Dawn Orbits Ceres

by Paul Gilster on March 6, 2015

I spent the morning working on an interesting paper about detecting ‘exorings’ — ring systems like Saturn’s around exoplanets — while switching back and forth to Twitter and various Web sources to follow events as the Dawn spacecraft became gravitationally captured by Ceres. I have problems with so-called ‘multi-tasking,’ which at least in my case means I do two things at once, performing each task less effectively than if I were tackling them separately. Fortunately, I have all weekend to tune up the exorings story, and I put it temporarily aside to work on Dawn’s historic arrival.

Congratulations to the entire Dawn team on the continuance of this splendid mission. We have much to look forward to as observations proceed and the orbit stabilizes. Similarly, we have the almost immediate prospect of following New Horizons in to Pluto/Charon, another case of a previously blurry object taking on breathtaking resolution as the days pass. The bounty of 2015 then opens into an uncertain future when it comes to exploring the outer system, but we can hope that the New Horizons extended mission will happen as anticipated and investigate a Kuiper Belt Object. We can also hope that the European Space Agency proceeds with its Jupiter Icy Moons Explorer without what would have been the NASA side of the mission.

Regarding Dawn, remember that the benefits of ion propulsion have never been more obvious thanks to this mission. The first spacecraft to reach orbit around a dwarf planet (at approximately 1239 UTC today), Dawn is also the first spacecraft to orbit more than one target, having explored the asteroid Vesta from 2011 to 2012 before moving on to its current location. That gives us quality data time at the two most massive asteroids in the main belt that stretches between Mars and Jupiter.


The just released image above was taken on March 1, before orbital insertion and before Dawn swung behind Ceres. The view is just a teaser for the scenery we’re going to be looking at as the spacecraft begins its orbital investigations. It took seven and a half years to get here (and 4.9 billion kilometers along the route), but we now have a healthy spacecraft at its target. This image was taken about 48,000 kilometers out, at a Sun-Ceres-spacecraft angle (phase angle) of 123 degrees. The image scale here is 2.9 kilometers per pixel. We’ll get new imagery in April as Dawn moves back around the near side of Ceres with respect to the Sun.



Planet in a Quadruple Star System

by Paul Gilster on March 5, 2015

Planets in multiple star systems intrigue us particularly when we try to imagine the view from the surface. Call it the ‘Tatooine Effect,’ made to order for visual effects specialists and cinematographers. But planets like these also raise interesting issues. Lewis Roberts (JPL) and colleagues have just published a new study of the 30 Ari system, demonstrating that it is a quadruple star system with a gas giant of about four times the mass of Jupiter in a 335 day orbit around its primary star.

We already knew about the planet in the 30 Ari system. What’s new is the discovery of the additional star. At 23 AU from the planet, the newly discovered fourth star would seem to be a factor in the orbital dynamics of the gas giant, but just what effects it has remain to be studied. The paper, which also reports the detection of a stellar companion to the exoplanet host system HD 2638, notes that 30 Ari is the second quadruple system known to host an exoplanet. And interestingly, both HD 2638 BC and 30 Ari BC have projected separations of less than 30 AU, so that the stellar companions may play a key role in the evolution of the exoplanets’ orbits.


Image: A gas giant orbiting a binary star. How planets interact with their primary and other stars in multiple-star systems like these is a question that will demand orbital computations over a long span of observation. Credit: NASA, E. Schwamb.

According to this news release from the University of Hawaii’s Institute for Astronomy, the view from the surface of the 30 Ari planet (or, let’s say, a moon around it) would involve the primary star and two other stars bright enough to be visible in daylight. One of the bright ‘stars’ would actually be a binary system if examined in a telescope. The other known planet in a quadruple system is Ph1b in a system designated KIC 4862625, from which a different view would emerge. Ph1b is on a circumbinary orbit, a giant planet of between 20 and 55 Earth masses orbiting an eclipsing binary made up of a G- and an M-class dwarf, with a second binary star at a distance of 1000 AU.

While quadruple star systems are somewhat unusual (Andrei Tokovinin of the Cerro Tololo Inter-American Observatory in Chile.estimates that about four percent of solar-type stars are in quadruple configurations), the scant number of planets we’ve found in such systems may be the result of how we observe. The Roberts paper, published in The Astronomical Journal makes the point, and notes the inherent observational difficulties:

Known close visual binaries are traditionally excluded from radial-velocity (RV) exoplanet programs because the presence of a visual companion degrades the RV precision. This intrinsic bias complicates statistical inferences about exoplanets in binaries. Moreover, a faint visual companion that is itself a close spectroscopic binary pair can produce periodic low-amplitude RV modulation in the combined light that can be mistaken for an exoplanet. False positive exoplanet detections caused by unrecognized hierarchical multiplicity of their hosts may reach 1–2% (Tokovinin 2014b) of the total exoplanet sample. This is yet another reason to observe exo-hosts with high angular resolution and deep dynamical range.

But despite their difficulties, the fact that exoplanets and the stars they orbit have a common origin means that the more we learn about the multiple star system in question, the more we learn about the exoplanet as well. Just what effects do multiple stars in their various configurations have on the planets around them? One possibility is that gravitational nudges from these nearby stars may affect the protoplanetary disk, producing massive planets on eccentric orbits as the disk is disrupted. To learn more, we need to increase the number of observed binary systems, especially those where the binary separation is small.

Thus far the pattern that has emerged is that the frequency of exoplanets among single stars is roughly the same as that around the components of wide binaries, the latter defined as those having a semimajor axis greater than 100 AU. The evidence suggests that wider binaries have little impact on exoplanet orbital dynamics. But when we get to binaries with separations of less than 100 AU, we find fewer exoplanets but in general more massive ones. No planet has yet been detected in a stellar binary with a separation of less than 10 AU. It’s worth keeping in mind that Centauri A and B, our nearest neighbors, close to within 11 AU at their closest approach.

Looking around for fictional descriptions of multiple star systems as viewed from one of their planets, I come back to a book I mentioned not long after this site began, Stanton A. Coblentz’s Under the Triple Suns (Fantasy Press, 1955). Here’s what Coblentz imagined some sixty years ago, a bit of fun to end this post:


The red sun glowed high in the copper heavens. It was as wide as a dozen moons, and of the color and brightness of smoldering embers; and it did not end sharply as a disk should, but terminated in a nebulous crimson fringe. It shed its rays like a dying fire over a great sweep of wooded, partly hilly country, terminated in the distance by saw-toothed mountains, and marked at closer range by the loop of a cascading river and the oval of a lake, and by a cluster of shimmering beehive structures that billowed and fluttered in the breeze.

