Chapter 20:

Near-Earth Asteroids
Near-Earth Asteroids at JPL
Asteroid Photo Gallery
Kuiper Belt Homepage
1999 Leonids, European Space Agency Page
Meteorite Collection
NEAR Shoemaker Web Site
Planetary Society's Descriptions of Comet and Asteroid Missions
Near-Earth Asteroid Tracking (Caltech)
Links from the Meteoritical Society
Asteroid links from Bill Bottke, Cornell
Recently discovered asteroids and asteroid satellites
American Meteor Society

Leonid Meteor Shower/Storm:

www.imo.net
www.amsmeteors.org
http://www.skypub.com/sights/meteors/leonids/9911leo.html
Dr. Donald Yeomans' Leonids Site

Loneos Discovers Asteroid with the Smallest Orbit

Flagstaff, AZ The ongoing search for near-Earth asteroids at Lowell Observatory has yielded another interesting object. Designated 2004 JG6, this asteroid was found in the course of LONEOS (the Lowell Observatory Near-Earth Object Search) on the evening of May 10 by observer Brian Skiff.

"I immediately noticed the unusual motion," said Skiff, "so it was certain that it was of more than ordinary interest." He quickly reported it to the Minor Planet Center (MPC) in Cambridge MA, which acts as an international clearinghouse for asteroid and comet discoveries. The MPC then posted it on a Web page for verification by astronomers worldwide. It happened that all the initial follow up observations, however, were obtained by amateur and professional observers in the Southwest US. The additional sky positions measured in the ensuing few days allowed an orbit to be calculated.

The official discovery announcement and preliminary orbit were published by the MPC on May 13. This showed that the object was located between Earth and Venus (presently the very bright "evening star" in the western sky). In addition, 2004 JG6 goes around the Sun in just six months, making it the asteroid with the shortest known orbital period. Ordinary asteroids are located between the orbits of Mars and Jupiter, roughly two to four times farther from the Sun than Earth, taking several years to go around the Sun.

Instead, 2004 JG6 orbits entirely within Earth's orbit, only the second object so far found to do so. "What makes this asteroid unique is that, on average, it is the second closest solar system object orbiting the Sun," said Edward Bowell, LONEOS Director. Only planet Mercury orbits closer to the Sun.

As shown in the included diagram (http://www.lowell.edu/press_room/2004JG6.pdf), JG6 crosses the orbits of Venus and Mercury, passing less than 30 million miles from the Sun every six months. The approximate average orbital speed of this asteroid is more than 30 km/sec, or 67,000 miles per hour. Depending on their locations, the asteroid may pass as close as 3.5 million miles from Earth and about 2 million miles from planet Mercury. In the coming weeks 2004 JG6 will pass between Earth and the Sun, just inside Earth's orbit. It will move through the constellations Cancer and Canis Minor low in the western sky at dusk. Because of the near-exact six-month period, the asteroid should be observable again in nearly the same spot in the sky next May, having gone around the Sun twice while Earth will have made only one circuit.

From present estimates, 2004 JG6 is probably between 500 meters and 1 km in diameter. Despite its proximity, the object poses no danger of colliding with Earth.

Asteroids with orbits entirely within the Earth's orbit have been informally called "Apoheles," from the Hawaiian word for orbit. Apohele also has Greek roots: "apo" for outside, and "heli" for Sun. Objects orbiting entirely within Earths orbit are thought by dynamicist William F. Bottke of Southwest Research Institute and colleagues to comprise just two percent of the total near-Earth object population, making them rare as well as difficult to discover. This is because they stay in the daylight sky almost all of the time. There may exist about 50 Apoheles of comparable size to or larger than 2004 JG6, but many of them are certain to be unobservable from the ground.

The first asteroid found entirely inside Earth's orbit was 2003 CP20, found just over a year ago by the NASA-funded Lincoln Laboratory Near-Earth Asteroid Research project, which observes near Socorro, New Mexico. Although larger than 2004 JG6, 2003 CP20 is a little more distant from the Sun.

LONEOS is one of five programs funded by NASA to search for asteroids and comets that may approach our planet closely. The NASA program's current goal is to discover 90 percent of near-Earth asteroids larger than 1 km in diameter by 2008. There are thought to be about 1,100 such asteroids.

JPL orbit diagram/animations:
http://neo.jpl.nasa.gov/cgi-bin/db_shm?sstr=2004+JG6

Static view of 2004 JG6 by Tom Polakis:
http://www.psiaz.com/polakis/misc/2004JG6.jpg

Diagram of 2004JG6 by Larry Wasserman, Astronomer, Lowell Observatory (a pdf)
http://www.lowell.edu/press_room/2004JG6.pdf

Meteorite Crater from 250-million-year-old Extinction Found

An impact crater believed to be associated with the "Great Dying," the largest extinction event in the history of life on Earth, appears to be buried off the coast of Australia.

NASA and the National Science Foundation (NSF) funded the major research project headed by Luann Becker, a scientist at the University of California, Santa Barbara (UCSB). Science Express, the electronic publication of the journal Science, published a paper describing the crater on May 13, 2004.

Most scientists agree a meteor impact, called Chicxulub, in Mexico's Yucatan Peninsula, accompanied the extinction of the dinosaurs 65 million years ago. But until now, the time of the Great Dying 250 million years ago, when 90 percent of marine and 80 percent of land life perished, lacked evidence and a location for a similar impact event.

Becker and her team found extensive evidence of a 125-mile- wide crater, called Bedout, off the northwestern coast of Australia. They found clues matched up with the Great Dying, the period known as the end-Permian. This was the time period when the Earth was configured as one primary land mass called Pangea and a super ocean called Panthalassa.

During recent research in Antarctica, Becker and her team found meteoric fragments in a thin claystone "breccia" layer, pointing to an end-Permian event. The breccia contains the impact debris that resettled in a layer of sediment at end- Permian time.

They also found "shocked quartz" in this area and in Australia. "Few Earthly circumstances have the power to disfigure quartz, even high temperatures and pressures deep inside the Earth's crust," Becker said.

Quartz can be fractured by extreme volcanic activity, but only in one direction. Shocked quartz is fractured in several directions and is therefore believed to be a good tracer for the impact of a meteor.

Becker discovered oil companies in the early 70's and 80's had drilled two cores into the Bedout structure in search of hydrocarbons. The cores sat untouched for decades. Becker and co-author Robert Poreda went to Australia to examine the cores held by the Geological Survey for Australia in Canberra. "The moment we saw the cores, we thought it looked like an impact breccia," Becker said. Becker's team found evidence of a melt layer formed by an impact in the cores.

In the paper, Becker documented how the Chicxulub cores were very similar to the Bedout cores. When the Australian cores were drilled, scientists did not know exactly what to look for in terms of evidence of impact craters.

Co-author Mark Harrison, from the Australian National University in Canberra, determined a date on material obtained from one of the cores, which indicated an age close to the end-Permian era. While in Australia on a field trip and workshop about Bedout, funded by the NSF, co-author Kevin Pope found large shocked quartz grains in end-Permian sediments, which he thinks formed as a result of the Bedout impact. Seismic and gravity data on Bedout are also consistent with an impact crater.

The Bedout impact crater is also associated in time with extreme volcanism and the break-up of Pangea. "We think that mass extinctions may be defined by catastrophes like impact and volcanism occurring synchronously in time," Becker said. "This is what happened 65 million years ago at Chicxulub but was largely dismissed by scientists as merely a coincidence. With the discovery of Bedout, I don't think we can call such catastrophes occurring together a coincidence anymore," she added.

The Meteoroid that Broke Up Over Chicago

The meteorites that punched through roofs in Park Forest, Ill., on the evening of March 26, 2003, came from a larger mass that weighed no less than 1,980 pounds before it hit the atmosphere, according to scientific analyses led by the University of Chicago's Steven Simon, who himself also happens to live in Park Forest.

Simon, a Senior Research Associate in Geophysical Sciences at the University of Chicago, and seven co-authors will publish these and other findings in the April issue of the journal Meteoritics and Planetary Science. Simon holds a unique distinction among scientists: his home sits in the middle of the strewnfield, the area from which the meteorites were recovered.

