[meteorite-list] NEAR Shoemaker Scratches Below the Surface

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:42:02 2004
Message-ID: <200101221738.JAA14585_at_zagami.jpl.nasa.gov>


          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
          the surface.
Received on Mon 22 Jan 2001 12:38:59 PM PST

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