Humanity is once again littering our nearest celestial neighbor. Early on Wednesday, the discarded upper stage of a Falcon 9 rocket built by Elon Musk’s SpaceX will unintentionally slam into the moon.
From Earth, the trajectory will appear to be coming from the west, hitting the moon’s side, near the dividing line between the near and far sides of the lunar surface. The crash is expected to occur around 2:35 a.m. Eastern time. It will be too dim to be seen with the naked eye, but it may be visible by telescope.
“The object is impacting at a point where there is at least some hope that we might observe it,” said Bill Gray, developer of Project Pluto, a suite of astronomical software used to calculate the orbits of asteroids and comets, who was the first to notice that there might be a collision.
The four-ton rocket stage will hit the moon at 5,400 miles per hour, more than twice the speed of a bullet shot from a rifle, generating enough heat to produce a flash of light. The sunlit plume of debris and dust will then rise and fall for several minutes.
However, Mr. Gray described the visibility as “quite iffy.” The flash may not be very bright, and the plume may not rise high enough, washed-out against the brightness of the moon’s surface.
In the coming years, SpaceX’s immense Starship spacecraft is to regularly ferry NASA astronauts and cargo to the surface of the moon. But so far, nothing built by SpaceX has made it to the moon or has even orbited it.
Parts of spacecraft have been pockmarking the moon for decades.
In the 1970s, NASA deliberately crashed into the moon the third stage of the Saturn V rockets that launched the Apollo astronauts. The first deliberate crash occurred during Apollo 13, the ill-fated mission that was cut short because of an oxygen tank explosion.
When mission control told the astronauts that vibrations of the impact were observed by a seismograph left on the surface by the Apollo 12 astronauts, Jim Lovell, the Apollo 13 commander, wryly said, “Well, at least something worked on this flight.”
More recently, the bombardment has accelerated. Private efforts to land spacecraft on the moon have produced new craters more often than not. In 2022, part of a Chinese rocket hit the far side of the moon. (Mr. Gray had initially misidentified that as a SpaceX Falcon 9.)
This Falcon 9 upper stage launched last year on Jan. 15, carrying two private lunar landers: Blue Ghost, built by Firefly Aerospace of Texas, and Resilience, built by Ispace of Japan. (Blue Ghost later landed on the near side of the moon, and Resilience crashed into it.)
After the Falcon 9 upper stage dropped off the two spacecraft on their moonward paths, it continued looping around Earth through largely empty space, where it posed little danger to people or other spacecraft. Amateur astronomers quickly located the rocket stage, and Mr. Gray kept tabs on it.
“It was clear that the object’s orbit would cross that of the moon,” Mr. Gray said. “So it was likely to eventually have a close flyby or impact.”
Mr. Gray said that by late summer last year, orbit calculations indicated that a collision with the moon was quite likely. But he did not say anything publicly until April, when he felt confident that he could determine where it would hit: near the 110-mile-wide Einstein crater in the moon’s northern hemisphere.
That piqued the interest of researchers like David Goldstein, a professor of engineering at the University of Texas at Austin, and his graduate student, William Jo. Mr. Jo did his doctoral thesis research on an earlier lunar collision, NASA’s LCROSS mission.
For the SpaceX rocket stage, Mr. Jo produced simulations of what would be kicked up if the stage came down vertically. “We’re treating this as a best-case, the case which will produce the most debris to be seen,” Dr. Goldstein said.
The simulation indicated that while a spike of debris would go straight up and down, much more of the debris would be kicked at a low angle, traveling up to perhaps a hundred miles before falling back to the surface a few minutes later. “It’s a pretty big plume,” Mr. Jo said.
Understanding the physics of such impacts could be crucial for people living on the moon in the future. If an impact occurred nearby, it could sandblast machines, habitats and astronauts, with debris traveling hundreds of miles per hour.
The high-speed particles could puncture an astronaut’s spacesuit, and all of the air inside would rush out, leaving nothing to breathe. “You’ll be sucking vacuum,” Dr. Goldstein said.
Computer simulations by another team of scientists led by Benjamin Fernando, a postdoctoral researcher at Los Alamos National Laboratory in New Mexico, came to similar conclusions. The Falcon 9 crash, they predicted, will create a new crater 65 to 100 feet wide.
Because the pull of the moon’s gravity is one-sixth that of Earth’s, some of the smaller particles will probably be accelerated into space. “I would be very surprised if some stuff didn’t end up in orbit,” he said.
Dr. Fernando said he was hopeful that the astronomers would see something. But even if not, the crash will give them new knowledge, because there are no natural, hollow metal cylinders zipping around the solar system.
“Either way, we learn something about the dynamics of this process,” he said.
Even if astronomers do not spot the impact on Wednesday morning, spacecraft orbiting the moon almost certainly will. Danuri, a Korean spacecraft, is expected to fly above the impact site about eight hours after the crash. NASA’s Lunar Reconnaissance Orbiter, which has successfully photographed most of the earlier crash sites, will pass over this new gash six days after impact.
“I think it will be a beautiful crater!” Mark Robinson, the principal investigator for the camera on the NASA orbiter, wrote in an email.
