Since 2005, NASA has observed 100 explosions on the surface of the Moon. The big booms, usually equivalent to a few hundred pounds of TNT, are caused by meteoroids smashing into the Moon's surface. The flashes of the bigger impacts are easily spotted by amateur astronomers on Earth using a backyard telescope. NASA launched the monitoring program in response to the recent plans to send humans back to the Moon.
These explosions don't require oxygen or combustion. Meteoroids hit the moon with tremendous kinetic energy, traveling 30,000 mph or faster. "At that speed, even a pebble can blast a crater several feet wide. The impact heats up rocks and soil on the lunar surface hot enough to glow like molten lava – hence the flash…"
Fortunately, says (the head of NASA's Meteoroid Environment Office, Bill) Cooke, astronauts are in little danger. "The odds of a direct hit are negligible. If, however, we start building big lunar outposts with lots of surface area, we'll have to carefully consider these statistics and bear in mind the odds of a structure getting hit."
Secondary impacts are the greater concern. When meteoroids strike the Moon, debris goes flying in all directions. A single meteoroid produces a spray consisting of thousands of "secondary" particles all traveling at bullet-like velocities. This could be a problem because, while the odds of a direct hit are low, the odds of a secondary hit may be significantly greater. "Secondary particles smaller than a millimeter could pierce a spacesuit," notes Cooke.