| 30 June 1997
For Immediate Release
Asteroid Mathilde Reveals Her Dark Past
More than 100 years after her discovery, asteroid 253 Mathilde has been sharing her secrets with scientists in the Science Data Center at The Johns Hopkins University Applied Physics Laboratory in Laurel, Md. A 25-minute flyby of the asteroid by the Near Earth Asteroid Rendezvous (NEAR) spacecraft on June 27 has resulted in spectacular images of a dark, crater-battered little world assumed to date from the beginning of the solar system.
The Mathilde flyby is the closest encounter with an asteroid to date and the first with a C-type asteroid. The asteroid's mean diameter was found to be 33 miles (52 kilometers), which is somewhat smaller than researchers originally estimated. A study of the asteroid's albedo (brightness or reflective power) shows that it reflects three percent of the sun's light, making it twice as dark as a chunk of charcoal. Such a dark surface is believed to consist of carbon-rich material that has not been altered by planet-building processes, which melt and mix up the solar system's original building block materials.
"The Mathilde encounter was one of the most successful flybys of all time," says Dr. Robert W. Farquhar, of the Applied Physics Laboratory, who serves as the NEAR Mission Director. "We got images that were far better than we thought possible, especially since the spacecraft was not designed for a fast flyby." Only the multispectral imager, one of six instruments on the spacecraft, was used during the flyby in order to conserve power provided by solar-powered panels. The spacecraft was 2.0 AU from the sun (AU=the mean distance between the Earth and sun: 93 million miles, or 149 million kilometers), too far to provide power for NEAR's other instruments.
"Even though this was a very difficult undertaking," says Dr. Stamatios M. Krimigis, head of the APL Space Department that managed the program for NASA, "the NEAR Operations Team was so well prepared there was little doubt that it would succeed; not only that but this was the smallest operations team of any planetary encounter, proving that the Discovery Program paradigm of 'smaller, faster, cheaper' is alive and well."
Although Mathilde proved to be rounder than asteroids such as Gaspra and Ida, Dr. Joseph Veverka of Cornell University, Ithaca, N.Y., who leads the mission's imaging science team, says, "Mathilde turned out to be more irregularly shaped than most of us expected. The degree to which the asteroid has been battered by collisions is astounding. At first glance there are more huge craters than there is asteroid."
The imager found at least five craters larger than 12 miles (20 kilometers) in diameter just on the lighted side of the asteroid. Scientists wonder how the asteroid can remain intact after having been hit by this many projectiles, each probably a few kilometers wide.
The craters reveal evidence of the asteroid's makeup. "We knew that C-asteroids are black, but we did not expect their surfaces to be as uniformly black and colorless as Mathilde's surface turned out to be," Veverka says. "This global blandness is an important clue telling us that asteroids such as Mathilde are made of the same dark, black rock throughout because none of the craters, which are punched deep into the asteroid, show evidence of any other kind of rock." Such uniformity seems to confirm that C-type asteroids are in fact pristine samples of the primitive building blocks of the larger planets.
Dr. Donald K. Yeomans of the Jet Propulsion Laboratory, Pasadena, Calif., who heads up the radio science team formed to determine Mathilde's mass says, "Mathilde is an asteroid with a very tortured past." By determining the bulk density of the asteroid researchers will have a clue to how it was formed. A composite of objects would have a lower density than a solid chunk from a larger asteroid. Data analysis to determine density will not be complete until later this year, but Dr. Yeomans says, "Preliminary results suggest that Mathilde is much less dense than we had thought."
The Mathilde flyby met all its initial goals: getting a clear image of the sunlit side of the asteroid, getting color images that will give clues to the types of rock that make up the asteroid, and getting images that will help researchers determine if Mathilde has any moons. In the next month scientists expect to complete initial analysis of their data and have improved measurements of Mathilde's volume, mass, and density.
One mystery that remains is Mathilde's extraordinarily slow (17.4 days) rotation rate. Its collision history could be a factor, but more research needs to be done to determine what role such collisions have played. The search for Mathilde moons continues; none has yet been discovered.
The next major event of the NEAR mission will occur on July 3, when the spacecraft's bi-propellant engine is fired to head NEAR back toward Earth. This deep-space maneuver will be the first time the engine has been fired and will keep both engineers and scientists in suspense for 11 minutes before they know if the maneuver was successful. An Earth gravity-assist maneuver on Jan. 23, 1998, will send the spacecraft toward its primary target, asteroid 433 Eros. NEAR will reach Eros nearly a year later and will remain locked in orbit around the asteroid until Feb. 6, 2000, when the mission ends.
Commenting on the success of the Mathilde flyby soon after the first images were received, Dr. Wesley T. Huntress Jr., NASA Associate Administrator, Office of Space Science, said, "It's today the Discovery Program really begins. NEAR was the first of our Discovery missions to be launched and it's the first to return scientific results." He said the APL-led team that managed the NEAR program proved the concept behind the Discovery Program: that exciting planetary missions can be done at low cost, in a short time.
The NEAR spacecraft was launched Feb. 17, 1996, from Cape Canaveral Air Station in Florida. NEAR Science Team Group Leaders are: Joseph Veverka, Cornell University; Jacob I. Trombka, NASA/Goddard Space Flight Center; Mario H. Acuna, NASA/GSFC; Maria T. Zuber, MIT and NASA/GSFC; and Donald K. Yeomans, NASA/Jet Propulsion Laboratory. Andrew Cheng, JHU/APL, is the Project Scientist. Mission operations have been the responsibility of The Johns Hopkins University Applied Physics Laboratory.
For more information contact
Helen Worth, JHU/APL Office of Public Affairs. Phone: 240-228-5113 or fax: 240-228-6123 or Donald Savage, NASA Headquarters Office of Space Science. Phone 202-358-1547; e-mail: email@example.com; or fax: 202-358-3093.
Mathilde flyby updates can be obtained on the Mathilde homepage at: http://sd-www.jhuapl.edu/NEAR/Mathilde. Photographs of the first Mathilde images, the NEAR spacecraft, and the NEAR launch are available upon request.