Moon and Earth clobbered at the same time

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This past March 17, an unusual number of deep-penetrating meteor fireballs tore through our atmosphere. Brilliant and eye-catching, their paths revealed that they’d been traveling together through space, along identical paths.

That same night, something extraordinary exploded on the Moon. Special cameras that NASA researchers have been using for the past eight years to monitor the Moon for signs of meteoroid impacts showed a bright flash. At fourth magnitude, it was ten times brighter than any explosion ever seen on the lunar surface.

This NASA program has already revealed hundreds of detectable impacts every year. But the one on March 17 was the biggest lunar explosion in history.

The impacting meteoroid must have been only a foot across – too small even quite to be called a boulder – but it was traveling at 56,000 miles per hour. Releasing all its kinetic energy onto a tiny spot on the smooth, dark surface of Mare Imbrium, some 700 miles east of the first Apollo landing site, its flash lasted for a full second. It would have been obvious on the dark portion of the Moon to anyone watching at that time. The impact energy was equivalent to five tons of TNT.

The meteoroid must have weighed about 80 pounds, and probably created a crater some 60 feet wide, large enough to surround an average house and front lawn. Here’s what’s cool: The impacting object had the same path as the meteoroids that struck Earth at that same time. Conclusion? Unless these were alien missiles fired in our direction, they were part of a short-duration cluster of material orbiting the Sun – a sort of miniature swarm of debris, like the meteor showers that we encounter every August 11 and December 13. This swarm, previously unknown, will surely again be encountered by the Earth/Moon system next year on that same date. (We always return whence we started at each anniversary, since our yearly calendar marches in tune with our orbital position.)

The lunar surface has already been turned into a fine powder by countless meteor impacts over the ages. It’s a sharp contrast with Earth’s protected surface, shielded by our atmosphere, which burns nearly all incoming meteoroids into dust. And 99 percent of even the rare ones that make it to the ground have been slowed by atmospheric friction to a terminal velocity of just 250 miles per hour: enough to penetrate a roof and maybe one additional story, but not pulverize surface rocks. And they certainly do not arrive frequently enough to present an ongoing hazard to hikers and strollers.

But now we must ask: Is the airless, unprotected Moon struck far more often than we ever realized by tiny missiles going dozens of times faster than bullets? Would merely being on its surface present a significant hazard to future astronauts or lunar colonists?

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