"I don't know of any other time when a meteoriticist was in the middle of a strewnfield," said Lawrence Grossman, Professor in Geophysical Sciences at the University of Chicago and one of Simon's co-authors.

In fact, Simon actually saw the flash the meteorite created. He had the drapes closed when the rock entered the sky over Illinois, but "the whole sky lit up," he said.

Grossman, who lives in Flossmoor, not far from Park Forest, also experienced the meteorite's arrival firsthand. He was awakened by the sound of the meteorite entering the atmosphere that night. "I heard a detonation," Grossman said. "It was sharp enough to wake me up."

The team calculated the projectile's size range based on measurements of the galactic cosmic rays that it absorbed. Measurements of a radioactive form of cobalt provided the projectile's minimum size. "If the object is too small the cosmic rays will just pass through and not make 60cobalt," Simon explained.

Simon and Grossman classify the meteorite as an L5 chondrite, a type of stony meteorite, one low in iron that was heated for a long period of time inside its parent body, probably an asteroid. "It's a fairly common type of meteorite," Simon said.

The Park Forest meteorite also showed signs that it had been highly shocked, probably when it was part of a rock that was broken from a much larger asteroid following a collision. The evidence for shock includes shocked feldspar. Apollo astronauts recovered shocked specimens of the mineral from the moon, as well, Simon said. Impact shock was common in the early history of the solar system because of the large quantity of interplanetary debris then in existence.

Witnesses in Michigan, Illinois, Indiana and Missouri reported seeing the fireball that the meteorite produced as it broke up in the atmosphere, Simon and his colleagues report. Local residents collected hundreds of meteorite fragments totaling approximately 65 pounds from an area extending from Crete in the south to the southern end of Olympia Fields in the north. Located in Chicago's south suburbs, "This is the most densely populated region to be hit by a meteorite shower in modern times," the authors write.

One meteorite narrowly missed striking a sleeping Park Forest resident after it burst through the ceiling of a bedroom. The meteorite sliced through some window blinds, cratered the windowsill, then bounced across the room and broke a mirror before coming to rest.

The meteorites were recovered from a track that trends southeast to northwest. Satellite data analyzed by Peter Brown of the University of Western Ontario indicates that the meteorite traveled from southwest to northeast, however.

"The meteorite broke up in the atmosphere, and the fragments encountered strong westerly winds as they fell," the authors write. "The smallest pieces were deflected the furthest eastward from the trajectory, and the largest pieces, carrying more momentum, were deflected the least."

Contributing to the paper in addition to Simon and Grossman were the University of Chicago's Robert Clayton and the late Toshiko Mayeda, Jim Schwade of the Planetary Studies Foundation in Crystal Lake, Ill.; Paul Sipiera of Harper College in Palatine, Ill.; John Wacker of Pacific Northwest National Laboratory in Richland, Wash.; and Meenakshi Wadhwa of the Field Museum of Natural History in Chicago.

Web-Based Program Calculates Effects of an Earth Impact

Related Web site

Next time an asteroid or comet is on a collision course with Earth you can go to a web site to find out if you have time to finish lunch or need to jump in the car and DRIVE.

University of Arizona scientists are launching an easy-to-use, web-based program that tells you how the collision will affect your spot on the globe by calculating several environmental consequences of its impact.

Starting today, the program is online at:
http://www.lpl.arizona.edu/impacteffects

You type in your distance from the predicted impact site, the size and type of projectile (e.g. ice, rock, or iron) and other information. Then the Earth Impact Effects Program calculates impact energies and crater size. It next summarizes thermal radiation, seismic shaking, ejecta deposition (where all that flying stuff will land), and air-blast effects in language that non-scientists understand.

For those who want to know how all these calculations are made, the web page will include "a description of our algorithm, with citations to the scientific sources used," said Robert Marcus, a UA undergraduate in the UA/NASA Space Grant Program. He discussed the project recently at the 35th Lunar and Planetary Science Conference meeting in Houston, Texas.

Marcus developed the web site in collaboration with planetary sciences Regents' Professor H. Jay Melosh and research associate Gareth Collins of UA's Lunar and Planetary Laboratory.

Melosh is a leading expert on impact cratering and one of the first scientists reporters call when rumors of big, Earth-smashing objects begin to circulate.

Reporters and scientists both want to know the same thing: how much damage a particular collision would wrack on communities near the impact site.

The web site is valuable for scientists because they don't have to spend time digging up the equations and data needed to calculate the effects, Melosh said. Similarly, it makes the information available to reporters and other non-scientists who don't know how to make the calculations.

"It seemed to us that this is something we could automate, if we could find some very capable person to help us construct the website," Melosh said.

That person turned out to be Marcus, who is majoring in computer engineering and physics. He applied to work on the project as a paid intern through the UA/NASA Space Grant Program.

Marcus built the web-based program around four environmental effects. In order of their occurrence, they are:

1) Thermal radiation. An expanding fireball of searing vapor occurs at impact. The program calculates how this fireball will expand, when maximum radiation will occur, and how much of the fireball will be seen above the horizon.

The researchers based their radiation calculations on information found in "The Effect of Nuclear Weapons." This 1977 book, by the U.S. Defense Department and U.S. Department of Energy, details "considerable research into what different degrees of thermal radiation from blasts will do," Melosh noted.

"We determine at a given distance what type of damage the radiation causes," Marcus said. "We have descriptions like when grass will ignite, when plywood or newspaper will ignite, when humans will suffer 2nd or 3rd degree burns."

2) Seismic shaking. The impact generates seismic waves that travel far from the impact site. The program uses California earthquake data and computes a Richter scale magnitude for the impact. Accompanying text describes shaking intensity at the specified distance from the impact site using a modified Mercalli scale This is a set of 12 descriptions ranging from "general destruction" to "only mildly felt."

Now suppose the dinosaurs had this program 65 million years ago. They could have used it to determine the environmental consequences of the 15-kilometer-diameter asteroid that smashed into Earth, forming the Chicxulub Crater.

The program would have told them to expect seismic shaking of magnitude 10.2 on the Richter scale. They also would have found (supposing that the continents were lined up as they are now) that the ground would be shaking so violently 1,000 kilometers (600 miles) away in Houston that dinosaurs living there would have trouble walking, or even standing up.

If the Chicxulub Crater-impact occurred today, glass in Houston would break. Masonry and plaster would crack. Trees and bushes would shake, ponds would form waves and become turbid with mud, sand and gravel banks would cave in, and bells in Houston schools and churches would ring from ground shaking.

3) Ejecta deposition. The team used a complicated ballistics travel-time equation to calculate when and where debris blown out of the impact crater would rain back down on Earth. Then they used data gathered from experimental explosions and measurements of craters on the moon to calculate how deep the ejecta blanket would be at and beyond the impact- crater rim.

They also determined how big the ejecta particles would be at different distances from impact, based on observations that Melosh and UA's Christian J. Schaller published earlier when they analyzed ejecta on Venus.

OK, back to the dinosaurs. Houston would have been covered by an 80.8-centimeter- (32-inch-) thick blanket of debris, with particles averaging 2.8 mm (about 1/8th inch) in size. They would have arrived 8 minutes and 15 seconds after impact (meaning they got there at more than 4,000 mph).

4) Air blast. Impacts also produce a shock wave in the atmosphere that, by definition, moves faster than the speed of sound. The shock wave creates intense air pressure and severe winds, but decays to the speed of sound while it's still close to the fireball, Melosh noted. "We translate that decreasing pressure in terms of decibels --from ear-and-lung-rupturing sound, to being as loud as heavy traffic, to being only as loud as a whisper."

The program calculates maximum pressures and wind velocities based on test results from pre-1960s nuclear blasts. Researchers at those blasts erected brick structures at the Nevada Test Site to study blast wave effects on buildings. The UA team used that information to describe damage in terms of buildings and bridges collapsing, cars bowled over by wind, or forests being blown down.

