The Planetary Society’s LightSail-A, launched on May 20 of this year, demonstrated sail deployment from a CubeSat despite software problems that plagued the mission. You’ll recall that communications were spotty and the upload of a software fix was compromised because of the spacecraft’s continued tumbling. After a series of glitches, the craft’s sail was deployed on the 7th of June, with LightSail-A entering the atmosphere shortly thereafter, a test flight that did achieve its primary objective, serving as a prototype for the upcoming LightSail-1.
Mixing CubeSats with solar sails seems like an excellent idea once we’ve ironed out the wrinkles in the technology, and as I’ve speculated before, we may one day see interplanetary missions carried out by small fleets of CubeSats propelled by solar sails. Although the LightSail-A demonstrator mission was in a low orbit, LightSail-1 will deploy its four triangular sails once it reaches an orbital altitude of 800 kilometers. A key reading will be what sort of increase in the spacecraft’s orbital speed is observed once it deploys its sail at altitude.
We’ll be watching this one with interest in April of next year, when it is scheduled to launch aboard a SpaceX Falcon Heavy, itself the object of great interest (this will be its first launch). Whether the launch goes on time will depend upon how well SpaceX recovers from the recent Falcon 9 launch failure. Whenever it launches, a successful LightSail-1 flight would lead to two more solar sail projects on The Planetary Society’s agenda, with LightSail-3 traveling to the L1 Lagrangian point, a useful position for monitoring geomagnetic activity on the Sun.
NASA, meanwhile, has CubeSat plans of its own, likewise dependent upon the health of a booster, in this case the Space Launch System (SLS) rocket. The first flight of the SLS, planned for 2018, will carry an uncrewed Orion spacecraft to a deep space orbit beyond the Moon and return it to Earth. It’s interesting to see that the first SLS mission, according to this NASA information sheet, has the ability to accommodate eleven 6U-sized CubeSats. The standard 10×10×11 cm basic CubeSat is a ‘one unit’ (1U) CubeSat, but larger platforms of 6U and 12U allow more complex missions (LightSail-1 is built around a 3U CubeSat format).
The future of CubeSats with NASA is confirmed by the Advanced Exploration Systems Division’s choice of three secondary payloads intended for SLS launch and a destination in deep space. BioSentinel is intriguing because it will mark the first time we’ve sent living organisms to deep space since the days of the Apollo missions. The organisms in question are yeast (S. cerevisiae), useful in studying DNA lesions caused by highly energetic particles. The idea is to operate in a deep space radiation environment for eighteen months, measuring the effects of radiation on living organisms at distances far beyond low Earth orbit. So far, the longest human mission in deep space was 12.5 days, accomplished by the crew of Apollo 17.
Image: Conceptual graphic of a radiation particle causing a DNA Double Strand Break (DSB). Credit: NASA.
Crews aboard the International Space Station have obviously spent far longer in space, but only in low-Earth orbit, leaving us with plenty to learn about the effects of deep space on biological systems. Another use of CubeSats will be to scout out important targets near our planet, which is the mission designed for NEA (Near-Earth Asteroid) Scout. Here we have another solar sail/CubeSat combination, allowing maneuvering during cruise for the approach to an asteroid. The plan is to study a small asteroid less than about 90 meters in diameter, homing in on a range of parameters including the asteroid’s shape, rotational properties, spectral class, local dust and debris field, regional morphology and properties of its regolith. Ideally these data will be used to resolve issues related to the eventual human exploration of NEAs.
Image: Near-Earth Asteroid Scout, or NEA Scout, will perform reconnaissance of an asteroid using a CubeSat and solar sail propulsion. Credit: NASA/JPL.
Lunar Flashlight is the third approved mission, a solar sail craft with a 6U CubeSat intended for insertion into lunar orbit to look for ice deposits and areas best suited for resource extraction by future human crews. So this one is likewise a scout, one whose sail will be able to reflect 50 kW of sunlight and light up dark craters at the lunar poles where surface water ice may be lurking. It’s also the first mission that will attempt to fly an 80 m2 solar sail. Repeated measurements will give us a chart of ice concentrations in these regions, while also building a catalog of places rich enough in materials to support in-situ resource utilization (ISRU).
