A dead Falcon 9 stage is heading for the Moon at seven times the speed of sound: what to expect on Wednesday

A spent SpaceX upper stage that has wandered through space for over a year is forecast to collide with the Moon on Wednesday, creating an impact scientists are treating as a rare calibration opportunity.

The collision

The Falcon 9 segment, about 12 metres long and weighing roughly 4.5 tonnes, is expected to strike near the Einstein crater on the Moon's sunlit western limb at approximately 6:34 am UTC. Space tracking expert Bill Gray projected an impact speed of about 8,700 km/h (5,400 mph), or seven times the speed of sound, releasing energy equivalent to three tonnes of TNT.

Things are getting crowded up there.

Benjamin Fernando of the Los Alamos National Laboratory said the resulting crater would measure roughly 27 metres in diameter and 5 metres in depth, too small to be seen from Earth but observable by lunar orbiters. NASA's Lunar Reconnaissance Orbiter and South Korea's Danuri spacecraft are positioned to collect before-and-after imagery of the crash site. Danuri will pass within a mile or two of the Falcon 9 stage roughly two minutes before impact, according to Fernando and his team.

Viewing prospects

The impact flash, expected to last less than a second, will probably be too dim to see with the naked eye. However, the ejected dust and rubble could stretch several kilometres into space and remain visible through telescopes for tens of minutes. The eastern United States and Canada, along with large portions of South America, are best placed for viewing.

The gravity on the Moon is low and there is no wind to blow the dust away.

No impact flash has ever been recorded on the sunlit face of the Moon, making this event a first if the plume is bright enough to capture.

The physics of the splash

New preprint research led by William Jo at the University of Texas at Austin modelled the crash using high-resolution simulations. The Falcon 9 upper stage, an empty shell drained of fuel, with a denser engine at one end, behaves differently from a solid meteorite. Rather than burying itself, the hollow cylinder is predicted to collapse inward, throwing up a broad curtain of lunar soil spreading as far as 183 kilometres wide, plus a thin, faster spike nearly straight up.

David Goldstein, the aerospace-engineering professor who supervised the work, described the simulation as the most optimistic scenario. The model assumes the stage hits engine-first, like a pin dropped on its head. In reality, the object is likely to arrive at an angle, producing a smaller and dimmer plume. Jo's team estimates the total displaced material at about 12,700 kilograms.

A serendipitous calibration tool

Unlike natural meteoroids that strike without warning, this impact arrives with its mass and velocity known in advance. Fernando said that makes it a unique calibration event for testing techniques used to study natural impacts.

Part of the reason for our interest in this event is to figure out how much of a hazard debris impacts pose to future astronauts.

Scientists also plan to analyse the plume's composition for clues about the lunar surface at the impact point.

Growing orbital clutter

The crash was never intended by SpaceX. After its 15 January 2025 mission, lifting Firefly Aerospace's Blue Ghost and ispace's Japanese lander toward the Moon, the upper stage ended up stranded on a highly elliptical Earth orbit instead of being directed into a solar orbit. It will be the second known accidental lunar impact by a dead rocket, following a Chinese rocket segment that dug two craters on the Moon's far side in 2022.