Centauri Dreams
Imagining and Planning Interstellar Exploration
Comet Storm Around ? Corvi?
Planetary migration can play a huge role in the evolution of a solar system, as witness the thinking that it was a migration of the gas giants Jupiter and Saturn that brought about the Late Heavy Bombardment, a time four billion years ago when impacts from space pock-marked the Moon and inner planets. The migration model, first proposed at the Observatoire de la Côte d’Azur and thus known as the Nice model, posits a long-lasting cometary bombardment caused when gravitational effects of the migration scattered icy bodies in the Kuiper Belt. Most of these would have been ejected from the system, but others would have been sent on planet-intersecting paths.
If the Nice model is correct, then we may have an explanation for at least part of the water that wound up on our blue and green planet today, with obvious consequences for the development of life. That makes events similar to the Late Heavy Bombardment of considerable interest when we can find them in other solar systems, and new work with Spitzer Space Telescope data says we’ve now done just that. The nearby bright star Eta Corvi is the site of a band of dust that shows up with the help of Spitzer’s infrared detectors. The star is approximately one billion years old, young enough to mimic what we believe was the situation around our own Sun in that era.
Image: This artist’s conception illustrates a storm of comets around a star near our own, called Eta Corvi. Evidence for this barrage comes from NASA’s Spitzer Space Telescope, whose infrared detectors picked up indications that one or more comets was recently torn to shreds after colliding with a rocky body. In this artist’s conception, one such giant comet is shown smashing into a rocky planet, flinging ice- and carbon-rich dust into space, while also smashing water and organics into the surface of the planet. A glowing red flash captures the moment of impact on the planet. Yellow-white Eta Corvi is shown to the left, with still more comets streaming toward it. Credit: NASA/JPL-Caltech.
What we see around Eta Corvi is telling. The researchers found evidence in the dust disc of water ice, organics and rock, all of which indicate a cometary source. From the paper:
We interpret this as demonstrating that the parent body for the ? Corvi warm circumstellar dust was a large object created early in the system’s history, that it formed outside the ice line of the ? Corvi system, and that it retained much of its icy volatiles and primitive material.
Moreover, the dust band in question is close enough to Eta Corvi that Earth-like planets could exist there, leaving the implication that we’re looking at the results of a collision between a planet and one or more comets. Is this what our own Solar System might have looked like during the Late Heavy Bombardment? It’s a distinct possibility. Lead author Carey Lisse (JHU/APL): “We believe we have direct evidence for an ongoing Late Heavy Bombardment in the nearby star system Eta Corvi, occurring about the same time as in our solar system.”
Two bands of dust surround Eta Corvi, the second being a much colder ring at about 150 AU in a region that bears obvious analogies to our Kuiper Belt and may be a reservoir for comets. It’s intriguing that the light signature emitted by the inner Eta Corvi dust band resembles the composition of the Almahata Sitta meteorite, fragments of which were recovered in the Sudan in 2008. The Kuiper Belt may well have been the origin of the Almahata Sitta object. Eta Corvi thus becomes a wonderful laboratory for the study of cometary bombardment because it contains both warm and cold dust reservoirs with strong spectral features, and its rarity may itself be telling us something, as the paper goes on to note (internal references omitted for brevity):
Large photometric studies of many debris disks and targeted spectroscopic studies of individual debris disks conducted with the Spitzer Space Telescope have constrained the typical locations of debris-producing collisions, the evolution of the emission from these collisions, and, in some cases, the chemical composition of and events identified by the colliding parent bodies. Most debris disks have dust temperatures colder than ~ 150—200 K, consistent with debris produced from icy colliding planetesimals… The frequency of warm (> 200 K) debris dust is low, < 5—10% regardless of age, indicating that planetesimal collisions in the terrestrial zone or asteroid belt-like regions are rare or have a small observational window, consistent with expectations from theory...