Image: Lunar Flashlight will map the lunar south pole for volatiles and demonstrate several technological firsts, including being the first CubeSat to reach the Moon, the first mission to use an 80 m2 solar sail, and the first mission to use a solar sail as a reflector for science observations. Credit: NASA/MSFC.
The CubeSats designed for the first SLS mission won’t get a lot of the publicity when the big rocket flies, but if they perform as expected, they’ll be pushing the small modular satellite concept into new areas. Particularly with regard to solar sails, NEA Scout and Lunar Flashlight should give us opportunities to navigate and maneuver with sails, building experience for the larger sail missions of the future. Meanwhile, Japan’s IKAROS sail, in its ten-month solar orbit, remains in hibernation, with its fifth wake-up call scheduled for winter of this year. Remember that a 50 meter sail, an ambitious successor to IKAROS designed for a mission to Jupiter and the Trojan asteroids, is in the works, with launch some time later in the decade.
Given the flow of new imagery from New Horizons, I began to realize that mission data were changing my prose. To be sure, I still lean to describing the system as Pluto/Charon, because given the relative size of the two bodies, this really seems like a binary object to me. I tend to call it a ‘binary planet’ among friends because I still think of Pluto as a planet, dwarf or not. But when New Horizons blew through the Pluto/Charon system, it was finally possible to start talking separately about Charon, because now we were seeing it, for the first time, up close.
Charon as a distinct object from Pluto is a fascinating thought, one I’ve mused over since the days of the smaller object’s discovery in 1978. An enormous moon hanging in the sky, never changing its position, over a landscape unknown — the imagination ran wild. In the event, New Horizons outdid anything I ever conceived, with imagery of both worlds we’ll be debating for a long time. But in some ways my favorite of the images so far is the one just below.
Image: Details of Pluto’s largest moon, Charon, are revealed in this image from New Horizons’ Long Range Reconnaissance Imager (LORRI), taken July 13, 2015, from a distance of 466,000 kilometers, combined with color information obtained by New Horizons’ Ralph instrument on the same day. The marking in Charon’s north polar region appears to be a thin deposit of dark material over a distinct, sharply bounded, angular feature; scientists expect to learn more by studying higher-resolution images still to come. (Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute).
What’s striking about Charon at first glance is the darkening at the north pole, a phenomenon Carly Howett (Southwest Research Institute, Boulder) discussed in a recent blog article for NASA. Redder and darker it is, a circumstance Howett explains by reference to the surface composition of the northern polar region. A working theory is that traces of Pluto’s atmosphere reach Charon on occasion, where the gases come into contact with polar regions with temperatures between -258 and -213° C. This is a range not a lot higher than absolute zero (?273.15° C, or ?459.67° Fahrenheit).
Gases arriving at Charon’s winter pole would simply freeze rather than escaping, so we have a deposit of Pluto’s atmospheric nitrogen, with methane and carbon monoxide, gradually building up. This would not occur at Charon’s somewhat warmer equator. When the winter pole re-emerges into sunlight, solar radiation on these ices produces tholins, which form when simple organic compounds like methane are irradiated. With their higher sublimation temperature, these tholins cannot escape back into space. Howett sums it up this way:
Charon likely has gradually built up a polar deposit over millions of years as Pluto’s atmosphere slowly escapes, during which time the surface is being irradiated by the sun. It appears the conditions on Charon are right to form red tholins similar to those shown, although we have yet to figure out exactly why. This is one of the many things I am looking forward to better understanding as we receive more New Horizons data over the next year and analyze it in conjunction with continued laboratory work.
Tholin color depends on the ratios of the molecules involved and the kind of radiation received — various shades have been produced in the laboratory. We don’t find them on Earth outside of our own laboratories, but tholins are thought to be abundant on the icy objects in the outer system, usually taking on a reddish brown hue. You may recall they’ve also been discussed in relation to Titan, where we’ve learned from Cassini measurements that tholins appear higher in the atmosphere than was once believed (see Titan’s Tholins: Precursors of Life?). Think of them not as a single specific compound but a range of molecules with a generally reddish color.
