A WELCOME success for NASA came on Monday as its Near Earth Asteroid
Rendezvous (NEAR) satellite arrived at the asteroid Eros and slipped into orbit
around it. As news of the feat reached the satellite control centre at Johns
Hopkins University in Baltimore, the mood was “euphoric”.
“It’s like Christmas for us,” said Joe Veverka, the leader of NEAR’s imaging
team. “The pictures show evidence of layering and other structures, like grooves
and rims. Eros will turn out to be an extremely interesting object.”
Launched on 17 February 1996, the $224-million NEAR faced tough
challenges. The satellite used Earth’s gravity to swing itself out past the
asteroid Mathilde and then on to Eros, an asteroid 33 kilometres long and 13
kilometres across. Because the asteroid’s orbit is tilted at 10 degrees to that
of Earth, NEAR’s trajectory was complex.
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The spacecraft had to “trap” itself within Eros’s puny gravitational
field—and no probe has ever orbited anything smaller than the Moon. NEAR
made a first attempt to orbit Eros in December 1998, but this failed due to a
27-hour loss of communication. Controllers were forced to let NEAR speed past
Eros, before an engine burn set the craft on course for a second attempt.
“It was quite a nice piece of work. Normally if you miss a rendezvous burn,
you’d fly by the object and you’d be in very bad trouble,” says Paul Weissman of
NASA’s Jet Propulsion Laboratory (JPL) in Pasadena.
On Monday, NEAR tried again. “We were sneaking up on the asteroid at just 10
metres per second,” says Don Yeomans of JPL. The fine control of the
craft—258 million kilometres from Earth—surprised even the
scientists when the final burn produced within 0.04 per cent of the planned
thrust. “That’s amazing,” says Yeomans.
NEAR will take 27 days to circle the asteroid, between 327 and 450 kilometres
away. The next major manoeuvre will push it into a lower orbit to see more
surface detail. The craft carries spectrometers that will measure the asteroid’s
composition.
The latest images indicate that Eros is a solid body rather than a “rubble
pile” of loose rocks, but other measurements hint that it is less dense than
observations during the 1998 flyby suggested. “The surface is covered with old
craters,” adds Yeomans. And the body of Eros is not wobbling, indicating it
hasn’t been hit by another object in the past couple of hundred thousand
years.
NEAR’s success follows depressing news from Japan. On Thursday last week, a
rocket failed to reach its correct altitude and place a joint Japanese-NASA
X-ray astronomy satellite called Astro-E into orbit. Less than a minute after
lift-off from the Uchinoura site in southern Japan, the three-stage M5 rocket
began to veer off its planned path because the first stage did not fire
properly.
Despite efforts to correct the problem, the rocket did not reach its intended
altitude. Controllers believe the M5 released the satellite at a sub-orbital
height, so it fell back to Earth and burnt up in the atmosphere.
Astro-E would have given astronomers unprecedented views of X-ray sources
such as galaxy clusters and supernova remnants. Its failure is a crushing blow.
“It’s a total loss and it’s very, very disappointing,” says Stephen Holt,
project scientist for Astro-E at the Goddard Space Flight Center in Baltimore.
The chances of getting a similar instrument into orbit soon are slim. “There’s
no way to do it in less than three years—even if a miracle happens.”
However, after a series of setbacks for NASA, Holt was glad to hear of NEAR’s
success. “I think it’s wonderful,” he says.