Hermes Is Found

"This re-sighting of Hermes is the Holy Grail of near-Earth asteroid discovery," said Edward Bowell, LONEOS Director. "Its orbit has been better calculated and observers have confirmed its re-appearance and also shown its binary nature... well, an asteroid's return just does not become more profound than this."

The binary object was some 19 million miles out at the time of re-discovery last Wednesday, nearly 66 years after it was first seen. Hermes, which poses no threat to Earth, will make its closest approach on November 4th. By then it will be 4 million miles away and bright enough for amateurs to see using backyard telescopes.

The same day Skiff captured the first images of Hermes, Discovery Communications, Inc. and Lowell Observatory announced a partnership to build the new Discovery Channel Telescope near Flagstaff, Arizona. (http://www.lowell.edu/press_room/releases/recent_releases/dct_rls.html) One research objective for this new $30-million, 4.3-meter telescope will be to significantly accelerate the search for near-Earth objects, including those smaller than Hermes.

First images of the kilometer-size asteroid were captured by a CCD camera during early morning observation through the LONEOS 24-inch Schmidt telescope. More than six decades ago, Hermes was discovered by Karl Reinmuth at Heidelberg, Germany on October 25, 1937. Fast forward to a few days ago when Andrea Boattini of Instituto di Astrofisica Spaziale, Rome, Italy, and Timothy Spahr of the Minor Planet Center in Cambridge, Massachusetts analyzed the new positions of Hermes and determined what it was: the long- lost asteroid.

"Since we find new near-Earth asteroids fairly regularly (I found, for instance, two near-Earth asteroids the same night), my only reaction upon finding it was that it was unusually bright," Skiff told BBC News Online on Friday.

Asteroid Images with Adaptive Optics

"Asteroid Davida was discovered 100 years ago, but this is the first time anyone has been able to see this level of detail on this object," said Dr. Al Conrad, scientist at the W.M. Keck Observatory. "With adaptive optics, we're finally able to transform asteroids like Davida from a single, faint point-source into an object of true geological study."

Ground-based observations of large, main-belt asteroids are made possible only through a powerful astronomical technique called adaptive optics, which removes the blurring caused by Earth's atmosphere. Without adaptive optics, critical surface information and details about the asteroid's shape are lost. The techniques used at the W.M. Keck Observatory allow astronomers to measure the distortion of light caused by the atmosphere and rapidly make corrections, restoring the light to near-perfect quality. Such corrections are most easily made to infrared light. In many cases, infrared observations made with Keck adaptive optics are better than those obtained with space-based telescopes.

The observations of asteroid (511) Davida were made with the 10-meter (400-inch) Keck II telescope on December 26, 2002. Images were taken over a full rotation period of about 5.1 hours, just a few days before its closest approach to Earth. At that time, Davida's angular diameter was less than one-ten-thousandth of a degree, about the size of a quarter as seen from a distance of 18 kilometers (11 miles). The high angular resolution allowed astronomers to see surface details as small as 46 kilometers (30 miles), about the size of the San Francisco Bay area. The next time Davida comes this close to Earth will be in the year 2030.

At the time of the observations, Davida's north pole faced Earth. While scientists could see the asteroid spinning, only the northern hemisphere was visible. Yet the profile of the asteroid is far from circular: At least two flat facets can be seen on its surface. Although scientists knew previously from light variations that Davida must have an oblong shape, details of that shape were not available until now. Initial evaluation of the images reveal some dark features, and scientists are still working to understand to what extent these are surface markings, topographical features, or artifacts of the image processing.

"Adaptive optics on large telescopes is allowing us to make detailed studies from the ground that were previously impossible or prohibitively expensive," said Dr. William Merline, principal scientist with the Southwest Research Institute, and a participant in this research. "We can now make observations that once required either the scarce resources of space telescopes or spacecraft missions to asteroids. While these space telescopes and space missions are still needed for complete study of the asteroids, ground-based observations such as these will help tremendously in planning the mission observations and focusing the resources where they will be most effective."

Asteroids are the collection of rocky objects orbiting between Mars and Jupiter. They were likely prevented from forming into a planet, partly due to Jupiter's massive gravitational influence.

"Although the asteroids began their lives colliding gently, in a way that would lead them eventually to form a planet, Jupiter's gravity eventually stirred up their orbits, and they began to collide at higher speeds," added participant Dr. Christophe Dumas, planetary astronomer with the Jet Propulsion Laboratory.

"These collisions tended to cause them to break up rather than gently stick together. The resulting fragments, numbering in the hundreds of thousands, are the asteroids we see today. They collide with each other and have impacted the Earth, Moon, and planets over time. One need only look at the scarred surface of our Moon to see the cumulative result. Study of the asteroid's shape, size, and surface features helps us understand how these collisions operate and thus how our planet was, and still is, being affected by these impacts."

Observations of the shapes of asteroids, such as those released today, can tell us about the types and severity of impacts that occurred, and possibly also give clues into the overall structure of an asteroid --- for example, whether it may be solid rock, or a jumble of smaller rocks. Surface features can reveal a history of large impacts or variations in the composition that should, in turn, further help us understand the asteroid's history.

Asteroid (511) Davida was discovered by R. S. Dugan in 1903 in Heidelberg, Germany. The (511) in Davida's name means it was the 511th asteroid to be discovered and included in the list of asteroids maintained by the International Astronomical Union.

Team members responsible for the observations are Al Conrad, David Le Mignant, Randy Campbell, Fred Chaffee, Robert Goodrich, Shui Kwok of the W.M. Keck Observatory; Christophe Dumas, Jet Propulsion Laboratory; William Merline, Southwest Research Institute; Heidi Hammel, Space Science Institute; and Thierry Fusco, Onera, France.

Muses-C is Hayabusa

MUSES-C Mission is Launched to an Asteroid

The Japanese MUSES-C mission was launched in May 2003 to asteroid 1998SF36. It is to bring a gram of dust back to Earth in 2007, after raising the dust by throwing steel projectiles at the asteroid.

MUSES-C Mission Launched to an Asteroid

Meteorites Land in Chicago

Meteorites Shower Chicago's South Sububrbs

From Steve Koppes s-koppes@uchicago.edu

University of Chicago meteorite experts Lawrence Grossman and Steven Simon usually commute from Chicago's south suburbs to their laboratory on the Hyde Park campus to study rocks that have fallen from space. But at midnight last night a shower of meteorites came to them.

"I heard a detonation," said Grossman, a Professor in Geophysical Sciences who lives in the south suburb of Flossmoor, Ill. "It was sharp enough to wake me up."

Simon, a Senior Research Associate in Geophysical Sciences, lives in nearby Park Forest, where much of the meteorite fell. Simon, who works in Grossman's laboratory, is spending the day collecting information from area residents who brought samples of the meteorite to the Park Forest police station.

Grossman said meteorites from the fall have been found in Park =46orest, which is approximately 30 miles south of downtown Chicago, as well as the nearby communities of Steger to the south and Olympia =46ields to the north. He said the meteorite is classified as a chondrite, a common type of meteorite.

The following is from spaceweather.com

CHICAGO FIREBALL: Sky watchers in Illinois, Michigan, and Indiana were surprised around local midnight on March 26th-27th when a brilliant fireball streaked across the sky and exploded. "It was a small space rock (perhaps only 1 or 2 meters wide) with a mass of about 10 metric tons," reports Bill Cook of the Marshall Space Flight Center. "Some 500 fragments scattered over a 10-km wide zone in the suburbs south of Chicago." Meteorites struck houses, cars, roads--but no people. Scientists are scouring the area now to collect debris for further study. News reports:

These events are surprisingly common. "We expect an asteroidal object one meter in diameter or larger to strike Earth's atmosphere about 40 times per year," says Donald Yeomans, manager of NASA's Near-Earth Object Program at JPL. Few are seen, however, because the fireballs usually appear over unpopulated areas.

How bright was this one'? "About half as bright as the Sun," says Robert Soltysik, who saw the fireball through the window of his house in Valparaiso, Indiana. "The flash lit up the room like daylight just before sunset."