Did tholin-rich comets play a role in delivering the precursor materials needed for life to develop on Earth? It’s a notion we can’t rule out, but for now what we can do is study tholins in the places they naturally occur, and in the case of Charon, we can see that they are part of a mechanism that can explain surface color variations. The object 28978 Ixion, a Kuiper Belt object in orbital resonance with Neptune, appears to be particularly rich in tholins.
Meanwhile, the flow of data from New Horizons in just the last few days has more than doubled what we can see of Pluto’s surface at the 400 meter per pixel level. We’re finding still more puzzles as we push deeper into the imagery, with features that appear to be dune-like, and what seem to be nitrogen ice flows and valleys that could have been carved by such flows over the surface. The processes at work should provide fodder for countless dissertations. Here’s one of the new images — for more, see New Pluto Images from New Horizons: It’s Complicated.
Image: This 350-kilometer wide view of Pluto from NASA’s New Horizons spacecraft illustrates the incredible diversity of surface reflectivities and geological landforms on the dwarf planet. The image includes dark, ancient heavily cratered terrain; bright, smooth geologically young terrain; assembled masses of mountains; and an enigmatic field of dark, aligned ridges that resemble dunes; its origin is under debate. The smallest visible features are 0.8 kilometers in size. This image was taken as New Horizons flew past Pluto on July 14, 2015, from a distance of 80,000 kilometers. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.
“The surface of Pluto is every bit as complex as that of Mars,” said Jeff Moore, leader of the New Horizons Geology, Geophysics and Imaging (GGI) team at NASA’s Ames Research Center in Moffett Field, California. “The randomly jumbled mountains might be huge blocks of hard water ice floating within a vast, denser, softer deposit of frozen nitrogen within the region informally named Sputnik Planum.”
Heavily cratered terrain next to young icy plains, with the suggestion of dunes on a place whose atmosphere should be too thin to produce them. No wonder William McKinnon (Washington University, St. Louis) calls the latter a ‘head-scratcher.’ The surfaces of Pluto and Charon have delivered complexities galore, and we’re only now learning that Pluto’s atmospheric haze is far more complex than earlier thought, offering a twilight effect that helps light nightside terrain. “If an artist had painted this Pluto before our flyby, I probably would have called it over the top,” says New Horizons principal investigator Alan Stern, “but that’s what is actually there.”
And then there’s this (brought to my attention by Larry Klaes):
Finding a biological marker in the atmosphere of an exoplanet is a major goal, but as Ignas Snellen argues in the essay below, space-based missions are not the only way to proceed. A professor of astronomy at Leiden University in The Netherlands, Dr. Snellen makes a persuasive case that technologies like high dispersion spectroscopy and high contrast imaging are at their most effective when deployed at large observatories on the ground. A team of European observers he led has already used these techniques to determine the eight-hour rotation rate of Beta Pictoris b. We’ll need carefully conceived space missions to study those parts of the spectrum inaccessible from the ground, but these will find powerful synergies with the next generation of giant Earth telescopes planned for operations in the 2020s.
by Ignas Snellen
While I was deeply involved by my PhD project, studying the active centers of distant galaxies, a real scientific revolution was unfolding in a very different field of astronomy. In the mid-1990s the first planets were found to orbit stars other then our Sun. For several years I managed to ignore it. Not impeded by any knowledge I was happy to join the many skeptics to dismiss the early results. But soon they could be ignored no more. And when the first transiting planet was found and a little later its atmosphere detected, I radically changed research field and threw myself, like many others, on exoplanet research. More than a decade later the revolution is still going strong.