That small observational window would certainly fit current thinking about the Late Heavy Bombardment as a period of intense activity that moderated following planetary migration. In any case, debris disks around young stars are proving their worth, suggesting as they do a reservoir of colliding planetesimals. Our own more mature system, of course, has two belts — relatively warm dust and metal-rich bodies in the asteroid belt between 2 and 4 AU and the cold dust and icy planetesimals of the Kuiper Belt beyond 30 AU. By studying such discs around other stars, we are looking at system evolution in action and learning more about our system’s past.
The paper is Lisse, et al., “Spitzer Evidence for a Late Heavy Bombardment and the Formation of Urelites in ? Corvi at ~1 Gyr,” accepted by the Astrophysical Journal (preprint).
The Joy of Extreme Possibility
Nuclear rocket designs are hardly new. In fact, it was clear as early as the 1950s that conventional chemical rocketry was inefficient, and programs like Project Rover, set up to study the use of nuclear reactors to heat liquid hydrogen for propulsion, aimed at the kind of rockets that could get us beyond the Moon and on to Mars. The NERVA rocket technology (Nuclear Engine for Rocket Vehicle Application) that grew out of all this showed great promise but ran afoul of political and economic issues even as the last Apollo missions were canceled. Nor is the public wariness of nuclear methods likely to vanish soon, yet another hurdle for future ideas.
But making people aware of what has done and what could be done is good practice, as Kenneth Chang does by example in his recent piece on the 100 Year Starship Symposium, which bears the optimistic title Not Such a Stretch to Reach for the Stars. In interstellar terms, propulsion is the biggest problem of all. Chang’s article suggests a pathway through conventional rocketry and into nuclear-thermal designs, with reference along the way to using nuclear engines to generate the electrical fields that power up an ion engine. The goal on this pathway is fusion, though Chang admits no one has yet built an energy-producing fusion reactor.
The Daedalus concept was fusion-based, and the ongoing Icarus project that followed is now examining Daedalus to note the effect of thirty years of new technology. But Chang has also talked to James Benford, whose interest in laser and microwave beaming remains strong. Leave the propellant behind and you’ve maximized payload, in addition to working with known physics and apparently achievable engineering. And there continue to be startling new concepts like those of Joseph Breeden, who finds a more extreme way to create an engineless vehicle:
From his doctoral thesis, Dr. Breeden remembered that in a chaotic gravitational dance, stars are sometimes ejected at high speeds. The same effect, he believes, could propel starships.
First, find an asteroid in an elliptical orbit that passes close to the Sun. Second, put a starship in orbit around the asteroid. If the asteroid could be captured into a new orbit that clings close to the Sun, the starship would be flung on an interstellar trajectory, perhaps up to a tenth of the speed of light.
“The chaotic dynamics of those two allow all the energy of one to be transferred to the other,” said Dr. Breeden, who came toting copies of a paper describing the technique. “It’s a unique type of gravity assist.”
What I call the ‘joy of extreme possibility’ has animated interstellar studies since the days of Robert Forward. It works like this: We know the distances between the stars are so vast as to dwarf the imagination. Indeed, most people have no notion of them, seeing an interstellar mission as merely a next step once we have explored the outer system, a kind of juiced-up Voyager. The scientists and engineers who work on these matters, knowing better, realize how far beyond our current technologies these journeys really are. So they’re not afraid to speculate even at the absolute far end of the plausible (and often beyond that). Work your way through interstellar papers like these and you pick up an infectious, jazzy brainstorming. It’s the kind of mental riffing on an idea that a John Coltrane or a McCoy Tyner does with a musical theme.
And by the way, Chang is careful to get those distances across to readers. I’m always interested in homely comparisons because you can use them to boggle audience minds when speaking about interstellar flight. This is useful, because a boggled mind often becomes a curious one, and while you can never predict these things, occasionally interstellar studies gain a new adherent. Chang cites a Richard Obousy analogy: If the Earth were Orlando and Alpha Centauri were in Los Angeles, then the Voyager spacecraft would have traveled but a single mile.
Even after all these years, that one still boggles my own mind. Chang again:
Another way of looking at the challenge is that in 10,000 years, the speed of humans has jumped by a factor of about 10,000, from a stroll (2.6 m.p.h.) to the Apollo astronauts’ return from the Moon (26,000 m.p.h.). Reaching the nearest stars in reasonable time — decades, not centuries — would require a velocity jump of another factor of 10,000.
