Previous Articles - 2002/03
[January-June 2002] [July-October 2002] [November 2002 - December 2003]

Moon discovered orbiting solar system's 10th planet

By Ron Baalke as posted to the MPML Mailing List
http://www.spaceflightnow.com/news/n0510/02xenamoon/

The newly discovered 10th planet, 2003 UB313, is looking more and more like one of the solar system's major players. It has the heft of a real planet (latest estimates put it at about 20 percent larger than Pluto), a catchy code name (Xena, after the TV warrior princess), and a Guinness Book-ish record of its own (at about 97 astronomical units-or 9 billion miles from the sun-it is the solar system's farthest detected object). And, astronomers from the California Institute of Technology and their colleagues have now discovered, it has a moon.

The moon, 100 times fainter than Xena and orbiting the planet once every couple of weeks, was spotted on September 10, 2005, with the 10-meter Keck II telescope at the W.M. Keck Observatory in Hawaii by Michael E. Brown, professor of planetary astronomy, and his colleagues at Caltech, the Keck Observatory, Yale University, and the Gemini Observatory in Hawaii. A paper about the discovery was submitted on October 3 to Astrophysical Journal Letters.

"Since the day we discovered Xena, the big question has been whether or not it has a moon," says Brown. "Having a moon is just inherently cool-and it is something that most self-respecting planets have, so it is good to see that this one does too."

Brown estimates that the moon, nicknamed "Gabrielle"-after the fictional Xena's fictional sidekick-is at least one-tenth of the size of Xena, which is thought to be about 2700 km in diameter (Pluto is 2274 km), and may be around 250 km across.

To know Gabrielle's size more precisely, the researchers need to know the moon's composition, which has not yet been determined. Most objects in the Kuiper Belt, the massive swath of miniplanets that stretches from beyond Neptune out into the distant fringes of the solar system, are about half rock and half water ice. Since a half-rock, half-ice surface reflects a fairly predictable amount of sunlight, a general estimate of the size of an object with that composition can be made. Very icy objects, however, reflect a lot more light, and so will appear brighter-and thus bigger-than similarly sized rocky objects.

Further observations of the moon with the Hubble Space Telescope, planned for November and December, will allow Brown and his colleagues to pin down Gabrielle's exact orbit around Xena. With that data, they will be able to calculate Xena's mass, using a formula first devised some 300 years ago by Isaac Newton.

"A combination of the distance of the moon from the planet and the speed it goes around the planet tells you very precisely what the mass of the planet is," explains Brown. "If the planet is very massive, the moon will go around very fast; if it is less massive, the moon will travel more slowly. It is the only way we could ever measure the mass of Xena-because it has a moon."

The researchers discovered Gabrielle using Keck II's recently commissioned Laser Guide Star Adaptive Optics system. Adaptive optics is a technique that removes the blurring of atmospheric turbulence, creating images as sharp as would be obtained from space-based telescopes. The new laser guide star system allows researchers to create an artificial "star" by bouncing a laser beam off a layer of the atmosphere about 75 miles above the ground. Bright stars located near the object of interest are used as the reference point for the adaptive optics corrections. Since no bright stars are naturally found near Xena, adaptive optics imaging would have been impossible without the laser system.

"With Laser Guide Star Adaptive Optics, observers not only get more resolution, but the light from distant objects is concentrated over a much smaller area of the sky, making faint detections possible," says Marcos van Dam, adaptive optics scientist at the W.M. Keck Observatory, and second author on the new paper.

The new system also allowed Brown and his colleagues to observe a small moon in January around 2003 EL61, code-named "Santa," another large new Kuiper Belt object. No moon was spotted around 2005 FY9-or "Easterbunny"-the third of the three big Kuiper Belt objects recently discovered by Brown and his colleagues using the 48-inch Samuel Oschin Telescope at Palomar Observatory. But the presence of moons around three of the Kuiper Belt's four largest objects - Xena, Santa, and Pluto - challenges conventional ideas about how worlds in this region of the solar system acquire satellites.

Previously, researchers believed that Kuiper Belt objects obtained moons through a process called gravitational capture, in which two formerly separate objects moved too close to one another and become entrapped in each other's gravitational embrace. This was thought to be true of the Kuiper Belt's small denizens-but not, however, of Pluto. Pluto's massive, closely orbiting moon, Charon, broke off the planet billions of years ago, after it was smashed by another Kuiper Belt object. Xena's and Santa's moons appear best explained by a similar origin.

"Pluto once seemed a unique oddball at the fringe of the solar system," Brown says. "But we now see that Xena, Pluto, and the others are part of a diverse family of large objects with similar characteristics, histories, and even moons, which together will teach us much more about the solar system than any single oddball ever would."

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Asteroids Caused the Early Inner Solar System Cataclysm

University of Arizona and Japanese scientists are convinced that evidence at last settles decades-long arguments about what objects bombarded the early inner solar system in a cataclysm 3.9 billion years ago.

Ancient main belt asteroids identical in size to present-day asteroids in the Mars-Jupiter belt -- not comets -- hammered the inner rocky planets in a unique catastrophe that lasted for a blink of geologic time, anywhere from 20 million to 150 million years, they report in the Sept. 16 issue of Science.

However, the objects that have been battering our inner solar system after the so-called Late Heavy Bombardment ended are a distinctly different population, UA Professor Emeritus Robert Strom and colleagues report in the article, "The Origin of Planetary Impactors in the Inner Solar System."

After the Late Heavy Bombardment or Lunar Cataclysm period ended, mostly near-Earth asteroids (NEAs) have peppered the terrestrial region.

Strom has been studying the size and distribution of craters across solar system surfaces for the past 35 years. He has long suspected that two different projectile populations have been responsible for cratering inner solar system surfaces. But there's been too little data to prove it.

Until now.

Now asteroid surveys conducted by UA's Spacewatch, the Sloan Digital Sky Survey, Japan's Subaru telescope and the like have amassed fairly complete data on asteroids down to those with diameters of less than a kilometer. Suddenly it has become possible to compare the sizes of asteroids with the sizes of projectiles that blasted craters into surfaces from Mars inward to Mercury.

"When we derived the projectile sizes from the cratering record using scaling laws, the ancient and more recent projectile sizes matched the ancient and younger asteroid populations smack on," Strom said. "It's an astonishing fit."

"One thing this says is that the present-day size-distribution of asteroids in the asteroid belt was established at least as far back as 4 billion years ago," UA planetary scientist Renu Malhotra, a co-author of the Science paper, said. "Another thing it says is that the mechanism that caused the Late Heavy Bombardment was a gravitational event that swept objects out of the asteroid belt regardless of size."

Malhotra discovered in previous research what this mechanism must have been. Near the end of their formation, Jupiter and the other outer gas giant planets swept up planetary debris farther out in the solar system, the Kuiper Belt region. In clearing up dust and pieces leftover from outer solar system planet formation, Jupiter, especially, lost orbital energy and moved inward closer to the sun. That migration greatly enhanced Jupiter's gravitational influence on the asteroid belt, flinging asteroids irrespective of size toward the inner solar system.