DARWIN, TPF, and SIM
Not all scientific endeavors were successful during this twenty-year period. Starting soon after the first exoplanet discoveries, enormous efforts were put in the design (and getting the political support) for a spacecraft that could detect potential biomarker gases in the atmospheres of nearby planet systems. European astronomers were concentrating on DARWIN. This mission concept was composed of four to five free-flying spacecraft carrying out high-resolution imaging using nulling interferometry, where the starlight from the different telescopes is combined in such way that it cancels out on-axis light, leaving the potential off-axis planet-light intact. After a series of studies over more than a decade, in 2007 the European Space Agency stopped all DARWIN developments – it was too difficult. Over the same time period, several versions of the Terrestrial Planet Finder (TPF) were proposed to NASA, including a nulling interferometer and a coronagraph. The latter uses a smart optical design to strongly reduce the starlight while letting any planet light pass through. Also these projects have subsequently been cancelled. Arguably an even bigger anticlimax was the Space Interferometry Mission (SIM), which was to hunt for Earth-mass planets in the habitable zones of nearby stars using astrometry. After being postponed several times, it was finally cancelled in 2010.
How pessimistic should we be?
Enormous amounts of people’s time and energy were spent on these projects, costing hundreds of millions of dollars and euros. A real pity, considering all the other exciting projects that could have been funded instead. We should set more realistic goals and learn from greatly successful missions such as the NASA Kepler mission, which was conceived and developed during that same period. A key aspect of the adoption of Kepler as a NASA space mission was the demonstration of technological readiness through ground-based experiments (by Bill Borucki and friends). A mission gets approved only if it is thought to be a guaranteed success. It is this aspect that killed Darwin and TPF, and it is this aspect that worries me about new, very smart spacecraft concepts such as the large external occulter for the New World Mission. Maybe I am just not enough of a (Centauri) dreamer.
In any case, lead times of large space missions, as the Kepler story has shown, are huge. This implies that it is highly unlikely that within the next 25 years we will have a space mission that will look for biomarker gases in the atmospheres of Earth-like planets. If I am lucky I will still be alive to see it happen. My idea is – let’s start from the ground!
The ground-based challenge
The first evidence for extraterrestrial life will come from the detection of so-called biomarkers – absorption from gases that are only expected in an exoplanet atmosphere when produced by biological processes. The prime examples of such biomarkers are oxygen and ozone, as seen in the Earth’s atmosphere. Observing these gases in exoplanet atmospheres will not be the ultimate proof of extraterrestrial life, but it will be a first step. These observations require high-precision spectral photometry, which is very challenging to do from the ground. First of all, our atmosphere absorbs and scatters light. This is a particular problem for observations of Earth-like planets, because their spectra will show absorption bands at the same wavelengths as the Earth’s atmosphere. In addition, turbulence in our atmosphere causes the light that enters ground-based telescopes to become distorted. Therefore, light does not form perfect incoming wavefronts, hampering high-precision measurements. Furthermore, when objects are observed for a longer time during a night, their light-path through the Earth atmosphere changes, as does the way starlight enters an instrument, making stability a big issue. These are the main reasons why many exoplanet enthusiasts thought that it would be impossible to ever probe exoplanet atmospheres from the ground.
Work over the last decade has shown that one particular ground-based technique – high dispersion spectroscopy (HDS) – is very suitable for detecting absorption features in exoplanet atmospheres. The dispersion of a spectrograph is a measure of the ‘spreading’ of different wavelengths into a spectrum of the celestial object. Space telescopes, such as the Hubble Space Telescope (HST), Spitzer, and the future James Webb (JWST) have instruments on board that are capable of low to medium dispersion spectroscopy, where the incoming light can be measured at typically 1/100th to 1/1000th of a wavelength. With HDS, precisions of 1/100,000th of a wavelength are reached – hence about two orders of magnitude higher than from space. For two reasons this can practically only be done from the ground: 1) the physical size of a spectrograph scales with its dispersion, meaning that HDS instruments are generally too big to launch to space. 2) At high dispersion the light is spread very thinly, requiring a lot of photons to do it right, hence a large telescope. For example, the hot Jupiter tau Bootis b required 3 nights on the 8m Very Large Telescope to measure carbon monoxide in its atmosphere. Scaling this to the HST (pretending it would have an HDS instrument) it would have cost on the order of 200 hours of observing time – more than was spent on the Hubble Deep Field. Hence, HDS is the sole domain of ground-based telescopes.
