Discovery Program, a NASA initiative for small planetary missions with a maximum 3-year development cycle and a cost capped at $150 million for construction, launch, and 30 days of operation. The NEAR mission was managed for NASA by the Johns Hopkins University Applied Physics Laboratory , Laurel, Maryland.
As the first spacecraft to orbit an asteroid, the NEAR mission promised to answer fundamental questions about the nature and origin of near-Earth objects, such as the numerous asteroids and comets in the vicinity of Earth's orbit.
The NEAR spacecraft touched down on Eros in February, 2001, returning the highest resolution pictures of an asteroid's surface.
Although XGRS was intended primarily for these compositional studies, it has also been used to make measurements of the diffuse X-ray and gamma-ray background during the cruise phase of the mission as part of the instrument's background calibration studies.
The X-ray fluorescence experiment used three gas-filled proportional counters, collimated to 5 degrees, observing X-ray line emissions from the asteroid. Balanced filters on two detectors (Al on one and Mg on the other) were used to separate Mg, Al, and Si lines; Ca, Ti, and Fe lines are resolved. The solar monitor used an additional gas-filled proportional counter with a pinhole active area observing the X-ray spectrum of the Sun. A high-resolution, solid-state solar monitor could be alternatively selected.
The gamma-ray spectrometer used a body-mounted NaI scintillator with BGO shield. This unique design eliminated the need for both a long boom and active cooling. This subsystem of the XGRS detected naturally radioactive elements-K, Th, U-by their gamma rays and those of their decay chain products. In addition, it could detect other elements-Fe, Si, O, H-by gamma rays produced by cosmic ray interactions. The gamma rays of interest are typically between 0.2 and 10 MeV.
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