NASA mission to end with twin moon smashes

Later today, two NASA spacecraft will smash into the moon. Individually named Ebb and Flow and together known as GRAIL (Gravity Recovery and Interior Laboratory), the twin craft have spent most of the past year making the most detailed study of the moon's gravity field to date. At 1728 EST (2228 GMT), that mission will come to a sudden end when the pair smack into the side of a lunar mountain. New Scientist takes a closer look.

What exactly will happen to the spacecraft?
First Ebb and then Flow will smack into a 2-kilometre-high mountain near the moon's north pole at about 1.7 kilometres per second. Mission scientists expect the twin washing-machine-sized craft to make two small craters, each about 3 metres in diameter and 20 to 40 km apart. The landing site was chosen partly to make the biggest crater, and partly to avoid hitting any historical spots like the Apollo landing sites ? though in any case, the chances of doing that were only 8 in a million, said project manager David Lehman of NASA's Jet Propulsion Laboratory in Pasadena, California in a press conference on 13 December.

Why such a violent end?
It's to squeeze as much science out of the mission as possible, says GRAIL principle investigator Maria Zuber of the Massachusetts Institute of Technology. Since the beginning of 2012, the twin craft have been orbiting the moon in single file, moving closer and farther from each other depending on slight variations in the moon's gravity due to the uneven distribution of matter inside it. GRAIL has obtained super-precise gravity measurements by orbiting very close to the moon's surface: since August, the spacecraft have spiralled down from an average altitude of 55 km to an average of 11 km on 6 December. "Our priority was getting to the lowest possible altitude and mapping as low as we could for as long as we could," Zuber said.

But nothing lasts forever, and the fuel is running out. "I'm hoping tonight a gas station will pull up next to our spacecraft and refuel it," joked Lehman. "But I don't think that's going to happen."

What will the impacts teach us?
The most exciting possibility is that the impacts will kick up a plume of dust and gas that can be analysed later by the Lunar Reconnaissance Orbiter as it flies by. The contents of the plume can help determine how the moon's atmosphere, which is so thin and tenuous that it is more properly known as an exosphere, interacts with the surface. The impact of cosmic rays, the solar wind and micrometeorites is thought to be ejecting particles of lunar soil into high-flying trajectories ? giving rise to the exosphere.

So what is left to discover?
Samples brought back by the Apollo moon missions contained elements like iron, magnesium and calcium, that have not been observed in the atmosphere. But none of these were observed by the LCROSS mission, which crashed a school-bus-sized rocket stage into the moon in 2009, revealing evidence of water ice in the resulting plume.

The Lunar Reconnaissance Orbiter has a spectrometer that can detect calcium. If the GRAIL plumes contain calcium, that would tell us two things, says LCROSS principal investigator Anthony Colaprete of NASA's Ames Research Centre in Moffett Field, California. First, it would confirm that one of the elements detected by Apollo really is in the lunar soil. It would also suggest that these elements can be kicked up by impacts. "That tells us something about how they're held in the mineral matrix, how they're liberated, and gives us a calibration point on our models," he says.

Could the GRAIL mission have ended in a different way?
"Not around the moon," Colaprete says. The moon's lumps and bumps mean it's difficult for spacecraft to maintain a stable orbit without burning a lot of fuel, especially at low altitudes. "What goes up must come down."

Can I watch?
Unfortunately, it won't be a very good show. The impact will happen in darkness, on the moon's far side, and the spacecraft are probably too small to make much of an explosion. The LCROSS plume was surprisingly wimpy, and the two from GRAIL will be even smaller. Still, NASA will describe the proceedings starting at 5 pm, Eastern Standard Time (2200 GMT). And if you gaze at the moon tonight, just think of the two new craters that will be taking shape on its dark side.

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