After a time, above the serrate edges of the far-off ranges, a white illumination began to spread; and the mist-banks about the peaks, ruddy before, took on a sheet-like glare as a globe that seemed of a hand’s width slowly swam into sight. Although much smaller than the red sun, it dominated the scene by its intense hot flame.

The white orb was about fifteen degrees above the horizon when another light began to emerge. Of an almost unbearable brilliance, it looked not much larger than a silver dollar; but its companions seemed almost pale beside its terrible sea-blue incandescence. Evidently the blue sun and the white belonged together, like the earth and the moon; and the three luminaries, along with a Saturn-like ringed fourth that had no fire of its own but glowed red, white or blue according to the influence of the moment, circled with a gradual movement from west to east.

The paper is Roberts et al., “Know the Star, Know the Planet. III. Discovery of Late-Type Companions to Two Exoplanet Host Stars,” The Astronomical Journal Vol. 149, No. 4 (2015), 118 (abstract / preprint).


Strategies for Life on Titan

by Paul Gilster on March 4, 2015

Back in September of 1961, Isaac Asimov penned an essay in Fantasy & Science Fiction under the title “Not As We Know It,” from which this startling passage:

…when we go out into space there may be more to meet us than we expect. I would look forward not only to our extra-terrestrial brothers who share life-as-we-know-it. I would hope also for an occasional cousin among the life-not-as-we-know-it possibilities.

In fact, I think we ought to prefer our cousins. Competition may be keen, even overkeen, with our brothers, for we may well grasp at one another’s planets; but there need only be friendship with our hot-world and cold-world cousins, for we dovetail neatly. Each stellar system might pleasantly support all the varieties, each on its own planet, and each planet useless to and undesired by any other variety.

Asimov’s idea, prompted by a monster movie excursion with his children, was to look at realistic ways that life much different from our own could emerge. Here he anticipated our discussions of habitable zones and just what they imply, for we usually speak of a world being habitable if liquid water can exist on its surface. Asimov would have none of that because he wanted to know what kind of life might emerge in the hottest and coldest places in the Solar System. Reprints of the essay inspired James Stevenson, a graduate student at Cornell University, whose recent work on the astrobiological possibilities on Titan has energized wide discussion.


Image: Are there ways life could emerge on Titan? A panorama of the shoreline where Huygens touched down, stitched from DISR Side-Looking and Medium-Resolution Imager Raw Data. Image credit: ESA / NASA / JPL / University of Arizona / Rene Pascal (panorama).

Collaborating at Cornell with astronomer Jonathan Lunine and chemical engineer Paulette Clancy, Stevenson went to work on a cell membrane that could function in a cold and methane-rich environment. Clancy specializes in chemical molecular dynamics, while Lunine’s background includes working on the Cassini mission. With non-aqueous life on the table (Lunine had received a grant from the Templeton Foundation to study the possibilities), Clancy’s expertise seemed made to order. She comments on the work in this Cornell news release:

“We’re not biologists, and we’re not astronomers, but we had the right tools. Perhaps it helped, because we didn’t come in with any preconceptions about what should be in a membrane and what shouldn’t. We just worked with the compounds that we knew were there and asked, ‘If this was your palette, what can you make out of that?’”

It’s an interesting palette in a very interesting place. Liquid methane is the only liquid other than water that forms seas on the surface of a planetary body in the Solar System. The paper also notes the intriguing fact that there is an unknown process at work on Titan’s surface that consumes hydrogen, acetylene and ethane — these reach the surface out of the atmosphere but do not accumulate. Finding a cell membrane mechanism for Titan’s methane seas becomes an exercise in astrobiology that we can hope one day to weigh against data from the surface.

Using molecular simulation strategies given the challenges of cryogenic experimentation, the researchers screened for the best candidates for self assembly into membrane-like structures. The result: A cell membrane the researchers call an azotosome, made out of nitrogen, carbon and hydrogen molecules already known to exist in Titan’s frigid seas. If Earth life is built around the phospholipid bilayer membrane — water-based vesicles made from this are known as liposomes — then a methane-based membrane like the azotosome could be the Titanian analog, a flexible and stable cell membrane able to function at temperatures of -180 °C. From the paper:


In a cold world without oxygen, we suggest that the vesicles needed for compartmentalization, a key requirement for life, would be very different to those found on Earth. Rather than long-chain nonpolar molecules that form the prototypical terrestrial membrane in aqueous solution, we find membranes that form in liquid methane at cryogenic temperatures do so from the attraction between polar heads of short-chain molecules that are rich in nitrogen. We have termed such a membrane an azotosome. We find that the flexibility of such membranes is roughly the same as those of membranes formed in aqueous solutions. Despite the huge difference in temperatures between cryogenic azotosomes and room temperature terrestrial liposomes, which would make almost any molecular structure rigid, they exhibit surprisingly and excitingly similar responses to mechanical stress.

Image: A representation of a 9-nanometer azotosome, about the size of a virus, with a piece of the membrane cut away to show the hollow interior. Credit: James Stevenson.

Could such membranes form on Saturn’s largest moon? We already know that a liquid organic compound called acrylonitrile can be found in the atmosphere there, and the researchers believe that an acrylonitrile azotosome compound would offer indigenous life the same kind of stability and flexibility that phospholipid membranes bring to life on Earth. Studying the metabolism and reproduction of the hypothesized cells is the next order of business, but Lunine talks of one day going well beyond theory to float a probe on Titan’s seas to sample its organics directly.

None of this demonstrates that life is present on Titan, but focusing on the availability of molecules that can form cell membranes helps us understand the kind of chemistries we need to look for under cryogenic conditions. In their conclusion, the authors talk about the ‘liquid methane habitable zone,’ a wonderful reminder of how our views on astrobiology are expanding.

The paper is Stevenson, Lunine & Clancy, “Membrane alternatives in worlds without oxygen: Creation of an azotosome,” Science Advances Vol. 1, No. 1 (27 February 2015), e1400067 (full text).



