Astronomy Picture of the Day:

Asteroid 253 Mathilde's Large Craters

NEAR Mathilde


June 30, 1997

Four more images of asteroid 253 Mathilde take by the NEAR spacecraft are now available on the NEAR home page.

The new images show several craters on the asteroid's surface including one particularly large crater about 10 km deep.

Ron Baalke

Applied Physics Laboratory
Johns Hopkins University, Laurel, Maryland

June 18, 1997


The Near Earth Asteroid Rendezvous (NEAR) spacecraft is closing in fast on the main-belt asteroid 253 Mathilde, as scientists prepare for the closest-ever study of an asteroid. On June 27, starting at about 8:50 a.m. EDT, NEAR will be streaking past Mathilde at 22,000 miles per hour (10 kilometers per second), just 750 miles (1,200 kilometers) from the asteroid.

For 25 minutes NEAR will take a series of 534 images using a multispectral imager. "The flyby is an important opportunity to learn more about asteroids in general and C-type [carbon-rich] asteroids in particular," says Dr. Scott L. Murchie, Instrument Scientist from The Johns Hopkins University Applied Physics Laboratory (APL), Laurel, Md. "With the data we get we will be able to determine Mathilde's size, shape, landforms, brightness, and color properties."

Using a combination of spacecraft radio tracking and imaging data as well as Earth-based radar imaging techniques, researchers will be able to determine, for the first time, accurate bulk density for an asteroid. "The bulk density will provide clues as to how the asteroid formed and whether it is a monolithic structure or a collection of smaller fragments," says Dr. Donald K. Yeomans of NASA's Jet Propulsion Laboratory, who heads up the radio science experiment.

As NEAR reaches its closest proximity to Mathilde a planned loss of signal will keep researchers in suspense until the captured data begins to flow into APL's Mission Operations and Science Data centers at about 9:40 a.m. The first complete image is expected to be available later that afternoon.

"This is the first science data return of NASA's Discovery Program," says Dr. Robert W. Farquhar, NEAR Mission Director at the APL, where the spacecraft was designed and built. "What makes it even more special is that it is 'bonus science' because it is an add-on to NEAR's primary mission to study asteroid 433 Eros, at virtually no additional cost."

Because the Mathilde flyby was conceived after spacecraft integration had begun, the multispectral imager was not designed for a fast flyby. Still, researchers expect to get images and take measurements that will provide significant new information about Mathilde, which they can compare with data the Galileo spacecraft obtained during its flybys of Gaspra (1991) and Ida (1993) for a better understanding of asteroids.

"Mathilde is a black asteroid made of carbon-rich rock, believed by many to be the most primitive -- least changed in the last 4.5 billion years -- material left in the asteroid belt," says Dr. Joseph Veverka of Cornell University, who leads the mission's Science Team. "Such material has never been studied up close by a spacecraft."

Located in the outer part of the asteroid belt, Mathilde will also be the largest asteroid ever visited by a spacecraft. It was discovered in 1885 and is believed to be named to honor the wife of astronomer Moritz Loewy, then-Vice Director of the Paris Observatory.

Interest in Mathilde was minimal until the NEAR flyby was announced in 1995. Since then ground-based telescopes have been used to determine, among other things, that Mathilde is a C-type asteroid and is one of the darkest objects in the solar system since it reflects only 4 percent of the light falling on it. It has been determined that Mathilde is approximately 38 miles (61 kilometers) across and has an amazingly slow rotation rate (17.4 days), which intrigues scientists since only two asteroids, 288 Glauke and 1220 Clocus, have longer rotation periods.

When NEAR encounters Mathilde it will be roughly 2.0 astronomical units from the sun and 2.2 AU from the Earth (AU=the mean distance between the Earth\ and sun). The spacecraft's great distance from the sun has resulted in the decision to use the limited power supplied by the solar cells frugally by activating only one of NEAR's six instruments, the multispectral imager.

The NEAR spacecraft, launched Feb. 17, 1996, from Cape Canaveral Air Station in Florida, is the first spacecraft powered by solar cells to operate beyond the orbit of Mars. Its encounter with Mathilde occurs as the spacecraft heads back toward Earth after a wide swing around the sun for a "slingshot" gravity assist in January 1998. The maneuver bends the NEAR trajectory nearly 11 degrees out of the ecliptic to put it in an orbit that will match Eros' orbital plane. The spacecraft will reach Eros Jan. 10, 1999, orbit the asteroid for a year, and end its mission Feb. 6, 2000, with a controlled landing onto its surface.

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.

Mathilde flyby updates can be obtained on the Mathilde homepage. Photographs of the first Mathilde images will be available sometime during the afternoon of June 27. Also available are photographs of the NEAR spacecraft, launch, and artists' concepts of the rendezvous at Eros, and a video of Mathilde flyby animation with B-roll footage of spacecraft development and testing.

The NEAR Mission

The Near Earth Asteroid Rendezvous (NEAR) spacecraft was the first launch in NASA's Discovery Program. The spacecraft, which was designed and built by The Johns Hopkins University Applied Physics Laboratory, was launched Feb. 17, 1996, from Cape Canaveral, Fla. NEAR will conduct the first long-term, close-up look at an asteroid's surface composition and physical properties. The spacecraft is scheduled to arrive at asteroid 433 Eros on January 10, 1999, and will scrutinize the asteroid's surface from as close as 9 miles before the mission concludes on Feb. 6, 2000.

Mathilde is a C-type asteroid, the most common type of asteroid in the outer belt. It was discovered in 1885 and is one of the largest asteroids known. NEAR's brief Mathilde flyby will provide researchers with a wealth of information on the asteroid and lay valuable groundwork for the later study of Eros.

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