The high dispersion is key to overcome the challenges that arise from observing through the Earth’s atmosphere. At a dispersion of 1/100,000th of a wavelength, HDS measurements are sensitive to Doppler effects due to the orbital motion of the planet. E.g. the Earth moves with nearly 30 km/sec around the Sun, while hot Jupiters have velocities of 150 km/sec or more. This means that during an observation, the radial component of the orbital velocity of a planet can change by tens of km/sec. While this makes absorption features from the planet move in wavelength, any Earth-atmospheric and stellar absorption lines remain stationary. Clever data analysis techniques can filter out all the stationary components of a time-sequence of spectra, while the moving planet signal is preserved. Ultimately, the signal from numerous individual planet lines can be added up together to boost the planet signal using the cross-correlation technique – weighing the contribution from each line by its expected strength.
Image: Illustration of the HDS technique, with the moving planet lines in purple.
So why does this work? Although the Earth atmosphere has a profound influence on the observed spectrum, the absorption and scattering processes are well behaved on scales of 1/100,000th of a wavelength and can be calibrated out. The signal of the planet can be preserved, even if variations in the Earth atmospheres are many orders of magnitude larger. In this way starlight reflected off a planet’s atmosphere can be probed, but also a planet’s transmission spectrum – when a planet crosses the face of a star and starlight filters through its atmosphere. In addition, a planet’s direct thermal emission spectrum can be observed. This is particularly powerful in the infrared. And it works well! In the optical, absorption from sodium has been found in the transmission spectra of several exoplanets. In the near-infrared, carbon monoxide and water vapor have been seen in both the transmission spectra as well as thermal emission spectra of several hot Jupiters – on par with the best observations from space. In the next two years new instruments will come online (such as CRIRES+ and ESPRESSO on the VLT) that will take this significantly further – allowing a complete inventory of the spectroscopically active molecules in the upper atmospheres of hot Jupiters, and extending this research to significantly cooler and smaller planets.
One step beyond
There is more. The HDS technique makes no attempt to spatially separate the planet light from that of the much brighter star – it is only filtered out using its spectral features. Hot Jupiters are much too close to their parent stars to be able to see them separately anyway. However, planets in wider orbits can also be directly imaged, using high-contrast imaging (HCI) techniques (also in combination with coronography). This technique is really starting to flourish using modern adaptive optics in which atmospheric turbulence is compensated by fast-moving deformable mirrors. A few dozen planets have already been discovered using HCI, and new imagers like SPHERE on the VLT and GPI on Gemini, which came online last year, hold a great promise. What I am very excited about is that HDS combined with HCI (let’s call it HDS+HCI) can be even more powerful. While HDS is completely dominated by noise from the host star, HCI strongly reduces the starlight at the planet position – increasing the sensitivity of the spectral separation technique used by HDS by orders of magnitude. Last year we showed the power of HDS+HCI by for the first time measuring the spin velocity of an extrasolar planet, showing beta Pictoris b to have a length of day of 8 hours. [For more on this work, see Night and Day on ? Pictoris b].
Image: HDS+HCI observations of beta Pictoris b.
The giants are coming
Both the US and Europe are building a new generation of telescopes that can truly be called giants. The Giant Magellan Telescope (GMT) will consist of six 8.4m mirrors, equivalent of one 24.5m diameter telescope. The Thirty Meter Telescope (TMT) will be as large as the name suggests, while the European Extremely Large Telescope (E-ELT) will be the largest with an effective diameter of 39m. All three projects are in a race with each other and hope to be fully operational in the mid-2020s.
