* Astronomy

NASA scientists have observed an explosion on the Moon. The blast, equal in energy to about 70 kg of TNT, occurred near the edge of Mare Imbrium (the Sea of Rains) (42.1W, 36.5N) on Nov. 7, 2005, when a 12-centimeter-wide meteoroid slammed into the ground travelling 27 km/s. The explosion was captured by Rob Suggs of the Marshall Space Flight Centre (MSFC) and Wes Swift whilst they were testing a new 10-inch telescope and video camera to monitor the moon for meteor strikes On their first night out they caught one. 

 The red dot marks the location of the November 7, 2005, meteoroid impact. Credit: NASA/MSFC/Bill Cooke. 

The object that hit the moon was probably part of the Taurid meteor shower that produce fireballs in late October and early November 2005. Unlike the Earth, the moon has no atmosphere to slowdown the meteoroids and turn them into harmless streaks of light. On the moon, meteoroids hit the ground--and explode.

"The flash we saw was about as bright as a 7th magnitude star" - Rob Suggs. That's two and a half times dimmer than the faintest star a person can see with their unaided eye.  Cooke estimates that the impact gouged a crater in the moon's surface "about 3 meters wide and 0.4 meters deep." As moon craters go, that's small. "Even the Hubble Space Telescope couldn't see it".

The moon is 384,400 km away. At that distance, the smallest things Hubble can distinguish are about 60 meters wide. This isn't the first time meteoroids have been seen hitting the moon. During the Leonid meteor storms of 1999 and 2001, amateur and professional astronomers witnessed at least half-a-dozen flashes ranging in brightness from 7th to 3rd magnitude. Many of the explosions were photographed simultaneously by widely separated observers. And in the early morning hours of Nov. 15, 1953, an amateur astronomer, Dr. Leon Stuart, in Oklahoma photographed what he believed to be a massive, white-hot fireball of vaporised rock rising from the centre of the Moon's face. Almost a half-century later, after numerous space probes and six manned lunar landings, what had become known in astronomy circles as "Stuart's Event" was proven to be correct.

There are many questions that need answering: "How often do big meteoroids strike the moon? Does this happen only during meteor showers like the Leonids and Taurids? Or can we expect strikes throughout the year from 'sporadic meteors?'" - Rob Suggs. Explorers on the moon are going to want to know. "The chance of an astronaut being directly hit by a big meteoroid is miniscule" - Bill Cooke. Though, the odds are not well known "because we haven't done enough observing to gather the data we need to calculate the odds." Furthermore, while the danger of a direct hit is almost nil for an individual astronaut, it might add up to something appreciable for an entire lunar outpost. Also, ground-shaking impacts could kick up moondust, possibly over a wide area. Moondust is electrostatically charged and notoriously clingy. Even a small amount of moondust can be a great nuisance: it gets into spacesuit joints and seals, clings to faceplates, and even makes the air smell when it is tramped indoors by moonwalkers. Could meteoroid impacts be a source of lunar "dust storms?" Another question for the future.... Suggs and his team plan to make more observations.

"We're contemplating a long-term monitoring program active not only during major meteor showers, but also at times in between. We need to develop software to find these flashes automatically. Staring at 4 hours of tape to find a split-second flash can get boring; this is a job for a computer"- Rob Suggs. With improvements, their system might catch lots of lunar meteors. "I'm ready for more surprises"- Rob Suggs.