Were There Planets Inside Mercury’s Orbit?

by Paul Gilster on March 3, 2015

With the Mercury Messenger mission now coming to its end, it seems an appropriate time to speculate on why our inner Solar System looks the way it does. After all, as we continue finding new solar systems, we’re discovering many multi-planet systems with planets — often more than one — closer to their star than Mercury is to ours. We have Kepler to thank for these discoveries, its data analyzed in a number of recent papers including one arguing that about 5 percent of all Kepler stars have systems with tightly packed inner planets. The awkward acronym for such systems is STIP.

Well, maybe it’s not all that awkward, and Kathryn Volk and Brett Gladman (University of British Columbia) have good cause to deploy it in their new paper, which focuses on this topic. They’re wondering why our Solar System lacks planets inside Mercury’s orbit, and they point to the paper I mentioned above (Lissauer et al, 2014) as well as another by Francois Fressin and colleagues that concludes that half of all Kepler stars have at least one planet in the mass range from 0.8 to 2 Earth masses with orbits inside Mercury’s distance from our Sun, which is 0.39 AU, or 58.5 million kilometers.


Image: The Caloris basin and adjacent regions on Mercury. Recent exoplanet discoveries raise the question of why our Solar System lacks planets inside Mercury’s orbit. Can instabilities in the early Solar System help us find the answer, while at the same time explaining some of the planet’s peculiarities? Image credit: JHU/APL.

Taking as an hypothesis that nearly all F, G and K-class stars originally form with planets well within Mercury’s orbital distance, Volk and Gladman ask whether the reason we find systems without such planets today is that instabilities have destroyed these worlds through generations of catastrophic collisions and gradual re-formation, leaving (in our case) Mercury as the surviving relic. It is true that STIPs can be dynamically stable over long time-frames (hence we see the Kepler examples), but the absence of tightly packed inner worlds around many stars is here taken as the result of a ‘metastable planetary arrangement’ that leaves one or no short period planets. The Kepler STIPs we see, then, are those that have survived this process.

The authors use the Kepler data to generate systems similar to those we have uncovered, allowing them to ‘evolve’ computationally to study system dynamics, taking some simulations well beyond the first collision to see how the instabilities multiply. An initial collision often produces second collisions at higher speeds. While low-speed impacts can occur in some systems, producing far smaller amounts of debris and subsequent accretion, a fraction of STIPs experience heavy perturbation that can lead to the destruction of their inner worlds. From the paper:

Our experiments show that instability timescales in these systems are distributed such that equal fractions of the systems go unstable (reach a first planetary collision) in each decade in time (Fig. 2). This logarithmic decay is not unknown in dynamical systems (eg., Holman & Wisdom (1993)) and is presumably related to chaotic diffusion and resonance sticking near the stability boundary. After a brief, relatively stable initial period, the systems hit instability at a rate of ∼20% per time decade, with half of the systems still intact at ∼100 Myr. The exact decay rate may be influenced by our usage of the current Kepler STIPs sample (perhaps the most stable); however if this decay rate held, at ∼5 Gyr 5–10% of STIPs would not yet have reached an instability, in rough agreement with the observed STIPs frequency.

Turning the results on our own Solar System, they find that the orbits of the three outer terrestrial planets (Venus, Earth, Mars) remain unaffected on 500 million year timescales by the presence of additional planets totaling several Earth masses, all of the latter inside a distance of 0.5 AU from the Sun. Dynamical instabilities would have initiated a sequence of collisions among these worlds that left Mercury as the sole survivor. The authors argue that it is possible for the orbits of the outer terrestrial planets to remain unperturbed as the inner planets fall victim to these events.

Various issues are explained by this scenario. A series of collisions concentrates iron into the surviving remnants, which accounts for Mercury’s high density. The authors also ask whether instabilities in the inner system approximately 4 billion years ago could account for the Late Heavy Bombardment (sometimes called the ‘lunar cataclysm’), when Mercury, Venus, Earth and Mars experienced a high number of impacts. From the paper:

Gladman & Coffey (2009) estimated that 10–20% of large (m to 100 km) debris originating near current Mercury would strike Venus, with 1–4% impacting Earth (∼0.1% strikes the Moon). The Earth’s impact rate would peak ∼1–10 Myr after the event and decay on ∼30 Myr timescales as Mercury and Venus absorb most of the debris; this is a plausible match for the cataclysm’s final stages (Cuk et al. 2010). A bottom-heavy size distribution for the 1–100 km debris could explain the recent finding (Minton et al. 2015) that a main-belt asteroid source would produce too many impact basins during the cataclysm.

Such an event might be studied by future sampling missions, for:

STIP debris would likely be mostly silicate-rich mantle material similar but not identical to main-belt asteroid compositions, consistent with cataclysm impactor compositions inferred via cosmochemical means (Joy et al. 2012). The smallest dust (being blown out hyperbolically) could impact the Earth-Moon system. We estimate that 10−11 of the departing dust would strike the Moon, at vimp ∼30 km/s. If any dust or meteoroid projectiles were retained, fragments might be found in regolith breccias compacted during the cataclysm epoch.

This is an interesting model, and the authors point out that it gets us out of the difficulty of assuming an inner protoplanetary disk edge that has to be adjusted to account for Mercury and the lack of worlds interior to its orbit. We have a model that would leave the orbits of the existing terrestrial-class worlds unaffected by the series of collisions and disintegrations that Mercury emerged from, although the recipients of catastrophic amounts of early debris. The model also accounts for the apparent instability of Mercury’s orbit on a 5 to 10 billion year time-frame.

The paper is Volk and Gladman, “Consolidating and Crushing Exoplanets: Did it happen here?,” submitted to Astrophysical Journal Letters (preprint). Thanks to Andrew Tribick for the pointer to this paper.



Seeing Ceres: Then and Now

by Paul Gilster on March 2, 2015

I’m interested in how we depict astronomical objects, a fascination dating back to a set of Mount Palomar photographs I bought at Adler Planetarium in Chicago when I was a boy. The prints were large and handsome, several of them finding a place on the walls of my room. I recall an image of Saturn that seemed glorious in those days before we actually had an orbiter around the place. The contrast between what we could see then and what we would soon see up close was exciting. I was convinced we were about to go to these worlds and learn their secrets. Then came Pioneer, and Voyager, and Cassini.

