The Saga of NEAR

Monday, a plucky little spacecraft called NEAR, for “Near Earth Asteroid Rendezvous,” is going to attempt to make space exploration history.

Back in February of 1996, the 805-kilogram spacecraft, a short, stubby cylinder with four solar panels forming a cross shape at one end, was launched to rendezvous and orbit the asteroid Eros, whose orbit brings it relatively close to Earth (hence, “Near Earth Asteroid”).

It’s almost didn’t make it. On Sunday, December 20, 1998, a signal was sent from Johns Hopkins University’s Applied Physics Laboratory ordering the main engine to fire to reduce the spacecraft’s speed from 3,500 kilometres per hour to 1,120 kilometres per hour. But the spacecraft detected a problem and shut down the engine after only one second. That left NEAR tumbling and going much too fast. The controllers lost contact with NEAR for 36 hours and thought they had lost the spacecraft for good.

Miraculously, though, they were able to re-establish contact and calculate a new trajectory. After chasing Eros around the sun for a year, NEAR finally achieved orbit a year ago.

This Monday, NEAR will attempt to gently drop down onto the cratered, boulder-strewn surface of Eros, currently more than 316 million kilometres from Earth, becoming the first spacecraft to land on an asteroid.

It’s probably a suicide mission; NEAR isn’t built to land, so the chance it will be able to continue transmitting data after contact is estimated at one percent or less by the scientists involved. Nevertheless, scientists hope to gather valuable data right up until the moment of impact. The digital images taken during the descent are expected to be five to 10 times better than any of the 150,000 photos of the potato-shaped, Manhattan-sized Eros NEAR has already taken, teaching us a lot about asteroids in the process.

Eros has no air and no water. It rotates once every 5.27 hours; during that time its surface temperature changes from 100 to -150 degrees C. It has very little gravity; an object that weighs 45 kilograms on Earth only weighs 28 grams on Eros.

Among the instruments NEAR turned on Eros, in addition to its cameras, were laser rangefinders and X-ray/gamma-ray spectrometers. The laser rangefinders allowed the spacecraft to map the shape of Eros with unprecedented accuracy, while the spectrometers have told us what Eros is made of.

It turns out Eros is a consolidated body rather than a rubble pile–that is, rather than being just a loose agglomeration of smaller pieces held together by gravity, it’s a single, solid chunk of metal and stone.

It’s also undifferentiated, made of material that has hardly been altered since the birth of the solar system; in fact, the proportions of rock-forming elements in Eros are similar to those found in the sun itself. That means Eros was never subjected to melting, which is what caused most of the metals that make up the Earth to sink to the center of the planet, leaving only a small portion behind in the crust.

The fact that Eros is a consolidated body and not a rubble pile is important to us in case we ever find ourselves faced with the need to try to deflect an asteroid on a collision course with Earth. The more we know about asteroids the more likely we are to succeed in deflecting one (if we have the time and resources). Blowing a big “rubble pile” asteroid into lots of smaller bits might actually be the worst thing we could do, simply ensuring that asteroids impacted all over the Earth with devastating force instead of hitting a single location.

These discoveries about Eros have, however, to more questions. Scientists, for instance, have discovered processes at work on the surface of Eros they haven’t seen anywhere else. Boulders seem to have disintegrated on the surface; no one knows why. Loose material has moved downhill, filling in low areas and craters, which doesn’t make sense, considering how low Eros’s gravity is.

The close-up photos NEAR will take on Monday may help answer some of these questions. NEAR’s last clear photo, taken at about 500 meters, could show surface features as small as 10 centimetres across.

Whatever happens Monday, it seems a respectful and fitting finish to the remarkable saga of NEAR, which has traveled 3.4 billion kilometres since its launch and taught us valuable lessons about our solar system.

In a way, it would be nice if NEAR could someday come home–but since its remains will be riding a giant asteroid, let’s hope it won’t.

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