A SpaceX rocket segment that has spent around seven years in space will smash into the moon in the coming weeks, according to experts.
The Falcon 9 rocket in question originally launched from Cape Canaveral, Florida, in February 2015, as part of a mission to deploy a space-weather satellite.
After completing a long burn, the second stage of the Falcon 9 released the National Oceanic and Atmospheric Administration's Deep Space Climate Observatory, which began its journey towards its final destination—a special location in space, known as a Lagrange point, located around a million miles away from Earth.
But after sending the satellite on its way, the second stage of the rocket lacked the energy to escape the gravity of the Earth-moon system and was high enough that it did not have sufficient fuel to be directed back into Earth's atmosphere, space expert and meteorologist Eric Berger wrote for Ars Technica. This left the second stage segment in a somewhat chaotic orbit.
Now, experts who track near-Earth objects say the rocket stage is on a collision course with the moon.
Bill Gray—a creator of software that tracks near-Earth objects—has predicted in a blog post that the rocket stage will very likely strike the far side of the moon near its equator on March 4, 2022.
Jonathan McDowell, an astronomer at the Harvard & Smithsonian Center for Astrophysics, also agreed with Gray's conclusion.
"For those asking: yes, an old Falcon 9 second stage left in high orbit in 2015 is going to hit the moon on March 4. It's interesting, but not a big deal," McDowell said in a tweet Tuesday.
For those asking: yes, an old Falcon 9 second stage left in high orbit in 2015 is going to hit the moon on March 4. It's interesting, but not a big deal.
— Jonathan McDowell (@planet4589) January 25, 2022
Some uncertainties remain in the predictions for exactly when and where the object will strike the moon, according to Gray.
"Space junk can be a little tricky," he wrote in the blog post. "I have a fairly complete mathematical model of what the earth, moon, sun, and planets are doing and how their gravity is affecting the object. I have a rough idea of how much sunlight is pushing outward on the object, gently pushing it away from the sun. This usually enables me to make predictions with a good bit of confidence.
"However, the actual effects of that sunlight are hard to predict perfectly. It doesn't just push outward; some of it bounces 'sideways.' The object is a long cylinder, spinning slowly; you can see the light from it vary as it tumbles, and you can plot a light curve for it indicating that it rotates about once every 180.7 seconds."
These "unpredictable effects" are very small, but they will accumulate between now and March 4. Nevertheless, Gray estimated that his prediction would only be wrong by a degree of two minutes or so. He said further observations would be needed in early February to refine the prediction and bring the uncertainty down.
Determining the impact location as precisely as possible will enable NASA's Lunar Reconnaissance Orbiter (LRO) and India's Chandrayaan-2 spacecraft to find the impact crater, and perhaps even image the impact "if we're lucky," although it is very likely to go unobserved.
Gray said "this is the first unintentional case" of space junk hitting the moon of which he is aware. According to the expert, if the LRO and/or Chandrayaan mission teams are able to image the impact crater very soon after the event, they may be able to learn something about the surface and subsurface material that is ejected from that part of the moon.
The rocket second stage, which weighs 4 metric tons, is expected to strike the moon at a velocity of around 5,700 miles per hour.