Stardust Images Asteroid Annefrank

JPL press releases, November 2, 2002, and November 4, 2002

NASA's Stardust spacecraft successfully completed a close flyby of asteroid Annefrank early today, Nov 2, as an opportunity for a full dress rehearsal of procedures the spacecraft will use during its Jan. 2, 2004, encounter with it primary science target, comet Wild 2.

Annefrank is about 4 kilometers (2.5 miles) across. Stardust passed within about 3,300 kilometers (2,050 miles) of the asteroid at 04:50 today, Universal Time (8:50 p.m. Nov. 1, Pacific Standard Time). Radio signals confirming the basic health of the spacecraft after the flyby were received about 30 minutes later via an antenna at the Canberra, Australia, complex of NASA's Deep Space Network, said Thomas Duxbury, project manager for Stardust at NASA's Jet Propulsion Laboratory, Pasadena, Calif.

Stardust visually tracked the asteroid for 30 minutes as it flew by at a relative speed of about 7 kilometers (4 miles) per second, a major goal of this test opportunity. Although no dust was anticipated near the asteroid, the spacecraft's dust instruments were in use as they will be at Wild 2: the dust collector was open and the dust counter from the University of Chicago and dust mass spectrometer from Germany were turned on.

Images and information from the flyby period are being transmitted from the spacecraft today and through the coming week. Stardust's scientists and engineers are analyzing the data to maximize the probability of success during the 2004 encounter with comet Wild 2.

November 4, 2002

Late Friday evening Pacific time on November 2, 2002 at the Jet Propulsion Laboratory (JPL) in Pasadena, California, and at Lockheed Martin Space Systems - Astronautics (LMA) near Denver, Colorado, the NASA STARDUST flight team pulled off a tremendously successful close flyby of the main belt asteroid Annefrank. This flyby was used as an engineering test of the ground and spacecraft operations that will be implemented at the primary scientific target, Comet Wild 2 (pronounced "Vilt" 2) just over one year from now.

STARDUST is a low-cost Discovery Mission that continues to perform as expected after more than three and a half years into a planned seven-year mission to rendezvous with Comet Wild 2 in January 2004. STARDUST will collect cometary dust samples, flowing from the nucleus just hours before spacecraft flyby, and return the samples to Earth in a Sample Return Capsule in January 2006. The close flyby of Annefrank offered a unique opportunity to thoroughly test all planned operations on the spacecraft and ground support operations which will be used during the rendezvous with Comet Wild 2.

"We performed a full dress rehearsal with the cometary dust collector deployed as we flew STARDUST within 3,300 kilometers of Annefrank," said Professor Donald Brownlee, the project's Principal Investigator from the University of Washington. "The spacecraft was poised in its flyby attitude with all the science instruments on. The flyby has exceeded all of our expectations and provided us with unexpected data about the asteroid," said Brownlee.

The approach geometry to Annefrank was much more difficult than will be the case for Comet Wild 2. The spacecraft was pointed over 60 degrees off of the normal Sun and Earth pointing attitude and was running on its battery in order to attempt to detect and capture images of Annefrank.

"The spacecraft performed every command perfectly and did everything asked of it," said Allan Cheuvront, Spacecraft Engineer at Lockheed Martin Space Systems near Denver. "We are thrilled with how well the entire operation went. We couldn't have asked for better performance from STARDUST and the images it captured of the asteroid exceeded everyone's expectations. The spacecraft's pointing, attitude and flight operations were excellent. This really adds to our level of confidence about how well the spacecraft will perform when we reach Wild 2," added Cheuvront. Cheuvront and a team of engineers at Lockheed Martin's spacecraft control center, known as the Mission Support Area, control the spacecraft in conjunction with JPL and the Deep Space Network.

The Navigation Camera was straining to see Annefrank during approach. "This camera was operating at its limit of performance and seeing very dim stars down to about 11th visual magnitude", said Ray Newburn, the Lead Scientist for the camera at JPL.

However, the brightness predicted by Drs. Stephen Synnott and Donald Yeomans of JPL was dimmer than 11th visual magnitude. "We tried everything we could think of including taking multiple long exposures and adding these on the ground", said Dr. T. S. Mike Wang, Optical Navigation Specialist at JPL, "but Annefrank was not cooperating. It was just too dim."

Because of the high probability of not seeing Annefrank during the approach, the flyby was designed to be successful without having to see it up to 20 minutes from encounter. "A flyby distance of 3,000 km (1,864 miles) was chosen so that there was no risk of the spacecraft flying near any possible dust environment or small satellites of Annefrank", said Ed Hirst, JPL Mission Design Manager. "We also wanted to ensure that Annefrank would be in the camera view at the start of the encounter sequence," added Hirst.

Since Annefrank was not seen in the approach images, the flight team felt that the asteroid was at least as dim as predicted and possibly even dimmer. The team decided to send up a new encounter configuration file and set the initial flyby exposures longer. "We had a planned uplink six hours before encounter for this very purpose," said Robert Ryan, Mission Manager at JPL. "We had some communications problems the day before that gave us some difficulty, but NASA's Deep Space Network gave us highest priority, and excellent communications on Friday, allowing us to play back earlier images we missed as well as sending our final encounter commands," added Ryan.

At 8:00 pm (PST) Friday evening, communications were established with the spacecraft to watch its pre-loaded sequence command turn the spacecraft away from the Sun and Earth into its flyby attitude. "We have built up over three years of flight experience and a tremendous amount of confidence and respect for our spacecraft to perform such operations routinely," said Joe Vellinga, STARDUST Program Manager at Lockheed Martin who led the development and manufacture of the spacecraft. "The spacecraft did not miss a beat during its flyby and it maintained all critical thermal, power, attitude, memory and reserves at or above design levels," added Vellinga.

The main function to be tested during flyby was a sophisticated flight computer program that would take over control of the spacecraft to keep the camera view locked on Annefrank during a 25-minute period around its closest encounter. "This software was a derivative of the nucleus tracking software successfully flown on the Deep Space 1 (DS1) flyby of Borrelly," said Dr. Shyam Bhaskaran, developer of the algorithms at JPL. "Based upon my previous experience on DS1, it performed up to my expectations with this encounter at Annefrank with over 60 successful images having Annefrank right in the middle of each image," added Bhaskaran. David Gingerich, Flight Software specialist at LMA who implemented and tested the nucleus tracking software said, "its performance was executed just like the coach drew it on the blackboard."

Over 70 encounter images were obtained that show a typical small solar system body, highly irregularly shaped and cratered. Annefrank is about twice as large as predicted, at least 6 kilometers in diameter, but darker than expected and therefore more difficult to detect in the early images. Not only did the camera perform well but the University of Chicago Dust Flux Measurement Instrument (DFMI) and the German Cometary and Interstellar Dust Analyzer (CIDA) performed as expected.

Professor Tom Economou, DFMI scientist from University of Chicago, stated "we ran for 28 minutes as we will at Wild 2 with DFMI performing all expected functions". Dr. Jochen Kissel, Lead Scientist for CIDA from Max Planck Institute in Garching, Germany, said "I will be able to put CIDA into an even better configuration at Wild 2 based upon the Annefrank experience." Both dust instrument teams are combing through their data to see if by chance they may have seen a dust particle.

"Performing such flight testing before the primary encounter is a critical part of reducing risks and significantly increasing the probability of success when we reach Wild 2", said JPL Project Manager, Thomas Duxbury. "We have performed exhaustive testing and training with LMA at their spacecraft test laboratory and through flight simulations, but these cannot totally replace actual flight operations testing. We learned a lot that will improve our operations at Wild 2 based upon the lessons learned at Annefrank. The bottom line is that if Annefrank had been Wild 2, we would have succeeded in every respect," added Duxbury.

"I applaud the entire flight team," said Don Brownlee. "We could not have asked for more, except possibly for Annefrank to be a little brighter. However, for everything that we could control with the spacecraft, we were nearly perfect.

Even though this was an engineering test, the flyby with Annefrank provided new information previously unknown about the asteroid about its size, shape, spin state and brightness as a function of viewing angle.

