Is this splat on the moon all that remains of a lost lunar probe?

In the late 1960s, NASA's Lunar Orbiter 2 spacecraft was circling the moon, spotting potential landing sites for the Apollo missions. In 1969, the probe was commanded to crash into the moon's far side, and we don't know for sure what happened after that. This new picture may be the answer.

What you're looking at here is a crater on the far side of the Moon. It's not the dark side of the Moon, as it's lit half the time by the Sun, but since the Moon rotates at the same rate as it revolves, us Earthlings never get a good look at what's going on back there. NASA's currently got the Lunar Reconnaissance Orbiter (LRO) buzzing around the Moon snapping pics, and this one seems to show a fresh impact crater in more or less exactly the same spot as the Lunar Orbiter 2 probe was supposed to hit when it was sent on a high-speed suicide run into the lunar surface. You may have noticed that the crater looks a little bit funky, though, so stay with me while I try to explain what the deal is.

If you've ever made an impact crater of your own (by, say, throwing a rock into sand), you've probably noticed that the only way to make a round crater is to throw the rock downward vertically. If you throw it at an angle, you get an oblong crater. This seems to suggest that since the vast majority of craters on the moon are also round, most impacts occur at a 90 degree angle, but they don't: since impact angles are random, the average impact angle is in fact 45 degrees.

So why aren't most of the craters on the moon oblong? For a long time, people had no idea, and it was assumed that all of those circular holes on the moon must therefore be volcanoes, not craters. But as soon as astronomers realized just how blisteringly fast things like asteroids travel (ten to 20 miles per second), they had the answer: impacts at those speeds aren't pushing stuff out of the way to make a crater (which is what happens when you throw a rock at sand), but instead, as soon as they hit the surface, most of the impacting body vaporizes, and that explosion (which is a point source of energy) is where the circular crater comes from.

The exception to this is if you've got an extremely shallow impact angle, where the impacting body just grazes the surface. In this case, you actually get different parts of the impactor contacting the surface at slightly different times. So instead of a single point source of energy, you've now got an elongated smear of energy, and instead of a circular crater, you get an oblong one, with material thrown out to the sides. The pattern formed by oblique impacts like this looks like a butterfly, and we've seen it on both the Moon and Mars:

butterfly_eject_mars.jpg

So anyway, getting back to poor old Lunar Orbiter 2, the crater we're looking at appears to be caused by a low-angle impact, which would be consistent with the final moments of the probe. And it's in the exact same spot that the probe was calculated to have ended up. The crater and ejecta appear to be quite a bit bigger than the spacecraft should have been able to produce, which is weird, so the LRO team is planning to come back and take another look in better light to try and solve this mystery once and for all.

NASA LRO, via Discovery

Full Disclosure: Evan Ackerman was, in his past life, an expert on impact cratering

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