Evidence that main belt asteroids pummeled the early inner solar system confirms a previously published cosmochemical analysis by UA planetary scientist David A. Kring and colleagues.

"The size distribution of impact craters in the ancient highlands of the moon and Mars is a completely independent test of the inner solar system cataclysm and confirms our cosmochemical evidence of an asteroid source," Kring, a co-author of the Science paper, said.

Kring was part of a team that earlier used an argon-argon dating technique in analyzing impact melt ages of lunar meteorites -- rocks ejected at random from the moon's surface and that landed on Earth after a million or so years in space. They found from the ages of the "clasts," or melted rock fragments, in the breccia meteorites that all of the moon was bombarded 3.9 billion years ago, a true global lunar cataclysm. The Apollo lunar sample analysis said that asteroids account for at least 80 percent of lunar impacts.

Comets have played a relatively minor role in inner solar system impacts, Strom, Malhotra and Kring also conclude from their work. Contrary to popular belief, probably no more than 10 percent of Earth's water has come from comets, Strom said.

After the Late Heavy Bombardment, terrestrial surfaces were so completely altered that no surface older than 3.9 billion years can be dated using the cratering record. Older rocks and minerals are found on the moon and Earth, but they are fragments of older surfaces that were broken up by impacts, the researchers said.

Strom said that if Earth had oceans between 4.4 billion and 4 billion years ago, as other geological evidence suggests, those oceans must have been vaporized by the asteroid impacts during the cataclysm.

Kring also has developed a hypothesis that suggests that the impact events during Late Heavy Bombardment generated vast subsurface hydrothermal systems that were critical to the early development of life. He estimated that the inner solar system cataclysm produced more than 20,000 craters between 10 kilometers to 1,000 kilometers in diameter on Earth.

Inner solar system cratering dynamics changed dramatically after the Late Heavy Bombardment. From then on, the impact cratering record reflects that most objects hitting inner solar system surfaces have been near-Earth asteroids, smaller asteroids from the main belt that are nudged into terrestrial-crossing orbits by a size-selective phenomenon called the Yarkovsky Effect.

The effect has to do with the way asteroids unevenly absorb and re-radiate the sun's energy. Over tens of millions of years, the effect is large enough to push asteroids smaller than 20 kilometers across into the jovian resonances, or gaps, that deliver them to terrestrial-crossing orbits. The smaller the asteroid, the more it is influenced by the Yarkovsky Effect.

Planetary geologists have tried counting craters and their size distribution to get absolute ages for surfaces on the planets and moons.

"But until we knew the origin of the projectiles, there has been so much uncertainty that I thought it could lead to enormous error," Strom said. "And now I know I'm right. For example, people have based the geologic history of Mars on the heavy bombardment cratering record, and it's wrong because they're using only one cratering curve, not two."

Attempts to date outer solar system bodies using the inner solar system cratering record is completely wrong, Strom said. But it should be possible to more accurately date inner solar system surfaces once researchers determine the cratering rate from the near-Earth asteroid bombardment, he added.

The authors of the Science paper are Strom, Malhotra and Kring from the University of Arizona Lunar and Planetary Laboratory, and Takashi Ito and Fumi Yoshida of National Astronomical Observatory, Tokyo, Japan.

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Ceres: Asteroid or Miniplanet?

As posted to the MPML Mailing List
http://www.news.cornell.edu/stories/Sept05/Ceres.to.html

When is a space rock more than just a space rock?

Ceres 1 was already holding the title of the solar system's largest asteroid. Now new observations show the space rock may be more worthy of the appellation "miniplanet."

On Sept. 7 NASA released photographs of Ceres that show the rock is a smooth ellipsoid, or oblong sphere, with an average diameter of approximately 590 miles -- about the size of Texas. A scientific paper on the findings, by a group led by Peter C. Thomas, senior research associate at Cornell University's Center for Radiophysics and Space Research, appeared in the Sept. 9 issue of the journal Nature.

Co-author Joel Parker, an astronomer at the Southwest Research Institute in Boulder, Colo., used the Hubble Space Telescope's Advanced Camera for Surveys to snap 267 images of Ceres on Dec. 28, 2003, during a nine-hour period -- one Ceres "day."

Being ellipsoid and smooth is special for a rock. It indicates that the body is heavy enough to possess gravity strong enough to suck its own surface smooth -- a process called "gravitational relaxing." Because the process typically requires a mass of many trillions of tons, depending on the temperature, the average pebble is not going to be gravitationally relaxed; even most asteroids aren't.

By combining the new information on Ceres' roundness with previous independent measurements of its mass, Thomas and his colleagues inferred that Ceres must have a "differentiated interior" similar to the terrestrial planets. Although this possibility had been previously predicted, it was not widely accepted. "We used the best telescope available to apply a basic geophysical test of other people's predictions," Thomas said.

Based on their own models and observations, Thomas and his colleagues believe Ceres contains a rocky silicate core and icy mantel covered by a crust of carbon-rich compounds and clays. Furthermore, they predict that the icy mantel may contain more frozen water than all of the fresh water on Earth.

Whenever water is mentioned, people ask about life. But Thomas says the possibility is "very remote," noting that even if the interior of Ceres were warm enough for some of the water to liquefy, Ceres probably lacked a sufficient energy source for life to develop.

Ceres was discovered in 1801 by Sicilian astronomer Giuseppe Piazzi and declared to be the "missing planet" predicted between Mars and Jupiter. However, the title was revoked in 1802 when Ceres was found to be a member of the hundreds of thousands of other rocks and debris of the Asteroid Belt.

Since the discovery of 2003 UB313 -- which some have hailed as the tenth planet -- in July, some astronomers (and many non-astronomers) have begun to question whether objects such as Ceres should also be enshrined as planets.

Thomas professes a lack of concern about Ceres' place in the solar system. "There are plenty of other interesting things and processes in Ceres to contemplate rather than whether or not it should be called a planet," he said. But for those who prefer a more definitive answer, Thomas offers: "You can call Ceres a 'minor planet' or 'miniplanet' if you'd like, but I would not call it a 'full-fledged planet.'"

The other authors of the Nature paper are L.A. McFadden of the University of Maryland, S.A. Stern and E.F. Young of the Southwest Research Institute, C.T. Russell of the University of California-Los Angeles and M.V. Sykes of the Planetary Science Institute, Tuscon, Ariz. Funding for the project was provided by NASA through the Space Telescope Science Institute.