"It was an exciting Friday evening for those of us involved in this mission," Brownlee said. "We captured images of a primitive asteroid with a highly significant name and one whose size turned out to be similar to the asteroid that likely killed the dinosaurs 65 million years ago. We have now validated STARDUST's systems and operations and we are eagerly awaiting our encounter with Comet Wild 2, just over one year from now".

Asteroid Annefrank images are available here:

http://stardust.jpl.nasa.gov/photo/annefrank.html
http://photojournal.jpl.nasa.gov/catalog/PIA02885
http://photojournal.jpl.nasa.gov/catalog/PIA02886

Working Group On Near Earth Objects

Triennnial Report for the International Astronomical Union, November 4, 2002

The past three years have seen a tremendous growth in the study of NEOs. This period includes the one-year orbital study of 433 Eros by the NASA spacecraft NEAR-Shoemaker, followed by a landing on the asteroid surface http://near.jhuapl.edu. This mission has effectively resolved in the affirmative the long-standing issue of the association between S-type asteroids and the primitive ordinary chondrite meteorites. New radar studies have provided images of NEAs and include the discovery of several binary objects, which permit the calculation of densities http://echo.jpl.nasa.gov. Automated orbital calculation and risk estimates are now continuously available on-line through the NEO Dynamics system at Pisa http://newton.dm.unipi.it/cgi-bin/neodys/neoibo and the Sentry system at JPL http://neo.jpl.nasa.gov/risk/. The Spaceguard Survey discovery programs, led by the LINEAR MIT system http://www.ll.mit.edu/LINEAR/, have found more than 600 of the estimated 1100 +/- 100 NEAs brighter than absolute magnitude H=18 (diameter approximately 1 km). The primary Spaceguard search programs are supported by the United States government (NASA and the U.S. Air Force), with an international team for astrometric follow-up. The goal of the Spaceguard Survey is to find 90% of the NEAs larger than 1 km diameter by the end of 2008.

Communication with the international scientific community and with the interested public represents an important part of the WG efforts. One tool for public communication is the Torino Impact Scale, which has been adopted by the WG and other NEO scientists for this purpose. The Torino Scale is a "Richter Scale" for categorizing the Earth impact hazard associated with newly discovered asteroids and comets. The scale is described at http://impact.arc.nasa.gov. Other websites, although not formally endorsed by the IAU, also provide a valuable communication functions. These include the NASA NEO Program Office http://neo.jpl.nasa.gov, the NASA impact hazard website http://impact.arc.nasa.gov, the UK NEO Information Centre http://www.nearearthobjects.co.uk, and the Spaceguard Foundation and its on-line magazine Tumbling Stone http://spaceguard.ias.rm.cnr.it/SGF/.

David Morrison, November 4, 2002

Causes of Chicxulub and Sudbury Craters

University of Illinois at Urbana-Champaign press release, October 25, 2002

Champaign, Illinois. -- Two of the three largest impact craters on Earth have nearly the same size and structure, researchers say, but one was caused by a comet while the other was caused by an asteroid. These surprising results could have implications for where scientists might look for evidence of primitive life on Mars.

Susan Kieffer of the University of Illinois at Urbana-Champaign, Kevin Pope of Geo Eco Arc Research and Doreen Ames of Natural Resources Canada analyzed the structure and stratigraphy of the 65 million-year-old Chicxulub crater in Mexico and the 1.8 billion-year-old Sudbury crater in Canada.

Chicxulub is well preserved, but buried, and can be studied only by geophysical means, remote sensing and at a few distant sites on land where some ejecta is preserved. In contrast, Sudbury has experienced up to 4-6 kilometers of erosion, and is well exposed and highly studied by mining exploration companies because of its rich mineral resources.

By working back and forth with data from the two craters, the researchers were able to re-create the structures and then estimate the amount of melt in each structure. The amount of melt is critical for determining if long-lived hot-water circulation systems that might host life formscould have been formed after the impacts.

In their field studies, the researchers found that both craters were about 200 kilometers in diameter. In addition, they identified five ring-shaped structures with similar character and dimensions. A sixth ring -- the peak ring in the central basin -- was present at Chicxulub, but had been eroded away at Sudbury.

"While the size and structure of the two craters were similar, they differed greatly in the amount of impact melt that was produced," said Kieffer, who presented the team's findings at the annual meeting of the Geological Society of America, held Oct. 27-30 in Denver.

"Through field studies, we determined that Chicxulub has about 18,000 cubic kilometers of impact melt, approximately four times the volume of water in Lake Michigan," Pope said. "Sudbury has about 31,000 cubic kilometers of impact melt, approximately six times the volume of lakes Huron and Ontario combined, and nearly 70 percent more than the melt at Chicxulub. These differences in volume have significant implications about the amount of heat available to drive hot-water circulation systems."

The researchers then used an analytical cratering model to examine possible causes for the huge difference in melt. According to the simulation results, the difference in melt volume could be readily explained if Chicxulub -- the impact crater that doomed the dinosaurs - -- was formed by an asteroid and Sudbury was formed by a comet.

"Our calculation of 18,000 cubic kilometers of impact melt at Chicxulub agreed well with model estimates for an asteroid striking at a 45 degree angle," said Kieffer, the Walgreen Professor of Geology at Illinois. "None of the comet impact examples came close to agreeing."

In contrast, the Sudbury impact melt volume of 31,000 cubic kilometers fell between model estimates for a comet striking at an angle of 30-45 degrees, Kieffer said. "Similarly, none of the asteroid impact examples came close to agreeing with the Sudbury melt volume."

Another clue to the craters' origins lies in the impact melts themselves. The majority of the excess melt at Sudbury is in the form of a melt-rich breccia -- called suevite -- inside the crater. This material tends to form in impacts where the crustal target rock contains a lot of water. Sudbury has much more suevite in the preserved crater than Chicxulub.

"The mystery was that there probably wasn't a lot of water in the original rocks at Sudbury to account for the excess suevite," Kieffer said. "But in a comet impact of this size, somewhere around 1,400-2,000 cubic kilometers of water from the comet gets mixed into the impact melt, and that could play a major role in disrupting the melt and creating the excess suevite."

There is other independent evidence for an asteroid impact at Chicxulub, the team said, including the purported find of an asteroid fragment in an oceanic drill core, the amount of iridium spread around the world at the time of impact, and a telltale chromium 53 isotopic signature.

By studying the origin and structure of large impact craters on Earth, scientists might narrow the search for life on Mars. At Sudbury, for example, "there is evidence of a huge hydrothermal system that was driven by the heat of the impact melt," Ames said. "As a result, there was widespread hot spring activity on the crater floor possibly capable of supporting life."

The researchers are interested in "extrapolating these conclusions about comet and asteroid impacts to Martian conditions and asking where we might go to look for similar hydrothermal systems that could have hosted primitive life forms on Mars," Kieffer said. "Our next step is to model these hot-water circulation systems that were set up by the impact melts with fluid flow controlled by structures (fractures) inside the crater, and then extrapolate the results to Martian conditions."

The National Aeronautics and Space Administration and the Natural History Museum of Los Angeles County funded this work.

A geological map and RADARSAT-1 image of the Sudbury impact crater is available at

http://www.ccrs.nrcan.gc.ca/ccrs/rd/apps/geo/sudbury/sudbury_e.html

Meteorite Hits Girl in Foot

Sept 1, 2002

A British schoolgirl, Siobhan Cowper ("Siobhan" is pronounced "shi-vann") was hit on her foot last week by something falling. She looked down and found what appears to be a meteorite. It was still warm. She is planning to take it to the nearby University of Durham to see if its status as a meteorite can be verified. I spoke to her on the phone to verify that it really just hit her in the foot.

J.M. Pasachoff

http://icnewcastle.icnetwork.co.uk/0100news/0100local/page.cfm?objectid=12151422&method=full&siteid=50081
However, most such "meteorite" finds have more Earthly explanations. See:
http://epswww.unm.edu/iom/Howto.htm/a>

Twice as many asteroids as previously believed

ESA Press Release, April 5, 2002

Asteroids in our Solar System may be more numerous than previously thought, according to the first systematic search for these objects performed in the infrared, with ESA's Infrared Space Observatory, ISO. The ISO Deep Asteroid Search indicates that there are between 1.1 million and 1.9 million 'space rocks' larger than 1 kilometre in diameter in the so-called 'main asteroid belt', about twice as many as previously believed. However, astronomers think it is premature to revise current assessments of the risk of the Earth being hit by an asteroid.

Despite being in our own Solar System, asteroids can be more difficult to study than very distant galaxies. With sizes of up to one thousand kilometres in diameter, the brightness of these rocky objects may vary considerably in just a few minutes. They move very quickly with respect to the stars - they have been dubbed 'vermin of the sky' because they often appear as trails on long exposure images. This elusiveness explains why their actual number and size distribution remains uncertain. Most of the almost 40,000 asteroids catalogued so far (1) orbit the Sun forming the 'main asteroid belt', between Mars and Jupiter, too far to pose any threat to Earth. However, space-watchers do keep a closer eye on another category of asteroids, the 'Near Earth Asteroids' or 'NEAs', which are those whose orbits cross, or are likely to cross, that of our planet.

The ISO Deep Asteroid Search (IDAS), the first systematic search for these objects performed in infrared light, focused on main belt asteroids. Because it is impossible to simply point the telescope at the whole main belt and count, astronomers choose selected regions of the belt and then use a theoretical model to extrapolate the data to the whole belt.

Edward Tedesco (TerraSystems, Inc., New Hampshire, United States) and Francois-Xavier Desert (Observatoire de Grenoble, France) observed their main belt selected areas in 1996 and 1997 with ESA's ISO. They found that in the middle region of the belt the density of asteroids was 160 asteroids larger than 1 kilometre per square degree - an area of the sky corresponding to that covered by four full moons as seen from Earth. Then, a model developed by Tedesco and the astronomers Alberto Cellino and Vincenzo Zappala (Osservatorio Astronomico di Torino, Italy), allowed them to estimate the whole asteroid population in the main belt: between 1.1 million and 1.9 million asteroids with a diameter larger than 1 kilometre.

"If you consider the average value of 1.5 million asteroids, the ISO result is about twice as high as estimated by two other recent studies in visible light," Tedesco says.

The study by Durda et al., published in 1998, gave an estimate of about 860 000 asteroids larger than 1 kilometre in the main belt. In 2001, Ivezic et al. obtained an even lower figure of 740 000 asteroids based on preliminary data from the Sloan Digital Sky Survey.

Why the discrepancy?

The fact that visually dark objects - such as asteroids - are better detected in the infrared might explain the discrepancy between visible and infrared results. For an optical telescope, the brightness of an asteroid depends on the visible light it reflects from the Sun. Observations with infrared telescopes, on the other hand, detect the 'heat' of the asteroid, which does not depend that much on the reflected sunlight, but on the absorbed sunlight.

As an example, let's consider two spheres of the same size, and located close to each other in the asteroid belt, one of which reflects ten times as much of the visible light striking it as the other. As seen by an optical telescope, the sphere which reflects more appears ten times brighter than the other sphere which might be even invisible. However, for ISO both spheres would be visible. Actually, the 'dark' sphere would appear brighter in the infrared because it would have a higher temperature (as it has absorbed more sunlight).

Expert's 'best estimate'

Tedesco assumes that both visible and infrared searches might have their own biases, which is the reason why the given results have an error margin. Considering both the visible and infrared results, the 'best estimate' would be "1.2 million asteroids larger than 1 kilometre in the main belt, give or take 500,000," Tedesco says.

The best strategy for finding the asteroid size distribution, according to this expert, is to combine near-simultaneous observations at infrared and visible light. "They provide different kinds of information and therefore play a complementary role in the search for the asteroid population's size distribution," he says.

The 'impact hazard'

A better knowledge of the number and size distribution of asteroids in the main belt is essential to understand the population of Near Earth Asteroids (NEAs), since most NEA are believed to be former main belt asteroids. In the main belt there are four 'special' regions where Jupiter's gravitational influence is especially disruptive; originally, most asteroids currently known as NEA suffered collisions which resulted in them ending up in one of those four key regions, and because of Jupiter's gravitational influence their orbits quickly evolved into Earth-crossing orbits. Therefore, by studying the asteroids near these so-called 'source regions' in the main belt astronomers can learn about NEA. About 500 NEAs have been found so far, and none of them pose any threat to Earth in this century.

The generally accepted impact rate by objects larger than 1 kilometre in diameter is one every 100,000 to 300,000 years. The new 'best estimate' of about 1.2 million asteroids of 1 kilometre or larger in the main belt will not change the current estimates of impact hazard, the IDAS astronomers say; at least not yet.

"IDAS has contributed to our knowledge of main belt asteroids. And, although we did not observe any NEAs, the ISO data will be used to improve our knowledge regarding asteroids currently near the NEA source regions. This, in turn, will allow us to better understand the population characteristics of the NEAs and so ultimately enable us to refine our estimates of the NEA impact frequency and the magnitude of the impact hazard," Tedesco says.

Note to editors
The European Space Agency's infrared space telescope, ISO, operated from November 1995 until May 1998. As an unprecedented observatory for infrared astronomy ISO made nearly 30 000 scientific observations.

This note is based on the paper "The Infrared Space Observatory Deep Asteroid Search" by Edward F. Tedesco and Francois-Xavier Desert, published in the April 2002 issue of The Astronomical Journal.

(1) 39,462 main belt asteroids were catalogued as of 28 March 2002. This number increases by about 2,000 per month at present.

Eros as Evaluated from Near Shoemaker Observations

Enough time has passed since Near Shoemaker's orbiting of and landing on Eros for scientists to evaluate the results.

A 8-km-wide crater, with the suggested name of Shoemaker, may be the source of many of the over 30,000 boulders that have been catalogued. 44% of the boulders are within the crater itself, and most of the boulders along Eros's equator may also come from the blast that created Shoemaker. But boulders have not been found from the two other large craters on Eros: Himeros and Psyche. It has been suggested (in an article by Peter Thomas and others in the September 27 issue of NATURE) that the boulders may never have been thrown out or had been buried or eroded.

SCIENCE Magazine's planetary-science expert, Richard A. Kerr, summarized (14 December 2001 issue) the attempts to link Eros with types of meteorites found on Earth. He summarized papers from the December 2001 issue of METEORITICS & PLANETARY SCIENCE. Eros is very uniform across its surface, indicating that it was undoubtedly never separated into rock and metal. Its ground-based spectrum resembled that of an ordinary chondrite, which is thought to be primordial material. But the reddish tint shared by a group of asteroids seemed to indicate that it might be differentiated, which would mean that Eros isn't an ordinary chondrite after all. Perhaps, though, the color is only a thin surface layer.

Kerr describes how the remote-sensing apparatus, especially the x-ray/gamma-ray spectrometer, on Near Shoemaker was supposed to resolve the problem. A thiin coating of red material wouldn't be expected to alter the composition that would be detected by examining x-rays and gamma-rays emitted. But the results are inconclusive. Neither chondrites nor achondrites match the iron content or other results. The voting is on the side of chondrites, but the conclusion is not robust.

One limitation, caused by the low-level of funding for missions within NASA's Discovery program, is that the gamma-ray spectrometer was mounted within the spacecraft's body instead of at the end of a long boom. Thus the background noise from cosmic rays hitting the spacecraft was too high. Perhaps the sample return hoped for in 2007 from the Japanese MUSES-C mission will give us sufficiently accurate data to categorize asteroids of Eros's type.

NASA "Dawn" Mission Slated for Asteroid Tour

NASA announced that "Dawn" will be one of the two new Discovery missions slated for launch in 2006. The ion-propulsion powered mission will make a nine-year journey to orbit Vesta and Ceres, the two most massive asteroids known. The asteroids, located in the main asteroid belt between Mars and Jupiter, have recorded what the early solar system was like when the terrestrial planets formed.

"This is terrific," said University of Arizona astronomer Mark V. Sykes, a scientist on the Dawn mission. "We've been proposing advanced propulsion technology missions for many years now, and it's great to have an opportunity to actually fly one."

