Late Sunday evening, as Earth’s shadow painted the full moon’s face a deep, rusty red, a wayward meteor smashed into the lunar surface. The collision’s aftermath—a brief flash of light at the bloody orb’s darkest edge—lasted just long enough for some sharp-eyed eclipse viewers to spot.
Such impacts are normally “hard to detect,” says Ryan Watkins of the Planetary Science Institute. “Most of the impacts are very small,” she notes, “and we can only see them on portions of the moon that are dark or only slightly illuminated.”
As far as scientists know, it is the first time a meteorite impact visibly coincided with a total lunar eclipse; but in general, these types of lunar collisions are extremely common. And in fact, recent observations on Earth and from space suggest the moon is pummeled at higher rates than previously anticipated. “It’s very normal to see something like this,” says Arizona State University’s Mark Robinson, who is in charge of a camera currently riding onboard a lunar-orbiting spacecraft. “The moon gets hit every day.”
As for as what that means? Well, if you are planning on building a moon base, it is desirable to know how often the cosmos rains sizable chunks on your roof. “If you imagine this rock falling on your head, it’s not so pleasant,” says Stephanie Werner of the University of Oslo, who studies lunar impact rates. “There is definitely uncertainty in how well we understand the small projectile rate. The more information we can collect, the more exciting it is.”
In the hours following initial reports of an impact—which viewers glimpsed on several live streams of the eclipse—astronomers, photographers and others scurried to confirm a collision had occurred. They replayed videos, sifted through photos and ruled out camera artifacts, orbiting satellites and other phenomena.
Finally Jose Maria Madiedo, an astronomer at the University of Huelva in Spain, weighed in. “The impact flash has been recorded by telescopes operating in the framework of MIDAS survey from Europe,” he announced. MIDAS—the Moon Impact Detection and Analysis System—involves three different observatories in southern Spain. Since 1997 Madiedo and his colleagues have aimed the triad’s telescopes at the moon and recorded impact flashes. In the early morning hours of January 21 two of the three observatories were clouded out, he says, but the weather at the Seville site cooperated. “In total, four telescopes at that observatory obtained images of the event,” he says.
Video of the lunar impact captured by a telescope in the Moon Impact Detection and Analysis System (MIDAS).
Now, based on the brightness and duration of the observed flash, which lasted for 0.3 second, Madiedo calculates the impactor weighed about 10 kilograms, slammed into the lunar ground at 61,000 kilometers per hour and created an impact crater between seven and 10 meters in diameter. “The most likely situation is that the impactor was a fragment of a comet,” he says, noting that the bulk of the debris in our space neighborhood is broken-off cometary chunks. “The explosion would be equivalent to 0.5 tons of TNT.”
On Earth only the largest impactors survive a descent to the planet’s surface whereas smaller fragments disintegrate, burning up as they are dragged through our protective atmosphere, sometimes leaving observable, flaming streaks.
But the moon is not similarly swaddled. There, objects hurtling toward it are not slowed on their downward journey and instead slam into its surface at incredibly high speeds. (The moon’s cratered surface, which holds a much more indelible record of impacts than Earth’s, speaks to the frequency with which this happens.) During such an impact event, surrounding materials are briefly superheated and the molten, vaporized rocks emit an observable glow, even in the absence of oxygen.
Large impacts are exceedingly rare, although Madiedo caught one in 2013 that lasted for more than eight seconds and briefly shone as brightly as Polaris, the North Star. “In theory it could have been seen from Earth without any instrument,” he says. But relatively small impacts, like Sunday’s event, happen all the time—roughly every two or three months, Madiedo notes. Even smaller impacts, which are only observable by dedicated telescopes, are much more frequent; last year MIDAS detected two lunar impacts in July, spaced almost exactly one day apart.
Seismometers placed on the lunar surface by Apollo astronauts recorded roughly 1,700 tremors produced by pancaked meteors. And Watkins, who as a student helped design an instrument that could detect impact flashes, notes a NASA group dedicated to monitoring lunar flashes detected roughly 200 impacts over four years. Plus, images snapped by the Lunar Reconnaissance Orbiter (LRO)—which has been looping around the moon for nearly a decade—suggest new impact craters emerge all the time, with 222 fresh ones appearing over seven years. “These new impacts can teach us a lot about the moon,” she says. “Understanding the impact rate can greatly inform the impact hazard for the Earth–moon system and for future surface missions to the moon.”
Robinson, who is in charge of the LRO’s camera, says the spacecraft will not intentionally target the region of Sunday’s impact—doing that would mean sacrificing the team’s goal of objectively measuring impact rates on the moon. “If we’re targeting it, then it’s no longer a statistically meaningful look at impact rates,” he says. “We can respond to social media and immediately try to take a picture of this and find it, or we can do good science.”
He notes, however, it is likely the spacecraft will snag an image of the moon’s newest crater at some point over the next several years. And if, by the end of its mission, the LRO still has not spotted the crater, the team might send it on over to snap a photo anyway.
“It is fun that it happened during an eclipse,” he says. “Anytime the moon gets more attention, it’s a good thing. But this is not a surprise or scary or unusual.”