On September 11 2013 the moon was struck by a meteorite estimated to weigh half a tonne and traveling at 61,000km/h. The explosion was so bright it would have been visible with the naked eye from the Earth had anyone been watching at the right moment.
"This is the largest, brightest impact we have ever observed on the Moon," said Prof Jose Madiedo, of the University of Huelva in south-western Spain. The next brightest recorded event had a third the luminosity.
However, Madiedo's statement refers only to events confirmed with modern instruments. In 1178 five monks reported an event, described in a chronicle of the day, “There was a bright New Moon, and as usual in that phase its horns were tilted towards the east. Suddenly, the upper horn split in two. From the midpoint of the division, a flaming torch sprang up, spewing out fire, hot coals, and sparks.” In his TV series Cosmos (22 minute mark), Carl Sagan pointed to evidence the event the monks described may have been the formation of the crater Giordano Bruno, although this theory remains controversial.
The event of last September was rather less dramatic, but had the advantage of being observed not by monks but the Spanish Moon Impacts Detection and Analysis System (MIDAS) and reported in The Monthly Notices of the Royal Astronomical Society. MIDAS have been watching the moon for collisions such as this since 2009.
Video of the event can be seen below, along with an animation MIDAS produced on lunar meteor strikes.
"Usually lunar impacts have a very short duration - just a fraction of a second. But the impact we detected lasted over eight seconds.,” said Prof Madiedo. The collision is thought to have been the equivalent of letting off 16 tonnes of TNT, and created a crater 50m wide – tiny compared to Bruno's 22km. "That's the estimation we have made according to current impact models. We expect that soon NASA could observe the crater and confirm our prediction," said Prof Madiedo.
The brightness peaked at magnitude 2.9±0.2, the same as Alcyone, the brightest of the Pleiades.
An object of this size would make a spectacular fireball in the Earth's atmosphere, but would burn up rather than creating a crater. MIDAS claims the frequency of lunar impacts they have observed indicates that approximately ten times as many objects of this size are hitting the Earth's atmosphere as previously thought.