The astronauts who swung around the moon aboard NASA's Orion capsule spotted five tiny sparks from space rocks slamming into the lunar surface, likely the faintest impact flashes humans have ever seen.
The Artemis II Commander Reid Wiseman caught the first one during the mission just six months ago.
"I just saw a flash, 100 percent," he called out, placing it at about the 7 or 8 o'clock position on the moon. Moments later, he and crewmate Jeremy Hansen both saw another. Hansen described a "pinprick" of light on the lower third of the moon.
"It's just like a bloop," Hansen said.
Those sightings stunned the mission control room back in Houston, literally causing Kelsey Young's jaw to drop. Young, the mission's lunar science operations lead, admitted she hadn't expected the crew to see any micrometeorites at all. Artemis II flew during a quiet stretch of the year, outside any major meteor shower.
"You probably saw the surprise and shock on my face," she later told reporters, who had watched a broadcast of the space-to-ground communications. "And though I was not in the science evaluation room, I heard that there were audible screams of delight."
Each flash tells scientists something about how often bits of comets and asteroids pummel the moon and where they hit. NASA wants those numbers before it sends astronauts to live and work there. Lori Glaze, a senior NASA official, said the agency will use such data to judge the risk that meteorites, even the tiniest of space rocks, pose to future moon bases.
A flash happens when a speeding space pebble smacks the moon. Because the moon has no oxygen, a strike doesn't create combustion fire. Instead, the burst of light comes from the rock's pure kinetic energy.
During the crash, the extreme force instantly vaporizes the rock and a small amount of the moon's surface. That causes a split-second plasma cloud to glow just before cooling in the vacuum of space.
The naked-eye observation of impact flashes during the Artemis II mission, published this week in the Artemis II Preliminary Lunar Science Report, lends credibility to earlier astronaut reports. Crews reported three such flashes from lunar orbit a half-century ago — one on Apollo 16 and two on Apollo 17. Telescopes on Earth have since logged over 400.
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NASA timed the Artemis crew's watch for when the moon blocked the sun from Orion's view. That darkness let the astronauts' eyes adjust enough to catch faint flickers. The moon's surface stayed dark except for a soft glow of sunlight bouncing off Earth.
All five flashes the Artemis astronauts saw happened on the moon's unlit side. None showed any color, and each vanished instantly. Based on how faint they looked, scientists think rocks only a few centimeters across may have caused them, perhaps the size of a grape or a walnut. The crew, which included Victor Glover and Christina Hammock Koch, sketched each location on a tablet while their memories were fresh.
The moonbound crew saw more but said they reported only the ones that clearly differed from look-alikes. Astronauts often see random bright flashes of light in space — even with their eyes closed — because of cosmic rays passing through the spacecraft and directly striking their optic nerves.
But because some of these flashes were seen by more than one pair of eyes, NASA feels confident in the anecdotal reports.
"It was not sunglint off of particulate from the thrusters or the purge tanks," Wiseman said. "It was definitely impact flashes on the moon."
Meanwhile, the machines got nothing. A camera on the outside of the capsule filmed about two minutes of the eclipse at 30 frames per second and caught no flashes. NASA also hoped its Lunar Reconnaissance Orbiter, a probe that has mapped the moon since 2009, could photograph the sites. The team concluded the craters are probably too small for the camera to see.
NASA's lunar science team recruited stargazing groups to film the moon through tracking telescopes during the flyby and upload the video to a shared database. But the moon's phase seemed to work against them. Earth-based observers need a dark slice of moon to record flashes. So far, no one on the ground has confirmed one of the flashes from the flyby.
The sightings paid off a bet NASA's science team placed long before launch. Planners carved out dedicated time for the crew to watch for flashes, and the team trained the astronauts to report any they might see at any point in the mission.
"This is absolutely everything we hoped for by integrating science into flight operations," Young said.
