On Sunday and Monday, those in the Western Hemisphere with clear skies were fortunate enough to see the last total lunar eclipse of the decade. As the moon took on a distinctly redder shade just before midnight Eastern Time, livestreams of the phenomenon showed a flash of light suddenly and briefly emanating from the lunar surface.
Anthony Cook, an astronomical observer at Los Angeles’ Griffith Observatory which streamed the eclipse, thought it could have just been the camera’s random electronic noise. Then astronomers and citizen scientists started to share their detection of the flash on Reddit and Twitter.
The only explanation was that something slammed into the lunar surface and obliterated itself.
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The moon is a multi-billion-year-old library of impact events, with fresh collisions still taking place frequently today. Capturing a lunar impact on video is rare enough, but this event — a collision during a total lunar eclipse — may be a first.
“I have not heard of anyone seeing an impact like this during a lunar eclipse before,” said Sara Russell, a professor of planetary sciences at the Natural History Museum in London.
Dr. Russell said that flashes can only be seen from Earth when the lunar surface is in shadow, which is normally just a few days before and after the new moon. A luminous full moon is anathema to such detections, but a total lunar eclipse, painting our pale celestial companion in darkness, changed that.
Justin Cowart, a graduate student in geosciences at Stony Brook University in New York who spotted the flash, managed to narrow down the impact location. Using images from an amateur astronomer, Christian Fröschlin, Mr. Cowart determined that the flash took place somewhere in the lunar highlands, south of Byrgius crater.
NASA’s Lunar Reconnaissance Orbiter, which studies the moon’s surface, may soon spot the fresh crater. But the impact flash itself can be used to roughly estimate the size of the object that crashed on the moon, and its momentum. Based on NASA and European Space Agency databases of impact flashes, Mr. Cowart guesses that “it was probably somewhere between the size of an acorn and tennis ball.”
Detection data from the Moon Impacts Detection and Analysis System, or MIDAS, found that the space rock was a pipsqueak of a meteoroid, only weighing something like 22 pounds, and measuring no more than 12 inches across.
The provenance of the meteoroid remains an open question. According to Jose Maria Madiedo, an astrophysicist at the University of Huelva in Spain who is a co-director of the MIDAS system, most objects that hit Earth come from comets and not asteroids, and the same applies to the moon
With that in mind, he said, “the most likely situation is that the impactor was a rock from a comet.”
Considering its tiny dimensions, it must have been moving at an incredible speed to create a flash of light visible from Earth. Indeed, Dr. Madiedo said that the impact velocity would be in the range of 38,000 miles per hour. That is just over twice the speed that NASA’s old space shuttle had to move after launch to escape Earth’s gravity.
A total lunar eclipse is always something to marvel at, but one being gate-crashed by a hypersonic cometary shard is something else entirely.
An earlier version of this article, using information from an astronomer, misstated where on the moon a flash of light was seen. It was south of the Byrgius crater, not west of Legendre crater.
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