Shoemaker Scratches Below the Surface
yearlong orbit is quickly drawing to a close, but it's the
best time of the mission for the NEAR X-Ray/Gamma-Ray Spectrometer
team. With NEAR Shoemaker circling about 22 miles above Eros
through most of January, XGRS scientists have been exploring
the elements on - and for the first time, below - the asteroid's
surface from the instruments' prime operating distance.
in the mission, reading X-ray emissions from Eros' uppermost
surface, scientists saw chemical similarities between Eros
and the chondritic meteorites, the primitive remnants from
the solar system's birth. Now the instrument's gamma-ray detector,
which measures emissions excited by cosmic rays and natural
radioactivity, will probe a bit deeper.
gamma-ray spectrometer allows us to see about four inches
below the surface," says Dr. Jack Trombka, the XGRS team leader
from NASA's Goddard Space Flight Center in Greenbelt, Md.
"This is helping us determine if the chemistry we've seen
so far is characteristic of the whole asteroid or just the
thin, top layer."
X-ray instrument gets its best readings when the sun is most
active - large solar flares emit more X-rays, making for brighter
reflections off the surface. But it can be too much of a good
thing. "Too much current will fry the detectors," says Trombka,
"like burning out the filament in a light bulb. Fortunately,
the spacecraft senses this and shuts the instrument down when
things get too hot. The on-board computer can automatically
turn XGRS back on when the current returns to safe levels,
or await word from operators on Earth to activate it. The
system has worked like a dream so far and we've had some excellent
Shoemaker stays in XGRS' optimal orbit until Jan. 24, when
it dips into a series of low flyovers just 3-4 miles (5-6
kilometers) over Eros' ends. The spacecraft then flies between
1-2 miles (2-3 kilometers) over Eros during an even lower
pass scheduled for Jan. 28, before a boost back up to 22 miles
(35 kilometers). The mission ends Feb. 12 with NEAR Shoemaker's
controlled descent to the surface.
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