The Dawn mission is led by Christopher T. Russell of the University of California - Los Angeles. The project is managed by NASA's Jet Propulsion Laboratory, Pasadena, Calif. Orbital Sciences Corp., Dulles, Va., will develop the spacecraft.

"Dawn will study the conditions and processes of planet formation during the earliest epoch of our solar system by orbiting and studying Ceres and Vesta," Sykes said. Dawn builds on decades of asteroid and meteorite studies, he added.

"Ceres is more than a quarter the diameter of the moon, is water-rich, and has retained its primitive composition and condition. Vesta, on the other hand, was dry, heated to the point of melting, and preserves a record of its subsequent differentiation.

"Almost all asteroids that we observe today are the fragments of larger asteroids like Vesta and Ceres that were destroyed by ancient catastrophic collisions. By studying Vesta and Ceres, we gain a much greater understanding of how these modern fragments were once put together," Sykes said.

Actually, scientists already have pieces of one of the asteroids within reach - as meteorites that landed on Earth.

"Cratering collisions have knocked off pieces of Vesta, which have been recovered as meteorites. They provide us with detailed information on geochemical processes that have occurred within specific sites on Vesta from the time of its formation at the beginning of the solar system," Sykes said.

"Going to Vesta will give us the big picture within which these hand-sized pieces fit. It will be like going from studying bits of hair, nail, and bone to seeing and studying the entire animal up close for the first time," he added.

Sykes, an associate astronomer at Steward Observatory, specializes in the study of asteroids, comets and interplanetary dust. He is the Chair of the Division for Planetary Sciences of the American Astronomical Society.

Dawn will carry a framing camera and mapping spectrometer provided by the German Aerospace Center, DLR, Institute of Sensor Technology and Planetary Exploration in Berlin; a laser altimeter experiment provided by the NASA Goddard Space Flight Center in conjunction with the Massachusetts Institute of Technology; a gamma ray/neutron spectrometer from the Department of Energy's Los Alamos National Laboratory; and a magnetometer provided by UCLA.

Ion engines will power the spacecraft to the asteroid belt, where it first orbits Vesta in an ever-tightening circle and then spirals outward and heads to its rendezvous with Ceres. The spacecraft will orbit as high as 800 kilometers (500 miles) to as low as 100 kilometers (about 62 miles) above the surface of the asteroids. Flybys of more than a dozen other asteroids along the way are planned.

NASA also selected the Kepler mission for 2006 launch. Kepler, a spaceborne telescope, will search for Earth-like planets around stars beyond the solar system.

"Kepler and Dawn are exactly the kind of missions NASA should be launching, missions that tackle some of the most important questions in science yet do it for a very modest cost," said Edward Weiler, associate administrator for space science at NASA Headquarters in Washington. "It's an indicator of how far we've come in our capability to explore space when missions with such ambitious goals are proposed for the Discovery Program of lower-cost missions rather than as major projects costing ten times as much."

Information about Dawn and images are available on the Internet at
http://www-ssc.igpp.ucla.edu/dawn/

Sloan Digital Sky Survey lowers estimate of asteroid impact risk

Princeton, N.J. -- The odds of earth suffering a catastrophic collision with an asteroid over the next century are about one in 5,000, which is less likely than previously believed, according to research published this month.

Astronomers using data from the Sloan Digital Sky Survey found that the solar system contains about 700,000 asteroids big enough to destroy civilization. That figure is about one-third the size of earlier estimates, which had put the number at around two million and the odds of collision at roughly one in 1,500 over a one hundred-year period.

"Our estimate for the chance of a big impact contains some of the same uncertainties as previous estimates, but it is clear that we should feel somewhat safer than we did before we had the Sloan survey data," said lead researcher Zeljko Ivezic of Princeton University.

The results were published in the November issue of the Astronomical Journal.

The new estimate draws on observations of many more asteroids, particularly small faint ones, than were available in previous impact risk estimates, said Ivezic. The ability to detect faint objects in large numbers is a hallmark of the Sloan survey, a multi-institutional collaboration that is mapping one-quarter of the sky. While its main purpose is to look at objects outside our galaxy, the survey also records images of closer objects that cross the view of its telescope, which is located at the Apache Point Observatory in New Mexico.

The survey data also allowed the astronomers to gauge the size of asteroids with improved accuracy, which required categorizing the objects by their composition. Asteroids with a surface of carbon -- looking like giant lumps of coal -- are darker than those made of rock. A small rocky asteroid therefore looks just as bright as a much larger one made of carbon.

"You don't know precisely the size of an object you are looking at unless you know what type it is," Ivezic said, noting that the Sloan survey provides information about the color of objects, which allows astronomers to distinguish between carbon and rock.

Based on observations of 10,000 asteroids, the researchers estimated that the asteroid belt contains about 700,000 that are bigger than one kilometer (six-tenths of a mile) in diameter, which is the minimum size thought to pose a catastrophic risk to humans and other species. The asteroid belt is the source for a smaller group of asteroids called "near- earth objects," which have broken from the belt and have the potential to collide with earth. Although they did not specifically observe near earth objects, the researchers believe that their census of main belt asteroids reveals the likelihood of collisions with similarly sized near-earth asteroids.

Ivezic noted that the new impact risk estimate, like most previous ones, relies on assumptions about a single event 65 million years ago when a 10-kilometer asteroid collided with earth and killed the dinosaurs. The researchers assumed that such impacts occur on roughly 100 million-year intervals and used that statistic to calculate the impact odds for the more common asteroids of smaller sizes. This calculation required knowing how much more common one-kilometer asteroids are than 10-kilometer ones, which was hard to measure before the Sloan data was available.

"There is a lot of uncertainty when you have a sample of only one event," Ivezic said, referring to the dinosaur-killing impact. "But this is the best information we have."

Previous studies could detect only asteroids five kilometers or larger, so astronomers had to extrapolate to estimate the number of smaller ones, said Ivezic. The Sloan researchers found that this approach produced high estimates. When they could actually observe them, the small asteroids were not as plentiful as had been expected from observations of large ones.

The reason for this reduced number of smaller asteroids is an open question, which, if answered, may offer important clues about the history of the solar system and the factors that shaped the asteroid belts, said team member Serge Tabachnik of Princeton.

Another valuable piece of information for scientists is the observation that the rock and carbon asteroids are separated into two bands, said co-author Tom Quinn of the University of Washington. The heart of the rocky asteroid belt is 260 million miles from the sun, while the other is 300 million miles from the sun. The sun and earth, by comparison, are 93 million miles apart.

The astronomers attribute much of the success of the study to software that automatically identifies asteroids from among the millions of images observed by the Sloan survey. Independent tests by Mario Juric from the University of Zagreb, Croatia, have shown that the Sloan software finds at least nine of every ten asteroids.

"We have only five minutes to follow the motion of an asteroid as it passes in front of the telescope," said Robert Lupton, a Princeton researcher who developed the software for automatic detection of asteroids. "But we have found that we detect them very efficiently and reliably." Lupton said the team benefited greatly from software for finding the positions and relative movements of objects, developed by Jeff Pier, Jeff Munn, Robert Hindsley and Greg Hennessy of the U.S. Naval Observatory.

"The Sloan study is a major advance in our understanding of the gross asteroid belt structure," said Robert Jedicke, an asteroid expert at the University of Arizona. "Their determination of the Earth impact rate for killer asteroids agrees with soon-to-be-published results based on data from the Spacewatch Project at the University of Arizona." The Arizona team based its risk estimate on a study of near-earth objects, rather than main belt asteroids.

The Sloan Digital Sky Survey (www.sdss.org) is a joint project of the University of Chicago, Fermilab, the Institute for Advanced Study, the Japan Participation Group, the Johns Hopkins University, the Max-Planck-Institute for Astronomy, the Max-Planck-Institute for Astrophysics, New Mexico State University, Princeton University, the United States Naval Observatory and the University of Washington.

Funding for the survey has been provided by the Alfred E. Sloan Foundation, the participating institutions, the National Aeronautics and Space Administration, The National Science Foundation, the U.S. Department of Energy, the Japanese Monbukagakusho and the Max Planck Society.

