural objects to achieve interstellar goals, so I took note when I saw Avi Loeb’s latest piece in Scientific American. Loeb (Harvard University), who chairs Breakthrough Starshot, is not one to avoid speculation, without a healthy dose of which we can hardly work with concepts involving SETI and highly evolved civilizations.

So let’s leave the realm of what humans can do with our small G-class star far behind as we consider what sufficiently advanced technologies might attempt. If a culture were to be a billion or more years old, how would we know where to look for it? One way into the problem is to consider the need for energy useful to Kardashev Type II and III civilizations, energy which is available in abundance around certain natural phenomena.

And given the recent attention to Betelguese and the question of when it might become a supernova, Loeb has been moved to consider the power such an event would unleash. The results make working with the flux from a G-class star seem trivial indeed. Even the best Sundiver maneuver at Sol yields velocities that would take hundreds of years to reach the nearest stars. A similar maneuver around the most luminous stars we know might reach 10 percent of lightspeed (not too shabby!). But for maximum kick, a lightsail riding the shockwave of a supernova from a star like Betelguese or Eta Carinae could be pushed to a high percentage of c.

Image: Hubble Space Telescope-Image of Supernova 1994D (SN1994D) in galaxy NGC 4526 (SN 1994D is the bright spot on the lower left). Credit: NASA/ESA.

Could a civilization time a supernova explosion accurately enough to ride the shockwave? That’s a question we can’t answer, nor can we say what kind of timeframes such a culture might operate within. Loeb imagines numerous lightsails parked around a star nearing the end of its life, perhaps placed by a civilization nearby, and perhaps left there indefinitely. If this civilization wanted to use the supernova for propulsion, it would still face numerous problems:

First, as in Starshot, the sails must be highly reflective so as not to absorb too much heat and burn up. Second,once the sails are placed in orbit around the massive star, they will be pushed away by the bright starlight or mass loss prior to the explosion. To avoid this danger, one could deploy the sails in a folded configuration and equip them with a switch that would open them up like umbrellas as soon as the explosion flash begins to rise. Third, even though the launch can start from a distance that is a hundred times larger than the size of the exploding star, care must be taken in selecting particularly empty acceleration paths – clear of any stellar debris.

Indeed, given dust particles with a relative speed close to the speed of light, such a sail would have to be folded to reduce the area it presents in the direction of flight as soon as it reached peak velocity. An even wilder prospect: A massive enough star (Loeb mentions Eta Carinae) collapsing into a black hole could produce gamma-ray bursts (GRBs) that would drive the Lorentz factor to extreme levels. Now we’re in range of Poul Anderson’s ‘Leonora Christine,’ the runaway starship in the novel Tau Zero that crosses galaxies in far less than a human lifetime as measured by the crew, while aeons pass outside their rest frame.

We’re in Dyson sphere country here, a reference Loeb himself makes, looking into ways advanced civilizations could harvest high energy sources around them. A Dyson sphere or array, gathering the maximum amount of stellar light, might be found by its infrared signature, so Dysonian SETI, which looks for evidence of ETI in our astronomical records, has a target.

Clément Vidal has also worked this notion in his book The Beginning and the End (Springer, 2014), looking into questions like extracting energy from the accretion disk around a rotating black hole or tapping the power of X-ray binaries. We are also in Olaf Stapledon territory, asking about extraterrestrial engineering that is inconceivable to ourselves, but possibly visible as a SETI signature in the ‘watering holes’ for energy the universe makes available.

It’s hard for me to imagine a civilization with the patience to wait out a supernova explosion to drive a lightsail, but when we’re dealing with technologies that may be several billion years beyond our own, we have no business imposing our own limitations on the cosmos. Anomalies in our observations of supernova remnants would be worth investigating, although Loeb admits to the difficulty of isolating artificial components within them. Even so, dying stars conceivably have reason to be pondered by civilizations advanced enough to make use of their energy.

New Horizons Parallax Program Targets Proxima Centauri, Wolf 359

In a few short months, New Horizons will be almost 8 billion kilometers out, a distance that still boggles the mind until we remember that Voyager 1 has reached 22.2 billion kilometers (over 148 AU). Then, of course, we’re humbled again with the thought that the inner Oort Cloud is thought to be between 2,000 and 5,000 AU from the Sun, with an outer edge that could extend as far as halfway to the nearest star. That star, Proxima Centauri, is 268,770 AU from us.

As New Horizons hunts Kuiper Belt objects for the next flyby, the spacecraft is now being used to perform parallax studies to detect the apparent ‘shift’ in the relative position of nearby stars as compared with what we see on Earth. Earth’s orbit is about 300 million kilometers in diameter, so we see that apparent shift by comparing observations taken half a year apart. That’s a pretty decent baseline, but if we extend the baseline, as now with New Horizons, we can see better parallax effects, and thus tighten the distance measurements we make from them.

The most celebrated name in parallax studies is German astronomer and mathematician Friedrich Bessel, who in 1838 obtained the first stellar parallax measurements to determine the distance of the star 61 Cygni. Here again it’s humbling to reflect that the distances to the nearest stars are almost half a million times greater than the baseline offered by Earth’s orbit, which makes Bessel’s feat all the more impressive. Bessell worked out a distance of about 10 light years, not all that far off the 11.4 light year distance modern astronomers have estimated for 61 Cygni.

I might mention that Bessell (1784-1846) was also the first astronomer to predict the existence of an unseen companion around another star, announcing from his study of Sirius that deviations in its motion indicated a companion that we now know as the white dwarf Sirius B.