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Hayabusa Arrives at Itokawa, Starts Hovering Operations

As posted to the MPML Mailing List
http://www.jaxa.jp/press/2005/09/20050914_hayabusa_e.html

The Hayabusa spacecraft successfully arrived at its target, the near Earth asteroid Itokawa (25143), at 01:00 UTC(10:00 JST) on September 12th. Hayabusa performed a short chemical thruster burn to slow its asteroid-relative speed by 7 centimeters per second to settle into the Gate Position, defined at a distance of 20 kilometers from Itokawa towards the Earth. Hayabusa is now hovering with respect to Itokawa and the project has made a great step toward its scientific observations of the asteroid. Hayabusa's main purpose is to demonstrate key technologies required for future planetary exploration. Hayabusa was launched in May 2003. In May 2004, the spacecraft performed an Earth gravity assist while using ion engine propulsion, this was the first time a spacecraft has flown such a powered flyby.

During its approach to rendezvous, the visible imager carried by Hayabusa succeeded in taking multi-band filter photographs of Itokawa, as the asteroid rotated. This press release contains the resulting synthesized pseudo-color image and a series of rotation pictures of Itokawa. In the images, sunlight illuminates the asteroid from behind the camera, making the terrain features appear more subdued. Most of the key images obtained by the mission to date are available on the JAXA main web-site (http://www.jaxa.jp) and the ISAS (Institute of Space and Astronautical Science) main web-site (http://www.isas.jaxa.jp).

The Hayabusa spacecraft is in good health and all its scientific instruments (consisting of a visible imager, a near infra-red and X-ray spectrometer and a laser altimeter) are functioning normally and have started their calibration observations. Hayabusa will stay at the asteroid until the end of November and plans to perform detailed remote sensing and mapping of the asteroid, followed by an attempt to collect surface samples of Itokawa. The project goals are then to return the sample back to Earth in 2007. Updated information and future press releases about the mission will be available at the JAXA and ISAS web-sites.

The Hayabusa project is a collaborative mission with participation by the National Aeronautics and Space Administration (NASA) of the United States. The Hayabusa project expresses its appreciation to NASA for its support of the mission.

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Work Continues on the Solar System's Three Recently Discovered Kuiper Belt Objects

By Ron Baalke as posted to the MPML Mailing List
http://pr.caltech.edu/media/Press_Releases/PR12734.html

When planetary scientists announced on July 29 that they had discovered a new planet larger than Pluto, the news overshadowed the two other objects the group had also found. But all three objects are odd additions to the solar system, and as such could revolutionize our understanding of how our part of the celestial neighborhood evolved.

To the discoverers, the objects still go by the unofficial code-names "Santa," "Easterbunny," and "Xena," though they are officially known to the International Astronomical Union as 2003 EL61, 2005 FY9, and 2003 UB313. The three objects were all detected with the 48-inch Samuel Oschin Telescope at Palomar Observatory by a team composed of planetary scientists from the California Institute of Technology, the Gemini Observatory, and Yale University. Xena is the object the group describes as one of sufficient size to be called the tenth planet.

"All three objects are nearly Pluto-sized or larger, and all are in elliptical orbits tilted out of the plane of the solar system," says Mike Brown, a professor of planetary astronomy at Caltech and leader of the effort.

"We think that these orbital characteristics may mean that they were all formed closer to the sun, and then were tossed around by the giant planets before they ended up with the odd orbits they currently have," Brown adds.

The other two members of the team are Chad Trujillo, a former postdoctoral researcher at Caltech and currently an astronomer at the Gemini Observatory in Hawaii, and David Rabinowitz of Yale University. Trujillo has led the spectrographic studies of the discoveries, while Rabinowitz is one of the builders of the instrument affixed to the Oschin Telescope for the study, and has led the effort to understand the color and spin of the objects.

Santa, Easterbunny, and Xena are all members of the Kuiper belt, a region beyond the orbit of Neptune that for decades was merely a hypothetical construct based on the behavior of comets, among other factors. But astronomers began detecting objects in the mid-1990s, and the Kuiper belt was suddenly a reality rather than a hypothesis.

Xena, which is currently about 97 astronomical units from the sun (an astronomical unit being the 93-million-mile distance between the sun and Earth), is at least the size of Pluto and almost certainly significantly larger. The researchers are able to determine its smallest possible size because, thanks to the laws of motion, they know very accurately the distance of the planet from the sun. And because they also know very precisely how much light the planet gives off, they can also calculate the diameter of the planet as if it were reflecting sunlight as a uniformly white ball in the sky. Hence, a perfectly round mirror at that distance would be the size of Pluto.

However, the question remains how well the new planet reflects light. The less reflective its surface, the bigger it must be to put out enough light to be detected here on Earth.

At any rate, the researchers hope that infrared data returned by the Spitzer Space Telescope over the weekend of August 27-28, in addition to recently obtained data from the 30-meter IRAM telescope in Spain, will help nail down Xena's size. In much the same way that the detected visible light sets a lower limit on the diameter, the infrared radiation detected by the Spitzer will ideally set an upper limit. That's because the Spitzer is capable of measuring the total amount of heat given off by the planet; and because the researchers know the likely surface temperature is about 405 degrees below zero Fahrenheit, they can infer the overall size of the body.

Brown predicts that Xena will likely be highly reflective, because the spectrographic data gathered by his colleague and codiscoverer Chad Trujillo at the Gemini Observatory show the surface to have a similar composition to that of the highly reflective Pluto. If indeed Xena reflects 70 percent of the sunlight reaching it, as does Pluto, then Xena is about 2700 kilometers in diameter.

And then there's the matter of naming the new planet, which is pretty much in the hands of the International Astronomical Union. Brown says the matter is in "committee limbo": while one IAU committee is taking its time deciding whether or not it is a planet, other committees have to wait until they know what it is before they can consider a name. So for the time being, the discoverers keep calling the new planet Xena, though the name will sooner or later change.

The second of the objects, currently nicknamed Santa because Brown and his colleagues found it on December 28, 2004, is one of the more bizarre objects in the solar system, according to Rabinowitz. His observations from a small telescope in Chile show that Santa is a fast-rotating cigar-shaped body that is about the diameter of Pluto along its longer axis. No large body in the solar system comes even close to rotating as fast as Santa's four-hour period. Observations by Brown and his colleagues at the Keck Observatory have shown that Santa also has a tiny moon, nicknamed Rudolph, which circles it every 49 days. The third new discovery is Easterbunny, so named because of its discovery earlier this year on March 1. Easterbunny is also at 52 astronomical units, and like Santa is probably about three-quarters the size of Pluto. Morever, Easterbunny is now the third known object in the Kuiper belt, after Pluto and Xena, which is known to have a surface covered in frozen methane. For decades, Pluto was the only known methane-covered object beyond Neptune, but "now we suddenly have three in a variety of sizes at a variety of distances and can finally try to understand Pluto and its cousins," says Kris Barkume, a PhD student working with Brown.

"With so many bright objects coming out at once it is hard to keep them all straight," says Brown, adding that the remote region beyond Neptune may present even more surprises in the future.

"We hope to discover a few more large objects in the outer solar system."