It’s good to see the 100 Year Starship Study steadily percolating in the news. Maybe one day these concepts will not seem as esoteric as they do today. I note as I write this, for example, that my word processor flags the word ‘starship’ as a spelling error. We need to set deeper roots into the culture than that. We can start by doing what conference organizer David Neyland told Chang he wants to do, to establish a bar high enough that people “will actually go start tackling some of these really hard problems.” Of course, the real bar is set by nature, and it’s the highest bar we as a species have faced in terms of travel times and distance. But the joy of extreme possibility only ignites the spirit when everything is on the table and the challenge is immense.
Habitable, Not Earth-like
I’ve put off writing about Wesley Traub’s paper on the frequency of planets in the habitable zone because I knew Adam Crowl had reservations about Traub’s method. We talked about this at the 100 Year Starship Symposium, which led to Adam’s agreeing to writing this piece for Centauri Dreams. How you define a habitable zone is, of course, a critical matter, especially when you’re dealing with a topic as compelling as extrasolar planets that can support life. Adam places Traub’s work in the context of earlier attempts at defining the habitable zone and finds HZ estimates different from Traub’s, though one is surprisingly similar to a much earlier study.
by Adam Crowl
The recent paper by Wesley Traub [reference below] has estimated the frequency of terrestrial (“Earth-like”) planets in the Habitable Zone (HZ) of their stars based on statistical analysis of the recent Kepler data release, but the frequency computed, of ~34(+/-14)% around FGK stars, is dubious due to the assumption of wide HZ limits. Before I discuss the specifics, let’s look at the modern history of the “Habitable Zone”.
The modern discussion really began with Stephen Dole’s “Habitable Planets for Man”, a RAND commissioned study from the early 1960s, eventually updated in 1970, and popularized with Isaac Asimov. Dole based his HZ limits on the criterion that a significant fraction of a terrestrial planet would experience a “hospitable climate”. He didn’t examine the effect of atmosphere, and derived the HZ limits of 0.725 – 1.25 AU, from just outside the orbit of Venus and a bit closer to the Sun than Mars at its closest. Applying statistical analysis to various features of the known planets, then extrapolating to other stars, Dole found that potentially 645 million Earth-like planets might exist in the Galaxy.
In the mid 1970s Michael Hart developed the first evolutionary models of the atmosphere of an Earth-like planet, finding Earth to be poised on a virtual knife-edge, tipping towards a Runaway Greenhouse if closer than 0.95 AU and Runaway Glaciation if further out from the Sun than 1.01 AU. When this criterion was applied to other stars, the frequency of Earth-like planets was less than 1 in a quarter million stars, or less than 400,000 Earths in a Galaxy of 100 billion stars.
Hart’s limits seemed overly sensitive to climate perturbations, and further work on the evolution of Earth’s atmosphere in the 1980s led to the paper “Habitable Zones Around Main Sequence Stars”, (Kasting, Whitmire & Reynolds, 1993) , which redefined the debate. What James Kasting and colleagues discovered was a powerful feedback loop between the levels of carbon dioxide in the atmosphere, geological weathering and the heat input from the Sun.
This creates a self-regulated surface temperature which can keep water in its liquid range out to a significant distance from the Sun. The chief uncertainty came from the complication of dry-ice clouds. Past 1.37 AU clouds of dry-ice begin forming and by 1.67 AU the cloud cover becomes total, negating the effectiveness of the carbon dioxide greenhouse effect. Some preliminary work on water clouds also suggests the inner radius of the habitable zone, just 0.95 AU, might be extended to closer to Venus.
Traub’s paper has somewhat more generous HZ limits. Traub examined three cases, with the ranges from 0.72-2.0 AU in the best case, a nominal HZ of 0.8-1.8 AU, and a “conservative” 0.95-1.67 AU. Using the observed planetary radii distribution and the orbital radii, Traub was able to compute the frequency of terrestrial planets in these HZ as 34(+/-14)%, with the extremes providing the error bar limits.
