Donald Savage
Headquarters, Washington, DC
(Phone: 202/358-1727)
Helen Worth
Johns Hopkins University Applied Physics Laboratory, Laurel, MD
(Phone: 240/228-5113)
RELEASE: 00-22
The NEAR spacecraft is straightening its orbit and
putting its best solar panels forward as it approaches
asteroid 433 Eros for a Valentine’s Day rendezvous. Its
intended is a near-Earth asteroid named for the Greek god of
love.
The NEAR (Near Earth Asteroid Rendezvous) mission, a NASA
Discovery Program being conducted by the Johns Hopkins
University Applied Physics Laboratory, Laurel, MD, is the
first mission to orbit an asteroid. For a year the spacecraft
will use its instruments to scrutinize the potato-shaped space
rock to learn about its chemical and physical features and
evolutionary history. The asteroid is known to be 21 by 8 by 8
miles (33 by 13 by 13 kilometers) — about twice the size of
Manhattan Island.
NEAR is less than 2,900 miles (4,700 kilometers) from
Eros and is slowly closing in at about 18 mph relative to the
asteroid. The spacecraft is alive with preparations for its
rendezvous. Its multispectral imager has been taking daily
images for the past few weeks to confirm that the spacecraft
is on track, to look for any moons orbiting the asteroid, and
to measure its brightness variations for clues to its
rotation.
The last scheduled rendezvous burn prior to orbit
insertion will take place Feb. 8 at 5 p.m. EST. On Feb. 13 at
about 11:33 p.m. EST, the spacecraft will fly directly between
the sun and the asteroid, enabling NEAR’s near-infrared
spectrometer to take critical observations of Eros’ northern
hemisphere under near-perfect lighting conditions, which will
allow it to distinguish the asteroid’s mineral composition.
In October a similar sweep will be made over its southern
hemisphere.
On Feb. 14, at 10:33 a.m. EST, when NEAR is 207 miles
(333 kilometers) from the center of Eros, it will fire its
hydrazine engines to slow it enough to be captured by the
asteroid’s weak gravitational pull. Confirmation of orbit is
expected to come at about 11:30 a.m. EST to waiting team
members in the Mission Operations Center on the Applied
Physics Laboratory campus.
During the first few weeks after achieving orbit the
spacecraft will slowly descend toward the asteroid. Because
the asteroid is irregularly shaped and rotating (it rotates
once every 5.27 hours), this early stage of the mission can be
very tricky, says Dr. Robert Farquhar, NEAR mission director.
“No one has ever orbited a small body in space,” Farquhar
says. “The orbital stability is rather tenuous, and as we
travel around Eros our navigation maneuvers must be perfect to
keep us from crashing into it.”
Using a multispectral imager, laser rangefinder, and
onboard radio science experiment, mission scientists and
engineers will acquire enough information on Eros’ shape, mass
and gravity field to allow the spacecraft to come closer.
“Soon after we go into orbit we should know the asteroid’s
mass and therefore its density to within 5 percent,” says Dr.
Andrew Cheng, mission scientist.
The onboard magnetometer will determine the strength of
the asteroid’s magnetic field — if there is one. “This will
give the scientific community the first definitive measurement
of an asteroid’s magnetism, which contains clues to its
thermal and geologic history,” Dr. Cheng says. “The results
of these measurements and others that we will take over the
next year will help us to determine the origin of the asteroid
and give us an unprecedented understanding of asteroids in
general.”
For the first two months NEAR will slowly descend to
within 31 miles (50 kilometers) from Eros. During this low-
orbit phase the x-ray/gamma-ray spectrometer will measure
elemental abundances — important information to help
determine the relationship between meteorites and asteroids.
In late August the spacecraft will begin to climb from 31
to 311 miles (50 to 500 kilometers) above the center of Eros.
During this ascent the multispectral imager will continue to
take images of the asteroid’s surface that will be compiled
into a complete map of the asteroid. In December the
spacecraft will descend, possibly to less than a mile, from
the surface of the asteroid. At that vantage point the near-
infrared spectrometer can collect extremely high resolution
data of the asteroid’s surface, making it possible to
distinguish the composition of rocks as small as a grapefruit.
Final events of the mission, which will end in February 2001,
will be determined sometime this summer.
NEAR was launched Feb. 17, 1996, from Cape Canaveral Air
Station, FL. Its original rendezvous date of Jan. 10, 1999,
was postponed when a firing of the spacecraft’s bipropellant
engine, designed to put the spacecraft on target for the
rendezvous, exceeded preset acceleration limits and caused the
spacecraft to retreat into safe mode. But valuable
information about the asteroid was collected by a hastily
programmed flyby of Eros on Dec. 23, 1998. Early images can
be found on the Internet at:
