The pictures that immediately began flooding into The Johns Hopkins University Applied Physics Laboratory, which is responsible for mission operations, offer the closest look ever at an asteroid in its pristine state. (Meteorites are most likely chips of asteroids, and offer some information about them, but they have been fragmented and heated by the time they reach the earth's surface.) Using a combination of radio tracking and imaging data, as well as earth-based radar techniques, researchers will for the first time make an accurate measurement of the bulk density of 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 Donald K. Yeomans of NASA's Jet Propulsion Laboratory, who heads up the radio science experiment.
Asteroids intrigue scientists because they are leftovers from the formation of the solar system some 4.6 billion years ago. Some of them also cross paths with the earth, occasionally leading to violent impacts. Most of the asteroids are clustered in a vast, doughnut-shaped "belt" between the orbits of Mars and Jupiter, though others stray within the orbit of Mercury and well into the outer solar system. One small body even circles the sun in step with the earth, making it a kind of second moon.
So far, astronomers have discovered more than 7,000 asteroids, and several hundred more are identified each year. Ceres, the largest known asteroid, is 914 kilometers in diameter, but the vast majority of them are far smaller. There are only 26 known asteroids larger than 200 kilometers in diameter. Altogether, there are probably hundreds of thousands of these objects, most of them too small to be seen from the Earth. Scientists estimate that the total mass of all asteroids is equivalent to a solid body approximately 1,500 kilometers (roughly 900 miles) across--less than half the diameter of the moon.
Until just a few years ago, nobody even knew what an asteroid looked like. Then the Galileo spacecraft snapped pictures of two bodies located in the main belt--Gaspra and Ida. Both of these are "S-type" bodies, meaning that they are stony in composition.
Now it is Mathilde's turn. This flying rock was discovered in 1885 and is believed to be named to honor the wife of astronomer Moritz Loewy, who at the time was vice director of the Paris Observatory. When the NEAR flyby was announced in 1995, researchers then began a crash course of study to prepare for the recent encounter. Observations of the asteroid's changing brightness have since revealed an unusually long rotation period of 418 hours, or approximately 17 days. Using data obtained by the Infrared Astronomy Satellite, astronomers established that Mathilde is approximately 61 kilometers (38 miles) in diameter, substantially larger than either Gaspra (16 kilometers) or Ida (33 kilometers).
And unlike the other asteroids visited so far, Mathilde is a carbonaceous, or C-type, asteroid. Carbonaceous asteroids inhabit the outer regions of the asteroid belt and account for more than 75 percent of the known asteroids. Their composition is broadly similar to that of the sun if it were depleted in hydrogen, helium, and other volatiles, or easily vaporized elements. C-type asteroids are thought to contain some of the most primitive (that is, unchanged from its initial state) material in the inner solar system. Mathilde is one of the darkest objects known, reflecting only 4 percent of the incoming light.
Joseph Veverka of Cornell University, who leads one of the NEAR science teams, describes Mathilde as "a black asteroid made of carbon-rich rock, believed by many to be the most primitive material left in the asteroid belt." He notes that "such material has never been studied up close by a spacecraft." The images returned by NEAR will reveal the shape, texture and colors of this astronomical relic.
The NEAR mission is part of an ongoing effort by the National Aeronautics and Space Administration to find "faster, better, cheaper" ways to explore the solar system. It is the first in a planned series of innovative, comparatively inexpensive Discovery missions. The spacecraft was completed for less than the $150 million budget cap.
After waltzing by Mathilde, NEAR will head back toward the earth in preparation for its main event. A wide swing around the sun for a "slingshot" gravity assist in January 1998 will put NEAR in an orbit that will head it toward its final meeting with Eros on January 10, 1999. Eros, named after the Greek god of love, is a strange, elongated object that measures 35 kilometers wide across its longest dimension. It has attracted particular attention because it is the largest of the "near-earth asteroids," objects whose orbits take them menacingly close to home.
NEAR's encounter with Eros will be a much more intimate affair than the spacecraft's brief flirtation with Mathilde. Over the course of a year, NEAR will orbit Eros and study its surface from an altitude as low as 24 kilometers (15 miles). The probe will end its mission February 6, 2000, with a daring, controlled landing onto Eros' surface.