NASA probes on crash course for the moon

Posted by Kate Taylor

NASA's twin Grail spacecraft, which have been circling the moon for almost year, are set to crash into the moon's surface on Monday.

The probes, named Ebb and Flow, have been studying the internal structure and composition of the moon, most recently delivering a gravity map revealing deep cracks and craters that will provide a better understanding of how Earth and other rocky planets in the solar system formed and evolved.

But their mission's now finished, and they are set to descend and impact on a mountain near the moon's north pole at about 5:28 pm EST on Monday, December 17.

The crash is entirely deliberate. In their current low orbit, they've been using up fuel fast, and there isn't enough left for any further scientific operations.

"It is going to be difficult to say goodbye," says Grail principal investigator Maria Zuber of MIT. "Our little robotic twins have been exemplary members of the Grail family, and planetary science has advanced in a major way because of their contributions."

The mountain where the two spacecraft will hit is located near a crater named Goldschmidt. The first probe to reach the moon, Ebb, also will be the first to go down, with Flow following about 20 seconds later.

Both spacecraft will hit the surface at 3,760 mph. Disappointingly, though, we won't be able to see anything as the region will be in shadow at the time.

Ebb and Flow have one more job to do before their mission ends. They will fire their main engines until their propellant tanks are empty to determine precisely the amount of fuel remaining in their tanks. This will help NASA engineers validate fuel consumption computer models to improve predictions of fuel needs for future missions.

"Our lunar twins may be in the twilight of their operational lives, but one thing is for sure, they are going down swinging," says Grail project manager David Lehman of NASA's Jet Propulsion Laboratory. "Even during the last half of their last orbit, we are going to do an engineering experiment that could help future missions operate more efficiently."

The burn that will put the spacecraft on their collision course is scheduled to take place this morning.

"Such a unique end-of-mission scenario requires extensive and detailed mission planning and navigation," says Lehman. "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."