ITHACA, N.Y. -- So many craters, so little asteroid.
Cornell University astronomer Joseph Veverka and a team of scientists arereleasing the first close-up images of a little-known C-class asteroid, 253Mathilde, to be published exclusively in the journal "Science" (Dec. 19).Until now, astronomers have been able to do little but gaze throughtelescopes and observe the minor planet, discovered 112 years ago. On June27 of this year, the Near Earth Asteroid Rendezvous (NEAR) spacecraftpassed within 1,212 kilometers of Mathilde and took images of the asteroid.Scientists didn't expect to find the minor planet so densely pocked withcraters and so porous, as it is made mostly of carbonaceous chondrite.
"Mathilde is very porous, and we still don't know if it was formed that wayoriginally," said Veverka. "This is the first time anyone has ever lookedat an asteroid like this and we were surprised at how 'underdense' it is onthe inside."
After reviewing 534 frames of images taken with a variety of equipmentduring the close flyby in June, scientists were surprised to find so manylarge craters packed so tightly on the relatively small surface ofMathilde. This means that large objects have been able to strike theasteroid's surface without destroying it, Veverka said. "Hitting Mathildeis like hitting a Styrofoam cup or packing material," he said.
"Even more remarkable than the simple existence of these large craters isthe degree to which their rim crests and basic shapes seem minimallyaffected by subsequent large impacts," scientists write in the articledescribing the images, "NEAR's flyby of Mathilde: Images of a C Asteroid."The article's authors include Veverka and Cornell scientists Peter Thomas,senior research associate; Ann Harch, research support specialist; BethClark, research associate; James F. Bell III, senior research associate;Brian Carcich, systems programmer, and Jonathan Joseph, programmer, all inthe astronomy department. In addition to the Cornell members of the NEARteam, astronomers from the Southwest Research Institute, Boulder, Colo.;Northwestern University; Space Science Systems, San Diego, Calif.; theUniversity of Maryland, College Park; and Johns Hopkins University'sApplied Physics Laboratory participated in the image analysis and thewriting of the paper.
Currently, the NEAR spacecraft is on its way to the S-class asteroid 433,named Eros. Enroute to Eros, NEAR flew by Mathilde on June 27 to gatherinformation. At a flyby speed of 9.93 kilometers a second, the spacecraftspent about 25 minutes relatively close to the asteroid. During its closestapproach, the spacecraft took 144 high-resolution images of the minorplanet's irregular shape and heavily cratered surface.
The slow-rotating Mathilde resides in the asteroid belt, which containsthousands of minor planets between Mars and Jupiter. At the time theimages were taken, Mathilde was approximately 203 million miles from Earth.Mathilde rotates once every 17.4 days; only two other known asteroidsrotate more slowly: 288 Glauke and 1220 Crocus.
"That's one of the other mysteries of Mathilde," said Veverka . "It is sosluggish and we still don't know why."
Launched aboard a Delta II rocket on Feb. 17, 1996, the NEAR spacecraftmission cost NASA about $122 million and is the first of NASA's DiscoveryMissions, which include a series of small-scale spacecraft designed toproceed from development to flight in under three years for a cost of lessthan $150 million each. In February 1999, the spacecraft will rendezvouswith Eros and orbit that minor planet for about a year.
The above story is based on materials provided by Cornell University. Note: Materials may be edited for content and length.
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