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 is managed for NASA by the Johns Hopkins University Applied Physics Laboratory , Laurel, Maryland.
As the first spacecraft to orbit an asteroid, the NEAR mission promises 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.
Although XGRS is 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 uses 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) are used to separate Mg, Al, and Si lines; Ca, Ti, and Fe lines are resolved. The solar monitor uses 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 can be alternatively selected.
The gamma-ray spectrometer uses a body-mounted NaI scintillator with BGO shield. This unique design eliminates the need for both a long boom and active cooling. This subsystem of the XGRS detects naturally radioactive elements-K, Th, U-by their gamma rays and those of their decay chain products. In addition, it detects 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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Last modified: Thursday, 26-Jun-2003 13:48:16 EDT