12. Asteroids and meteorites

Shape and form

For more than two centuries, no one knew what an asteroid looks like. Since they are so small and far away, the surfaces of asteroids cannot be distinguished with telescopes on Earth, although modern technology has been used to image one of the largest ones, Vesta, with some success. The shape of an asteroid can nevertheless be inferred from the form and amplitude of its periodic light variations. They have told us that most asteroids are at least slightly elongated, chipped and pummeled into irregular shapes by eons of collisions. The stretched out, irregular shapes of some asteroids have also been determined from radar observations of near-Earth asteroids that travel close enough for scientists to detect the echoes of radio waves bounced off them. During their close approach, these asteroids speed by the Earth at distances of several hundred million meters, permitting brief, high-resolution radar images before they move on and fade from view.

Fig. .. 

The radar data indicate that the overall shape of some asteroids is dominated by two irregular, lumpy components that touch each other, something like a dumbbell. Each asteroid is a double object, that is, two bodies in contact. Examples are 4179 Toutatis, pronounced too-TAT-is and 4769 Castalia. The two pieces probably merged after a past catastrophic collision of a larger body; they may have been thrown apart and subsequently came together under their mutual gravity. Or they might be two former asteroids that joined in a gentle encounter.

Fig. .. 

The inquisitive eyes of spacecraft were nevertheless required for the full resolution of the surface details of asteroids, and to turn these moving points of light into real places. The first glimpse was provided when the Galileo spacecraft flew close by two asteroids, 951 Gaspra and 243 Ida on its way to Jupiter, revealing details of these ravaged, misshapen worlds.

Fig. .. 

Another sideways glance was obtained when the NEAR Shoemaker spacecraft flew past 253 Mathilde on its way to a rendezvous with the asteroid, 433 Eros. Like Gaspra and Ida, asteroid Mathilde has survived blow after blow of cosmic impacts. Its surface is covered with the crater scars of past collisions that have disfigured the asteroidís shape, like the battered and scarred face of a professional boxer who has just lost a fight. Huge pieces have been removed from Mathilde, leaving four enormous craters tens of thousands of meters across.

(page 4 of 7)

Copyright 2010, Professor Kenneth R. Lang, Tufts University