Solving a moon mystery helps game out future landings
U-M, Johns Hopkins partnership explains a consistent pattern in the dust under moon landings.
5 minutes
Experts
Jesse Capecelatro
Multiple moon landings, conducted over several decades, produced a remarkably similar pattern of dust on the lunar surface. The recurrence of this pattern, created by rockets firing during touchdown, is a long-running mystery that now has an answer, provided by a research team including researchers at NASA, Johns Hopkins and U-M.
It matters because the ability to explain phenomena that occur during landing validates simulations of dust particles during landings, used to ensure that both crewed and uncrewed missions arrive safely.
“Trial-and-error isn’t an option when designing spacecraft for other planets. These findings give us greater confidence in the simulations guiding future missions,” said Jesse Capecelatro, a U-M associate professor of aerospace engineering and mechanical engineering, and a co-author of the study in Nature Communications.
The team determined what’s behind the repeating patterns, which appear as streaks radiating outward from the landing area. The team discovered that a fluid dynamics phenomenon called Görtler instability is at play.
Video transcript
On-screen text: When the Apollo missions landed on the Moon they kicked up more than just history. They stirred up clouds of high-speed lunar dust that’s still puzzling scientists decades later.Researchers from Michigan and Johns Hopkins are revisiting that mystery with new tools and old data. The team uncovered what causes these strange dust patterns.A rare fluid instability triggered by rocket exhaust, called Görtler instability. The team used a vacuum chamber, simulated lunar soil, and high-speed cameras to study this phenomenon. They hope this will influence the design of future landers and limit damage caused by these dust clouds.
“We discovered that the strikingly regular streak patterns seen during landings aren’t caused by the chosen landing sites,” said Rui Ni, associate professor of mechanical engineering at Johns Hopkins Whiting School of Engineering. “Instead, they result from the behavior of the supersonic rocket plume as it imprints on the granular surface. This effect is extremely pronounced on the Moon due to its near-vacuum environment.”
Determining the cause required recreating the Moon’s low-pressure conditions on Earth. Johns Hopkins researchers mounted six cameras inside a 15-foot vacuum chamber at NASA’s Marshall Space Flight Center in Alabama. The goal—to view the erosion caused by a gas jet running over a bed of simulated lunar soil under near-vacuum conditions.
Those cameras were able to see how craters formed and tracked the trajectory and velocity of the small soil particles. Ni and his team analyzed the laboratory data, while U-M researchers performed complementary simulations to determine whether the streaks originate in the gas jet above the surface, which the cameras could not pick up.
“The lower the pressure, the more you see the pattern,” said Capeceltro, who led the simulations. “At higher pressures, it’s almost not even visible. On Mars, with its higher pressure, the jet is collimated, almost like a light saber that drives into the surface, creating deep craters. You barely see the patterns there.
“But on the Moon, you have much lower pressure, so the jets blow outward more, causing this erosion instead of cratering.”
Capecelatro and Meet Patel, a U-M graduate student research assistant, first thought the patterns might be the result of turbulence. But the fact the streaks were found to be more pronounced at lower pressures discounted the idea. Patel began exploring a different explanation that doesn’t rely on turbulence, pointing to a type of fluid instability that generates persistent, streak-like vortices.
Görtler instability describes the creation of vortices, secondary flows in fluid dynamics, that appear in a boundary layer along a concave surface.
“The streaks are mainly caused by something that is happening very close to the engine itself,” Patel said “When the flow of exhaust gases comes out from the engine, there is a curvature to the flow—a bending that occurs due to the Moon’s extremely low pressure.
“We figured there must be something in the jet, like the gas, that’s actually generating these vortices that imprint on the surface.”
Johns Hopkins’ laboratory experiments visualized the streaks on the granular surface, but didn’t capture the gas flow just above it—something that could verify that the vortices originate upstream near the nozzle. U-M’s supercomputer, the Great Lakes HPC cluster, provided high-resolution simulations of the high-speed jet. That allowed the gas flow to be visualized away from the surface, and a wider range of pressure ratios to be studied beyond the experimental conditions.
Not only were counter-rotating vortices observed, but the number of vortices matched the theory predicted by the Görtler instability, providing evidence that this dynamic created the pattern.
“Understanding the high-speed ejecta produced by exhaust plumes is crucial because it’s something that poses serious risks to space missions,” Capecelatro said. “A theory that predicts the number of streaks created by a rocket engine can help validate numerical models.”
Funding for the research was provided by a NASA Space Technology Graduate Research Opportunity grant, as well as NASA’s