Observations reveal curiosities on the surface of asteroid Ceres

SwRI Press Release, November 1

Boulder, Colorado -- October 19, 2001 -- An international team led by scientists at the Southwest Research Institute (SwRI) has discovered some curious properties of the largest asteroid, Ceres. The astronomers observed Ceres with the Hubble Space Telescope (HST) at ultraviolet wavelengths using a resolution higher than previously attained. The resulting images are the first to resolve detail on the surface of Ceres and show features as small as 50 kilometers across.

Led by Principal Investigator Dr. Alan Stern of SwRI, the team detected a dark spot on the surface of Ceres, which it nicknamed "Piazzi" in honor of the discoverer of Ceres. "Although we can't determine the nature of the spot with these data, whether it is an area of different coloration or possibly a crater from an impact by another asteroid, it is pretty big," says Dr. Joel Parker, also of SwRI, who led the team in the analysis of the images. "The Piazzi feature has a diameter of about 250 kilometers, which is more than a quarter the size of Ceres. If it resulted from an impact, the object that hit Ceres would have been about 25 kilometers across. It must have really shaken things up."

The high-resolution images allowed the team to refine measurements of Ceres. Although Ceres is the largest known asteroid -- estimated to contain more than one-third of the total mass of all other asteroids combined -- researchers still dispute its size, even after 200 years of observations. The new HST measurements indicate that the asteroid is slightly flattened, with a diameter ranging from 930 to 970 kilometers. Spinning objects can have a flattened or "squashed" shape depending on how big they are, how fast they spin, and what kind of material they are made of. However, the amount of flattening seen on Ceres is more than expected and may indicate that the inner structure is not as homogeneous as previously assumed.

"These results are very tantalizing," says Stern. "What we need to be definitive are observations with better resolution and frequent enough to follow Ceres through a nine-hour rotation period to track surface features. This 'movie' would allow us to finally map the surface of Ceres and figure out what the Piazzi feature is." The team has already proposed such an experiment with a new instrument to be installed on HST next year.

The analysis of the Ceres images will be published in the January 2002 issue of The Astronomical Journal. Authors include researchers from SwRI, the Massachusetts Institute of Technology, Cornell University, the University of Arizona, and the Observatoire Midi-Pyrenees in France.

In addition to being the largest asteroid, Ceres was also the first asteroid to be discovered. In the latter part of the 18th century, astronomers noted a regular spacing in the planets of the solar system, but with a gap between Mars and Jupiter where they expected to find a planet. On January 1, 1801, the Sicilian astronomer Giuseppe Piazzi at the Palermo Observatory discovered a moving object in the region. Researchers at the time assumed that this object, Ceres, was the missing planet. However, early observations indicated that Ceres was too small to be a planet, and as more such objects were discovered in the region, they became known as "asteroids" or "minor planets." Ceres orbits the sun once every 4.6 years at a distance of 41 million kilometers, and it spins on its axis once every nine hours.

EDITORS: The Ceres images are available for viewing and download at www.swri.org/press/ceres.htm.

The Source of Rocks on Eros

Cornell Press Release, September 28, 2001

ITHACA, N.Y. -- The first detailed global mapping of an asteroid has found that most of the larger rocks strewn across the body were ejected from a single crater in a meteorite collision perhaps a billion years ago.

"One big impact spread all this debris," says Peter Thomas, senior researcher in Cornell University's Department of Astronomy. "This observation is helping us start answering questions about how things work on the surface of an asteroid."

Thomas' report on the crater -- which has the proposed name of Shoemaker -- as a major source of ejected rocks on asteroid 433 Eros appears in the latest issue (Sept. 27) of the journal Nature. Thomas' fellow authors are Joseph Veverka, professor of astronomy at Cornell; Mark Robinson of Northwestern University and Scott Murchie of Johns Hopkins University. The paper is one of three detailing the first findings from the controlled landing of the spacecraft NEAR-Shoemaker on the surface of Eros on Feb. 12, 2001.

Before the landing, the spacecraft had orbited Eros for a year, taking thousands of high-resolution images of the 21-mile-long asteroid. From the global map of the surface that was assembled, Thomas and his colleagues were able to count 6,760 rocks larger than about 16 yards across (15 meters) strewn over the asteroid's 434 square miles (1,125 square kilometers). They found that nearly half (44 percent) of these rocks were inside the Shoemaker crater, positioned near one end of the potato-shaped asteroid. And most of the rocks of this size along the asteroid's equator appear to have been ejected from Shoemaker, Thomas says.

"We know they came from Shoemaker because the mapping of the geography of the pattern [of the rocks] on the surface closely matches the predicted paths from the one impact event that made Shoemaker," he says. Eros is estimated to be about 4 billion years old, probably the remnant of a larger asteroid broken up by a collision with another asteroid. Perhaps a billion years ago, Eros itself was struck by an object -- a meteorite or small comet -- creating a crater nearly 5 miles (7.6 kilometers) wide and shattering into rocks of all sizes. Some of these rocks "went straight up and straight down," says Thomas. Most of the remainder traveled as far as two-thirds of the way around the rotating asteroid in either direction (the asteroid rotates once every 5 1/4 hours), finally coming to rest on the surface. The mystery posed by the Eros maps for the researchers is why the same thing didn't happen with two other large craters on Eros: Himeros, on the body's convex side, and Psyche, on the concave side. Either the rocks have been buried, have been eroded or weren't made in the first place, says Thomas.

One of the big surprises from the maps, Robinson reports in his Nature paper, is that Eros' surface appears to have a global cover of "loose fragmental debris." The surface appears to be blanketed with a fine material, some of which has created flat deposits, particularly in depressions, such as craters. These fine deposits, Robinson's paper reports, appear to have been "sorted" from the upper portion of the asteroid's regolith, or soil.

These so-called "ponded" deposits were visible in the final images transmitted by the spacecraft before it hit the asteroid's surface. Indeed, in his paper Veverka reports, "A strong argument is that the last image shows that the spacecraft landed on or within a few meters of a pond, a landform known to occur predominantly on the floors of craters."

How has this sorting occurred? Robinson's paper postulates an electrostatic effect, similar to that indicated on the moon's surface by the Surveyor spacecraft. Particles can build up photoelectric charges with long exposure to the sun, and this charge might separate out finer particles, says Thomas. But he concedes, "This requires a lot of assumptions, and does not explain all the mechanisms."

The big question for researchers is: Do these observations of the surface mechanics of Eros indicate that similar processes are under way on other astronomical bodies? In his paper, Veverka notes it is difficult to make comparisons because no other such distant body has been so closely mapped. There are high-resolution views of the asteroids Gaspra and Ida and of Phobos, a satellite of Mars. Phobos, he writes, does show groupings of rocks in the vicinity of the crater Stickney that are comparable to those on Eros. "Nothing comparable to the flat 'pond' deposits has been noted on Gaspra, Ida or Phobos, even though Phobos coverage is certainly adequate to show such features if they were present," he writes. In making his assessment of rock distribution on Eros, Thomas counted about 30,000 rocks. He was able to do this by using software created by Cornell analyst Jonathan Joseph. The software allows a researcher to mark a rock in an image, then calculate from a shape model where the rock is and its size and then to record this information in a data file.

Thomas's report in Nature is titled "Shoemaker Crater: A major source of ejecta on asteroid 433 Eros." Veverka's report, which has several co-authors, is titled "The landing of the NEAR-Shoemaker spacecraft on asteroid 433 Eros." (Veverka was the principal investigator on the multispectral imager, or camera, and the NEAR infrared spectrometer, two of the five instruments on board the spacecraft.) Robinson's report, co-authored by Thomas, Veverka, Murchie and Brian Carcich of Cornell, is titled "Morphology, Distribution and Origin of Ponded Deposits on Eros." The research was supported by NASA.

The web version of this release, with accompanying photos, may be found at
http://www.news.cornell.edu/releases/Sept01/Eros.Nature.deb.html

Well Preserved Meteorite Yields Clues to Carbon Evolution in Space

Arizona State University Press Release, 8/25/2001