Size is everything in this game – in particular for HDS and HDS+HCI observations. HDS benefits from the number of photons that can be collected, which scales with the diameter squared. Taking into account also other effects, the E-ELT will be >100 times faster than the VLT (in particular using the first-light instrument METIS, and HIRES). This will bring us near the range needed to target molecular oxygen in the atmospheres of Earth-like planets that transit nearby red dwarf stars. We have to be somewhat lucky for such nearby transiting systems to exist, but simulations show that the smaller host star makes the transmission signal of molecular oxygen from an Earth-size planet similar to the carbon monoxide signals we already have detected in hot Jupiter atmospheres – it is just that the systems will be much fainter than tau Bootis requiring the significantly bigger telescopes. The technology is already here, but it is all about collecting enough photons. This could also be solved in a different way if even the ELTs turn out not to be large enough. HDS observations of bright stars do not require precisely shaped mirrors and this could be achieved by arrays of low-precision light collectors, but this is something for the more distant future.
Image: Artist impression of the E-ELT – ready in 2024! (credit: ESO).
Even more promising are the high-contrast imaging capabilities of the future ELTs. Bigger telescopes not only collect more photons, but also see sharper. This makes their capability to see faint planets in the glare of bright stars scale with telescope size up to the fifth power, making the E-ELT more than a 1000 times faster than the VLT. Excitingly, rocky planets in the habitable zones of nearby planets become within reach. Again, simulations show that their thermal emission can be detected around the nearest stars, while HDS+HCI at optical wavelengths can target their reflectance spectra, possibly even including molecular oxygen signatures.
Realistic space missions
Whatever happens with space-based exoplanet astronomy, ground-based telescopes will push their way forward towards characterizing Earth-like planets. This does not mean there is no need for space missions. First of all, I have not done justice to the fantastic, groundbreaking exoplanet science the JWST is going to provide. Secondly, a series of transit missions, TESS from NASA (launch 2017), and CHEOPS and PLATO from ESA (Launch 2018 & 2024), will discover all nearby transiting planet systems, a crucial prerequisite for much of the science discussed here.
Above all, ground-based measurement will not be able to provide a complete picture of a planet’s atmosphere – simply because large parts of the planet’s spectrum are not accessible from the ground. This will mean that the ultimate proof for extraterrestrial life will likely have to come from a space mission type DARWIN or TPF. Imagine how a ground-based detection of say water in an Earth-like atmosphere would open up political possibilities, but the right timing for such missions is of upmost importance. Aiming too high and too early means that lots of time and money will be wasted, at the expense of progress in exoplanet science. It is good to dream, but we should not forget to stay realistic.
Snellen et al. (2013) Astrophysical Journal 764, 182: Finding Extraterrestrial Life Using Ground-based High-dispersion Spectroscopy (http://xxx.lanl.gov/abs/1302.3251).
Snellen et al. (2014), Nature 509, 63: Fast spin of the young extrasolar planet beta Pictoris b (http://xxx.lanl.gov/abs/1404.7506).
Snellen et al. (2015), Astronomy & Astrophysics 576, 59: Combining high-dispersion spectroscopy with high contrast imaging: Probing rocky planets around our nearest neighbors (http://xxx.lanl.gov/abs/1503.01136).
In Stephen Baxter’s novel Ultima (Roc, 2015), Ceres is moved by a human civilization in a parallel universe toward Mars, the immediate notion being to use the dwarf planet’s volatiles to help terraform the Red Planet. Or is that really the motive? I don’t want to give too much away (and in any case, I haven’t finished the book myself), but naturally the biggest question is how to move an object the size of Ceres into an entirely new orbit.
Baxter sets up an alternate-world civilization that has discovered energy sources it doesn’t understand but can nonetheless use for interstellar propulsion and the numerous demands of a growing technological society, though one that is backward in comparison to our own. That juxtaposition is interesting because we tend to assume technologies emerge at the same pace, supporting each other. What if they don’t, or what if we simply stumble upon a natural phenomenon we can tap into without being able to reproduce its effects through any known science?
Something of the same juxtaposition occurs in Kim Stanley Robinson’s Aurora (Orbit, 2015), where we find a society that has the propulsion technologies to enable travel at a pace that can get a worldship to Tau Ceti in a few human generations. We’ve discussed Aurora in these pages recently, looking at some of the problems in its science — I’ll let those better qualified than myself have the final word on those — but what I found compelling about the novel was its depiction of what happens aboard that worldship.
