A chunk of asteroid or comet whizzing through space slammed into the moon and left a 728-foot-wide pit, the largest known fresh crater in the solar system.
Nobody saw it happen. No alarm sounded. No telescope caught the flash. The rock hit the moon's eastern edge at roughly 33,000 mph, punching a hole 2 1/2 football fields wide into the lunar surface. Then, the moon went on about its business for more than a year.
The discovery came from housekeeping. NASA's Lunar Reconnaissance Orbiter has photographed the entire moon month after month since 2010, and researchers periodically line up two images of the same terrain, shot under matching light, to see whether anything has changed. Robert Wagner, an image-processing specialist, ran that comparison and found something wholly unexpected in the spring 2024 images.
"I just stopped, dropped everything, and started looking into what that spot was," Wagner said in a statement.
But the crater is the least of it. The impact reached out roughly 30 times its own radius, fluffing dust over an area you could see from orbit by looking at how the material cools at night. Beyond that, it scuffed the surface faintly for 75 miles. A strike that would barely register as a dot on a globe had the power to rearrange a region the size of a city.
Astronomers have named this new crater McGetchin, in honor of lunar scientist Tom McGetchin, whose mathematical model in 1973 predicted how debris scatters after a major space collision.
That ties into what scientists have learned from this crash. A thermal analysis shows the original impact left a sort of bruise that spans four miles across — a patch of ground that cools some 15 degrees Fahrenheit below its surroundings every lunar night. The cold spot occurs because the strike aerated the top powdery soil until it could no longer retain heat, according to research. Findings from the crater appear in two Science Advances papers.
When researchers checked the catalog of other recent craters, nearly every impact bigger than about 100 feet appeared to have done the same thing. If those events happen often, then the reach of these impacts loosens the moon's topsoil about as much as the impacts themselves. That means the things scientists have been counting — the holes in the ground — only cover a fraction of the damage that impacts actually inflict.
Within a few hundred feet around McGetchin, a blanket of pulverized rocks buried nearly every smaller existing crater, wiping out traces of older impacts in one fell swoop. Boulders the size of delivery trucks now ring the rim. At the first moment of contact, before the crater had even finished forming, material sprayed out sideways fast enough to clear a mountain ridge and mark the plain on the other side.
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In retrospect, Apollo 16 provided evidence for what happened beyond the debris field. When the crew landed near South Ray crater in 1972, the astronauts' bootprints pressed deeper into the moondust than at other landing sites. The ground was softer there, for reasons unclear. Decades later, the Lunar Reconnaissance Orbiter began mapping nighttime temperatures and found that the site sits inside a faded patch that cools faster than the land around it.
Cold spots fade somewhere between 100,000 and 2 million years, so nearly every one available for study is already half-erased, buried under whatever has happened since. But McGetchin hands scientists a control case, with before and after pictures.
The research may have implications for other worlds in the solar system. Mars has patches like these around its young craters. Mercury is expected to have them, too, and a heat-sensing instrument aboard BepiColombo, the European and Japanese spacecraft now closing in on the planet, may be the first to spot one there.
Meanwhile, McGetchin Crater, which sinks about 141 feet deep, is already beginning to age. Sunlight and the steady rain of micrometeorites will dull its brightness, and the cold spot will warm toward the temperature of the ground around it.
As NASA maps out its future Artemis missions to the moon, some scientists might advocate for a future visit to McGetchin. The team would like to send a lunar rover to the site while there is still something left. A rover could evaluate how loose the soil is and how deep that fluffy layer goes. An orbiting spacecraft can only make inferences.
The previous record-holder for largest fresh crater in the solar system measured about 230 feet, according to NASA, carrying roughly one-tenth the energy of the collision creating McGetchin. A lunar impact of McGetchin's scale is expected only about once every 132 years.