The research is funded by NASA. For more information see
http://www.gps.caltech.edu/~mbrown

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Study Indicates Ceres May Have Water-Ice Mantle

By Ron Baalke as posted to the MPML Mailing List
http://www.swri.org/9what/releases/2005/ceres.htm

A new study of Ceres, the largest known object in the asteroid belt between Mars and Jupiter, shows evidence of a planet-like round shape and a surprisingly complex internal structure, with a rocky core possibly surrounded by a mantle rich in water-ice.

The study, to be reported September 8 in a letter to the journal Nature by investigators from Cornell University, Southwest Research Institute (SwRI), the University of Maryland, University of California at Los Angeles and the Planetary Science Institute in Tucson, Ariz., used Hubble Space Telescope images made during observations in December 2003 and January 2004 to examine surface features of the asteroid in reflected sunlight, study its rotational properties and search for any small moons. The paper is led by Dr. Peter C. Thomas of the Center for Radiophysics and Space Research at Cornell University. The project was funded by NASA through the Space Telescope Science Institute.

"This is the first time we have seen Ceres in such detail and can even say something about its interior," said the project team's leader, Dr. Joel Parker, assistant director of SwRI's Space Studies Department. "You can watch it rotate in our observations, and you get the feeling of it being a whole new world, not just a bit of rocky debris."

Differences in surface features among asteroids preserve a record of how the asteroid belt evolved between the earliest days of the solar system and about 4.6 billion years ago, when Jupiter's growing gravitational pull halted the accretion of bodies in the asteroid belt.

Some asteroids, like 4 Vesta, the second-most massive body in the asteroid belt, have a crust, mantle and core, indicating that they experienced sufficient thermal evolution to differentiate into layered structures.

Other asteroids appear more homogeneous, similar to carbonaceous meteorites that have undergone only minimal thermal processing. In the past, Ceres -- about the size of Texas at 580 miles across -- was placed in this latter category, in part because of its low density, its low albedo or heat signature, and its relatively featureless visible reflectance.

However, the new study indicates that Ceres' round shape and smoothness resemble more that of a gravitationally relaxed object, or one whose shape is determined by hydrostatic equilibrium. If so, it would be the only asteroid thus far to be characterized in that way. Also, its shape is more flattened than would be expected of a homogeneous object, but consistent with a central mass concentration indicating a layered makeup.

The relaxed state, differentiated structure and mean density observed on Ceres strongly suggest water-ice as the primary mantle constituent. Unlike the icy moons of Jupiter and Saturn, however, the higher heating available at Ceres' distance from the sun makes water-ice unstable at the surface. If water-ice has been at the surface of Ceres, it may currently hide just below a thin residual layer of clay and dark carbonaceous materials.

Editors: A movie showing the rotation of Ceres is available at
http://www.swri.org/press/2005/ceres.htm .

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Ceres May Be 'Mini Planet' with Water Ice

By Ron Baalke as posted to the MPML Mailing List
http://hubblesite.org/newscenter/newsdesk/archive/releases/2005/27/text/

Observations of 1 Ceres, the largest known asteroid, have revealed that the object may be a "mini planet," and may contain large amounts of pure water ice beneath its surface.

The observations by NASA's Hubble Space Telescope also show that Ceres shares characteristics of the rocky, terrestrial planets like Earth. Ceres' shape is almost round like Earth's, suggesting that the asteroid may have a "differentiated interior," with a rocky inner core and a thin, dusty outer crust.

"Ceres is an embryonic planet," said Lucy A. McFadden of the Department of Astronomy at the University of Maryland, College Park and a member of the team that made the observations. "Gravitational perturbations from Jupiter billions of years ago prevented Ceres from accreting more material to become a full-fledged planet."

The finding will appear Sept. 8 in a letter to the journal Nature. The paper is led by Peter C. Thomas of the Center for Radiophysics and Space Research at Cornell University in Ithaca, N.Y., and also includes project leader Joel William Parker of the Department of Space Studies at Southwest Research Institute in Boulder, Colo.

Ceres is approximately 580 miles (930 kilometers) across, about the size of Texas. It resides with tens of thousands of other asteroids in the main asteroid belt. Located between Mars and Jupiter, the asteroid belt probably represents primitive pieces of the solar system that never managed to accumulate into a genuine planet. Ceres comprises 25 percent of the asteroid belt's total mass. However, Pluto, our solar system's smallest planet, is 14 times more massive than Ceres.

The astronomers used Hubble's Advanced Camera for Surveys to study Ceres for nine hours, the time it takes the asteroid to complete a rotation. Hubble snapped 267 images of Ceres. From those snapshots, the astronomers determined that the asteroid has a nearly round body. The diameter at its equator is wider than at its poles. Computer models show that a nearly round object like Ceres has a differentiated interior, with denser material at the core and lighter minerals near the surface. All terrestrial planets have differentiated interiors. Asteroids much smaller than Ceres have not been found to have such interiors.

The astronomers suspect that water ice may be buried under the asteroid's crust because the density of Ceres is less than that of the Earth's crust, and because the surface bears spectral evidence of water-bearing minerals. They estimate that if Ceres were composed of 25 percent water, it may have more water than all the fresh water on Earth. Ceres' water, unlike Earth's, would be in the form of water ice and located in the mantle, which wraps around the asteroid's solid core.

Besides being the largest asteroid, Ceres also was the first asteroid to be discovered. Sicilian astronomer Father Giuseppe Piazzi spotted the object in 1801. Piazzi was looking for suspected planets in a large gap between the orbits of Mars and Jupiter. As more such objects were found in the same region, they became known as "asteroids" or "minor planets."

Hubble Tracks Ceres

http://hubblesite.org/newscenter/newsdesk/archive/releases/2005/27/image/a

NASA's Hubble Space Telescope took these images of the asteroid 1 Ceres over a 2-hour and 20-minute span, the time it takes the Texas-sized object to complete one quarter of a rotation. One day on Ceres lasts 9 hours.

Hubble snapped 267 images of Ceres as it watched the asteroid make more than one rotation. By observing the asteroid during a full rotation, astronomers confirmed that Ceres has a nearly round body like Earth's. Ceres' shape suggests that its interior is layered like those of terrestrial planets such as Earth. Ceres may have a rocky inner core, an icy mantle, and a thin, dusty outer crust inferred from its density and rotation rate.

The bright spot that appears in each image is a mystery. It is brighter than its surroundings. Yet it is still very dark, reflecting only a small portion of the sunlight that shines on it.

Ceres is approximately 580 miles (930 kilometers) across and is the largest known asteroid. It resides with tens of thousands of other asteroids in a region between the orbits of Mars and Jupiter called the main asteroid belt. Besides being the largest asteroid, Ceres also was the first to be discovered, in 1801.

Astronomers enhanced the contrast in these images to bring out important features on Ceres' surface. The observations were made in visible and in ultraviolet light. Hubble took the snapshots between December 2003 and January 2004.