But back to New Horizons. On April 22 and 23, the spacecraft will take images of Proxima Centauri and Wolf 359. These can be used in combination with Earth-based images taken on the same dates to yield what the New Horizons team is calling ‘a record-setting parallax measurement,’ one that will be made in coordination with Earth-based observatories and a public observing campaign. Amateur astronomers with a camera-equipped, 6-inch or larger telescope are invited to participate. Says New Horizons principal investigator Alan Stern:

“These exciting 3D images, which we’ll release in May, will be as if you had eyes as wide as the solar system and could detect the distance of these stars yourself. It’ll be a truly vivid demonstration of the immense distance New Horizons has traveled, and a cool way to take advantage of the spacecraft’s unique vantage point out on the very frontier of our solar system!”

Image: Color images of the Wolf 359 (top) and Proxima Centauri star fields, obtained in late 2019. The large proper motions of both stars (at the center of each image) will cause them to shift by over an arcsecond by April 2020, when NASA’s New Horizons spacecraft, nearly five billion miles (8 billion kilometers) from Earth, will image them. A green circle provides a rough estimate of where both stars will appear in the New Horizons images. Credit: William Keel/University of Alabama/SARA Observatory.

A new way to find our way around nearby interstellar space? New Horizons science team member Tod Lauer (National Science Foundation Optical-Infrared Astronomy Research Laboratory) points out the method’s history and its future promise:

“For all of history, the fixed stars in the night sky have served as navigation markers. As we voyage out of the solar system and into interstellar space, how the nearer stars shift can serve as a new way to navigate. We will see this for the first time with New Horizons.”

More details are available here for those interested in participating in the New Horizons Parallax program. I’m anxious to see the results, and also reminded how limited parallax measurements have historically been when confined to Earth’s orbit around the Sun. New Horizons points the way to a future in which we extend the baseline even further. Imagine a mission like Claudio Maccone’s FOCAL, an observing instrument collecting light at distances greater than 550 AU, where the gravitational lensing effects of the Sun become observable. And who knows, perhaps one day we’ll have a baseline as far as the Alpha Centauri stars to help map the Orion Arm.

As Spitzer’s Mission Ends, First Light for CHEOPS

Farewell to Spitzer after more than 16 years of infrared observations of the universe. We’ve recently looked at the observatory’s accomplishments (see Looking Back at the Spitzer Space Telescope), but I want to run the photo below to celebrate the team that managed it.

Image: Spitzer Project Manager Joseph Hunt stands in Mission Control at NASA’s Jet Propulsion Laboratory in Pasadena, California, on Jan. 30, 2020, declaring the spacecraft decommissioned and the Spitzer mission concluded. Credit: NASA/JPL-Caltech.

Meanwhile, we have the good news that the European Space Agency’s CHEOPS (CHaracterising ExOPlanet Satellite), which was launched from Kourou (French Guiana) on December 18, has completed its early orbit phase, involving instrument tests and calibration, and has now opened its telescope cover, exposing the focal plane to starlight.

The space observatory carries a 95-cm long baffle that shields its telescope from stray light and minimizes light contamination from sources like the nearby Earth (CHEOPS is in a 700 kilometer heliosynchronous orbit, passing over any given point on Earth’s surface at the same local time). What has been removed is the baffle cover — ESA likens it to the lens cap of a camera — which had protected the instrument from dust and bright light during the initial phases of in-orbit commissioning. The procedure was considered mission-critical, and its successful completion means the telescope cover is now permanently locked in the open position.

Francesco Ratti is a CHEOPS instrument engineer for ESA:

“The opening mechanism is known to be extremely reliable, as it was extensively tested on the ground and already flown on previous space missions, but it was still quite a nerve-wracking moment to witness, and we are all very excited now that the telescope has opened its eye to the Universe.”

Image: The telescope baffle cover of the Cheops satellite, pictured here during spacecraft testings in the cleanroom at Airbus Defence and Space Spain, Madrid, protected the mission’s science instrument from dust and bright light during testing, launch and the early phases of in-orbit commissioning. The cover lid consists of an aluminium frame covered with copper-coloured, multi-layer insulation (MLI) sheeting. The spring-loaded hinge mechanism that connects the cover lid to the baffle is clearly visible in the foreground of the image. On the opposite side of the baffle (not visible in this photo), a locking mechanism held the cover closed. Credit: ESA.

Bear in mind that 80 percent of the observing time on CHEOPS is set aside for a program defined by the science team, but the remaining 20 percent is available to the astronomical community through ESA’s guest observer program, so proposals can be submitted for peer review in a competitive selection process. The observatory will study bright stars already known to host planets, using transit techniques to investigate their physical and chemical properties. For more, see CHEOPS Enters the Game, a Centauri Dreams post from late December.

Voyager 2 Recovers

When one of our Voyagers experiences a blip of any kind, it gets my attention. It’s not like we have any other options outside the heliosphere right now.

Both Voyagers have fault protection software that allows the spacecraft to protect themselves if problematic situations occur. And a problem did indeed surface aboard Voyager 2 on January 25, when there seems to have been a delay in the onboard execution of commands for a scheduled maneuver. The latter was a 360 degree rotation to be executed as a way of calibrating the craft’s magnetic field instrument, and the result of the delay was that two systems that consume power at relatively high levels were operating at the same time.

