Pair of NASA spacecraft to suicide into lunar mountain on Monday

For the past year, two spacecraft the size of washing machines have been orbiting the Moon in formation, mapping out gravitational anomalies by precisely measuring the distance between them. Now they're all done, and to celebrate, NASA is about to slam 'em both into a lunar mountain at a couple thousand miles an hour.

As missions to space go, the job of the GRAIL spacecraft was a fairly simple one. They orbited all around the Moon, with the first spacecraft (named Ebb) closely following the second spacecraft (named Flow). Using Ka band radar, the two spacecraft keep track of how far they are from each other with enough sensitivity to detect changes in distance on the order of the width of one red blood cell, which is pretty neat, and this allows them to create a gravitational map of the Moon.

When there's (say) a mountain on the lunar surface, its mass will attract a spacecraft slightly, causing it to speed up. This attraction happens to the first GRAIL spacecraft before it happens to the second one, opening up a bit of a gap that the radar can detect. The really cool bit is that this technique works not just with mountains, but with any variations of mass and/or density, including subsurface phenomena that we're otherwise unable to see. Here's the map that the GRAIL spacecraft put together:


Cool! So now let's smash them to tiny bits, right? Right!

On Monday at 5:28 PM EST, Ebb will plow into an unnamed mountain near the Moon's north pole, followed 20 seconds later by Flow. Generally, NASA prefers controlled endings to missions like this, rather than risk the spacecraft running out of fuel and ending themselves in some way that's messier or more dangerous, and this final act will use up all the probes' fuel and help NASA to check and see how good its estimates of fuel use were.


Ebb and Flow will not be cratering at some steep angle like you're probably picturing. Instead, they'll be sent skimming over the lunar surface at 3,760 mph until they end up running into the side of this mountain, up near the peak. The probes will have a spectacular view in their final hours and minutes, but they don't have cameras on them, and the actual impact will take place in shadow, so we won't be able to see a thing.



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