Credit: NASA, ESA, J. Parker (Southwest Research Institute), P. Thomas (Cornell University), and L. McFadden (University of Maryland, College Park)

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NASA Finds Evidence Some Comets May Have Become Asteroids

By Ron Baalke as posted to the MPML Mailing List
http://www.nasa.gov/centers/ames/research/exploringtheuniverse/comets.html

Some asteroids that have comet-like orbits may actually be comets that have lost gases and other easily vaporized substances, according to a NASA research team.

The team will present its findings at the American Astronomical Society's Division for Planetary Sciences annual meeting in Cambridge, England, on Sept. 5.

"Several objects classified as asteroids have orbits that are dynamically similar to those of comets," said Dale Cruikshank, an astronomer at NASA Ames Research Center in California's Silicon Valley and a member of the research team. These asteroids may be comets that have lost gases and other materials by repeated passages through the inner solar system, according to Cruikshank.

The team studied infrared light from 55 asteroids using NASA's Spitzer Space Telescope to "better understand possible links between asteroids and comets," according to the authors. In addition to co-author Cruikshank, Joshua Emery who also works at NASA Ames and is an employee of the SETI Institute, Mountain View, Calif. is the principal author; and Jeffrey Van Cleve of Ball Aerospace, Boulder, Colo., is the other co-author.

"The suggestion that some asteroids originated as comets has been made before, but the new Spitzer Space Telescope observations provide the first chance to really test this suggestion," Emery noted. "Most of the objects observed in our program appear to be typical asteroids, but a few have surface compositions and textures that are more similar to comets," Emery added.

"The infrared light we are studying gives us information about the composition and surface textures of solid bodies in the solar system." Cruikshank said.

The research team reports that some of the asteroids have very fine-grained surfaces. "We think this fine-graininess is a characteristic of comets," Cruikshank explained.

The Jet Propulsion Laboratory in Pasadena, Calif., manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology, also in Pasadena. Caltech manages JPL for NASA.

For information on the Spitzer Space Telescope visit:

http://www.spitzer.caltech.edu/spitzer/

For high-resolution images of comets, please visit:

http://www.nasa.gov/centers/ames/multimedia/images/2005/comets1.html

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Asteroid's Near-Miss May Be Home Run for Scientists

By Ron Baalke as posted to the MPML Mailing List
http://www.umich.edu/news/?Releases/2005/Aug05/r081605c

A University of Michigan-led research team has discovered that for the first time in history, scientists will be able to observe how the Earth's gravity will disrupt a massive asteroid's spin.

Scientists predict a near-miss when Asteroid 99942 Apophis passes Earth in 2029. An asteroid flies this close to the planet only once every 1,300 years. The chance to study it will help scientists deal with the object should it threaten collision with Earth.

Only about three Earth diameters will separate Apophis and Earth when the 400-meter asteroid hurtles by Earth's gravity, which will twist the object into a complex wobbling rotation. Such an occurrence has never been witnessed but could yield important clues to the interior of the sphere, according to a paper entitled, "Abrupt alteration of the spin state of asteroid 99942 Apophis (2004 MN4) during its 2029 Earth flyby," accepted for publication in the journal Icarus.

The team of scientists is led by U-M's Daniel Scheeres, associate professor of aerospace engineering, and includes U-M's Peter Washabaugh, associate professor of aerospace engineering.

Apophis is one of more than 600 known potentially hazardous asteroids and one of several that scientists hope to study more closely. In Apophis' case, additional measurements are necessary because the 2029 flyby could be followed by frequent close approaches thereafter, or even a collision.

Scheeres said not only is it the closest asteroid flyby ever predicted in advance, but it could provide a birds-eye view of the asteroid's "belly."

"In some sense it's like a space science mission 'for free' in that something scientifically interesting will happen, it will be observable from Earth, and it can be predicted far in advance," Scheeres said.

If NASA places measuring equipment on the asteroid's surface, scientists could for the first time study an asteroid's interior, similar to how geologists study earthquakes to gain understanding of the Earth's core, Scheeres said. Because the torque caused by the Earth's gravitational pull will cause surface and interior disruption to Apophis, scientists have a unique opportunity to observe its otherwise inaccessible mechanical properties, Scheeres said. Throwing the asteroid off balance could also affect its orbit and how close it comes to Earth in future years.

"Monitoring of this event telescopically and with devices placed on the asteroid's surface could reveal the nature of its interior, and provide us insight into how to deal with it should it ever threaten collision," Scheeres said.

The asteroid will be visible in the night sky of Europe, Africa and Western Asia.

The asteroid was discovered late last year and initially scientists gave it a 1-in-300 chance of hitting the Earth on April 13, 2029. Subsequent analysis of new and archived pre-discovery images showed that Apophis won't collide with Earth that day, but that later in 2035, 2036, and 2037 there remains a 1-in-6,250 chance that the asteroid could hit Earth, Scheeres said. Conversely, that's a 99.98 percent chance that the asteroid will miss Earth.

The asteroid is relatively small, about the length of three football fields. If it hit it wouldn't create wide-scale damage to the Earth, but would cause major damage at the impact site, Scheeres said.

The team of scientists also includes Lance Benner and Steve Ostro of NASA's Jet Propulsion Laboratory, Alessandro Rossi of ISTI-CNR, Italy, and Francesco Marzari of the University of Padova, Italy.

The complete graphical animation (director's cut), with some details,is available from this page:
http://orsa.sourceforge.net/atwork.html

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Hayabusa's Contributions Toward Understanding the Earth's Neighborhood

By Don Yeomans/Ron Baalke as posted to the MPML Mailing List
http://neo.jpl.nasa.gov/missions/hayabusa.html

Beginning in early September 2005, the Japanese Hayabusa spacecraft will rendezvous with near-Earth asteroid (25143) Itokawa. Itokawa, a 600 meter sized, potato-shaped asteroid, is named after Hideo Itokawa, a Japanese rocket pioneer. Although the primary objectives of the Hayabusa mission are to test new technologies, the mission will also provide a wealth of scientific returns. For the three month period from September through November 2005, the science instruments on board the Hayabusa spacecraft will undertake an intensive study of near-Earth asteroid Itokawa. After closely observing the asteroid for several weeks, a few pellets will be fired from the spacecraft at close range into the asteroid's surface and about a gram of the pellet's impact ejecta will be collected into a sample capsule. This capsule will then be brought back to Earth and parachuted into the Australia outback in June 2007 so that some of the asteroid's surface minerals can be studied in Earth-based laboratories. This will be the first asteroid sample return mission.