Here’s where just what is computed and why is important. The ranges used by Traub for the HZ apply to specifically liquid water compatible planets with extensive greenhouse gas atmospheres. Such worlds, with up to several bars of carbon dioxide for atmosphere, are only distantly “Earth-like”, much like Mars or Venus can be called Earth-like. The Earth we know, with an oxygen rich, carbon dioxide poor, atmosphere is somewhat more sensitive to climatic instability. If more conservative HZ ranges are used a quite different result is obtained.
The HZ, inside of the CO2 cloud limit found by Kasting, et.al., is the more restrictive 0.95-1.37 AU. This gives a frequency of just 13.3%. If we use the Continuously Habitable Zone (0.95-1.15 AU), also from Kasting, et.al., then the frequency drops to a mere 6.3%. Using Hart’s even more restricted range drops the frequency to less than 2%. Another caveat is that the planet frequency estimated is limited to stars in the mass-range 1.13-1.01 solar masses and is yet to be extended into the wider population of stars which make up ~80% of the Galaxy.
The HZ limits derived by Kasting et.al. assumed ocean-dominated terrestrial planets. The broader range of land dominated “desert planets” (Abe et.al., 2011), with water bodies limited to circum-polar lakes/ice-caps, increases the HZ range to 0.75-1.3 AU, and a corresponding frequency of 17.3%. Incidentally this range is equivalent to that derived by Dole’s (1964) ground-breaking study.
So, in conclusion, the high frequency of “Earth-like” planets derived by Traub, is tempered somewhat when a more precise Earth-like Habitable Zone range is used. Planets warm enough for liquid water thanks to multi-bar atmospheres of carbon dioxide, methane or hydrogen, while probably conducive to extremophiles, aren’t “Earth-like” as usually understood, and this caveat should be more widely appreciated when making such estimates.
The paper is Traub, “Terrestrial, Habitable-Zone Exoplanet Frequency from Kepler,” available online as a preprint. Other references:
Y.Abe, A.Abe-Ouchi, N.H.Sleep, and K.J.Zahnle. “Habitable Zone Limits for Dry Planets”, Astrobiology, Volume 11, Issue 5, pp. 443-460 (2011).
S.H. Dole, Habitable Planets for Man, Blaisdell, New York (1964).
M.H. Hart, “Habitable zones about main sequence stars”, Icarus, 37: 351-357 (1978).
J.F. Kasting, D.P. Whitmire and R.T. Reynolds. “Habitable Zones Around Main Sequence Stars” Icarus 101: 108-128 (1993).
Building the Brown Dwarf Census
About a month ago we were looking at the work of Ray Jayawardhana and team on the brown dwarf 2MASS 2139, an interesting case because Jayawardhana (University of Toronto) thinks he has spotted a giant storm raging on the object, or perhaps holes in the cloud deck that allow a glimpse of deeper layers of the atmosphere within. At issue is the striking 30 percent change in brightness of the star within a mere eight hours, seeming to indicate atmospheric changes we can pick up as the brown dwarf rotates. Unlike a normal star, a brown dwarf is hot when young but gradually cools to the point where it has an atmosphere similar to that of a gas giant.
All this is part of a survey program called SONYC – Substellar Objects in Nearby Young Clusters – that uses data from the Subaru Telescope in Hawaii and the Very Large Telescope (VLT) in Chile. SONYC may change the way we look at brown dwarfs, and now it is back in the news. The latest word is that the same team has found over two dozen free-floating brown dwarfs within two young star clusters, including one object just six times more massive than Jupiter. Have a look at the image below, which depicts brown dwarfs in the cluster NGC 1333.
Image: Brown dwarfs in NGC 1333. This photograph combines optical and infrared images taken with the Subaru Telescope. Brown dwarfs newly identified by the SONYC Survey are circled in yellow, while previously known brown dwarfs are circled in white. The arrow points to the least massive brown dwarf known in NGC 1333: it is only about six times heftier than Jupiter. Credit: SONYC Team/Subaru Telescope.
What to make of the smallest of the brown dwarfs yet identified in this cluster? Aleks Scholz of the Dublin Institute of Advanced Studies comments:
“Its mass is comparable to those of giant planets, yet it doesn’t circle a star. How it formed is a mystery.”
