100 explosions recorded on the Moon during the past 2.5 years

Ceres, the Moon and the Earth.

Space.com tells me that there have been a total of 100 explosions recorded on the Moon over the past 2.5 years since we started paying close attention:

"We started our monitoring program in late 2005 after NASA announced plans to return astronauts to the Moon," says team leader Rob Suggs of the MSFC. If people were going to be walking around up there, "it seemed like a good idea to measure how often the Moon was getting hit."

"Almost immediately, we detected a flash."

That first detection — "I'll never forget it," he says — came on Nov. 7, 2005, when a piece of Comet Encke about the size of a baseball hit Mare Imbrium. The resulting explosion produced a 7th magnitude flash, too dim for the naked eye but an easy target for the team's 10-inch telescope.

A common question, says Cooke, is "how can something explode on the Moon? There's no oxygen up there."

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."

and:

"Even when no meteor shower is active, we still see flashes," says Cooke.

These "off-shower" impacts come from a vast swarm of natural space junk littering the inner solar system. Bits of stray comet dust and chips off old asteroids pepper the Moon in small but ultimately significant numbers. Earth gets hit, too, which is why on any given night you can stand under a dark sky and see a few meteors per hour glide overhead — no meteor shower required. Over the course of a year, these random or "sporadic" impacts outnumber impacts from organized meteor showers by a ratio of approximately 2:1.

and lastly:

Fortunately, says 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.

At present, no one knows how far and wide secondary particles travel. To get a handle on the problem, Cooke, Suggs and colleagues are shooting artificial meteoroids at simulated moon dust and measuring the spray. This work is being done at the Vertical Gun Range at NASA's Ames Research Center in Mountain View, CA.

This is actually one of the reasons why proponents of a Mars first strategy don't want to return to the Moon first - radiation, lack of atmosphere making a settlement vulnerable to asteroid strikes like this, and the weird 14-day day and 14-day night. You can't beat the Moon for its location though, and Mars has terrible launch windows so there are just as many arguments for not going to Mars first. Venus has an Earthlike temperature and atmosphere 50 km above the surface in the cloudtops which originally made me favour there but it turns out that there is lightning there, which is another factor to take into account. Perhaps Ceres will turn out to be the best location, who knows.