The
NEAR Shoemaker spacecraft has scored two important firsts in the exploration
of asteroids: it's the first to orbit an asteroid, Eros, and the first to
encounter a denizen of the outer reaches of the asteroid belt, the C-type
asteroid Mathilde. In a scheme that reflects how they have historically
been a topic for astronomy, not geology, asteroids are classified into groups
based on their colours as observed through telescopes.
The two major classes of asteroids are called S-types and C-types. S-types,
whose colours are consistent with 'stony' or rocky compositions, prevail
among asteroids that orbit closer to the Sun than the mid-point of the asteroid
belt. Eros and the two asteroids encountered briefly by the Galileo spacecraft
on its way to Jupiter -- Gaspra and Ida -- are all S-types. C-types like
Mathilde have a dark grey colour consistent with a 'carbonaceous' composition,
rich in carbon compounds and other dark materials. They prevail in the outer
part of the asteroid belt. |
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In
this montage, Mathilde (at left) and Eros (at right) are shown at the same
scale, as they were imaged by NEAR Shoemaker from about 1,800 km on 27 June
1997, and 12 February 2000, respectively. Mathilde is 56 km across, and
Eros is 33 km long and 13 km wide. However, Mathilde's brightness is greatly
exaggerated for viewing purposes -- it's actually six times darker than
Eros, with about the same reflectivity as soot! |
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