Two Spacecraft Crash into Moon Mountain!

Yes, really! But not by accident.

On December 17 2012, NASA’s twin GRAIL (Gravity Recovery and Interior Laboratory) spacecraft were steered into a mountain near the Moon’s north pole. Both were about the size of a washing machine with a mass of around 200 kg (440 lbs). The aim here was to squeeze one last bit of science out of the spacecraft and take a look at the Moon’s interior.

The crashes alone could not achieve that. The twin impacts created twin plumes, but another spacecraft had to be on hand to analyse what the nature of these implied for the Moon’s composition. In this case it was NASA’s Lunar Reconnaissance Orbiter (LRO).

In the case of the NASA team driving the LRO it was a mad scramble to get the spacecraft into position to observe the impacts. LRO’s team had only three weeks’ notice of the ultimate position of the two GRAIL’s resting places and had to make sure their baby was on hand to focus on the columns of ejecta.

LRO was about 160 km (100 mi) from the lunar surface when the two spacecraft made impact. Because the site was in shadow at the time of the impact the LRO had to wait until the plumes rose high enough to be in sunlight before making its observation. In this case, the LRO used the LAMP instrument (Lyman Alpha Mapping Project), which is an ultraviolet imaging spectrograph. The LAMP saw mercury and enhancements of atomic hydrogen in the plume.

The results are interesting because the presence of mercury was also noted from the LCROSS (Lunar Crater Observation and Sensing Satellite) impact in October 2009, however that impact was at the bottom of the Moon’s Cabeus crater, which has not seen sunlight in an estimated billion years and is likely to be quite cold.

Now two craters around 4-6m in diameter dot the side of the unnamed mountain at an elevation of around 700m above the surrounding plain, around a third of the way up the 2,500m tall massif. Each has a faint, dark ejecta pattern. The dark ejecta is unusual since  impact craters on the Moon are usually bright. One theory is that the dark pattern is a result of the spacecraft remnants being mixed with the local materials.

If Elon Musk can get the price of space travel down by his claimed factor of 100, then maybe I can send my old washing machine up to the Moon for a scientifically relevant impact. Now that would be something.

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