[meteorite-list] NEAR Shoemaker Scratches Below the Surface
From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:42:02 2004
NEAR Shoemaker Scratches Below the Surface
January 22, 2001
The 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
Earlier 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.
"The 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."
But even with time and proximity to Eros on its side,
XGRS still faces a tough ride. The 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. That the sun is at the peak
of its 11-year activity cycle boosts the potential for
quality data, but the energy generated from especially
large solar flares and coronal mass ejections - which
blast high-energy particles and solar materials into
space at millions of miles per hour - can overload NEAR
Shoemaker's X-ray and gamma-ray detectors.
Trombka says it's almost like too much of a good thing.
"Too much current will fry the detectors - 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 results."
NEAR 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
Received on Mon 22 Jan 2001 12:38:59 PM PST