Indeed, but the implication is that free-floating objects not a great deal larger than a Jupiter-class planet can form the same way stars form, emerging from contracting gas clouds, although there is also the possibility that some of the smaller brown dwarfs formed around a star and were later ejected from the system. SONYC is all about building a more complete census of brown dwarfs in star-forming regions to resolve questions like this, the ultimate goal being to understand how the early development of stars depends on object mass, which will in turn illuminate models of dynamical interactions and accretion as the nascent objects form. These considerations make brown dwarfs in a mass range that overlaps with massive planets a key area for research.
The SONYC census is developing our database in this area, as described in one of the two papers recently made available on this work:
We find 10 new likely brown dwarfs in this cluster, including one with a spectral type ~L3 and two more with spectral type around or later than M9. These objects have estimated masses of 0.006 to 0.02M [solar masses], the least massive objects identified thus far in this region. This demonstrates that the mass function in this cluster extends down to the Deuterium burning limit and beyond. By combining the findings from our SONYC survey with results published by other groups, we compile a sample of 51 objects with spectral types of M5 or later in this cluster, more than half of them found by SONYC. About 30-40 of them are likely to be substellar.
The astronomers studied both NGC 1333 and the Rho Ophiuchi star cluster with Subaru at both optical and infrared wavelengths. NGC 1333, a young cluster thought to be no more than a million years old, turns out to have a higher population of brown dwarfs than the average young cluster, a fact that may offer clues to different conditions within the cluster that affect brown dwarf formation. The cluster houses half as many brown dwarfs as normal stars.
The paper continues:
The star vs. brown dwarf ratio in NGC1333 is significantly lower than in other nearby star forming regions, possibly indicating environmental differences in the formation of brown dwarfs. We show that the spatial distribution of brown dwarfs in NGC1333 closely follows the distribution of the stars in the cluster. The disk fraction in the brown dwarf sample is < 66%, lower than for the stellar members, but comparable to the brown dwarf disk fraction in 2-3 Myr old regions. The substellar members in NGC1333 show a large fraction of highly flared disks, evidence for the early evolutionary state of the cluster.
The paper cited above is Scholz et al., “Substellar Objects in Nearby Young Clusters (SONYC) IV: A census of very low mass objects in NGC1333,” accepted for publication in the Astrophysical Journal (preprint). The second paper on this work is Muzic et al., “Substellar Objects in Nearby Young Clusters (SONYC) V: New brown dwarfs in rho Ophiuchi,” also accepted by the Astrophysical Journal (preprint).
Remembering Collier’s and Looking Ahead
We’ve been talking lately about space missions designed to maximize science vs. those that are at least partly geared toward public relations. But most missions will have both components, the need for public support being woven into the fabric of our ambitions. As we try, then, to ramp up the scientific return, what can we also do to keep the public engaged and instill interest in space exploration? One answer came from Wernher von Braun’s massive project for space exploration, described in a series of articles on space presented by Collier’s magazine from March of 1952 to April, 1954.
Let’s put aside all the technical problems of the von Braun concept and concentrate on it as an incentive for space missions. Al Jackson, with whom I enjoyed dinner and several good conversations in Orlando at the 100 Year Starship Symposium, recently sent me The Ugly Spaceship and the Astounding Dream, an article he wrote for the AIAA’s Horizons magazine. Al is completely upfront about the fact that it was the Collier’s series that brought him into space research in the first place. Now a visiting scientist at the Lunar and Planetary Institute, Al has seen aerospace from the government side (as astronaut trainer on the Lunar Module Simulator) to industry, knew Robert Bussard as well as Robert Forward, and wrote a number of papers on interstellar matters for JBIS, including work in the late 1970s on a laser-powered interstellar ramjet that I want to discuss soon in these pages.
Image: Chesley Bonestell’s take on the von Braun lunar lander, an ugly spaceship with a mind-blowing cargo of ideas.
