12. Asteroids and meteorites

NEAR embraces Eros

On Valentine’s day 14 February 2000, the Near Earth Asteroid Rendezvous, abbreviated NEAR, spacecraft became the first to orbit an asteroid, arriving at 433 Eros after a four-year journey from Earth. The NEAR craft was the first in NASA’s Discovery Program of no-frills, scientifically focused, low-cost missions, designed to do quality science in a “faster, better, cheaper” mode. The mission took just 26 months from start to launch, at a bargain total cost of $224 million. The car-sized vehicle circled Eros for a year, landing on the asteroid on 12 February 2001, another historic first. In the meantime, NASA renamed the spacecraft NEAR Shoemaker in honor of the astronomer-geologist Eugene M. Shoemaker (1928-1997), a pioneering expert on asteroid and comet impacts.

Radio tracking of the orbiting spacecraft was used to determine the mass of Eros, which weighed in at 6.687 million billion, or 6.7 x 1015, kilograms, about one-billionth the mass of Earth. That means that most adults would weigh less than a few ounces if standing on Eros, about as much as a bag of airline peanuts. And on Eros you could jump thousands of meters high, never to return. The gravity is so slight that NEAR Shoemaker had to keep its speed down to about 5 thousand meters per hour to stay in orbit, moving about as fast as a casual bicyclist. If it moved at a faster speed, the spacecraft would escape the asteroid’s feeble gravity and move into interplanetary space.

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Eros is a warped and misshapen world, with heavily cratered expanses abutting relatively smooth areas. The asteroid’s biggest crater measures 5.5 thousand meters across, and most of the surface is peppered with smaller craters. The NEAR scientists spotted 100 thousand of them.

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Eros has also been smoothed and rounded by glancing blows during its catastrophic past. This cosmic sculpture rivals the smaller bronze and marble forms of Constantin Brancusi (1876-1957) and Henry Moore (1898-1986). Equally beautiful is the broad, curved, saddle-like depression that connects two mountains on Eros, each thousands of meters high.

Far from being a barren lump of rock, Eros has a dusty, boulder-strewn landscape. Despite its weak gravity, the diminutive asteroid has managed to hold on to about 7 thousand boulders larger than 15 meters, forced out of craters and pulled back to the surface during the relentless bombardment of its past. Some of the isolated stones are as large as a house, and up to 100 meters across. The positions of the boulders on Eros indicate that at least 3 thousand of them were scoured out of a single crater by a colliding projectile a billion years ago. Some of these boulders went straight up and straight down. Most of the reminder traveled as far as two-thirds of the way around the asteroid, in all directions, before finally coming to rest on the surface.

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Smaller rocks and a loose layer of dirty debris came into view when the NEAR Shoemaker spacecraft moved in to land on the boulder-strewn surface of 433 Eros. It took pictures as close as 125 meters above the surface, showing features as small as a golf ball. They indicate that, Eros is something between a very big rock and a planet, large enough to hold onto its pieces yet too small to lose its odd, distorted shape.

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Copyright 2010, Professor Kenneth R. Lang, Tufts University