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　　Every day, 100 tons of space dust rain down on the Earth's atmosphere. By night, we know this material as shooting stars. And our neighbor the moon is likewise exposed to that debris—but without an atmosphere to stop it. So all those particles strike the moon's surface, at 12 miles per second, like tiny bullets. And each impact kicks up a thousand times its weight, in moondust.

　　Mihály Horányi, a physicist at the University of Colorado, makes this analogy: "Imagine you are making pasta and there's flour on the table, and you get impatient and upset and you smack the table. There's going to be a whole cloud of tiny particles, you know, flying off the table."

　　Horányi and his colleagues sampled that cloud of particles with NASA's Lunar Atmosphere and Dust Environment Explorer, or LADEE, as it orbited the moon, slurping up dust. Judging by the shape and density of the dust cloud, the scientists say the particles striking the moon and kicking up the cloud must be mostly high-speed comet grains, rather than slower bits of asteroid. And, as you might expect, the moon's halo of dust increases during heavy bombardment—the same time that we have meteor showers here on Earth. The findings are in the journal Nature.

　　There is one mystery these measurements do not solve: the source of the strange "horizon glow" the Apollo astronauts observed on the moon. Because Horányi says that, based on these samples, there just isn't enough dust to create the visual effect the astronauts saw. Not that this is 'case closed.' "There is not a single paper or a single measurement that I can claim to be the last word on anything, so, no, as always, there are more questions and open issues. But one way to figure out is to try it again. Send people. Orbit the moon again and see if we have better instruments and take better images, see if we could resolve this." But if NASA's current plans are any indication, it might be a while before any American astronauts see that glow firsthand.

　　—Christopher Intagliata

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