And, of course, Dawn. As we discover more and more about Ceres, the process repeats itself, as it will again when New Horizons reaches Pluto/Charon. Below is a page from a book called Picture Atlas of Our Universe, published in 1980 by the National Geographic. Larry Klaes forwarded several early images last week as a reminder of previous depictions of the main belt’s largest asteroid, or dwarf planet, or whatever we want to call it. Here the artwork isn’t all that far off the mark for Ceres, though Vesta would turn out to be a good deal less spherical than predicted. No mention of a possible Ceres ocean in the depictions of this time; all that would come later.


Image: Ceres and other asteroids as seen through the eyes of artist Davis Meltzer in 1980, with the Moon as a background.

The recent Dawn imagery has us buzzing about the two bright spots on Ceres that, of course, were unknown to our artist in 1980. From 46,000 kilometers, all we can do is admit how little we know, which is more or less what Andreas Nathues, lead investigator for the framing camera team at the Max Planck Institute for Solar System Research (Gottingen) does:

“The brightest spot continues to be too small to resolve with our camera, but despite its size it is brighter than anything else on Ceres. This is truly unexpected and still a mystery to us.”


Image: This image was taken by NASA’s Dawn spacecraft of dwarf planet Ceres on Feb. 19 from a distance of nearly 46,000 kilometers. It shows that the brightest spot on Ceres has a dimmer companion, which apparently lies in the same basin. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA.

Chris Russell, principal investigator for Dawn, speaks of a possible “volcano-like origin” of the two bright spots, but adds that we have to wait for better resolution to make any serious geological interpretations. The wait won’t be all that long (for better resolution, at least) given that we’re just days away from entering orbit on March 6. Could there be a better approach to this small world than this one, already focusing on something no one had expected to see?

In 1961, in an illustration from The Universe (New York: Morrow), we find Ceres again displayed with companion objects like Vesta and Pallas (I’m afraid I don’t know the name of the artist). Here the round, cratered Ceres is reasonably accurate, and you’ll note the size comparison, with Ceres tucked up inside Texas. At the bottom of the image is Eros, shown here as an object the size of Manhattan and described in the caption as “Flying end over end through space like an island torn from its moorings…”


And here is Eros as it appeared in the Astronomy Picture of the Day in 2001.


Image: Orbiting the Sun between Mars and Earth, asteroid 433 Eros was visited by the robot spacecraft NEAR-Shoemaker in February of 2000. High-resolution surface measurements made by NEAR’s Laser Rangefinder (NLR) have been combined into the above visualization based on the derived 3D model of the tumbling space rock. NEAR allowed scientists to discover that Eros is a single solid body, that its composition is nearly uniform, and that it formed during the early years of our Solar System. Credit: NEAR Project, NLR, JHUAPL, Goddard SVS, NASA.

When we get cameras in the vicinity of objects that for so long were just smudges in even the best telescopes, we sometimes find ourselves surprised and delighted, as witness the volcanoes of Io or, for that matter, the cryovolcanism and ‘canteloupe terrain’ on Neptune’s moon Triton. Sharpening the view takes us out of the realm of the artist’s imagination and into the world of concrete measurement. Giving up earlier visions can be poignant, as we learned with Mariner 4’s 1965 flyby of Mars, when a vegetative and even fertile Mars suddenly became a fantasy forever lost. But as Ceres is proving right now, the discovery of the unexpected is a much greater reward.



Astrobiology: A Cautionary Tale

by Paul Gilster on February 27, 2015

We’re discovering planets around other stars at such a clip that moving to the next step — studying their atmospheres for markers of life — has become a priority. But what techniques will we use and, more to the point, how certain can we be of their results? Centauri Dreams columnist Andrew LePage has been mulling these matters over in the context of how we’ve approached life on a much closer world. Before the Viking landers ever touched down on Mars, a case was being made for life there that seemed compelling. LePage’s account of that period offers a cautionary tale about astrobiology, and a ringing endorsement of the scientific method. A senior project scientist at Visidyne, Inc., Drew is also the voice behind Drew ex Machina.

by Andrew LePage


Every time I read an article in the popular astronomy press about how some new proposed instrument will allow signs of life to be detected on a distant extrasolar planet, I cannot help but be just a little skeptical. For those of us with long memories, we have already been down this road of using remote sensing techniques to “prove” life existed on some distant, unreachable world, only to be disappointed when new observations became available. But instead of a distant extrasolar planet, over half a century ago that planet was our next door neighbor, Mars.

Back when I was in high school in the late 1970s, I enjoyed spending time during study hall going through science books and magazines, old as well as new, in the school library. Among the interesting tidbits I read about were spectral features known as “Sinton bands” and how in the early 1960s these were considered the latest evidence of life on Mars. Of course by the time I was reading this, I knew from the then-recent results from the Viking missions that the explanation for these and other observations was simply incorrect. So what ever happened to these Sinton bands and the interpretation they were evidence of life on Mars?

In the years leading up to the beginning of the Space Age, the general consensus of the scientific community was that Mars was a smaller and colder version of the Earth that supported primitive plant life akin to lichen. This view was based on a large body of observational evidence gathered over the first half of the 20th century. A firmly established wave of darkening was observed spreading over the spring hemisphere of Mars each Martian year which was widely seen as being the result of plants coming out of their winter slumber much as happens on Earth each spring. This interpretation was bolstered by visual observations that the dark regions of Mars appeared to have a distinct green hue just as one would expect from widespread plant life.

Other observations of Mars during this period lent further support to the view that the Red Planet could support simple life forms. The general consensus of the astronomical community at this time based on analyses of decades of photometric and polarimetric measurements of Mars indicated that the surface pressure of the Martian atmosphere was about 85 millibars or about 8.4% of Earth’s surface pressure. Carbon dioxide and water vapor were detected and nitrogen was widely expected to be the major atmospheric constituent just as it was on Earth. No large bodies of water were visible on the surface and the climate was certainly colder than on Earth as a whole owing to Mars’ greater distance from the Sun, but the surface temperatures at the equator easily exceeded the freezing point of water during the summer so that liquid water was expected to be available. While not an ideal environment by terrestrial standards, it seemed that Mars had conditions that would be expected to support life much like the high arctic here on Earth.


Image: This was the best photograph of Mars available before the Space Age taken at the Mt. Wilson observatory in 1956 – the same year Sinton bands were discovered. Credit: Mt. Wilson Observatory.