The Collier’s series, tapping the artistic genius of Chesley Bonestell, produced what Al calls ‘the most influential feat of popular science writing ever,’ with its depiction of a complete manned space program ranging from Earth-circling space stations to lunar landings and that massive expedition to Mars. And ‘massive’ is the right word, for von Braun imagined a flotilla of ten spaceships with a crew of 70 making the Mars journey, fifty of them landing on the surface. 950 ferry flights were needed to assemble the spaceships, but von Braun’s vision included the space infrastructure to make that happen, all based on ideas he had been working on in 1947 and 1948, published first in a German journal and then brought out in a hardcover edition: Marsprojekt; Studie einer interplanetrischen Expedition. Sonderheft der Zeitschrift Weltraumfahrt (1952).
But it was the October 18, 1952 issue of Collier’s that Al, then almost twelve years old, found the most fascinating, the one with the ugliest, most un-aerodynamic spaceship he had ever seen shown on final descent to the Moon. It was in the pages of this issue that he learned that aerodynamic shapes count for little in space, and as he tells us in his article, the entire series would wind up changing his life:
I came to love that ugly space ship! It cemented itself to my soul. It led me to a life in science, a B.A. degree in Mathematics, an M.A. degree in Physics and a Ph.D. in physics. Most startling, to me, that series led me to 35 years of work in spaceflight, first Apollo, the Shuttle program, and now the ISS. All due to the romance of space expressed by Chesley Bonestell, Wernher von Braun and Willy Ley.
Collier’s produced spectacle and soaring ideas, doubtless contributing to the careers of numerous scientists and engineers, even if we now find the technical specifications of the von Braun program daunting. But I sometimes wonder what we might do today to recapture some of the spectacle produced by the Apollo landings, which similarly energized broad portions of the population. Space-minded people like myself are fascinated with the Dawn mission to Vesta and Ceres, and equally drawn to the allure of the Pluto/Charon encounter coming up in 2015. But what would have the most impact on those less obsessed with deep space?
I think I may have found the right concept. It’s the notion of exploring another planet by balloon, exemplified by MGA, the Mars Geoscience Aerobot, studied by a JPL team in the 1990s under the acronym MABVAP – Mars Aerobot Validation Program. It’s an idea with an international pedigree: A balloon system for Mars designed for a Soviet space probe was also under development through the work of Jacques Blamont (CNES), although the project was eventually dropped for financial reasons.
In essence, though, the ideas are similar. The idea is to deliver a superpressure balloon system to Mars, an aerobot that would remain aloft for as much as three months, circumnavigating Mars more than 25 times. Along with an infrared spectroscopy system, a magnetometer, instruments for studying Martian weather and a radar sounder, the balloon would be equipped with an ultrahigh resolution stereo imager. Robert Zubrin and a team at Martin Marietta have more recently been working on the Mars Aerial Platform (MAP), a plan to use eight balloons to map the global circulation of Mars’ atmosphere, examine its surface and subsurface with remote sensing techniques, and return thousands of high resolution images of the Martian surface.
The views returned by an aerial circumnavigation of Mars would be little short of spectacular, as Zubrin notes in the new edition of his The Case for Mars (2011):
Today, nearly five hundred years since Copernicus and Kepler, Brahe and Galileo, most people still think of Earth as the only world in the universe. The other planets remain mere points of light, their wanderings through the night sky of interest to a select few. They are abstractions, notions taught in schools. The MAP cameras offer the possibility of taking humanity’s eyes to another planet in a way that has never been done before. Through the gondola’s cameras we will see Mars in its spectacular vastness: its enormous canyons, its towering mountains, its dry lake and river beds, its rocky plains and frozen fields. We will see that Mars is truly another world, no longer a notion but a possible destination. And, just as the New World entranced and enticed mariners here on Earth, so can Mars entice a new generation of voyagers, a generation ready to fashion the ships and sails proper for heavenly air.
Remember, I’m thinking in terms of how to kindle public interest in space and keep those fires burning, and to me the ability to see the topography of Mars through close-up imaging of its entire surface could create an experience as breathtaking for some budding scientists as the Collier’s series was for Al Jackson. We go from looking at rover tracks and landscapes limited by a rover’s range to the ability to move freely over Mars, from Olympus Mons to the Valles Marineris.
