If a big meteorite struck the moon, would we notice it here on earth?

July 12, 2013

Dear Cecil:

I’m curious about the effect of a sizable (say, one-kilometer) meteorite striking the moon. Assuming it hit the side facing us, would we be able to see the impact or the aftermath (plumes of dust) with the naked eye?

Cecil replies:

This is another example of why the world, or anyway NASA, needs to put me in charge.

Obviously you don’t realize a meteorite struck the moon on March 17, producing an incandescent flash readily visible with the naked eye from earth. Readily visible, that is, to anyone looking directly at it during the approximately one second it lasted. Evidently no one was — it wasn’t until two months later that a NASA analyst spotted the impact while reviewing the telescope videos.

What telescope videos, you ask? The ones NASA has been making since 2005, when it started keeping telescopes trained on the moon ten to twelve nights a month (whenever the moon is 10 to 55 percent full) looking for meteorites crashing into it. Astronauts may camp on the lunar surface for extended missions some day, the space agency reasoned, and since the moon has no protective atmosphere, getting brained by incoming meteorites is a nontrivial risk.

To get an idea of how nontrivial, NASA began counting visible strikes. So far it’s tallied more than 300. The one on March 17 was the biggest so far, ten times brighter than anything seen previously, although nowhere near the hypothetical one-kilometer catastrophe you’re talking about. This rock was more like a foot in diameter and weighed maybe 90 pounds.

Still, it was traveling close to 56,000 miles per hour and had an impact equivalent to five tons of TNT, gouging a crater perhaps 65 feet across. NASA has asked the scientists operating the Lunar Reconnaissance Orbiter, now mapping the moon’s surface, to take a picture of the March 17 crater, and they expect to get around to it later this year.

That’s why I need to be running things. I concede that as a practical matter there’s no need to get crater photos right this second, and if meteorites slam into the moon in March and we don’t hear about it till May, that’s likewise no big deal. By the same token, if there were a two-month delay announcing the results of the Stanley Cup finals — to pick another realm where objects move at 56,000 miles an hour and you can’t tell what happened till you see the replay — would it really matter in the grand scheme? No. However, that attitude doesn’t make for happy fans.

Same with the lunar meteorite watch. The main benefit the space program offers the average civilian is the chance to see cool pictures, and here we have the cosmos putting on a continuous fireworks show for free. But all we get are some fuzzy shots taken through an earthbound 14-inch telescope (Google “lunar meteorite video” to see for yourself), and they arrive two months late.

If NASA wants to keep those appropriations rolling in, it’ll have to do better than that. The Hubble Space Telescope can take extraordinarily detailed photos of objects in space due to its vantage point in low earth orbit, where it’s free of atmospheric distortion. The Hubble, I acknowledge, is too important to waste on shooting videos of lunar car wrecks and is too clunky for moon photography anyway. (Among other shortcomings, it can’t shoot the whole lunar surface at once, but rather must piece it together from 130 separate shots, which won’t work for fleeting phenomena like meteorite strikes.)

The meteorite watch, however, gives us a plausible scientific excuse to launch a spacecraft optimized for high-res lunar videography. I’m happy to make the case for this to the relevant parties. (“Congressman, do you want to see our astronauts squashed like bugs because we were too cheap to take decent pictures of incoming rocks?”) If NASA’s interested, I’m available for a reasonable fee.

Make no mistake, there’s plenty to see. In its first 18 months, examining less than an eighth of the lunar surface, the NASA monitoring program recorded 54 meteorite impacts big enough to produce flashes of light visible through an earth telescope. Sixteen hours of monitoring during one meteor shower captured 27 visible impacts. Given some skillful editing, imagine what that would look like on HDTV.

But you asked what would happen if a kilometer-wide rock hit the moon. Not only would it be readily visible on earth, it’d leave quite a gouge on the lunar surface — a crater 9.5 miles across, assuming the meteorite came straight down at 35,700 miles an hour. If it struck at the same speed as the May 17 boulder, it’d release the equivalent of 7.8 trillion tons of TNT, which would probably be visible in broad daylight.

We’re not likely to see anything like that soon. Even for the earth, a larger target, a one-kilometer asteroid strike occurs once every million years. But video of more typical lunar impacts, if we were set up to record them properly? Don’t know about you, but I’d definitely click “play.”

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References

“Bright Explosion on the Moon” NASA Science – Science News Online edition, 17 May, 2013. Accessed 22 June, 2013. http://science.nasa.gov/science-news/science-at-nasa/2013/16may_lunarimpact/

Chapman, Clark R. "The hazard of near-Earth asteroid impacts on earth." Earth and Planetary Science Letters 222.1 (2004): 1-15.

Ivanov, Boris A. "Mars/Moon cratering rate ratio estimates." Space Science Reviews 96.1-4 (2001): 87-104

“Meteor crashes into moon's surface causing flash – video” The Guardian Online edition, 18 May, 2013. Accessed 23 June, 2013. http://www.guardian.co.uk/science/video/2013/may/18/meteor-crashes-moons-surface-flash-video

Neukum, Gerhard, Boris A. Ivanov, and William K. Hartmann. "Cratering records in the inner solar system in relation to the lunar reference system." Space Science Reviews 96.1-4 (2001): 55-86.

Ortiz, J. L., et al. "Detection of sporadic impact flashes on the Moon: Implications for the luminous efficiency of hypervelocity impacts and derived terrestrial impact rates." Icarus 184.2 (2006): 319-326.

Suggs, Robert M., et al. "The NASA lunar impact monitoring program." Earth, Moon, and Planets 102.1-4 (2008): 293-298.

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