To further test this view, American astronomer William Sinton (1925-2004) decided to use the latest technological advancements in infrared (IR) spectroscopy to obtain observations of Mars during its especially favorable 1956 opposition. On seven nights during the fall of 1956, Dr. Sinton used the 1.55-meter Wyeth Reflector at the Harvard College Observatory to make IR spectral measurements using a lead sulfide detector cooled using liquid nitrogen to vastly improve its sensitivity. He made repeated measurements between the wavelengths of 3.3 to 3.6 μm in order to sample the spectral region where resonances from the C-H bonds of various organic molecules would create distinctive absorption features. His analysis found a dip in the IR spectrum of Mars near 3.46 μm which resembled his IR spectrum of lichen. This finding and his conclusions were published in highly respected, peer-reviewed astronomical publication The Astrophysical Journal.

Encouraged by these initial results, Dr. Sinton repeated his measurements using an improved IR detector on the 5-meter Hale Telescope at the Mt. Palomar Observatory (then, the largest telescope in the world) during the following opposition of Mars in October 1958. His new observations had ten times the sensitivity of his original measurements and now covered wavelengths from as short as 2.7 μm out to 3.8 μm. In addition to absorption features attributable to methane and water vapor in Earth’s atmosphere, Dr. Sinton identified absorption features centered at 3.43, 3.56 and 3.67 μm that appeared to be weaker or absent in the brighter areas of Mars. Dr. Sinton concluded that inorganic compounds like carbonates could not produce the observed features. Instead they must be produced by organic compounds selectively concentrated in the dark areas of Mars that were already known to be greener. While the features he observed were not a perfect match for any known plant life on Earth, he concluded that they were due to organic compounds such as carbohydrates produced by plants on the surface of Mars. These findings and conclusions were again published in a well-regarded, peer-reviewed scientific journal, Science.

While there was naturally some healthy skepticism about the findings, they were seen by many as supporting the generally held view that Mars was the home of simple, lichen-like plant life. In order to better observe what became known as “Sinton bands”, the Soviet Union even planned to include IR instrumentation to measure these spectral features from close range on the first pair of spacecraft they launched towards Mars in October 1960. Unfortunately, both Mars probes succumbed to launch vehicle failures during ascent and never even made it into Earth orbit. Soviet engineers attempted it again with a pair of much more capable flyby probes of which only Mars 1 survived launch on November 1, 1962. Unfortunately, Mars 1 suffered a major failure in its attitude control system during its cruise and contact was lost three months before its encounter with Mars on June 21, 1963. As a result, there were no close-up IR observations of the Sinton bands at this time.


Image: The earliest Soviet Mars probes carried IR instrumentation to observe Sinton bands at close range including Mars 1 launched in November 1962. Credit: RKK Energia.

But even as the Soviet Union was struggling to reach Mars with their first interplanetary probes, the case for there being plant life on Mars and the Sinton bands being evidence for it was already beginning to unravel. Donald Rea, leading a team of scientists at the University of California – Berkeley, published the results of their work on Sinton bands in September 1963. They examined the IR spectra of a large number of inorganic and organic samples in the laboratory and could not find a match for the observed Sinton bands. While they could not find a satisfactory explanation for the bands, they found that the presence of carbohydrates as proposed by Dr. Sinton was not a required conclusion.

Another major blow was landed in a paper by another University of California – Berkeley team headed by chemist James Shirk which was published on New Year’s Day 1965. Their laboratory work suggested that the Sinton bands could be caused by deuterated water vapor – water where one or both of the normal hydrogen atoms, H, in H2O are replaced with the heavy isotope of hydrogen known as deuterium, D, to form HDO or D2O. Shirk and his team speculated that the deuterated water vapor was present in the Martian atmosphere with the implication that the D:H ratio of Mars greatly exceeded that of the Earth.

The final explanation for the Sinton bands came in a paper coauthored by Donald Rea and B.T. O’Leary of the University of California – Berkeley as well as William Sinton himself published in March of 1965. Based on a new analysis of Dr. Sinton’s data from 1958, observations of the solar IR spectrum from Earth’s surface and the latest laboratory results, it was found that the absorption features in the Martian spectrum now identified as being at 3.58 and 3.69 μm were the result of HDO in Earth’s atmosphere. The feature at 3.43 μm was, in retrospect, a marginal detection in noisy data and was probably spurious. The mystery of the Sinton bands was solved and, unfortunately, it had nothing to do with life on Mars.

Sinton bands were not the only causality of advances in technology and remote sensing techniques at this time. As more detailed ground-based observations of Mars were made during the 1960s and the first spacecraft reached this world, it was eventually found that all of the earlier observations that had been taken as evidence of life on Mars were either inaccurate or had non-biological explanations. After a half century of observations from space and on the surface, we now know that the Martian environment is simply too hostile to support even hardy lichen-like plants as had been widely believed before the Space Age.

This story about the rise and fall of the view that Mars harbors plant-like life forms should not be taken as an example of the failure of science. Instead, it is a perfect example of how the self-correcting scientific process is supposed to work. Observations are made, hypotheses are formulated to explain the observations and those hypotheses are then tested by new observations. In this case, the pre-Space Age view that Mars supported lichen-like plants was disproved when new data no longer supported that view. And our subsequent experience with the in situ search for life on Mars by the Viking landers in 1976 is further evidence not that Mars is necessarily lifeless, but that detecting extraterrestrial life is much more difficult than had been previously believed. These lessons need to be remembered as future instruments start to scan distant extrasolar planets and claims are made that life has been found because of the alleged presence of one compound or another. Past experience has shown that such interpretations can easily be incorrect especially when dealing with new observing techniques of distant worlds with unfamiliar environments.



A Laser ‘Comb’ for Exoplanet Work

by Paul Gilster on February 25, 2015

It’s been years since I’ve written about laser frequency comb (LFC) technology, and recent work out of the Max Planck Institute of Quantum Optics, the Kiepenheuer Institute for Solar Physics and the University Observatory Munich tells me it’s time to revisit the topic. At stake here are ways to fine-tune the spectral analysis of starlight to an unprecedented degree, obviously a significant issue when you’re dealing with radial velocity readings of stars that are as tiny as those we use to find exoplanets.

Remember what’s happening in radial velocity work. A star moves slightly when it is orbited by a planet, a tiny change in speed that can be traced by studying the Doppler shift of the incoming starlight. That light appears blue-shifted as the star moves, however slightly, towards us, while shifting to the red as it moves away. The calibration techniques announced in the team’s paper show us that it’s possible to measure a change of speed of roughly 3 cm/s with their methods, whereas with conventional calibration techniques, the best measurement is roughly 1 m/s (although see the citations below for HARPS calibration of an LFC that reaches 2.5 cm/s). Detecting an Earth-mass planet in an Earth-like orbit around a solar-type star involves observing velocity changes of 10 cm/s or less, so we’re clearly entering the right range here.