Because it’s not at all inconceivable that we might solve the propulsion problem before we solve the closed-loop life support problem, and that is more or less what we see happening in Aurora. A worldship could house habitats of choice, and if you think of some visions of O’Neill cylinders, you’ll recall depictions that made space living seem almost idyllic. But Robinson shows us a ship that’s simply too small for its enclosed ecologies to flourish. Travel between the stars in such a ship would be harrowing, as indeed it turns out to be in the book. Micro-managing a biosphere is no small matter, and we have yet to demonstrate the ability.
Image: The O’Neill cylinder depicted here is one take on what might eventually become an interstellar worldship. Keeping its systems and crew healthy is a skill that will demand space-based experimentation, and plenty of it. Credit: Rick Guidice/NASA.
In Baxter’s Ultima, what happens with Ceres is compounded by the fact that just as humans don’t fully understand their power source, they also have to deal with an artificial intelligence whose motives are opaque. Put the two together and you can see why the movement of Ceres to a new position in the Solar System takes on an aura of menace. Various notions of a ‘singularity’ posit a human future in which our computers are creating entirely new generations of themselves that are designed according to principles we cannot begin to fathom. What happens then, and how do we ensure that the resulting machines want us to survive?
With Ceres very much in mind, I was delighted to receive the new imagery from the Dawn spacecraft at the present-day Ceres (in our non-alternate reality), showing us the bright spots that have commanded so much attention. Here we’re looking at a composite of two different images of Occator crater, one made with a short exposure to capture as much detail as possible, the other a longer exposure that best captures the background surface.
Image: Occator crater on Ceres, home to a collection of intriguing bright spots. The images were obtained by Dawn during the mission’s High Altitude Mapping Orbit (HAMO) phase, from which the spacecraft imaged the surface at a resolution of about 140 meters per pixel. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.
We’re looking at the view from 1470 kilometers, with images offering three times better resolution than we had from the spacecraft’s previous orbit in June. Two eleven-day cycles of surface mapping have now been completed at this altitude, with the third beginning on September 9. All of Ceres is to be mapped six times over the next two months, with each cycle consisting of fourteen orbits. Changing angles in each mapping cycle will allow the Dawn researchers to put together 3-D maps from the resulting imagery.
So we’re learning more about the real Ceres every day. Given our lack of Baxter’s ‘kernels’ — the enigmatic power sources that energize his future civilization as well as the unusual but related culture they encounter — we may do better to consider this dwarf planet as a terraforming possibility in its own right, rather than a candidate for future use near Mars. On that score, I remind you of Robert Kennedy, Ken Roy and David Fields, who have written up a terraforming concept that could be applied to small bodies in or outside of the habitable zone (see Terraforming: Enter the ‘Shell World’ for background and citation).
It will be through myriad experiments in creating sustainable ecologies off-world that we finally conquer the life support problem. It always surprises me that it has received as little attention as it has in science fiction, given that any permanent human presence in space depends upon robust, recyclable systems that reliably sustain large populations. Our earliest attempts at closed-loop life support (think of the BIOS-3 experiments in the 1970s and 80s, and the Biosphere 2 attempt in the 1990s) have revealed how tricky such systems are. Robinson’s faltering starship in Aurora offers a useful cautionary narrative. We’ll need orbital habitats of considerable complexity as we learn how to master the closed-loop conundrum.
Extending missions beyond their initial goals is much on my mind as we consider the future of New Horizons and its possible flyby past a Kuiper Belt Object. But this morning I’m also reminded of EPOXI, which has given us views of the Earth that help us study what a terrestrial world looks like from a distance, characterizing our own planet as if it were an exoplanet. You’ll recall that EPOXI (Extrasolar Planet Observation and Deep Impact Extended Investigation) is a follow-on to another successful mission, the Deep Impact journey to comet Tempel 1.
As is clear from its acronym, EPOXI combined two extended missions, one following up the Tempel 1 studies with a visit to comet Hartley 2 (this followed an unsuccessful plan to make a flyby past comet 85P/Boethin, which proved to be too faint for accurate orbital calculations). The extrasolar component of EPOXI was called EPOCh (Extrasolar Planet Observation and Characterization), using the craft’s high resolution telescope to make photometric observations of stars with known transiting exoplanets. But the spacecraft produced observations of Earth that have been useful for exoplanet studies, as well as recording some remarkable views.
