After launching aboard a Delta II rocket from Cape Canaveral Air Force Station on September 10, 2011, at 9:08 a.m., and orbiting the Moon for almost a year, the two GRAIL spacecraft, which came to be named Ebb and Flow, have ended their mission with an impact into the lunar surface.
The two GRAIL spacecraft were washing machine-sized craft meant to explore the gravity field of the Moon. Looking at the changes in the intensity of the gravity field offers insight on what lies underneath the surface of the Moon.
The spacecraft, named Ebb and Flow by elementary school students from Bozeman, Mont., entered orbit on New Year’s Eve of 2011 and New Year’s Day of 2012.
They flew in close formation over the Moon’s surface at an altitude of about 55 km. They were able to sense the changes in the Moon’s gravity field by how the gravity affected the distance between the two spacecraft. Their nominal mission was completed early on May 29, and they were granted an extended mission.
However, the extended mission could not begin immediately. Sub-optimal lighting conditions would have prevented the spacecraft from maintaining enough power to operate continuously. The worst period for the spacecraft would have been the July 4 lunar eclipse.
Instead, the spacecraft were turned off until August 30, when their extended mission really began. During this extended mission, Ebb and Flow flew only 23 km over the lunar surface, clearing some of the tallest lunar features by only 8 km.
The extended mission lasted from August 30 to December 3. However, the spacecraft were still in excellent condition, so the GRAIL team continued to take data at successively lower altitudes until their fuel began to run dangerously low. The team selected a crater ridge near the north pole of the Moon for an impact site to end the mission. This site would allow GRAIL to continue operating its mapping systems virtually right up until the moment of impact, providing the maximum amount of data possible.
But even the destruction of the spacecraft may yet provide science data. The Lunar Reconnaissance Orbiter’s orbit has been modified slightly to permit a fly-over of the area to try to image the impact sites. Examining images of the impact features might reveal information about the mechanical properties of the lunar material making up the crater rim into which the spacecraft crashed.
The team is hopeful, while still recognizing that the possibility is remote, that the impact will liberate volatile molecules, like water, buried in the lunar regolith. Current thinking holds that the only volatiles on the Moon exist in permanently shadowed regions of certain craters near the lunar poles. Finding volatiles in an area that sees sunlight frequently would be an incredible revelation.
Ebb impacted the unnamed crater ridge at about 5:28 p.m. ET, followed by Flow about 30 seconds later and a few meters away.
After being asked by the GRAIL team, NASA consented to name the impact site after a former GRAIL team member who had passed away earlier, astronaut Sally Ride.