Not a good idea. Right now, with power dwindling inexorably, the Voyager missions are both dominated by power management. Hence the shutdown of Voyager 2’s science instruments to make up for the power deficit, as reported by the Voyager team on Twitter:

Here's the skinny: My twin went to do a roll to calibrate the onboard magnetometer, overdrew power and tripped software designed to automatically protect the spacecraft.

Voyager 2's power state is good and instruments are back on. Resuming science soon. https://t.co/4buDM32bap pic.twitter.com/4T856Lpxjm

— NASA Voyager (@NASAVoyager) January 29, 2020

The Voyager team was able to turn off one of the high power systems on January 28, and has now turned the science instruments back on, although data acquisition has not resumed yet. What’s needed is a review of spacecraft status as a check to ensure that returning to normal operations is warranted. What I want to focus on here is the distance this diagnostic work is being conducted at, about 18.5 billion kilometers. Bear in mind that one way communications require 17 hours, with another 17 hours for the response.

Thus 34 hours are required between sending a command and finding out whether it has had the desired effect on the spacecraft. We’re dealing with 1970s technology here, a fact that emphasizes the role that spacecraft autonomy will increasingly play as we push into the immediate interstellar neighborhood, when such operations will need to be handled aboard the spacecraft from start to finish. It’s a tribute to the quality of the Voyager design that we are still able to make the craft function given communications delays that were never anticipated.

We’re also working with two spacecraft whose power supply is heading for exhaustion. The radioisotope thermoelectric generator (RTG) that powers up the Voyagers turns heat from radioactive decay into electricity, but such decay reduces Voyager 2’s power budget by about 4 watts per year. We’ve looked before in these pages at the balancing act this requires of controllers, who last year turned off the primary heater for the Voyager 2 cosmic ray subsystem to compensate for the decrease in power, although the instrument is still in operation.

Image: This artist’s concept depicts NASA’s Voyager 1 spacecraft entering interstellar space, or the space between stars. Interstellar space is dominated by the plasma, or ionized gas, that was ejected by the death of nearby giant stars millions of years ago. The environment inside our solar bubble is dominated by the plasma exhausted by our Sun, known as the solar wind. Credit: NASA/JPL-Caltech.

Imagine trying to keep all these balls in the air at the same time, and then factor in the need to manage the temperature of the various systems, which can be manipulated through heaters or excess heat from the various instruments and systems onboard. The critical computer system, radio transmitter and receiver electronics and other instruments would quickly cool to mere tens of degrees above absolute zero without heat management, and now, with the interplay of instruments being shut down, power savings have to be weighed against temperature.

We’re in what NASA calls the Voyager Interstellar Mission, and we have scant time remaining, despite the heroic efforts of controllers, to continue receiving data before the science instruments have to be turned off in their entirety. But for now our first interstellar craft, even if not remotely designed for that purpose, are still communicating and capable of sending data.

Keeping Voyager alive is a tale worth telling and will one day produce a book of its own. For now, though, I recommend Jim Bell’s The Interstellar Age and Stephen Pyne’s Voyager: Seeking Newer Worlds in the Third Great Age of Discovery. Bell slants more toward science and engineering, while Pyne opts for context and philosophy. Someone in the Voyager interstellar team is bound to produce the next book, which will doubtless become a testament to making missions achieve the seemingly impossible.

DART & Hera: Changing an Asteroid’s Trajectory

Asteroids are objects of obvious scientific interest, not only for their intrinsic properties but also our need to understand how we can change their motion in space in case one looks like it will come dangerously close to Earth in the future. OSIRIS-REx is extracting all kinds of valuable data from asteroid 101955 Bennu, but we should also keep in mind that Bennu itself is a potentially hazardous object, with a small chance (1-in-2700, according to current estimates) of striking the Earth between 2175 and 2199. Thus the second ‘S’ in OSIRIS, which stands for ‘security’, and is all about measuring the factors that affect the object’s trajectory.

When we get samples from Bennu, we’ll have a better idea about the asteroid’s chemistry and morphology, useful for understanding the early Solar System as well as assessing how hazardous such an object is. But we need to know more, which is where NASA’s Double Asteroid Redirection Test (DART) mission comes in. Here the purpose is planetary defense from the start, for DART will demonstrate how a kinetic impactor can change the motion of an asteroid in space. The target is a binary near-Earth asteroid called (65803) Didymos.

Image: Simulated image of the Didymos system, derived from photometric lightcurve and radar data. The primary body is about 780 meters in diameter and the moonlet is approximately 160 meters in size. They are separated by just over a kilometer. The primary body rotates once every 2.26 hours while the tidally locked moonlet revolves about the primary once every 11.9 hours. Almost one sixth of the known near-Earth asteroid (NEA) population are binary or multiple-body systems. Credit: Naidu et al., AIDA Workshop, 2016.

DART will carry an imaging instrument called DRACO (Didymos Reconnaissance & Asteroid Camera for OpNav), which is based on the now familiar LORRI high-resolution imager that flew on New Horizons, and will use roll-out solar arrays (each 8.6 meters by 2.3 meters) and a NEXT-C ion engine for propulsion. The plan is simplicity itself: DART will crash into the Didymos moonlet at 6.6 kilometers per second, which should change the moonlet’s orbital speed around the main body by a fraction of one percent, and the orbital period by several minutes.

