Sen— The twin Gravity Recovery and Interior Laboratory (GRAIL) probes named Ebb and Flow are being prepared for the final stage of their mission, a controlled descent and impact on a mountain near a crater called Goldschmidt, close to the Moon's north pole. The impact will take place on Monday, December 17 at 22:28 UTC.
The twin probes launched back in September 2011, arriving in Lunar orbit at New Year 2012, spending two months reshaping and merging their orbits. From March to May the two flew in formation at the same low-altitude, about 55 km (34 miles) above the surface, in a near-circular and near-polar orbit. On August 30th their altitude was lowered to 23 km (14 miles) for their extended mission.
The final flight path for NASA's twin GRAIL probes. Credit: NASA/JPL-Caltech/GSFC/ASU
By measuring changes in the distance between the two spacecraft down to a few microns, about the diameter of a red blood cell, they produced the most complex and detailed map of the Moon's gravity field, in fact the most detailed gravity map of any celestial body.
Their mission accomplished, and running low on fuel, the probes have one more task to perform. To determine precisely the amount of fuel remaining in their tanks, they will fire their main engines until their propellant tanks are empty. This will help NASA engineers improve predictions of fuel needs for future missions.
"Such a unique end-of-mission scenario requires extensive and detailed mission planning and navigation," said GRAIL project manager David Lehman of NASA's Jet Propulsion Laboratory, "We've had our share of challenges during this mission and always come through in flying colors, but nobody I know around here has ever flown into a moon mountain before. It'll be a first for us, that's for sure."
NASA map of "lunar heritage sites" and the path NASA's GRAIL probes will take on their final flight. Credit: NASA/JPL-Caltech
Ebb and Flow will descend gradually for several hours and skim the surface of the moon until the elevated terrain of the target mountain gets in their way. The twin probes will hit the surface at 3,760 mph (1.7 kilometers per second). The first probe to reach the Moon, Ebb, also will be the first to go down, with Flow following about 20 seconds later. Unfortunately the region will be in shadow at the time so no imagery of the impact is expected.