Spent Falcon 9 upper stage to crash into moon Aug 5 in first visible impact flash

A spent Falcon 9 upper stage is expected to crash into the moon on Aug 5, potentially producing the first-ever visible impact flash on the sunlit side. Scientists say the event offers a rare chance to study lunar debris and highlights dust hazards for future missions.

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A spent Falcon 9 upper stage is on course to crash into the moon on August 5, and researchers say the impact could produce the first flash ever recorded on the sunlit side of the lunar surface. The collision is expected to occur at 06:34 UTC near the Einstein crater on the moon's western limb, with the rocket hitting at a speed of more than 5,400 miles per hour.

According to a study posted on arXiv on July 27, the impact could kick up a plume of debris that may be visible from Earth. The research, led by William Jo, a doctoral candidate at the University of Texas at Austin's Cockrell School of Engineering, and supervised by aerospace engineering professor David Goldstein, simulates how the 3,900-kilogram hollow metal stage would behave on impact. The simulation shows the shell collapsing inward at the edges, throwing out a debris curtain up to 183 kilometers wide, with a thin, faster spike rising nearly straight up. In total, about 12,700 kilograms of lunar material would be displaced.

The stage is the leftover from a Falcon 9 launch in January 2025 that carried two landers to the moon. While the simulation assumes the rocket comes straight down, engine-first, the real object is likely to arrive at an angle, which would produce a smaller and dimmer plume.

Observing the flash from Earth is possible but uncertain. If the plume reaches 50 kilometers past the lunar limb, it would span about 28 arc seconds, which a small telescope could resolve. However, the plume is expected to be 30 to 300 times fainter than the moon itself, requiring a very steady telescope mount. The peak brightness would occur about 90 seconds after impact. The best viewing location is the US Southeast; observers north of a line from Massachusetts to Texas would see the moon low on the horizon, through more than three times the atmosphere's thickness.

Bill Gray of Project Pluto first flagged the impact and is urging astronomers to try to catch it. Historical precedent exists: in 1999, NASA's Lunar Prospector probe was deliberately crashed into a crater to search for water, and in 2009, a spent rocket was slammed into the south polar region, where water ice was detected in the plume. A 2022 upper-stage impact widely attributed to a Falcon 9 was likely a misidentification, with Gray and others saying it was almost certainly a discarded Chinese booster.

Goldstein described the stage as "like an empty eggshell, because it had all the fuel in it, and there is a rocket engine at one end that's denser." He added, "I would view our study as the most optimistic in terms of generating sprays." The research underscores concerns for future lunar exploration, as NASA aims to return astronauts to the moon within a few years and China races to put humans there. Ejecta particles traveling at 300 to 400 meters per second could be abrasive and harmful. "We're going to be colonizing the moon apparently pretty soon," Jo said. "Dust control during lunar operations is going to be one of the biggest problems that's faced," Goldstein added.