Let’s back up and consider how a laser frequency comb works. Below is an image from the European Southern Observatory explaining the ‘comb’ analogy — as you can see, the graph resembles a fine-toothed comb, one built around short, equally spaced pulses of light created by a laser. The different colors of the pulsed laser light are separated based on their individual frequencies. Combining an ultrafast laser as a calibration tool with an external source of light allows scientists to measure the frequency of the external light to a high degree of precision.


Image: This picture illustrates part of a spectrum of a star obtained using the HARPS instrument on the ESO 3.6-metre telescope at the La Silla Observatory in Chile. The lines are the light from the star spread out in great detail into its component colours. The dark gaps in the lines are absorption features from different elements in the star. The regularly spaced bright spots just above the lines are the spectrum of the laser frequency comb that is used for comparison. The very stable nature and regular spacing of the frequency comb make it an ideal comparison, allowing the detection of minute shifts in the star’s spectrum that are induced by the motion of orbiting planets. Note that in this image, the colour range is for illustrative purposes only, as the real changes are much more subtle. Credit: ESO.

The laser frequency comb is, then, a standard ‘ruler’ that can measure the frequency of light to extreme precision. In the case of the recently announced findings, the researchers worked with sunlight averaged over the complete solar disk, as captured by the ChroTel solar telescope (located at the Vacuum Tower Telescope installation in Tenerife, Canary Islands). They combined this light with the light from the laser frequency comb, injecting both into a single optical fiber. The result was sent on to a spectrograph for analysis, with striking results. Lead author Rafael Probst (Max Planck Institute of Quantum Optics) comments:

“Our results show that if the LFC light and the sunlight are simultaneously fed through the same single-mode fibre, the obtained calibration precision improves by about a factor of 100 over a temporally separated fibre transmission. We then obtain a calibration precision that keeps up with the best calibration precision ever obtained on an astrophysical spectrograph, and we even see considerable potential for further improvement.”

Probst goes on to say that although the technique is currently restricted to solar spectroscopy, it should be workable even for faint astronomical targets as it is perfected. He comments in this news release from the Institute of Physics that a key aspect of the work is the clean and stable beam at the output that results from using single-mode fiber, a kind of fiber common in laser applications but relatively little used thus far in astronomy. The LFC at the Vacuum Tower Telescope is the first installation for astronomical use based on single-mode fiber.

These refinements of laser frequency comb technique point toward future measurements of Doppler shifts that will make detecting Earth-sized planets with radial velocity methods more likely. The laser frequency comb seems poised to become a major tool. “In astronomy, frequency combs are still a novelty and non-standard equipment at observatories,” the authors write in their conclusion. “This however, is about to change, and LFC-assisted spectroscopy is envisioned to have a flourishing future in astronomy.”

The paper is Probst et al., “Comb-calibrated solar spectroscopy through a multiplexed single-mode fiber channel,” New Journal of Physics Vol. 17 (February 2015) 023048 (abstract). See also this video abstract of the work. Laser frequency comb work at HARPS reaching into the cm/s range is reported in Wilken et al., “A spectrograph for exoplanet observations calibrated at the centimetre-per-second level,” Nature Vol. 485, Issue 7400 (May, 2012), 611-614 (abstract).



Soft Robotics for a Europa Rover

by Paul Gilster on February 23, 2015

Approaching problems from new directions can be unusually productive, something I always think of in terms of Mason Peck’s ideas on using Jupiter as a vast accelerator to drive a stream of micro-spacecraft (Sprites) on an interstellar mission. Now Peck, working with Robert Shepherd (both are at Cornell University) is proposing a new kind of rover, one ideally suited for Europa. The idea, up for consideration at the NASA Innovative Advanced Concepts (NIAC) program, is once again to exploit a natural phenomenon in place of a more conventional technology. What Peck and Shepherd have in mind is the use of ‘soft robotics’ — autonomous machines made of low-stiffness polymers or other such material — to exploit local energy beneath Europa’s ice.

We’re at the edge of a new field here, with soft robotics advocates using principles imported from more conventional rigid robot designs to work with pliable materials in a wide range of applications, some of which tie in with the growth in 3D printing. The people working in this area are developing applications for everything from physical therapy to minimally invasive surgery, with energizing inputs from organic chemistry and soft materials science. If the average robot is modeled around metallic structures with joints based on conventional bearings, soft robotics looks to the natural world for models of locomotion through terrain and innovative methods of energy production.

Peck and Shepherd are proposing what they call a ‘soft-robotic rover with electromagnetic power scavenging,’ a device capable of moving in the seas of Europa that is anything but the submarine-like craft some have envisioned to do the job. The closest analog in the natural world is the lamprey eel, the soft robotics version of which would use electrodynamic tethers to scavenge energy. The rover moves by swimming, powered not by solar or nuclear power but by the use of expanding gases. The rover under the ice sends data to a Europa orbiter using VLF wavelengths, like a submarine. Let me quote from the NIAC proposal on this mechanism:

The electrical energy scavenged from the environment powers all rover subsystems, including one that electrolyzes H2O. Electrolysis produces a mixture of H2 and O2 gas, which is stored internally in the body and limbs of this rover. Igniting this gas expands these internal chambers, causing shape change to propel the rover through fluid or perhaps along the surface of a planetary body.

Power Beneath the Ice

This is the first time I have encountered locomotion based on electromagnetic power scavenging, with accompanying reliance on soft robotic structures that could change the way we look at designing probes that will operate in ocean environments like that on Europa. It’s interesting to take this one step further, as the proposal itself notes, and remember that a rover inspired by biology may here point toward astrobiology, in that electromagnetic energy is considered to be a possible source of energy for any native life under Europa’s ice.


Image: Water electrolyzer and gas generation subsystem. Credit: Mason Peck, Robert Shepherd.

We know from experience in Earth orbit that tethers work, a result of the fact that a conductor moving through a magnetic field experiences an induced current. The power available at Jupiter gives us interesting options:

In Jupiter’s orbit, where the magnetic field can be up to 10,000 times more powerful and the ionosphere denser than Earth’s, the power can be even higher. A NASA/MSFC study characterized the power available for an EDT [electrodynamic tether] near Jupiter. The authors report that at least 1W would be available in Europa’s orbit but did not consider the much higher conductivity of the ocean or whatever tenuous atmosphere may exist near the surface.