After the successful launch of the spacecraft on May 9, 2003 from the Japanese Kagoshima Launch site, the mission name was changed from MUSES-C to Hayabusa. Hayabusa, which is Japanese for "falcon," will act much like its namesake, descending to the asteroid's surface, capturing its prey and returning it to Earth. While the scientific knowledge of near-Earth asteroids will be significantly advanced by the Hayabusa mission, the primary goals are to test four advanced technology systems: the electric propulsion (ion drive) engines; an autonomous navigation system; the sample collection system; and the sample capsule that re-enters the Earth's atmosphere.

A year after launch, on May 19, 2004, the spacecraft returned to Earth and made a close approach (altitude = 3725 km), thereby gaining the extra velocity it needed to reach the near-Earth asteroid Itokawa. During the Earth swing-by, the spacecraft also took images of the Earth and moon to test and calibrate the on board camera called AMICA (Asteroid Multi-band Imaging Camera). These Earth and lunar images can be viewed at: http://www.isas.ac.jp/e/snews/2004/0519.shtml. Because the efficiency of the solar panels were slightly degraded as a result of a solar flare in late 2003, the ion engines no longer receive quite as much electricity as they should so the spacecraft's arrival at the asteroid was delayed from mid-summer until September of 2005.

Upon arriving at the asteroid, the Hayabusa spacecraft will spend about three months hovering above the asteroid with its high gain antenna pointed toward Earth and its science instruments pointed toward the asteroid's surface. Using the spacecraft camera, the entire surface of the asteroid will be mapped so that its size, shape and volume can be determined. The Hayabusa spacecraft carries infrared and X-ray spectrometers that will identify the asteroid's most common minerals and chemical constituents. In mid-September, the spacecraft will evolve down to its so-called "gate position," 20 kilometers above the asteroid's surface, where it will begin the global mapping of the surface features and determine its surface composition. Toward the end of September, the spacecraft will move to its "home position," which is only seven kilometers above the surface. At this home position, a more detailed surface map will be generated and the surface composition differences will be examined as the asteroid rotates underneath the hovering spacecraft.

In the second half of November, the spacecraft will collect up to three surface samples as its sample horn captures small pieces of the asteroid ejected when tantalum pellets are fired into its surface at 300 meters per second. With these surface samples tucked safely into the spacecraft's sample capsule, the spacecraft will return to Earth, arriving in June 2007, and the sample capsule will parachute to the ground in Australia. The samples will be analyzed in various laboratories to study their detailed chemical composition and determine which meteorite examples in Earth-based collections provide the best match for Itokawa's particular composition. Once this question is answered, then future Earth based observations can be used to identify the likely minerals in other asteroids that share the same spectral characteristics as Itokawa.

During the first descent to fire a pellet into the surface, a small coffee-can-sized surface hopper, called MINERVA, will be dropped slowly onto the asteroid's surface. For one to two days it will slowly leap about the asteroid taking surface temperature measurements and high-resolution images with each of its three miniature cameras.

Hayabusa's observations will address each of three major issues concerning asteroids: 1.) their role as the building blocks of the solar system, 2.) their potential for impacting Earth and 3.) their future use as raw materials for building space structures.

1. The scientific interest in asteroids is due largely to their status as the remnant debris from the inner solar system formation process that occurred some 4.6 billion years ago. Since the chemical compositions of asteroids have remained relatively unchanged since their formation, knowledge of their elemental makeup would provide an understanding of the chemical mix from which the inner planets, including Earth, formed.
   
   
2. From time to time, near-Earth asteroids collide with Earth. Should one of them be found upon an Earth threatening trajectory, scientists would need to understand its composition and structure before a successful strategy could be undertaken to deflect the object away from Earth.
   
   
3. Some of the near-Earth asteroids that are potentially the most hazardous because they can closely approach the Earth are also the objects that could be most easily reached and exploited for raw materials. The minerals, metals and water ices on near-Earth asteroids and comets could be used to manufacture the space structures and rocket fuel that will be required to explore and colonize our solar system in the 21st century. We need to examine the chemical composition of some of these objects to understand which among them are richest in mineral wealth and other raw materials.

The Hayabusa asteroid sample return mission is the next giant step forward in understanding the role of near-Earth asteroids in the origin of the solar system, their potential threat to Earth and the future use of their raw materials to expand the human presence beyond Earth.

Additional information:

Hayabusa Project (JAXA main site)
http://www.isas.ac.jp/e/enterp/missions/hayabusa/index.shtml

SPACE NEWS (JAXA) -- Hayabusa acquired images of the earth and the moon.
http://www.isas.jaxa.jp/e/snews/2004/0519.shtml

Planetary Society:
http://planetary.org/news/2004/hayabusa_earth-swingby_preview.html

Hayabusa Science Objectives:

AMICA - Asteroid Multiband Imaging Camera

• Map surface morphology including surface features to one 1-mresolution
• Determine spin state, colors, size, shape, volume, and rotation characteristics
• Search for possible asteroid satellites and dust rings
• Establish a global map of surface features and colors
• Reveal history of impacts from other asteroid and comet fragments
• Determine optical parameters of regolith particles using polarization degree vs. phase curve at large phase angles
• Map mineralogical composition of asteroid and identify rock types present
• Determine most likely meteorite analog for composition of asteroid

Near-IR Spectrometer

• Map mineralogical composition of asteroid and provide main evidence for rock types present on surface at scales as small as 20 m
• Characterize surface heterogeneity
• Together with elemental composition measurements provided by (XRS)and color imagery from camera, IR spectrometer will provide link between this asteroid and a meteorite type

X-Ray Spectrometer (XRS)

• Map the major elemental composition of the surface as the asteroid rotates under the spacecraft
• Determine the major elemental composition at localized areas during asteroid approach phases
• Measure surface composition accurately enough to establish relationship between asteroids and meteorites and identify type of meteorite to which asteroid is linked
• Provide elemental abundance maps to investigate inhomogeneity of regolith

Sample Return Analysis

• Samples returned to Earth will provide a detailed and definitive elemental composition analysis of the asteroid's surface materials and hence forge an unambiguous link between the asteroid's composition and a meteorite type

LIDAR

• Provide accurate shape and mass determinations for asteroid
• Map asteroid's surface with a maximum resolution of about 1-meter

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"10th Planet" Discovered

From: http://science.nasa.gov/headlines/y2005/29jul_planetx.htm?list148527

"It's definitely bigger than Pluto." So says Dr. Mike Brown of the California Institute of Technology who announced today the discovery of a new planet in the outer solar system.

The planet, which hasn't been officially named yet, was found by Brown and colleagues using the Samuel Oschin Telescope at Palomar Observatory near San Diego. It is currently about 97 times farther from the sun than Earth, or 97 Astronomical Units (AU). For comparison, Pluto is 40 AU from the sun.

This places the new planet more or less in the Kuiper Belt, a dark realm beyond Neptune where thousands of small icy bodies orbit the sun. The planet appears to be typical of Kuiper Belt objects--only much bigger. Its sheer size in relation to the nine known planets means that it can only be classified as a planet itself, Brown says.

