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.