New study finds giant impacts aren’t periodic | Bad Astronomy

New study finds giant impacts aren’t periodic | Bad Astronomy: "

A giant impact from an asteroid or comet can ruin your whole day. Or year. Or, if you’re a dinosaur, your existence.

So astronomers do what they can to understand this menace from space. We look for rocks on orbits that intersect ours, we think about ways of moving them out of the way should we find one, and we also think about the record we do have of past impacts to see what we can learn from them.

There are about 180 impact craters known on our planet, ranging from tens of millennia in age to billions of years. They also vary in size from a few kilometers across to monsters so big they can only be detected from space. Sometimes it’s hard to measure their size (they can have multiple concentric rings, or be underground — covered up due to extreme age — making definite sizes hard to figure out) or hard to get their age. But we do have some statistics on them, and there have been many studies about them.

A big question is: are impacts periodic? That is, do they happen with some repeating period? If so, ...

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