Flying with DART will be LICIA, the Light Italian CubeSat for Imaging of Asteroid, which will observe the impact ejecta in the early phase of crater formation following the impact. The dynamic changes DART’s impact produces will be measured partly by what LICIA learns about the fallback ejecta on both asteroids and the subsequent Hera observations of unweathered fresh material on the two objects. NASA describes LICIA this way:

The LICIA Cube is a 6U CubeSat provided by the Italian Space Agency. It will be carried along with DART to Didymos and released approximately 2 days before the DART impact. LICIA Cube will perform a separation maneuver to follow behind DART and return images of the impact, the ejecta plume, and the resultant crater as it flies by. It will also image the opposite hemisphere from the impact. LICIA Cube is 3-axis stabilized and has a propulsion capability of 56 m/s. The onboard imager has a 7.6 cm aperture, F/5.2 telescope, and an IFOV of 2.9 arcsec/pixel.

Image: Two different views of the DART spacecraft. The DRACO (Didymos Reconnaissance & Asteroid Camera for OpNav) imaging instrument is based on the LORRI high-resolution imager from New Horizons. The left view also shows the Radial Line Slot Array (RLSA) antenna with the ROSAs (Roll-Out Solar Arrays) rolled up. The view on the right shows a clearer view of the NEXT-C ion engine. Credit: NASA.

The European Space Agency’s Hera mission — powered by solar arrays with a hydrazine propulsion system — is to be the follow-up, making a post-impact survey using high-resolution visual, laser and radio science to map what will be the smallest asteroid yet visited by spacecraft. The DART collision is scheduled for 2022, with immediate results probably hidden by an expected dust cloud. Hera will investigate the asteroid impact crater and surrounding surface in 2026, allowing scientists to refine their numerical models of the impact process. All of this works toward building a deflection technique for planetary defense.

Earth-based observations of the Didymos system gathered during its close approach in February-May 2017 were analyzed at a workshop in Prague in 2018, allowing constraints to be placed on the strength of the YORP effect, which results from uneven heating on the surface that can alter the object’s spin state. Further observations will tighten these constraints, making the effects of the DART impact easier to separate from the pre-impact state of the system.

A key Hera role in all this will be to measure Didymos’ mass, which will help scientists calculate the efficiency of the impact momentum transfer once we’ve also measured the change in the small moon’s orbital period. Thus the two missions will result in accurate modeling of the response to the impact as well as the likely internal structure of the asteroid. Hera will be carrying two six-unit cubesats of its own to provide spectral measurements of the surface of both asteroids, with a Cubesat called APEX (Asteroid Prospection Explorer) actually landing on one of them. A second CubeSat (Juventas) will measure the gravitational field and internal structure of the small moon, doing a low-frequency radar survey of the asteroid interior.

The DART launch window opens in late July of 2021, with launch aboard a SpaceX Falcon 9, with intercept of the Didymos moonlet in late September of 2022, when the system is about 11 million kilometers from Earth. Earth-based telescopes and planetary radar will be able to measure the effects of the impact to back up the findings of the spacecraft on the scene. The results should be small but highly useful in giving us data on how impacts affect asteroids of this size, with the added benefit of enhancing international cooperation on a matter of global importance.

OSIRIS-REx: Asteroid Sample Site Flyover

The latest operations of the OSIRIS-REx spacecraft at asteroid Bennu remind me how powerful a wave we’ve unleashed in the coupling of robotics and ever more capable spacecraft components. We’re not exactly at the stage of ‘routine’ asteroid missions, but Hayabusa2 and OSIRIS-REx when seen in the context of upcoming missions like NASA’s DART experiment and the European Space Agency’s Hera are part of our renaissance of this class of object, with results beneficial to science but also practically useful in terms of future impact mitigation. More on DART and Hera tomorrow.

Small objects have plenty to say about our future in space, and I haven’t even mentioned Lucy, which will be studying multiple Jupiter trojans, or the Psyche mission targeting what may be the exposed core of a planetary embryo, or for that matter, the remarkably successful Dawn, which unlocked so many mysteries at Vesta and Ceres. It goes without saying that having an operational spacecraft in the Kuiper Belt is likewise a sign that we are getting pretty good at doing robotic exploration even as we continue to wrestle with the human role on the Moon and Mars.

OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer) has now completed a 620 meter flyover of the site called ‘Nightingale’ on its target asteroid as part of the mission’s analysis of the primary sample collection site. To make this happen, the spacecraft left a 1.2 kilometer home orbit and performed an 11-hour transit over the asteroid, accumulating data about the 16-meter wide sample site and returning to safe orbit.

Image: During the OSIRIS-REx Reconnaissance B flyover of primary sample collection site Nightingale, the spacecraft left its safe-home orbit to pass over the sample site at an altitude of 0.4 miles (620 m). The pass, which took 11 hours, gave the spacecraft’s onboard instruments the opportunity to take the closest-ever science observations of the sample site. Credit: NASA/Goddard/University of Arizona.

The spacecraft has been compiling a Natural Feature Tracking image catalog by way of mapping the tiniest details among the boulders and craters of the landing site. The OSIRIS-REx team is also studying observations from the spacecraft’s Thermal Emissions Spectrometer (OTES), the OSIRIS-REx Visual and InfraRed Spectrometer (OVIRS), the OSIRIS-REx Laser Altimeter (OLA), and the MapCam color imager.

So there’s a lot happening at Bennu, including an upcoming flyover, scheduled for February 11, of the backup sample site, which has been given the name ‘Osprey.’ Further flybys in March (Nightingale) and May (Osprey) will take OSIRIS-REx even closer to the surface as the spacecraft goes into Reconnaissance C phase, operating at 250 meters. Assuming all goes well, a multi-hour sampling maneuver will begin in August, using the spacecraft’s robotic arm and sampler head to make contact with the asteroid. We will eventually gain between 60 and 2000 grams of surface material, with return to Earth scheduled for September of 2023.

