The spacecraft, called Near Earth Asteroid Rendezvous, or NEAR, is the first to orbit an asteroid and instruments on the craft have probed the chemistry, density and surface features of Eros. The results show that the asteroid is very old, perhaps as old as the Earth itself.
"We know now that the asteroid is a primitive body," said Andrew F. Cheng, the NEAR project scientist at the Johns Hopkins University Applied Physics Laboratory. "It has never been melted, never separated into core, crust and mantle the way the Earth and the other inner planets have."
Papers summarizing the results of NEAR appear on Friday in the journal Science.
Eros is a near-Earth asteroid, an orbiting space rock that passes close, but never crosses, the Earth's orbital path. Eros' orbit dips to within 105 million miles of the sun and then loops out to some 165 million miles from the sun. The Earth is about 93 million miles from the sun.
The NEAR was launched in 1996 and settled into an orbit of Eros, an asteroid named for the Roman god of love, on Feb. 14, St. Valentine's Day. The $224 million mission is scheduled to end in February when the NEAR will be deliberately flown into Eros.
One of the fundamental questions to be answered by NEAR was whether it was a solid, undifferentiated rock, or a collection of smaller rocks held together by gravity.
"Some asteroids are like a pile of gravel floating in space, not really tightly stuck together. But not Eros," said Cheng. "Eros is a consolidated body, not a rubble pile."
Cheng said the solid character of Eros suggests it may have broken off as one chunk from a much larger body, perhaps another asteroid. But that scenario is still being debated. The origin of the asteroid is unknown.
Eros' density, determined by measuring its effect on the orbit of NEAR, is about like that of a similar-sized chunk of the Earth's crust. Based on this, researchers estimate that its gravity is 1,000 to 2,000 times less than that of Earth.
Cheng said this means that the force required to move or lift 1 or 2 pounds on Earth would be enough to move or lift 1 or 2 tons on Eros.
Researchers have said that a person who can jump 3 feet on Earth could jump more than a mile on Eros and actually risk flying into orbit.
The asteroid is scarred and marked by heavy cratering. A very large crater in the center forms a saddle that gives Eros a shape variously described as like a telephone instrument, or like a ballet slipper, or like an unshelled peanut.
In February, when NEAR is sent falling toward Eros, engineers hope to make a sliding-type of soft landing.
"We're hoping that it will touch down gently enough that it will continue to function," said Cheng. But in ay case, the first controlled landing on an asteroid will send back images of unprecedented clarity.
"The resolution will be higher than for any other celestial object but the moon," said Cheng.
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