Wednesday, June 10, 2009

Rock


Antoine de Saint-Exupery's Little Prince lived on an asteroid scarcely larger than himself. As readers of the childhood classic will remember, his companions were a sheep and a rose, and some baobab seedlings that he carefully weeded, lest they grow into giant trees that would split his tiny world. The asteroid had three volcanoes, two of which were active, and all of which the Little Prince assiduously cleaned.

A charming little world, but of course scientifically implausible. An asteroid the size of the Little Prince's would not have enough internal heat to cause volcanic activity, nor enough gravity to hold an atmosphere. Water too would be absent, and surface temperatures would be either too hot or too cold for comfort.

Where children fly in their imaginations, NASA takes us in reality. On Feb. 17, 1996, the NEAR (Near Earth Asteroid Rendezvous) spacecraft was launched on voyage to the asteroid, Eros.

Eros is not so far away. It doesn't circle in the asteroid belt between Mars and Jupiter, but on an eccentric orbit that takes it nearly as close to the sun as Earth, and out just past Mars. Eros was the first near-Earth asteroid to be discovered, and the second biggest. It is a potato-shaped chunk of rock about the size of Martha's Vineyard. Not as small as The Little Prince's world, but small enough to circumnavigate in a brisk day's walk.

NEAR's journey to Eros took four years. A three-year journey was planned, but the first attempt to put the spacecraft into orbit around the asteroid failed. An extra year's travel gave engineers time to trim their skills and calculations. And it allowed NEAR to rendezvous with an asteroid named for the god of love on Valentine's Day 2000.

An object as small as Eros doesn't have much gravity to hold a spacecraft in orbit. The Little Prince would weigh about an ounce on Eros, and he could launch a stone into space with a swing of his arm. The orbiting NEAR was bound to Eros by a slender gravitational thread, and slipping the spacecraft into the thrall of the asteroid was a tour de force of remote navigation.

NEAR orbited just above the lumpy surface of Eros for a year, sending back stunning pictures of a gray and lifeless world without air or water. It was then crashed onto the surface, surviving well enough to send back an analysis of the surface debris.

No sheep or baobabs, no volcanoes, but lots of impact craters and scattered boulders. We catch a glimpse into the early history of the solar system, when scattered dust and gas was gathered into larger and larger chunks of rock, some of which would eventually coalesce to form the planets, and other potato-shaped clumps destined to drift through space like gloomy Flying Dutchmans.