Anyone looking at the moon at the right time on St. Patrick’s Day with a small telescope would have seen it. A pinpoint flash of light as bright as a 4th magnitude star suddenly appeared that evening in the lunar sea Mare Imbrium and faded away one second later.
“On March 17, 2013, an object about the size of a small boulder hit the lunar surface in Mare Imbrium,” says Bill Cooke of NASA’s Meteoroid Environment Office. “It exploded in a flash nearly 10 times as bright as anything we’ve ever seen before.”
“It jumped right out at me, it was so bright,” said Ron Suggs, an analyst at the Marshall Space Flight Center. He was the first to notice the explosion in recordings made with a 14-inch telescope maintained by NASA’s lunar impact observation program. Observers there have been video monitoring the moon for the past 8 years looking for signs of explosions caused by meteoroids hitting the surface. Since 2005 the team has detected more than 300 strikes, but the St. Pat’s Day impact was the biggest and brightest to date.
Based on the flash brightness and duration, the space boulder measured between a 1 foot long and a foot and a half long (0.3-0.4 m) and hit the moon traveling at 56,000 mph with a force of 5 tons of TNT.
Remember, the moon has no atmosphere, so there’s nothing to slow down or break apart an incoming meteoroid as happens on Earth so even a small stone can dig a significant hole; its energy of motion transforms it into a powerful little bomb. NASA estimates the crater could be as wide as 65 feet (20 m) across. By the way, the flash of light comes from molten rock and white-hot dust vapor created at the moment of the strike.
That good news because the Lunar Reconnaissance Orbiter should have no trouble spotting the crater once it targets the impact region. Comparing the size of the crater to the brightness of the flash would give researchers a valuable “ground truth” measurement to validate lunar impact models.
In an interesting twist, the very night the moon got smacked, all-sky cameras operated by NASA’s and the University of Western Ontario recorded several bright fireballs streaking through Earth’s skies from the same direction in space as the lunar meteoroid.
“My working hypothesis is that the two events are related, and that this constitutes a short duration cluster of material encountered by the Earth-Moon system,” says Cooke. Who knows. Maybe we’ll see a similar show next March.