Eros, a telephone-shaped
asteroid that has been studied close-up for months by a satellite,
is a solid, primitive chunk of rock scarred by craters and not
a ``pile of rubble'' like some other asteroids, researchers say.
A spacecraft
called Near Earth Asteroid Rendezvous, or NEAR, became the first
to orbit an asteroid last February and instruments on the craft
have probed the chemistry, density and surface features of the
21-mile-long rock. The results show that Eros is very old, perhaps
as old as the Earth itself.
``We know
now that the asteroid is a primitive body,'' 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 next February when
the NEAR will be deliberately flown into Eros. Mission scientists
hope for a soft landing.
Instruments
on NEAR show that Eros is solid, and not a collection of small
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.''
That was one
of the fundamental questions that NEAR was to answer about Eros,
he said.
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 one or two
pounds on Earth would be enough to move or lift one or two tons
on Eros.
Researchers
have said that a person who can jump three 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.
Cheng said
that because of Eros' light gravity, it may have lost some of
mass over the billions of years it has been orbiting the sun.
He said that each time the asteroid is hit by a smaller asteroid,
some material would be jarred away at speeds great enough to fly
independently into space. In effect, small asteroids could knock
a chip off the ole Eros.
Some of the
crater material did settle back on the asteroid, however, say
the researchers, because Eros appears to have a fine layer of
dust and smaller rocks churned to the surface by impacts.
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 touchdown gently enough that it will continue to
function,'' said Cheng. But in any 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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