I got an interesting phone call yesterday morning. It was from a man who claimed he saw something odd. On October 22 of last year he was driving along Interstate 50 in California, heading east away from Sacramento. The Moon was rising over the mountains, and he saw what looked like a giant asteroid impact on the Moon. He described it to me in some detail, with rising plumes arcing away from the Moon and everything. He said he saw people stopped by the side of the road, watching, with some taking pictures.
I told him I was sure it wasn't something that had actually hit the Moon. If it had been, every astronomer on the planet would have been going nuts, especially the amateur astronomers (who, in truth, would be more likely to see it before any professional astronomers). We're talking major impact here, and the whole planet would have seen it.
If what he said is true then I suspect what he saw was an atmospheric effect, given that it was only seen locally. Since the Moon was rising at the time, that makes it even more likely; when the Moon is low there you see it through more air, and this can cause all kinds of weird effects.
However, out of scientific honesty, I'll add that he claimed the Moon was full that night, when it was actually full five days earlier. Either he got the date wrong, or the phase wrong. I suspect it was the phase: on October 22 the five-days-past-full Moon would have risen at 9, and by 11:00 p.m. would have been low over the Sierra Nevada mountains. To be fair, it's been my experience that people commonly think a less-than-full Moon is full, and even though this was really not full, it was rising and may have looked distorted.
Anyway, if anyone saw this event last October, please leave a comment here! I'd like to know.
Now, stuff like this does in fact happen. Just look at the Moon's surface: it's a testament to a history of very seriously pummeling. Even now it still gets hit, though really big impacts are very unlikely. Still, a ton or so of asteroid shrapnel and dust hits the Moon every day.
By coincidence, I was literally reading an article about lunar impacts when I got that call! Weird. Anyway, give that a read; it's fascinating. It says that something massing 1100 kg (over a ton!) hit the Moon back in 1972. If it was a rock, it would have been about a meter across, and with a typical impact speed of 30 km/sec, it would have released about as much energy as 100 tons of TNT. Not bad for something smaller than a washing machine. I had no idea there had been an impact that large in the recent past. Pretty cool.
If you're interested in this, check out the page on Wikipedia about transient lunar phenomena. They have good links from there, too.'