NASA's About to Crash Satellites into the Moon

Ebb and Flow, NASA’s washing machine-sized twin Gravity Recovery and Interior Laboratory (GRAIL) satellites, are about to meet their demise. But it's a planned demise. The pair have returned a detailed map of the Moon’s gravity field that far outstrips anything we have of another celestial body. And now, at the end of their extended mission with little fuel on board and a low orbit, there’s nothing left but to smash into the Moon.

The GRAIL satellites (rendered above) launched in September 2011 with an interestingly specific goal: map the Moon’s gravity field and determine its interior nature from crust to core. This information would fill a gap in our understanding of the satellite’s thermal evolution and ultimately provide scientists with a better understanding of how the Earth and other rocky planets in the solar system formed and evolved.

Ebb and Flow got to the Moon about a year ago – last New Years Eve and New Years Day respectively. Then they got to work. Flying in formation about 125 miles apart, they orbited the Moon at an average altitude of 34 miles. From this height, they were able to resolve features as small as 8 miles across, a far better resolution than any previous mission.

As the satellites moved over mountains, craters, and areas of dense rock, they recorded changes in strength of the local gravity field by measuring changes to their own velocities. Passing over high-gravity regions caused them to speed up while low-gravity regions made them slow down. This data, combined with the simultaneous change in distance between the two, scientists have been able to work out the specifics about the Moon’s gravity. And from there, they’ve made inferences about the Moon’s geological structure. It’s a pretty nifty bit of science.

GRAIL’s 90-day primary mission designed to run from March to May of 2012 was extended. Starting on August 30, the satellites’ orbits were lowered to just 14 miles above the surface, allowing them to gather a trove of additional data. But the low orbit wasn’t without risks; at times the spacecraft were just a few miles above some of the Moon’s taller surface features.

Now, some of those tall features are Ebb and Flow’s targets. Both satellites will collide with a mountain near Goldschmidt crater near the Moon’s north pole. But the twin probes will go down in a blaze of scientific glory, running one last experiment in their final minis gone, allowing mission scientists to determine just how much fuel was left in their tanks at the end of the mission. This data will help NASA engineers validate fuel consumption computer models and improve predictions about fuel needs for future missions.

These final burns that will change the satellites' orbits and put them on a steady course for impact happened Friday morning. But even though it’s a crash landing, there’s some extensive and detailed mission planning and navigation involved. No one has ever deliberately flown into a mountain on the Moon.

The spacecraft will go down on Monday, December 17. Ebb, which was the first of the twin probes to reach the Moon, will also be the first to hit it at 2:28:40 p.m.; Flow will follow about 20 seconds later. Both spacecraft will hit the surface traveling 3,760 miles per hour, a speed that will tear them apart on impact.

While they're being flown into the Moon because there's nothing else NASA can feasibly do, the ejecta both spacecraft are expected to kick up will be another benefit to the mission’s science; water ice or other volatiles could be detected by the Lunar Reconnaissance Orbiter. LRO might see something neat, but we won’t. Ebb and Flow are too small and without fuel on board won’t be making a spark visible from the Earth.

And in other sort of related GRAIL news, Flow used its MoonKam to see the Lunar Reconnaissance Orbiter circling the Moon. It’s a little hard to see – it’s quite small and faint – but it’s there and awesome.