A new theory of how rockets erode moon soil makes a startling prediction: powered lunar landings may fling around four to 10 times more material than previously thought. The work suggests that without sufficient precautions, rocket-lofted lunar dust would pose a serious sandblasting hazard to cargo and crew on the moon.
The physicist behind the new calculations, Phil Metzger, is one of the world’s leading experts on how rocket plumes interact with planetary surfaces. Now that his research has been published in twostudies in the journal Icarus, he is calling for more global cooperation on the problem as space agencies plan out long-term lunar infrastructure—including human outposts.
“The amount of damage [that lunar dust] might cause to a spacecraft could be an order of magnitude worse than we believed,” says Metzger, director of the University of Central Florida’s Stephen W. Hawking Center for Microgravity Research and Education. “The international community needs to work on protocols and international agreements so multiple parties can operate on the moon.”
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“These [studies] are important, at least for the engineering and safety issues,” adds Mihály Horányi, a lunar physicist at the University of Colorado Boulder, who wasn’t involved with the studies. “People at least should be prepared for the possible outcome that these plumes are really more serious than previously estimated.”
Pulverized into existence by rock-shattering meteoroid impacts, moon dust is nasty stuff. The jagged material can snarl up spacesuits’ joints, clog up radiators, and irritate astronauts’ eyes and lungs—and that’s just the stuff stirred up at low speeds. The moon lacks an atmosphere, so when a rocket lands there, no air slows the material that’s kicked up. Small dust particles accelerated by rocket exhaust can travel many kilometers or even escape the moon entirely.
Researchers have known for decades that rocket-flung dust can harm lunar equipment. In November 1969 the Apollo 12 lunar module landed about 160 meters (520 feet) from a NASA robotic probe called Surveyor 3, fulfilling a goal to demonstrate a pinpoint landing. But when Apollo 12 astronauts inspected Surveyor 3, they found that it was caked in dust. Samples of the probe returned to Earth showed severe sandblasting damage, including literal craters.
Concerns around sandblasting have already shaped NASA policy. The agency issued nonbinding guidance in 2011 that recommended small lunar landers should not touch down within two kilometers of the Apollo landing sites to protect the areas from dust. That guidance was co-drafted by Metzger, who worked at NASA at the time, and was based on thousands of simulations. But the two-kilometer cutoff itself was an arbitrary placeholder, to be revisited as the underlying theory of lunar landings improved. The 2011 buffer was just the apparent distance of the lunar horizon from a six-foot-tall person’s point of view.
Sandblasts from the Past
Metzger’s new research reexamines an influential theory developed by Leonard Roberts, a brilliant Welsh-born NASA researcher who worked on the Apollo program. Roberts developed sophisticated equations to calculate how a rocket’s exhaust would behave as the plume descended toward the moon’s dusty surface.
In 1963 Roberts also estimated how the lunar soil would respond in turn to the rocket’s plume. As Roberts envisioned it, a rocket’s exhaust streaming across the lunar surface would erode the moon’s soil by shearing off individual grains, like a strong wind uprooting a tumbleweed. Roberts also theorized a feedback loop that would limit the erosion rate. As the gas picked up soil grains, it would impart momentum to the grains, reducing the gas’s momentum in turn.
For decades, Roberts’s theory has been a key tool for calculating the erosion—and the blast radius—caused by a lunar landing. Metzger and colleagues have, for instance, repeatedly used data-guided elaborations of Roberts’s approach to calculate just how much lunar soil the Apollo 12 lander sent aloft, most recently arriving at an estimate of 2.6 metric tons in 2015.
But for years Metzger had noticed some nagging issues with Roberts’s theory, including a contradiction with experimental data. On airplanes flying parabolic arcs to simulate lunar gravity, Metzger performed experiments where a gas jet fired into soil for about 10 seconds at a time. As the jets formed craters, Metzger found that the primary erosion rate at the craters’ center stayed constant, even though ever more dust grains built up within the gas—an accumulation that Roberts’s theory said should have controlled the erosion rate.
According to Metzger’s new theory, as rocket exhaust moves with incredible speed parallel to the moon’s surface, it does very little to stir up the soil directly. Instead a small fraction of the exhaust’s gas molecules diffuse into the soil, imparting some kinetic energy to the grains there. Erosion takes place only when the flux of kinetic energy into the soil is enough to push a fairly hefty soil grain up and over its neighbors—in the moon’s case, a height of roughly a quarter of a millimeter. Once the grains clear this height, the exhaust accelerates them away.
Although this may seem like a minor technical revision, it has huge implications for how lunar blast radii are calculated. In one of the new studies, Metzger tested his new theory against footage of the Apollo 16 moon landing that was filmed out of one of the lander’s windows. He found that his theory nicely explained how the dust flung out by the lander’s rocket exhaustduring descent blocked the crew’s view of nearby craters. But his calculations also imply that the Apollo 16 lander flung out between 11 and 26 metric tons of lunar soil—an amount at least four times larger than previous estimates, with much of the remaining uncertainty tied to the soil’s poorly constrained properties.
This upswing “shows how much we don’t know and how much uncertainty is in this prediction,” says Michelle Munk, acting chief architect for NASA’s Space Technology Mission Directorate at NASA Headquarters. “I’m glad that [Metzger] continues to work in this area; we really rely on his insight.”
Old Laws in a New Era
Without precautions, lunar sandblasting would now pose far greater risks than it did during the Apollo era, when landers were smaller and access to the moon’s surface was more limited. By the end of the Apollo program, the Apollo lunar lander had a touchdown mass of about 7.5 metric tons. Much larger vehicles are now in the picture, with SpaceX’s Starship potentially capable of landing 100 metric tons or more on the moon. There’s also the issue of cadence. The Apollo missions never returned to the same landing site, but both NASA and the China National Space Administration envision building lunar outposts that human crews will visit repeatedly. Sandblasting damage is cumulative, building up with every launch or landing.
Complicating matters, many international agencies and private companies are now angling to visit the lunar surface, suggesting a need to coordinate launches and landings. But as great as the need is for international cooperation, Metzger says, any diplomatic quick fix could create other issues, too. Under the Outer Space Treaty of 1967, the foundation of international space law, countries can’t claim territory on the moon or any other celestial body. But what if a country landed a sensitive instrument in the middle of a lunar region particularly well-suited to human habitation, just to declare that other countries’ moon landings within a wide radius would risk damaging the device? “There’s a concern that there could be a bad actor who wants to sidestep the Outer Space Treaty and claim de facto territory,” Metzger says.
There are potential engineering work-arounds. For example, infrastructure could be built behind berms or protective barriers, and landing areas could be covered in durable, dust-dampening landing pads. Landers could also make their final descent using thrusters placed higher up on the vehicles to reduce their stirring of the soil. Metzger, who has consulted with SpaceX, notes that based on publicly available renders, SpaceX seems to be pursuing this strategy with its current lunar Starship design.
More data from the moon could be coming soon. Munk is principal investigator on Stereo Cameras for Lunar Plume Surface Studies (SCALPSS), a NASA camera suite designed to map the soil directly beneath a moon lander before and after touchdown. The first version of SCALPSS flew on Odysseus, the Intuitive Machines lander that touched down in February, but it wasn’t able to collect any data. An upgraded version may fly as soon as the end of this year aboard the Blue Ghost lander built by U.S. company Firefly Aerospace.
“Gathering data from an in situ landing is the best validation,” Munk says.