Impact: Moon!

By Phil Plait

SLATE

Posted Sunday, May 26, 2013, at 8:00 AM

A program designed to monitor the Moon detected a fairly large flash of light on the evening of Mar. 17. What they had seen was an actual impact of a small asteroid hitting the Moon’s surface!

The flash of light reached fourth magnitude, which means that if you had been looking at the Moon at the time, you might have actually seen the impact with your own eyes! Here are frames from the video sequence showing the impact:

On Mar. 17, 2013, a small rock hit the Moon at high speed, creating a bright flash of light.
Photo by NASA

You can also see them as an animation. NASA also put together a short video discussing what happened.

The object that hit the Moon was probably less than a half-meter in size, not much bigger than a basketball, and had a mass of about 40 kilograms (90 pounds). But it was moving at high speed, something like 25 kilometers per second (15 miles per second), which is fast. The energy of motion, called kinetic energy, is converted into light and heat instantly when an object moving that rapidly hits something, which is why impacts like that create an explosion. This one detonated like the equivalent of several tons of TNT exploding.

It’s the brightest impact seen by the monitoring program in its eight years of lunar observations. The Moon is probably hit all the time, but most are too faint to catch, or happen on the far side where we can’t see them, or happen where the Moon is lit by the Sun and are drowned out by the bright landscape. That means, statistically speaking, for every one impact seen, many more must be happening. More than 300 impacts have been seen by the program.

A telescope caught the asteroid impact getting bright enough to see with the naked eye. Note the darkened edge of the Moon at the top.
Photo by NASA

Even more interesting, this flash happened around the same time meteor monitoring stations on Earth reported a flurry of meteors burning up in Earth’s atmosphere, and it’s possible these events were connected; a stream of debris may have been passing by and impacted both us and the Moon (and let me note that it’s not like we’re suddenly being attacked from space; we’re just getting better at observing them). This type of information is important to understand since our presence in space is growing; if we put bases on the Moon it’ll be very useful indeed to know how often the Moon is hit, and how large the impactors are!

I’ll note that new craters have also been seen by the Lunar Reconnaissance Orbiter, which has been orbiting the Moon since 2009. It’s detected some that are new at least since the 1970s, because they weren’t seen in Apollo photos. In fact, several craters have been identified that didn’t exist even in earlier LRO images, meaning they are less than four years old! Here’s one, seen before (left) and after (right):

Before (left) and after (right) showing a new crater on the Moon, centered in both frames.
Photo by NASA/GSFC/Arizona State University

The new crater is centered in both images, it happened on top of an older crater. You can see the spray of brighter, fresher material in the “after” image (there’s also an animated GIF that toggles between the two images).

I’m pleased to see these programs successfully capturing new cratering events! Objects this small are almost impossible to observe before they hit the Earth, so having good evidence of lunar impacts gives us a better understanding of the size range of objects in space. As Chelyabinsk showed us, even smallish objects can be pretty, um, dramatic, so we really need to have a firmer grip on what’s out there.