A routine data-quality check turned up the solar system's largest fresh crater 15 months late

Robert Wagner was running a quality check on a mosaic of the Moon when a bright patch ringed by a dark halo stopped him. "I just stopped, dropped everything, and started looking into what that spot was," Wagner said ^[8]. The halo mattered because it implied material at the surface had been shaken up ^[9]. Confirmation came from comparing images of the same ground before and after the impact ^[10].

The Lunar Reconnaissance Orbiter has been circling the Moon for more than 17 years with seven instruments aboard ^[15]. Models of lunar impact frequency put a crater this size at about one every 132 years ^[13]; NASA's own estimate is once a century or longer ^[14]. Divide 17 by 132 and a mission of this length should have expected about 0.13 such events ^[16]. The crater, named for Thomas McGetchin, a former director of the Lunar and Planetary Institute in Houston, is three times bigger than the previous record holder, which the same orbiter found a decade ago ^[38][37].

Robinson's team traced churned surface more than 100 kilometres from the rim, according to the Associated Press account of the paper ^[21]. New Scientist reports the same team's figure as more than 120 kilometres ^[22]. Take the smaller one: 100,000 metres divided by a 222-metre crater is about 450 crater diameters of modified ground ^[23]. Robinson said the dust and rocky soil left at a higher angle than expected ^[24].

Diviner, the orbiter's thermal instrument, found a patch about 7 kilometres wide around the crater that runs roughly 16 degrees Fahrenheit cooler at night than the ground beside it ^[26][25]. The second paper attributes the cooling to fluffed-up regolith, which is less dense and so holds less heat ^[27]. "It's really striking that a crater can modify the lunar surface to distances far larger than the crater itself," said Tyler Powell of Johns Hopkins University, an author of that paper ^[28]. That cold spot spans about 31 times the crater's own diameter ^[29].

David Paige of UCLA, a co-author, said the team is "now thinking of impacts as a way to garden the regolith" ^[30]. Robinson puts the overturn rate of the Moon's top two centimetres at once every 80,000 years, faster than previously thought ^[31], and said the Apollo footprints will "definitely be long gone in that time frame" ^[32].

The 132-year number comes out of models of how often impacts occur ^[13]. Powell said having "a baseline for a pristine crater that can serve as our calibration point for how these processes start is really incredible" ^[35]. Neither paper says whether debris landing 100 kilometres out arrives with enough energy to damage a structure. Robinson said the next step is to calculate the risk of ejected crater material striking NASA's planned moon base ^[33]. "That information will help engineers harden structures so one won't have to worry about damage, or maybe worry less," he said ^[34].

At the small end, the smallest craters the orbiter can distinguish are about 30 feet across, made by rocks about 43 inches wide, and the team estimates roughly 140 of those form across the Moon each year ^[18]. Over the mission it has identified at least 1,000 new impact craters and flagged 100,000 other surface changes ^[17].