MESSENGER's gamma-ray spectrometer gears up for Mercury flyby
Mon Sep 22, 2008 at 22:12 UTC
Two weeks from today, the MESSENGER spacecraft will fly by Mercury for the second time. As part of the final preparations for this encounter, the Gamma-Ray Spectrometer (GRS) was placed in an "anneal mode" to prepare its detector for optimal performance during the flyby.
"The detector material itself is a high-purity crystal made of the element germanium," explains GRS Instrument Engineer John O. Goldsten. "In space, the crystal develops defects-atoms knocked out of place-when bombarded by high-energy cosmic radiation, and this degrades the instrument's performance. Heating the detector to high temperatures promotes realignment in the crystal, a process called annealing."
This annealing process increases the detector temperature to 84°C for a period of time before lowering it to an operating temperature of -183°C. The annealing will last for two weeks in preparation for Mercury flyby 2 to improve energy resolution and signal-to-background ratio. The GRS detector will be annealed once again prior to the third flyby of Mercury in September 2009, and once every Mercury year (88 Earth days) during the orbital phase of the mission.
Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington.
This is the last image MDIS took of Mercury during MESSENGER's first flyby of the planet before the spacecraft turned its antennas to begin transmitting the flyby data to Earth. Even at this great distance, the giant Caloris basin can be identified as a brighter circular region in the upper right of the planet. Two weeks from today, on October 6, 2008, MESSENGER will fly by Mercury again.
The geometry of the second Mercury flyby is different from the first encounter, in that the point of closest approach will be nearly on the opposite side of the planet. As a result, MESSENGER will view about 30% of Mercury's surface previously never before seen by spacecraft. This new territory is located just to the left of the day/night terminator in this image.
During the second encounter, 1287 MDIS images are planned. MESSENGER's Magnetometer will also be making the first magnetic field measurements over the western hemisphere of the planet. The observations of Mercury's tenuous atmosphere and neutral sodium tail will be more extensive than during the first flyby, and the angular resolution of the plasma spectrometer has been improved since the first flyby as a result of new software.
The laser altimetry profile during the second flyby will be over areas that have been imaged by Mariner 10 or MESSENGER, which will permit the correlation of topographic characteristics and imaged features such as craters and faults to a degree never before possible.
| Source: Johns Hopkins University Applied Physics Laboratory | |
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