Image: This artist’s concept shows OSIRIS-REx contacting asteroid Bennu with its sample return instrument. Credit: NASA’s Goddard Space Flight Center.

Dwarf Novae: Mining Kepler Data for New Discoveries

Cataclysmic variable stars (CVs) are binary phenomena, usually consisting of a white dwarf that is accreting material out of a nearby companion star. As you would imagine, a wide range of CVs in various stages of accretion and subsequent outburst can be detected. When the accretion disk around the white dwarf becomes unstable, we get what is known as a dwarf nova (DN), and in systems with orbital periods less than two hours, there can be much more violent outbursts, feeding off orbital resonances in the orbit of the two stars.

Now we have a newly discovered cataclysmic variable (KSN:BS-C11a) in an interesting configuration, a white dwarf apparently feeding off a brown dwarf companion that is about 10 times less massive. The ‘super-outburst’ from the dwarf nova turned up in data from the decommissioned Kepler Space Telescope. Grad student Ryan Ridden-Harper (Australian National Observatory), lead author of the paper on this work, likes to refer to this cataclysmic variable as ‘a vampire star system.’ Whatever we call it, the level of activity here is noteworthy:

“The incredible data from Kepler reveals a 30-day period during which the dwarf nova rapidly became 1,600 times brighter before dimming quickly and gradually returning to its normal brightness. The spike in brightness was caused by material stripped from the brown dwarf that’s being coiled around the white dwarf in a disk. That disk reached up to 11,700 degrees Celsius at the peak of the super-outburst.”

Image: An artist’s impression of a star ‘feeding’ off a nearby brown dwarf. Credit: NASA and L. Hustak (STScI)

Still unexplained is the slow rise in brightness that preceded the outburst. Ridden-Harper has been working with colleagues at ANU as well as the Space Telescope Science Institute (STScI) and the University of Notre Dame. What catches my eye about the discovery of this bright transient is the data mining that turned it up. The team had been searching for new transients in the K2 and Kepler campaigns in a project known as K2: Background Survey. The idea here is that each science target in the data is accompanied by background pixels that have been observed at high cadence, meaning a short time period before re-observing the same target.

These background pixels, the authors note, can contain transient signals that have thus far gone undetected. The paper describes the K2: Background Survey as:

…a systematic search for transients in K2/Kepler background pixels. K2:BS independently analyses each pixel to detect abnormal behaviour. This is done by searching for pixels that rise above a brightness threshold set from the median brightness and standard deviation through a campaign. Telescope motion presents a challenge in candidate detection as science targets may drift into background pixel, triggering false events. False triggers are screened by vetting of events that last < 1 day, chosen for candidates with the 6 hourly telescope resets. Coincident pixels that pass the vetting procedure are grouped into an event mask. All candidate events are checked against the NASA/IPAC Extragalactic Database (NED) 1 and the SIMBAD database (Wenger et al. 2000) to identify potential hosts.

As to the parameters of the system, the brown dwarf orbits the white dwarf every 83 minutes at a distance of approximately 400,000 kilometers, close enough to allow the rapid growth of the accretion disk as material spirals from the brown dwarf inward toward the host star. The Kepler cadence of 30 minutes turns out to have been the key to making these observations, which are particularly useful because only about 100 such dwarf novae systems have been catalogued.

Image: This is Figure 3 from the paper. Caption: The K2/Kepler light curve of the transient KSN:BS-C11a shown with the 30-minute cadence. The time axis is shown in barycentric Julian days and the flux has been converted to Kepler magnitudes. Credit: Ridden-Harper et al.

Thus Kepler/K2 keeps on producing good science, in this case finding a transient whose rarity stems partially from the years or decades such a system can spend between outbursts. We shouldn’t be too surprised that Kepler data can produce such results — after all, the telescope was designed to study the transits of planets across the face of a star, making it ideal for spotting any objects that brighten or dim over time — but now we can see the potential for detecting other rare transient events both in archival data and data from ongoing missions like TESS.

The paper is Harper et al., “Discovery of a new WZ Sagittae-type cataclysmic variable in the Kepler/K2 data,” Monthly Notices of the Royal Astronomical Society,” Vol. 490, Issue 4 pp. 5551-5559 (abstract).

From Spitzer to JWST’s Early Targets

Yesterday’s post on the Spitzer Space Telescope leads naturally to the targets it produced for its successor. For when Spitzer’s mission ends on January 30, we have the far more powerful James Webb Space Telescope, also operating at infrared wavelengths, in queue for a 2021 launch. In many ways, Spitzer has been the necessary precursor for JWST, for it was the need to operate a telescope at extremely low temperatures in order to maximize infrared sensitivity that drove Spitzer design. JWST must maintain its gold-coated beryllium mirror at similarly precise temperatures.

With over 8,700 scientific papers published based on Spitzer findings, a number that will continue to grow for many years, a path has been charted that JWST will follow in the form of observations early in its mission. Consider WASP-18b, a gas giant of ten times Jupiter mass in a tight orbit around its star. Data from both Spitzer and Hubble showed in 2017 that the planet is laden with carbon monoxide and all but devoid of water vapor. No other extrasolar planet can match this one in the way carbon monoxide dominates its upper atmosphere.