Because higher conductivity produces greater current, the authors argue that far more power should be available on Europa, and that tethers no more than meters long should be sufficient to power the rover. According to the NIAC proposal, one end of the tether would be attached to the rover’s power systems while the other would be kept above the rover by a gas-filled balloon to maintain the necessary configuration as the rover conducted operations, and to ensure a predictable level of current. The tether itself also serves as an antenna to relay science data (possibly through an umbilical) to the surface. The proposed Phase I study would investigate tether configurations and the ability of an EDT to produce the power for transmission.

So we have energy harvested (or scavenged) by electrodynamic tethers being used to power up an electrolyzer that can split water into gaseous H2 and O2, an efficient way to use local resources in a domain where solar and perhaps nuclear power would be unusable (remember in relation to nuclear options that NASA has cancelled development of the Advanced Stirling Radioisotope Generator – ASRG – technologies, although some testing at NASA Glenn is to continue). The gases produced by the electrolysis would then be stored for energy usage, tapped as a combustible fuel/oxidizer mixture, and as a pressurant.

A Natural Model for Locomotion

This last point deserves a second look. In previous work, Robert Shepherd has created pneumatically powered silicon-based robots that can move and navigate obstacles, using onboard air compressors and lithium battery packs. Work at MIT has demonstrated a pneumatically powered swimming robot with a soft tail using onboard compressed CO2. Shepherd has also demonstrated the use of hydrogen combustion to increase the range and speed of soft robots, a model he and Peck propose for further study in the Europa concept, one that might be used both below the ice and on the surface.


Image: Water jetting actuated by ignition of H2/O2 gas and subsequent shape change. Credit: Mason Peck, Robert Shepherd.

Here again the model from nature is instructive, for as the report notes, “[t]he design of the combustion powered hydro-jetting mechanism is analogous to the morphology of an octopus’ mantle cavity.” The report anticipates using jetting methods to allow the rover to range widely at long distances and also for precision operations over short distances. Peck and Shepherd are also hoping to study grasping operations that would be modeled on biology, using ‘an array of teeth-like grippers positioned around the water jet area (mouth) of the synthetic lamprey.’

This is fascinating work that offers us solutions for powering an underwater robot but also provides mechanisms for movement in this environment (one that we may find in other gas giant moons) through the use of a form of robotics that mimics the natural world. Solutions inspired by biology help us move beyond the use of solar arrays, nuclear power or batteries to keep our rover operational and to give it what would seem to be a robust and lengthy lifetime. I would say that getting soft robotics into the picture for future space operations is a very wise idea, certainly one that justifies continued study and investment as we look toward the outer planets.



Beta Pictoris: New Analysis of Circumstellar Disk

by Paul Gilster on February 20, 2015

Our discovery of the interesting disk around Beta Pictoris dates back all the way to 1984, marking the first time a star was known to host a circumstellar ring of dust and debris. But it’s interesting how far back thinking on such disks extends. Immanuel Kant’s Universal Natural History and Theory of the Heavens (1755) proposed a model of rotating gas clouds that condensed and flattened because of gravity, one that would explain how planets form around stars. Pierre-Simon Laplace developed a similar model independently, proposing it in 1796, after which the idea of gaseous clouds in the plane of the disk continued to be debated as alternative theories on planet formation emerged.

Today we can view debris disks directly and learn from their interactions. Out of the Beta Pictoris discovery have grown numerous observations including the new visible-light Hubble images shown below. The beauty of this disk is that we see it edge-on and, because of the large amount of light-scattering dust here, we see it very well indeed. Beta Pictoris, a young star about twenty million years old, is a relatively close 63 light years from Earth. It also offers the only directly imaged debris disk that is known to have a giant planet, imaged in infrared wavelengths by the European Southern Observatory’s Very Large Telescope in 2009.


Image: The photo at the bottom is the most detailed picture to date of a large, edge-on, gas-and-dust disk encircling the 20-million-year-old star Beta Pictoris. The new visible-light Hubble image traces the disk in closer to the star to within about 1 billion kilometers of the star (which is inside the radius of Saturn’s orbit about the Sun). When comparing the latest images to Hubble images taken in 1997 (top), astronomers find that the disk’s dust distribution has barely changed over 15 years despite the fact that the entire structure is orbiting the star like a carousel. The Hubble Space Telescope photo has been artificially colored to bring out detail in the disk’s structure. Credit: NASA, ESA, and D. Apai and G. Schneider (University of Arizona).

As the paper on this work notes, the new images give us the most detailed view of the disk at optical wavelengths that we’ve ever had, with the opportunity as in the image above to study its characteristics over a fifteen-year period. This is helpful because the estimated orbital period of the planet here is between 18 and 22 years, giving astronomers the ability to study a large degree of planetary and disk motion in a relatively small timeframe. The comparison shows that the dust distribution has changed little over the past fifteen years despite the disk’s rotation. A key issue is how the disk is distorted by the presence of the massive planet embedded within it.

The new high-contrast images provide an inner working angle that is smaller than earlier images by a factor of 2, allowing astronomers to image the disk at the location where the gas giant planet was located in 2012. Changes in brightness within the disk indicate an asymmetry that may be the mark of an inner inclined disk projecting into the outer disk material. The image below combines data from Hubble and the ALMA array, highlighting the dust and gas asymmetry.


Image: This is a color composite image of the disk encircling Beta Pictoris. The image shows a curious asymmetry in the dust and gas distribution. This may be due to a planetary collision within the disk, which may have pulverized the bodies. Radio data from the Atacama Large Millimeter/submillimeter Array (ALMA) shows the dust (1.3 millimeter is colored green) and carbon monoxide gas (colored red). Credit for Hubble Data: NASA, ESA, D. Apai and G. Schneider (University of Arizona). Credit for ALMA Data: NRAO and W.R.F. Dent (ALMA, Santiago, Chile).

