Leonids Hit the Moon!

Week of November 29, 1999

Well, the Leonid meteor shower came and went a couple of weeks ago as I write this, and it left me a wee bit disappointed. I was with a group of a dozen or so folks that went out to a dark site in Virginia in hopes of catching a storm, and all we got was a drizzle. The best meteor we saw was just after we arrived; it shot across half the sky and left a glowing train that lasted for several seconds. Unfortunately, that was about it for us. I saw a total of fifteen, though one little girl in our group claims she saw 40. Did I mention it was about -2 Celsius outside? Brrr! Next time, I'll wait for the Perseid shower. It peaks in Autumn.

Not everyone was as unlucky as we were. A clever person put out an alert: watch the Moon! This year, the Moon was favorably placed to get bombarded by Leonids. The phase of the Moon was just past first quarter, which means about half the Moon was not lit up by the Sun. A meteor impact on the surface of the Moon might be bright enough to see, so a message went out to keep monitoring the dark terrain (hmmm, lunain?) of the Moon. Although the Mon has no atmosphere, the impact itself causes a flash of light. The meteor may be small, but the impact velocity is so large -- perhaps 70 kilometers per second -- that the impact energy can be tremendous, like a small bomb. If the impactor were big enough the energy could easily rival an atomic bomb. That energy is converted into light and heat when the meteor hits the surface of the Moon. The message was sent to astronomers (both pro and amateur) just in case.

Evidently, it paid off. At least two separate observers caught what appear to be several impacts on the Moon's dark portion. One flash got as bright as roughly 3th magnitude; bright enough to see with just your eye! This is possibly the first time such an event has ever been seen; it is certainly the first time it has been photographed and independently confirmed. One reason this is so exciting is that actually very little is known about the Leonid meteors. In this case we may learn a great deal; since we know the distance to the impact (the distance to the Moon), how bright it was and the impact speed, we can make much better estimates of the mass of the objects impacting the Moon. This in turn may tell us about just how comets lose material as they round the Sun.

Another reason this is so cool is that flashes have been seen on the lunar surface for decades, or perhaps even hundreds of years, but none has ever been confirmed, and no one knows just what causes them. Are they a meteor impact or perhaps the release of subsurface gas? Transient Lunar Phenomena (or TLPs) as they are known are an interesting mystery. Perhaps now we are finally closer to solving them.