Similar missions have been proposed for Titan, with a certain scientific return as well as the potential for serious public engagement as the aerobot probes the surface of the mysterious moon. An autonomous flying robot was actually delivered to Venus on each of the two Soviet Vega probes, entering the atmosphere in June of 1985. Both balloons operated for almost two Earth days until their batteries batteries failed [see comments below]. Deployed onto the darkside of the planet at an altitude of about 50 kilometers, the balloons operated for their brief lives in an altitude where pressure and temperature were not dissimilar to those of Earth.
While the scientific return from the Vega balloons was minimal, the concept was validated. Using aerostats equipped with high-definition imaging capabilities on Mars, we may be able to re-create some of the sizzle of exploration that the Voyagers had, giving a boost to science-minded young people and providing vistas for public viewing through remote sensing that could one day be looked back on as the key players in creating new careers in science. We may or may not one day get to the stars, but if we do, it will be because the right people came across the right incentives, leading them into careers that could change the way we do space exploration.
Huge Mountain Among Early Vesta Results
So much has been happening in recent weeks that I haven’t had the chance to keep up with all the stories in the queue, and that’s not a bad thing considering that a high level of activity usually means we’re learning new and interesting things. Consider the Dawn mission, which has been orbiting the asteroid Vesta since the middle of July. The Dawn team has been sharing results about Vesta in multiple locations, including the European Planetary Science Congress and the Division of Planetary Sciences Joint Meeting 2011 in Nantes and the annual meeting of the Geological Society of America in Minneapolis. As expected, Vesta turns out to be an intriguing place.
The image below is a look at Vesta’s topography in the southern polar region, with the overall curvature of the tiny world removed, so you’re seeing what it would look like on a flat surface. You wouldn’t have this view on Vesta because many of the features would wrap around below the horizon, but the image gets across the scale of the asteroid’s south polar mountain, which rises a good 22 kilometers above the average height of the terrain around it. And note the large cliff on the right side of the image, which bounds part of Vesta’s south polar depression.
Image: This image of the asteroid Vesta, calculated from a shape model, shows a tilted view of the topography of the south polar region. The image has a resolution of about 1,000 feet (300 meters) per pixel, and the vertical scale is 1.5 times that of the horizontal scale. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI.
Everything we see here is consistent with a giant impact in Vesta’s past. The surface is rougher than most asteroids we’ve observed in the main belt, and dating methods based on the crater count imply that some areas in the southern hemisphere are 1 to 2 billion years old, quite a bit younger than areas to their north. Dawn has continued in its closing, spiraling orbit around Vesta since July, reaching an orbital altitude of 2700 kilometers in August for mapping with its framing camera and infrared mapping spectrometer. In late August, the spacecraft began to move into its High Altitude Mapping Orbit, reaching 680 kilometers above the surface on September 29.
Carol Raymond (JPL), deputy principal investigator for Dawn, told an October 12 press conference that the fundamental dichotomy between northern and southern hemispheres was striking. She also pointed to the diversity in color on Vesta’s surface:
“We expected to see some variation in reflected light from Vesta, because that has been seen before with telescopes, and we also knew something about the composition of the asteroid from meteorites. But the diversity we saw with Dawn exceeds what we would have expected. The dichotomy in the morphology of the surface is also apparent in the color of the surface. In the southern hemisphere we see colors that are consistent with the basaltic morphology you would expect from meteorite studies, and in the northern hemisphere we see a more spectrally neutral surface punctuated by very distinct color variations that appear to be associated with impacts.”
A long process of analysis lies ahead as the Dawn team works to integrate these findings with the higher resolution observations it is now collecting. When operations in the High Altitude Mapping Orbit have been completed, the spacecraft will spiral into its Low Altitude Mapping Orbit in early December, with results from that regime reported in March of 2012. Dawn will spend a year orbiting the asteroid before departing in July of 2012 for Ceres. 2015 should be a lively year, for not only will Dawn reach Ceres but New Horizons will encounter Pluto/Charon. More on Dawn’s mission in this JPL news release.
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