What’s going on in the atmosphere of this planet merits close study because it’s extreme even for ‘hot Jupiters’ in being so close to its star that it may not survive another million years. Expect a long look from JWST into the processes at work here. Nikku Madhusudhan (University of Cambridge) was a co-author on the 2017 paper describing the WASP-18b findings:

“The only consistent explanation for the data is an overabundance of carbon monoxide and very little water vapor in the atmosphere of WASP-18b, in addition to the presence of a stratosphere. This rare combination of factors opens a new window into our understanding of physicochemical processes in exoplanetary atmospheres.”

The image below implies the method: Transmission spectroscopy. We can look at the light of the star passing through the atmosphere of the planet as it moves around the star in its orbit.

Image: A NASA-led team of scientists determined that WASP-18b, a “hot Jupiter” located 325 light-years from Earth, has a stratosphere that’s loaded with carbon monoxide, or CO, but has no signs of water. Credit: Goddard Space Flight Center.

At TRAPPIST-1, expect the fourth planet, TRAPPIST-1e, to receive early JWST scrutiny because of its density and surface gravity, both similar to Earth’s, in combination with incoming stellar flux sufficient to keep temperatures in the range needed for water on the surface. JWST should be able to tell us whether this planet does indeed have an atmosphere, and assuming it does, whether molecules like carbon dioxide or water vapor are present.

Here again Spitzer helped set the table, working with ground-based telescopes to confirm the first two candidates (found by the Transiting Planets and Planetesimals Small Telescope in Chile) and discover the other five. Our ideas of what these planets look like will change with the new data JWST, 1000 times more powerful than Spitzer, will bring in. The Hubble instrument has not been able to detect evidence for a hydrogen-dominated atmosphere on TRAPPIST-1d, e and f, making rocky composition likely. But it’s going to take JWST to further clarify the presence of atmospheres on the seven worlds and begin the study of their chemistry.

All of that should take what is a very fanciful image (below) and help us determine how far from reality it actually is. At present we’re simply injecting sparse data into the realm of art. As Nikole Lewis (Cornell University) says, “The diversity of atmospheres around terrestrial worlds is probably beyond our wildest imaginations. Getting any information about air on these planets is going to be very useful.”

Image: This artist’s concept shows what the TRAPPIST-1 planetary system may look like, based on available data about the planets’ diameters, masses and distances from the host star, as of February 2018. Credit: NASA/JPL-Caltech.

Spitzer’s work on 55 Cancri e will also inform early JWST studies of the system. Spitzer was able to produce data leading to the first temperature map of a super-Earth, in this case an apparently rocky world about twice the size of our own. Lava flows may be the cause of the extreme temperature swings between one side of the planet and the other, as noted in 2016 by Brice Olivier Demory (University of Cambridge), who was lead author of the paper in Nature:

“Our view of this planet keeps evolving. The latest findings tell us the planet has hot nights and significantly hotter days. This indicates the planet inefficiently transports heat around the planet. We propose this could be explained by an atmosphere that would exist only on the day side of the planet, or by lava flows at the planet surface.”

Spitzer put in 80 hours of infrared telescope time in the 55 Cancri e work, watching this tidally locked world move about its star and allowing the construction of the temperature map. Mission scientists pushed Spitzer hard in accumulating their data, using novel calibration techniques to extract maximum results from a detector that had not been design to work at such high precision. Now JWST will help sharpen the map’s focus to explain its unusual temperature swings, which argue against a thick atmosphere and global temperature distribution.

Image: 55 Cancri e is tidally locked to its star, just as our moon is to Earth, which means that one side always sizzles under the heat of its star while the other side remains in the dark. If the planet were covered in lava, then the hot, sun-facing side of the planet would have liquid lava flows, while the colder, dark side would see solidified lava rock. The hardened lava would be unable to transport heat across the planet, explaining why Spitzer detected that the cold side of the planet is much colder than the hot side. Such a lava planet, if it exists, would have dust streaming off of it, as illustrated here. Radiation and winds from the nearby star would blow off the material. Scientists say that future observations with NASA’s upcoming James Webb Space Telescope should provide more details about the nature of this exotic world. Credit: NASA/JPL-Caltech.

Both Spitzer and Webb are sensitive to the infrared glow of gas and dust orbiting in circumstellar rings around stars, which means JWST will be able to extend our knowledge of planetary formation. The same sensitivity will make JWST the instrument of choice in the study of brown dwarfs, an area where Spitzer has already been able to examine clouds in brown dwarf atmospheres. What’s interesting here are the differences between the distribution and motion of clouds on brown dwarfs and the atmospheric boiling seen on true stars. JWST will investigate winds that seem reminiscent of the belts of Jupiter, Saturn, Uranus and Neptune.

I’ve focused on exoplanetary targets here, but of course the handoff from Spitzer to JWST will also involve using the large surveys of Spitzer and the Hubble instrument to furnish JWST with targets like GN-z11, now the most distant galaxy ever measured. Sean Carey is manager of the Spitzer Science Center at Caltech/IPAC in Pasadena:

“Spitzer surveyed thousands of galaxies, mapped the Milky Way and performed other groundbreaking feats by looking at large areas of the sky. Webb won’t have this capability, but it will revisit some of the most interesting targets in the Spitzer surveys to reveal them in amazing clarity.”

JWST’s higher sensitivity should make it possible to find galaxies that are even older. It will also home in on luminous infrared galaxies (LIRG), which Spitzer found to be producing far more energy per second than typical galaxies, most of it in the form of far-infrared light. Star formation and galactic mergers come into focus, as does the growth of supermassive black holes. All of this depends, of course, on getting a fabulously complex telescope plagued by cost overruns into its future home at the L2 Lagrangian point 1.5 million kilometers from the Earth.