The disk asymmetry issue will be explored by future Hubble work as well as by observations from the James Webb Space Telescope, which should give us a better indication of planet/disk interactions in the system. There is much to learn here in relation to disk warping and the origins of the planet’s orbital inclination, a tilt from the main disk that previous work had estimated at about 1 degree. The paper refines this estimate:

The warped disk – as seen in projection – subtends an angle larger than the best-fit orbital inclination of β Pic b, suggesting that planetesimals may be perturbed to higher inclinations than that of the perturbing giant planet. This finding is consistent with the predictions of dynamical simulations of a planetesimal system influenced by secular perturbations of a planet on inclined orbits… The fact that the warp is seen at angles 4, would taken on face value, then suggests that β Pic b’s inclination is – within uncertainties – underestimated by current measurements and it may be close to ∼ 2.


Image: Key structures in the β Pic system, as derived from multi-wavelength imaging. Credit: Daniel Apai et al. (Figure 15 from the paper).

We need to learn how the massive gas giant in the Beta Pictoris system ended up with an inclined orbit, and what has caused the asymmetry in the disk itself. Learning how the process works around this nearby star will help us detect the evidence of exoplanets in other circumstellar disks. While we continue to use the dusty Beta Pictoris system as the model for debris disks around young stars, we’re learning that the structure of disks may be intimately related to the planets moving within them. Thus each circumstellar disk will likely have its own signature. “The Beta Pictoris disk is the prototype for circumstellar debris systems, but it may not be a good archetype,” says co-author Glenn Schneider (University of Arizona).

The paper is Apai et al., “The Inner Disk Structure, Disk-Planet Interactions, and Temporal Evolution in the β Pictoris System: A Two-Epoch HST/STIS Coronagraphic Study,” in press at the Astrophysical Journal (preprint).



Scholz’s Star: A Close Flyby

by Paul Gilster on February 19, 2015

The star HIP 85605 until recently seemed more interesting than it may now turn out to be. In a recent paper, Coryn Bailer-Jones (Max Planck Institute for Astronomy, Heidelberg) noted that the star in the constellation Hercules had a high probability of coming close enough to our Solar System in the far future (240,000 to 470,000 years from now) that it would pass through the Oort Cloud, potentially disrupting comets there. The possibility of a pass as close as .13 light years (8200 AU) was there, but Bailer-Jones cautioned that distance measurements of this star could be incorrect. His paper on nearby stellar passes thus leaves the HIP 85605 issue unresolved.

Enter Eric Mamajek (University of Rochester) and company. Working with data from the Southern African Large Telescope (SALT) and the Magellan telescope at Las Campanas Observatory in Chile, Mamajek showed that the distance to HIP 85605 has been underestimated by a factor of ten. As Bailer-Jones seems to have suspected, the new measurement takes the star on a trajectory that does not bring it within the Oort Cloud. But in the same paper, the team names an interesting system called Scholz’s Star as a candidate for a close pass in the past.

Studying the star’s tangential velocity (motion across the sky) as well as radial velocity data, the team found that despite being relatively close at 20 light years, Scholz’s Star shows little tangential velocity. That would imply an interesting encounter ahead, or one that had already happened. Mamajek explains:

“Most stars this nearby show much larger tangential motion. The small tangential motion and proximity initially indicated that the star was most likely either moving towards a future close encounter with the solar system, or it had ‘recently’ come close to the solar system and was moving away. Sure enough, the radial velocity measurements were consistent with it running away from the Sun’s vicinity – and we realized it must have had a close flyby in the past.”

Red and Brown dwarf binary system

Image: Artist’s conception of Scholz’s star and its brown dwarf companion (foreground) during its flyby of the solar system 70,000 years ago. The Sun (left, background) would have appeared as a brilliant star. The pair is now about 20 light years away. Credit: Michael Osadciw/University of Rochester.

The paper on this work, recently published in the Astrophysical Journal, determines the star’s trajectory, one that shows that about 70,000 years ago, it would have passed some 52,000 AU from the Sun. This works out to about 0.82 light years, or 7.8 trillion kilometers, quite a bit closer than Proxima Centauri, and probably close enough to pass through the outer Oort Cloud. The star was within 100,000 AU of the Sun for a period of roughly 10,000 years.

Scholz’s star (W0720) is a low-mass object in the constellation Monoceros also tagged WISE J072003.20-084651.2 and only recently discovered (by Ralf-Dieter Scholz in 2014) thanks to its dimness in optical wavelengths, its proximity to the galactic plane and its low proper motion. Adaptive optics imaging and high resolution spectroscopy has demonstrated that the star is actually a binary, an M-dwarf with a companion at 0.8 AU that is probably a brown dwarf.

The question that immediately comes to mind is what kind of object the Scholz’s star system would have presented in the night sky some 70,000 years ago. The answer is not dramatic, for at its closest approach the binary would have had an apparent magnitude in the range of 11.4 (note: there is a typo in the paper, as noted here, which had specified an apparent magnitude of 10.3). This is five magnitudes, or a factor of 100 times, fainter than the faintest naked eye stars. But the paper notes that M-dwarfs like this one are often given to flare activity that might have made Scholz’s star a brighter object. From the paper:

If W0720 experienced occasional flares similar to those of the active M8 star SDSS J022116.84+194020.4 (Schmidt et al. 2014), then the star may have been rarely visible with the naked eye from Earth (V < 6; ∆V < −4) for minutes or hours during the flare events. Hence, while the binary system was too dim to see with the naked eye in its quiescent state during its flyby of the solar system ∼70 kya, flares by the M9.5 primary may have provided visible short-lived transients visible to our ancestors.

And take a look at this graph, which Eric Mamajek published on Twitter yesterday.


As you can see, Scholz’s Star was moving out. If it had been visible, what would ancient skywatchers have made of it? We also have to wonder what other close encounters our Solar System may have had with other stars. Note this point from the paper about M-dwarfs:

Past systematic searches for stars with close flybys to the solar system have been understandably focused on the Hipparcos astrometric catalog (García Sánchez et al. 1999; Bailer-Jones 2014), however it contains relatively few M dwarfs relative to their cosmic abundance. Searches in the Gaia astrometric catalog for nearby M dwarfs with small proper motions and large parallaxes (i.e. with small tangential velocities) will likely yield addition candidates.

So much still to learn about M-dwarfs!

The paper is Mamajek et al., “The Closest Known Flyby of a Star to the Solar System,” Astrophysical Journal Letters 800 (2015), L17 (preprint). The Bailer-Jones paper discussed above is “Close Encounters of the Stellar Kind,” in press at Astronomy & Astrophysics (preprint). For more on Bailer-Jones, see Stars Passing Close to the Sun.