For a brief moment last September, a flash on the moon shone about as bright as the North Star, Polaris, giving away the biggest crash from a space rock hitting the lunar surface ever caught on camera, astronomers say.
The discovery -- "the brightest and longest confirmed impact flash," according to the study authors -- was detailed in the Monthly Notices of the Royal Astronomical Society and reveals that perhaps 10 times as many small rocky bodies as we thought are reaching Earth.
Space rocks, fragments broken from larger asteroids or comets, are constantly bombarding the Earth. But most of them never make it to the ground, because they're burned up upon entry into the planet's thick, protective atmosphere. So what few meteorite fragments actually touch down are often baseball-sized chunks that can fit into the palm of your hand. (Larger ones, like the Chelyabinsk meteor last year, do occasionally make it through, with dramatic and destructive consequences.)
The moon, as the Earth's faithful companion, suffers the same constant battering. But unlike the Earth, it lacks a thick, cushiony atmosphere to slow down projectiles, which shrink as the friction from pushing through all that air burns them up. The moon has only the most tenuous "exosphere" -- still interesting to study, but no good against incoming objects. So rocks of all sizes, from very small to very large, can make it to the moon's surface, which is why it's so heavily cratered.
Still, even with the constant barrage, it's hard to catch a crash in the act. That's why scientists have set up a network of small telescopes, called the Moon Impacts Detection and Analysis System (or MIDAS for short) to spot them.
Lead author Jose Madiedo, an astronomer at the University of Huelva in Spain, was working with two such telescopes last fall when a bright spot suddenly flared in the western part of Mare Nubium, a dark, ancient, lava-filled basin. A rock must have just hit the surface, the scientists said.
Their analysis showed that the interplanetary missile had probably weighed about 0.44 tons and stretched from a little under 2 feet to just over 4.5 feet long. It hit the dark basin at a speed of roughly 37,900 miles per hour and probably created a new crater about 131 feet wide in an explosion with the energy of 15 kilotons of TNT -- three times greater than the last record-holder. The bright spot lasted a total of 8.3 seconds, and for a tiny fraction of that, if you were staring at the moon at just the right moment, it would have been visible to the naked eye.
Understanding how many times these missiles hit the moon is important because it gives researchers a sense of how often they may hit the Earth. And in their analysis, the astronomers showed that small, approximately meter-sized objects may visit us 10 times as often as previously estimated. But don't worry -- these missiles would be small enough that they'd probably just result in fireballs in the sky rather than damage on the ground. The scientists said it's time to be on the lookout for more such fireballs in Earth's skies.
"A systematic monitoring of moon impact flashes but also of fireballs in the Earth's atmosphere would provide a more reliable impact frequency," the authors wrote.