All launches are scary, but this one more than most. We need Spitzer’s successor to fly.

Looking Back at the Spitzer Space Telescope

The Spitzer Space Telescope, which is to end its mission on January 30, has a special place in my memory. I was making a trip to the Jet Propulsion Laboratory as part of the research for my Centauri Dreams book when I noticed on a monitor a countdown — still in days — for the launch of Spitzer, then known as the Space Infrared Telescope Facility (SIRTF).

The observatory was launched on the 25th of August, 2003. I remember hot Pasadena weather, a conversation with aerospace legend Adrian Hooke (he was a member of the Kennedy Space Center launch team for Apollo 9, 10, 11 and 12, among much else), a rousing talk with Humphrey “Hoppy” Price about interstellar possibilities. So many good conversations, some serious interviews, and a growing enthusiasm for interstellar flight.

But Spitzer had my attention because it was the next mission, one of the Great Observatory missions which included the Hubble Space Telescope, the Compton Gamma Ray Observatory, the Chandra X-ray Observatory, and Spitzer itself. The infrared observatory lasted longer than anyone expected and continued to function even after exhausting its helium coolant in 2009, at which point the ‘cold mission’ ended, but the ‘warm mission’ would last over a decade.

‘Warm’ and ‘cold’ are obviously relative terms when we talk about objects in space. With a full stock of coolant, the spacecraft could maintain temperatures as low as minus 267 degrees Celsius, so yes, that’s ‘cold.’ But Spitzer functioned a long way from Earth, so that even the ‘warm’ mission could operate at minus 244° C, and that enabled continued observations in two infrared wavelengths. We got more than 16 years of use out of Spitzer, another case among many of a mission’s science team working all the angles to keep their craft alive.

Image: The Spitzer Space Telescope (formerly the Space Infrared Telescope Facility or SIRTF) is readied for launch at Cape Canaveral Air Force Station, in 2003. Credit: NASA.

The beauty of working at infrared wavelengths is that you can see things that are too cold to emit much visible light. For our purposes, that includes brown dwarfs and exoplanets, but the observatory has also excelled at studying distant galaxies, including (in conjunction with Hubble observations) the most distant galaxy yet observed, GN-z11, which is seen in the direction of Ursa Major as it was 13.4 billion years in the past. Again of direct relevance to Centauri Dreams readers is Spitzer’s work on interstellar dust, whose chemical composition, studied via spectroscopy, can help us learn more about what goes into planet formation.

This is the instrument that, in 2007, gave us the first identification of molecules in the atmosphere of an exoplanet — two of them, actually. HD 209458b and HD 189733b are ‘hot Jupiters’ and the use of transmission spectroscopy to study their atmospheres points us toward the future prospect of searching for biosignatures on small, rocky planets like Earth. In 2010, Spitzer was part of the discovery of a planet 13,000 light years away, found via gravitational microlensing. Back in 2005, it made direct observations of the ‘hot Jupiters’ HD 209458b and TrES-r1, the first telescope to directly observe light from a planet in another stellar system.

Image: Spitzer was the first telescope to directly observe light from a planet outside our solar system. Prior to that, exoplanets had been observed only indirectly. This accomplishment kicked off a new era in exoplanet science, and marked a major milestone on the journey toward detecting possible signs of life on rocky exoplanets. Two studies released in 2005 reported direct observations of the warm infrared glows from two previously detected “hot Jupiter” planets, designated HD 209458b and TrES-r1. Hot Jupiters are gas giants similar to Jupiter or Saturn, but are positioned extremely close to their parent stars. From their toasty orbits, they soak up ample starlight and shine brightly in infrared wavelengths. Credit: NASA/JPL-Caltech.

I won’t even try to go through all of Spitzer’s accomplishments, but I do need to mention the seven Earth-sized planets around TRAPPIST-1, which Spitzer data, over 500 hours worth, helped scientists identify via transit methods. And I have to include the ‘weather map’ of HD 189733b, a chart of temperature variations over the surface of a gas giant. Sean Carey, manager of the Spitzer Science Center at Caltech, has this to say about the instrument:

“When Spitzer was being designed, scientists had not yet found a single transiting exoplanet, and by the time Spitzer launched, we still knew about only a handful. The fact that Spitzer became such a powerful exoplanet tool, when that wasn’t something the original planners could have possibly prepared for, is really profound. And we generated some results that absolutely knocked our socks off.”

So true! From its Earth-trailing orbit, Spitzer was a long way from our planet’s heat and could offer greater sensitivity than other instruments, especially considering that it could detect some infrared wavelengths that could not be seen through the filter of our atmosphere. What a marvelous run for this observatory, whose decommissioning leads us directly into the launch of the infrared James Webb Space Telescope. We can only hope that the highly complex JWST makes it safely into space and that it lives to see as many mission extensions (5) as the Spitzer Space Telescope.

Image: This dazzling infrared image from NASA’s Spitzer Space Telescope shows hundreds of thousands of stars crowded into the swirling core of our spiral Milky Way galaxy. In visible-light pictures, this region cannot be seen at all because dust lying between Earth and the galactic center blocks our view. In this false-color picture, old and cool stars are blue, while dust features lit up by blazing hot, massive stars are shown in a reddish hue. Both bright and dark filamentary clouds can be seen, many of which harbor stellar nurseries. The plane of the Milky Way’s flat disk is apparent as the main, horizontal band of clouds. The brightest white spot in the middle is the very center of the galaxy, which also marks the site of a supermassive black hole. Credit: NASA/JPL-Caltech.