Backyard astronomers with large telescopes can see the new planet. But don't expect to be impressed: It looks like a dim speck of light, visual magnitude 19, moving very slowly against the starry background. "It is currently almost directly overhead in the early-morning eastern sky in the constellation Cetus," notes Brown.

The planet was discovered by, in addition to Brown, Chad Trujillo, of the Gemini Observatory in Mauna Kea, Hawaii, and David Rabinowitz, of Yale University, New Haven, Connecticut. They first photographed the new planet with the 48-inch Samuel Oschin Telescope on October 31, 2003. The object was so far away, however, that its motion was not detected until they reanalyzed the data in January of this year. In the last seven months, the scientists have been studying the planet to better estimate its size and its motions.

"We are 100 percent confident that this is the first object bigger than Pluto ever found in the outer solar system," Brown adds.

Telescopes have not yet revealed the planet's disk. To estimate how big it is, the astronomers must rely on measurements of the planet's brightness. Like all planets, this new one presumably shines by reflecting sunlight. The bigger the planet, generally speaking, the bigger the reflection. The reflectance, the fraction of light that bounces off the planet, is not yet known. Nevertheless, it is possible to set limits on the planet's diameter:

"Even if it reflected 100 percent of the light reaching it, it would still be as big as Pluto," says Brown. Pluto is 1400 miles (2300 km) wide. "I'd say it's probably [about] one and a half times the size of Pluto, but we're not sure."

The size of the planet is further limited by NASA's Spitzer Space Telescope, which has already proved its mettle in studying the heat of dim, faint, faraway objects such as the Kuiper-belt bodies. Because Spitzer has been unable to detect the new planet, the overall diameter must be less than about 2000 miles (3200 km), says Brown.

The planet's temporary name is 2003 UB313. A permanent name has been proposed by the discoverers to the International Astronomical Union, and they are awaiting the decision of this body before announcing the name. Stay tuned!

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Team Catches Rare Light Show with Charon

On a clear summer night, the stars aligned for MIT researchers watching and waiting for one small light in the heavens to be extinguished, just briefly.

Thanks to a feat of both astronomical and terrestrial alignment, a group of scientists from MIT and Williams College succeeded in observing distant Pluto's tiny moon, Charon, hide a star. Such an event had been seen only once before, by a single telescope 25 years ago, and then not nearly as well.

The MIT-Williams consortium spotted it with four telescopes in Chile on the night of July 10-11.

The team expects to use data from this observation to assess whether Charon has an atmosphere, to measure its radius and to determine how round it is.

The data and results from the observation will be presented at the 2005 meeting of the American Astronomical Society's Division of Planetary Sciences meeting to be held in Cambridge, England, in September.

MIT team leader James L. Elliot headed the group at the Clay Telescope at Las Campanas Observatory in Chile.

"We have been waiting many years for this opportunity. Watching Charon approach the star and then snuff it out was spectacular," said Elliot, a professor in the Department of Earth, Atmospheric and Planetary Science and the Department of Physics at MIT in Cambridge, Mass.

Jay M. Pasachoff, team leader from Williams College in Williamstown, Mass., and a professor in that school's Department of Astronomy, said, "It's amazing that people in our group could get in the right place at the right time to line up a tiny body 4 billion miles away. It's quite a reward for so many people who worked so hard to arrange and integrate the equipment and to get the observations."

With the Clay Telescope's 6.5-meter mirror (more than 21 feet across, the size of a large room) the researchers were able to observe changes throughout the event, which lasted less than a minute. While their electronic cameras sensitively recorded data, the light of the faint star was seen to dim and then, some seconds later, brighten. This kind of disappearance of a celestial body behind a closer, apparently larger one is known as an occultation.

Studying how the light dimmed and brightened will let the MIT-Williams consortium look for signs that Charon has an atmosphere. It has very little mass and thus little gravity to hold in an atmosphere, but it is so cold (being some 40 times farther from the sun than the Earth) that some gases could be held in place by the small amount of Charon's gravity.

Other telescopes around Chile used by the MIT-Williams consortium included the 8-meter (more than 26 feet across) Gemini South on Cerro Pachon, the 2.5-meter (more than 8 feet across) DuPont Telescope at Las Campanas Observatory and the 0.8-meter (almost 3 feet across) telescope at the Cerro Armazones Observatory of Chile's Catholic University of the North near Cerro Paranal.

The team had searched for a distribution of telescopes along a north-south line in Chile since the predictions of the starlight shadow of Charon were uncertain by several hundred kilometers. Since the star that was hidden is so far away, it casts a shadow of Charon that is the same size as Charon itself, about 1,200 kilometers in diameter. To see the event, the distant star, Charon and the telescopes in Chile had to be perfectly aligned. All of these telescopes had clear views of the event.

Other MIT affiliates involved in the observation were MIT graduate students Elisabeth Adams, Michael Person and Susan Kern and postdoctoral associate Amanda Gulbis.

The images from three telescopes in Chile, including the Clay Telescope, and one in Brazil, were taken with new electronic cameras and computer control obtained by MIT and Williams with an equipment grant from NASA. The expeditions were sponsored by NASA's Planetary Astronomy Program.

A video showing the star dimming as Charon passes in front of it and then brightening again is posted on the Web at: http://occult.mit.edu/research/occultations/Charon/C313.2/C313OccMovie.html.

Teams from the Observatory of Paris at Meudon and from the Southwest Research Institute in Boulder, Colo., also observed the occultation.

_________________________

Announcement from IAU Circular No. 8570:

                                                   Circular No. 8570
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
Mailstop 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
IAUSUBS@CFA.HARVARD.EDU or FAX 617-495-7231 (subscriptions)
CBAT@CFA.HARVARD.EDU (science)
URL http://cfa-www.harvard.edu/iau/cbat.html  ISSN 0081-0304
Phone 617-495-7440/7244/7444 (for emergency use only)
.....................

OCCULTATION BY PLUTO I (CHARON)
      L. A. Young, C. B. Olkin, and E. F. Young, Southwest Research
Institute; and R. G. French, Wellesley College, report their
observations of an occultation on July 11 of the star 2UCAC
26257135 by Pluto I (Charon).  E. F. Young and K. Shoemaker
observed with 0.2-s integration times using the visitor instrument
PHOT (unfiltered) on the 4.2-m SOAR telescope at Cerro Pachon.  C.
Olkin, C. Ruhland, and L. A. Young observed via a 240-s I-band
exposure trailed at 1".5/s with the 4-m Blanco telescope (+ Mosaic
II camera) at Cerro Tololo.  R. French, B. Gregory, L. A. Young,
and R. Galvez observed via 0.5-s I-band exposures using the 0.9-m
SMARTS telescope (+ Tek2K No. 3 CCD camera) at Cerro Tololo.  All
three sites report durations of 55.3 ± 0.2 s.  The chord length
is 1179 ± 4 km, which serves as a lower limit on the diameter
from these telescopes.  No obvious atmosphere was detected.
Immersion and emersion times (+/- 0s.01) at SOAR were 3h36m16s.19
and 3h37m11s.26 UT.

