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 distance.

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."

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. 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 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.

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