Spitzer was a pathfinder in many areas of astronomical research, but a kind of personal pathfinder for me, as I’ve always associated its observations with my work as a chronicler of this field. An observatory designed to study “the cold, the old and the dusty,” as NASA likes to say, taught us about objects as close as nearby asteroids and as far as the oldest galaxies. The frustrations of human spaceflight in the post-Apollo era have to be balanced against the brilliance of science missions like Spitzer, which have charted a path forward that should one day lead to direct observations of planets where life may even now be flourishing.

An Impact-Driven End to ‘Snowball Earth’?

The oldest preserved impact structure on Earth appears to be at Yarrabubba in Western Australia, where a magnetic anomaly about 20 kilometers in diameter has been interpreted to be a remnant of an original impact crater 70 kilometers across. Here, what had been an approximate age of 2.65 to 1.075 million years has now been constrained to 2.229 billion years, making Yarrabubba 200 million years older than the next oldest impact.

A team led by Timmons Erickson (Curtin University) analyzed the minerals zircon and monazite at the site. Their sample showed shock recrystallization (in the form of so-called neoblasts) from an asteroid strike, the analysis of which allowed them to pin down the structure’s age. A paper just out in Nature Communications reports on the team’s use of uranium-lead (U–Pb) dating to investigate the age of the shock features and impact melt.

A global climate change may have occurred as a result of this impact, perhaps one with consequences for so-called ‘snowball Earth,’ a hypothesis of almost planet-wide ice cover in one or more periods before 650 million years ago. This impact conceivably ended the ice era.

Image: This is Figure 1 from the paper. Caption: Composite aeromagnetic anomaly map of the Yarrabubba impact structure within the Yilgarn Craton, Western Australia, showing the locations of key outcrops and samples used in this study. The image combines the total magnetic intensity (TMI, cool to warm colours) with the second vertical derivative of the total magnetic intensity (2VD, greyscale) data. The demagnetised anomaly centred on the outcrops of the Barlangi granophyre is considered to be the eroded remnant of the central uplift domain, which forms the basis of the crater diameter of 70 km. Prominent, narrow linear anomalies that cross-cut the demagnetised zone with broadly east-west orientations are mafic dykes that post-date the impact structure. Credit: Erickson et al.

The result is intriguing because the Yarrabubba crater was made at a time when rocks on many continents provide evidence of glacial conditions, and oceans were becoming more oxygenated. Thus co-author Nicholas Timms (Curtin University):

“The age of the Yarrabubba impact matches the demise of a series of ancient glaciations. After the impact, glacial deposits are absent in the rock record for 400 million years. This twist of fate suggests that the large meteorite impact may have influenced global climate.

“Numerical modelling further supports the connection between the effects of large impacts into ice and global climate change. Calculations indicated that an impact into an ice-covered continent could have sent half a trillion tons of water vapour – an important greenhouse gas – into the atmosphere. This finding raises the question whether this impact may have tipped the scales enough to end glacial conditions.”

An end to a period of snowball Earth? Perhaps so. Certainly the work is a reminder of how much we have to learn about crater structures on our own planet, and how much we need to acquire precise ages for them. The impact’s effects, assuming a continental ice sheet, would have been powerful. The team’s numerical models of a 70-kilometer impact crater driven into a granitic target with overlying ice sheet (modeled at from 2 to 5 kilometers in thickness) show the almost instantaneous vaporization of huge amounts of ice. From the paper:

The vapourised ice corresponds to between 9 × 1013 and 2 × 1014 kg of water vapour being jetted into the upper atmosphere within moments of the impact… Impact-generated water vapour in the lower atmosphere would have condensed and rapidly precipitated as rain and snow with no significant long-term climate effects, or could have even triggered widespread glacial conditions via cloud albedo effects during interglacial periods. However, ejection of high-altitude water vapour has potential for greenhouse radiative forcing, depending critically on atmospheric residence time.

Image: Zircon crystal used to date the Yarrabubba impact. Credit: Erickson et al.

The authors are quick to note how difficult it is to model the impact’s effects due to our uncertainties about the composition of the Earth’s atmosphere in the period in question. But they note that the atmosphere would have contained only a fraction of current levels of oxygen, making it likely that huge amounts of H2O vapor released instantaneously into the atmosphere would have had global ramifications.

While the ‘snowball Earth’ hypothesis dominates coverage of this paper, I think the broader significance is in the nature of ongoing research. We can inspect the impact record on surfaces like the Moon due to the lack of atmospheric erosion, but on Earth we face the latter as well as the obscuring effects of tectonics. Until we have a better record of terrestrial impacts, we won’t understand the links between impacts and changes to global climate. At least we have the notable exception of the Chicxulub crater in the Gulf of Mexico’s Yucatán Peninsula, a feature under intense study.

Going back much farther in time, however, we’re looking at poorly constrained and ambiguous impact evidence, a problem that this paper addresses for a period that was hitherto lacking in impact analysis at this level of detail. More broadly, we’re reminded as well of the shaping effect of bombardment as young terrestrial planets evolve, and should always be mindful of the contingencies forced upon nascent worlds by system debris.

The paper is Erickson et al., “Precise radiometric age establishes Yarrabubba, Western Australia, as Earth’s oldest recognised meteorite impact structure,” Nature Communications 11, article no: 300, published online 21 January 2020. Full text.