Image: Four images from a sequence of photos taken by the Deep Impact spacecraft when it was 50 million km from the Earth. Africa is at right. Notice how much darker the moon is compared to Earth. It reflects only as much light as a fresh asphalt road. Credit: Donald J. Lindler, Sigma Space Corporation, GSFC, Univ. Maryland, EPOCh/DIXI Science Teams.
Although communications with EPOXI were lost in the summer of 2013, the mission lives on in the form of the data it produced, some of which are again put to use in a new paper out of the University of Washington. Edward Schwieterman, a doctoral student and lead author on the work in collaboration with the university’s Victoria Meadows, reports on Earth observations from EPOXI that have been compared to three-dimensional planet-modeling data from the university’s Virtual Planet Laboratory. The comparison has allowed confirmation of the signature of nitrogen collisions in our atmosphere, a phenomenon that should have wide implications.
The presence of nitrogen is significant because it can help us determine whether an exoplanet’s surface pressure is suitable for the existence of liquid water. Moreover, if we find nitrogen and oxygen in an atmosphere and are able to measure the nitrogen accurately, we can use the nitrogen as a tool for ruling out non-biological origins for the oxygen. But nitrogen is hard to detect, and the best way to find it in a distant planet’s atmosphere is to measure how nitrogen molecules collide with each other. The paper argues that these ‘collisional pairs’ create a signature we can observe, something the team has modeled and that the EPOXI work has confirmed.
Nitrogen pairs, written as (N2)2, are visible in a spectrum at shorter wavelengths, giving us a useful tool. The paper explains how this works:
A comprehensive study of a planetary atmosphere would require determination of its bulk properties, such as atmospheric mass and composition, which are crucial for ascertaining surface conditions. Because (N2)2 is detectable remotely, it can provide an extra tool for terrestrial planet characterization. For example, the level of (N2)2 absorption could be used as a pressure metric if N2 is the bulk gas, and break degeneracies between the abundance of trace gases and the foreign pressure broadening induced by the bulk atmosphere. If limits can be set on surface pressure, then the surface stability of water may be established if information about surface temperature is available.
It’s interesting as well that for half of Earth’s geological history, there was little oxygen present, despite the presence of life for a substantial part of this time. The paper argues that given Earth’s example, there may be habitable and inhabited planets without O2 we can detect. Moreover, atmospheres with low abundances of gases like N2 and argon are more likely to accumulate O2 abiotically, giving us a false positive for life.
A water dominated atmosphere lacks a cold trap, allowing water to more easily diffuse into the stratosphere and become photo-dissociated, leaving free O2 to build up over time. Direct detection of N2 through (N2)2 could rule out abiotic O2 via this mechanism and, in tandem with detection of significant O2 or O3, potentially provide a robust biosignature. Moreover, the simultaneous detection of N2, O2, and a surface ocean would establish the presence of a significant thermodynamic chemical disequilibrium (Krissansen-Totton et al. 2015) and further constrain the false positive potential.
Combining the EPOXI data with the Virtual Planetary Laboratory modeling demonstrates that nitrogen collisions that are apparent in our own atmosphere should likewise be apparent in exoplanet studies by future space telescopes. EPOXI, then, demonstrated that nitrogen collisions could be found in a planetary spectrum, and the VPL work modeling a variety of nitrogen abundances in an exoplanet atmosphere shows how accurately the gas can be measured. “One of the interesting results from our study,” adds Schwieterman, “is that, basically, if there’s enough nitrogen to detect at all, you’ve confirmed that the surface pressure is sufficient for liquid water, for a very wide range of surface temperatures,”
The paper is Schwieterman et al., “Detecting and Constraining N2 Abundances in Planetary Atmospheres Using Collisional Pairs,” The Astrophysical Journal Vol. 810, No. 1 (28 August 2015). Abstract / preprint.