NASA astronomers monitoring the moon for signs of meteor impacts say they've identified the biggest explosion in the history of the observation program.

The scientists, who have spent 8 years looking for explosions caused by meteoroids hitting the lunar surface, said Friday the impact would have been visible to the naked eye to anyone looking at the moon at the right moment.

"On March 17, 2013, an object the size of a small boulder hit the lunar surface in Mare Imbrium," Bill Cooke of NASA's Meteoroid Environment Office said in a release from the space agency. "It exploded in a flash nearly 10 times as bright as anything we've ever seen before."

The impact site glowed with a brightness equivalent to a fourth magnitude star for about 1 second, astronomers said.

Ron Suggs, an analyst at the Marshall Space Flight Center, was the first to notice the impact in a digital video recorded in March by one of the monitoring program's 14-inch telescopes.

"It jumped right out at me, it was so bright," he said in a NASA release.

The meteor, estimated to weigh 80 pounds and measuring several feet in diameter, hit the moon traveling 56,000 mph, creating an explosion equivalent to 5 tons of TNT.

The moon, unlike the Earth, is without an atmosphere to protect it from meteor strikes.

Since the monitoring program began in 2005, NASA said, the lunar impact team has detected more than 300 strikes, although the one in March was by far the biggest.