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Seismic Shaking Erased Small Impact Craters on Asteroid Eros

University of Arizona scientists have discovered why Eros, the largest near-Earth asteroid, has so few small craters.

When the Near Earth Asteroid Rendezvous (NEAR) mission orbited Eros from February 2000 to February 2001, it revealed an asteroid covered with regolith -- a loose layer of rocks, gravel and dust -- and embedded with numerous large boulders. The spacecraft also found places where the regolith apparently had slumped, or flowed downhill, exposing fresh surface underneath.

But what NEAR didn't find were the many small craters that scientists expected would pock Eros' landscape.

"Either the craters were being erased by something or there are fewer small asteroids than we thought," James E. Richardson Jr. of UA's planetary sciences department said.

Richardson concludes from modeling studies that seismic shaking has obliterated about 90 percent of the asteroid's small impact craters, those less than 100 meters in diameter, or roughly the length of a football field. The seismic vibrations result when Eros collides with space debris.

Richardson, Regents' Professor H. Jay Melosh and Professor Richard Greenberg, all with UA's Lunar and Planetary Laboratory, report the analysis in the Nov. 26 issue of Science.

"Eros is only about the size of Lake Tahoe -- 20 miles (33 kilometers) long by 8 miles (13 kilometers) wide," Richardson said. "So it has a very small volume and a very low gravity. When a one-to-two-meter or larger object hits Eros, the impact will set off global seismic vibrations. Our analysis shows how these vibrations easily destabilize regolith overlaying the surface."

A rock-and-dust layer creeps, rather than crashes, down shaking slopes because of Eros' weak gravity. The regolith not only slides down horizontally, but also is launched ballistically from the surface and 'hops' downslope. Very slowly, over time, impact craters fill up and disappear, Richardson said.

If Eros were still in the main asteroid belt between Mars and Jupiter, a 200-meter crater would fill in about 30 million years. Because Eros is now outside the asteroid belt, that process takes a thousand times longer, he added.

Richardson's research results match the NEAR spacecraft evidence. Instead of the expected 400 craters as small as 20 meters (about 70 feet) per square kilometer (three-fifths mile) on Eros' surface, there are on average only about 40 such craters.

The modeling analysis also validates what scientists suspect of Eros' internal structure.

"The NEAR mission showed Eros to most likely be a fractured monolith, a body that used to be one competent piece of material," Richardson said. "But Eros has been fractured throughout by large impacts and is held together primarily by gravity. The evidence is seen in a series of grooves and ridges that run across the asteroid's surface both globally and regionally."

Large impacts fracture Eros to its core, but many smaller impacts fracture only the upper surface. This gradient of big fractures deep inside and numerous small fractures near the surface is analogous to fractures in the upper lunar crust, Richardson said. "And we understand the lunar crust -- we've been there. We've put seismometers on the moon. We understand how seismic energy propagates through this kind of structure."

The UA scientists' analysis of how impact-induced seismic shaking has modified Eros' surface has a couple of other important implications.

"If we eventually do send spacecraft to mine resources among the near-Earth asteroids or to deflect an asteroid from a potential collision with the Earth, knowing internal asteroid structure will help address some of the strategies we'll need to use. In the nearer future, sample return missions will encounter successively less porous, more cohesive regolith as they dig farther down into asteroids like Eros, which has been compacted by seismic shaking," Richardson noted.

"And it also tells us about the small asteroid environment that we'll encounter when we do send a spacecraft out into the main asteroid belt, where Eros spent most of its lifetime. We know the small asteroids -- those between the size of a beachball and a football stadium -- are out there. It's just that their 'signature' on asteroids such as Eros is being erased," Richardson said.

This finding is important because the cratering record on large asteroids provides direct evidence for the size and population of small main-belt asteroids. Earth-based telescopic surveys have catalogued few main-belt asteroids that small. So scientists have to base population estimates for these objects primarily on visible cratering records and asteroid collisional history modeling, Richardson said.

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Kissing in the Kuiper Belt

By Robert Naeye of Sky and Telescope.

Among the strangest denizens of the solar system are contact binaries. In these systems, two minor planets orbit each other so closely that they literally or nearly touch end-to-end -- resulting in a peanut-like overall shape. Until now, astronomers had found only two possible contact binaries of relatively large size: the main-belt asteroid 216 Kleopatra and the Trojan asteroid 624 Hektor. Now Scott S. Sheppard (Carnegie Institution of Washington) and David C. Jewitt (University of Hawaii) may have found a third example: a Kuiper-Belt object orbiting beyond Pluto.

The object, 2001 QG298, orbits so far from the Sun that even the Hubble Space Telescope has no chance of resolving a peanut shape. But after measuring the object's changing brightness in 2002 and 2003 with the University of Hawaii's 2.2-meter telescope and the 10-meter Keck I telescope, Sheppard and Jewitt noticed something unusual. Its brightness varies by a whopping 1.14 magnitude every 6.89 hours. Yet the object's colors do not change, which suggests that dark spots rotating in and out of view are not causing the brightness changes.

Only three other solar-system objects larger than 50 kilometers across range in brightness by more than 1 magnitude. Two of them are Kleopatra and Hektor; the third is Saturn's peculiar moon Iapetus, which displays a very dark leading hemisphere and a very bright trailing hemisphere. But Iapetus represents an unusual case because it is locked in synchronous rotation with Saturn, so its leading edge may be sweeping up dark material kicked off from Saturn's outer satellites.

With an average diameter of about 180 kilometers, 2001 QG298 is large enough that it should be nearly spherical. But the object is not spinning fast enough for rotation to whirl it into an elongated shape. The simplest explanation for the brightness variations is that two roughly spherical and equal-sized bodies eclipse each other periodically every 6.89 hours, which means they must be very close together. We view them along their equators, which maximizes the eclipsing effect.

"We believe 2001 QG298 is a contact binary," said Sheppard, as he reported the team's findings at the November meeting of the American Astronomical Society’s Division of Planetary Sciences in Louisville, Kentucky.

Sheppard and Jewitt have found other possible contact-binary Kuiper Belt objects (KBOs). Given the number of KBOs they have observed, and the fact that other KBO contact binaries might be viewed pole-on (which makes them harder to categorize), Sheppard and Jewitt estimate that at least 10 to 20 percent of all large KBOs might be contact binaries with similarly-sized components. "The number of contact binaries with one component much larger than the other is probably much higher," says Sheppard, "but these don't make as large brightness variations and thus are not as easy to distinguish."

These close pairs probably formed early in the solar system's history when two bodies approached each other and went into mutual orbit after exchanging orbital energy with other bodies nearby.

For full story with diagrams refer to: http://SkyandTelescope.com/news/article_1394_1.asp

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