A 25-minute flyby of asteroid 253 Mathilde, the largest asteroid ever visited by a spacecraft, is providing scientists with a spectacular look at a small planet more irregularly shaped than they imagined and heavily scarred with craters. Moving past Mathilde on June 27 - at a speed of 10 kilo meters a second (22,000 miles per hour) the Near Earth Asteroid Rendezvous (NEAR) spacecraft came within 1,200 kilometers of the asteroid and obtained hundreds of clear, color images of the asteroid's surface. The images will be analyzed to determine Mathilde's density, volume, and mass.
"Mathilde is an asteroid with a very tortured past," Dr. Donald K. Yeomans of the Jet Propulsion Laboratory (JPL) in Pasadena, Calif., announced a few days after the flyby. Yeomans directs the radio science team that is calculating Mathilde's mass. Using a combination of spacecraft radio tracking and imaging data as well as Earth-based radar imaging techniques, researchers will be able to accurately determine the bulk density of an asteroid for the first time.
A multispectral imager, one of six instruments mounted on the spacecraft, detected at least five craters as wide as 20 kilometers in diameter just on the sunlit side of the asteroid. "The degree to which the asteroid has been battered by collisions is astounding," says Dr. Joseph Veverka of Cornell University, Ithaca, N.Y., who leads the mission's imaging science team. "At first glance, there are more huge craters than there is asteroid."
Figure 1. Mathilde, seen from a distance of about 1,200 kilometers (748 miles) on June 27. Numerous impact craters, ranging from over 30 kilometers to less than 0.5 kilometers in diameter, are shown. Raised crater rims suggest that some of the material ejected from these craters traveled only short distances before falling back to the surface. Straight sections of some crater rims indicate the influence of large faults or fractures on crater formation. Figure 2. NEAR trajectory profile for Mathilde flyby. |
Although scientists knew that carbon-rich (C-type) asteroids such as Mathilde were black, they did not expect Mathilde's surface to be as uniformly black and colorless as it is. None of the craters, which are punched deep into the planet, show evidence of any other kind of rock. Such uniformity, adds Veverka, seems to confirm that C-type asteroids are pristine samples of the primitive building blocks of the larger inner planets.
During the flyby, the spacecraft was 2.0 astronomical units (AU) from the sun and 2.2 AU from Earth. (An astronomical unit represents the mean distance between Earth and the sun 149 million kilometers or 93 million miles.)
REVEALING MATHILDE
Located in the outer part of the asteroid belt, Mathilde was discovered in 1885 and is thought to be named for the wife of astronomer Moritz Loewy, then vice director of the Paris Observatory. Scientists did not pay much attention to Mathilde until the NEAR flyby was announced in 1995. By the time of the June 27 flyby, ground-based telescopes had determined that Mathilde was one of the darkest objects in the solar system, reflecting only 3 or 4 percent of the light falling on it from the sun.
"Asteroids are classified by their albedos and colors, as determined by spectrographic observation," says NEAR's mission director, Dr. Robert W. Farquhar of The Johns Hopkins University Applied Physics Laboratory. The laboratory manages the NEAR mission for the National Aeronautics and Space Administration (NASA).
The flyby allowed scientists to slightly revise their estimates of Mathilde's size, now determined to be approximately 52 kilometers across (previous ground-based measurements had suggested the asteroid was about 61 kilometers across). The reason for the asteroid's extraordinarily slow rotation rate of 17.4 days, noted before the flyby, remains a mystery. Mathilde's collision history may be a factor, but more research needs to be done to determine what role such collisions have played.
EARLY SOLAR HISTORY
Clues to the early solar system's processes and conditions are preserved on small planetary bodies such as asteroids, comets, and meteorites. As the larger terrestrial planets continued to evolve, those earliest records were altered or destroyed. Scientists believe that the asteroids closest to Earth still hold evidence that will help explain how the inner planets, including Earth, were formed.
But asteroids also intrigue researchers because they are the primary source of large objects that collide with Earth, thus influencing the evolution of the atmosphere and life on this planet. Many scientists now believe that an asteroid impact in the Yucatan peninsula created a catastrophic chain of events that eventually wiped out the dinosaurs and many other forms of life 65 million years ago. An impact in 1908 destroyed thousands of square kilometers of forest near Tunguska, Siberia. These collisions, say some scientists, may be related to the fact that the orbits of asteroids can be easily destabilized by gravitational interactions with major planets, nudging them into orbits that cross the paths of other planetary bodies.
NEAR MISSION TO EROS
Eleven years ago, when NASA was first planting the missions to Mars that began last year, some researchers proposed a parallel mission to study a near-Earth asteroid in the inner asteroid belt between Earth and Mars. Small, unmanned spacecraft sent to a near-Earth asteroid could provide an ideal first test of whether low-cost exploration programs - the hallmark of the Discovery program's "smaller, faster, cheaper" credo - could successfully yield valuable scientific data. The NEAR spacecraft's liftoff on Feb. 17, 1996, from Cape Canaveral, Fla., was the first launch in the Discovery program.
The Mathilde flyby, though highly successful, was never the priority objective of the mission. Scientists had targeted another asteroid, Eros, the first near-Earth asteroid ever discovered, for their orbital study. Only later did they discover that the planned flight path would take the probe past Mathilde as well.
In February 1999, the spacecraft will finally rendezvous with Eros. During its year-long orbit, the NEAR probe will conduct the first quantitative and comprehensive measurements ever taken of an asteroid's composition and structure, including bulk, surface, and internal properties. Eros has been identified as an S-type asteroid, a more common variety than C-types like Mathilde. S-type asteroids are believed to be composed of iron- and magnesium-bearing silicates (pyroxene and olivine) mixed with metallic nickel and iron, say Farquhar and Veverka. Scientists hope that NEAR's rendezvous with Eros will also reveal whether the asteroid is made up of solid fragments of rock with densities like those of meteorites, or of porous piles of collisionally fragmented rubble. They also hope to determine if Eros possesses a magnetic field.
