It looks like Elon Musk will be getting his dream of sending something to the Moon a little early, although not quite in the way he planned. A Falcon 9 booster rocket, which has been in a chaotic orbit since shortly after launch in 2015, recently made a close approach to the Moon, and is now predicted to collide in March. Sadly, we won't get to witness the event from Earth.
In February 2015 SpaceX launched the National Oceanic and Atmospheric Administration's (NOAA) Deep Space Climate Observatory to an orbit near L1, on the opposite side of the Earth from the JWST's L2. Although the launch was a success, the second stage of the rocket became one more piece of space junk albeit at 4 tonnes (4.4 tons) an unusually large one. Its orbit was too high for friction with the outermost parts of the atmosphere to bring the rocket down, but not high enough to escape the Earth's gravity well entirely.
The initial momentum of the rocket's launch, combined with the gravitational forces of Earth, Sun, and Moon made its orbit chaotic, meaning it could not be predicted in the long term.
That doesn't rule out short term predictions, however, and Bill Gray, who tracks near-Earth objects, is quite confident about the rocket's future. With assistance from amateur astronomers who located it during close approaches, Gray concludes on his blog: “With all the data, we've got a certain impact at 2022 March 4 12:25:39.”
Gray has calculated the expected site of the crash landing; lunar “latitude +4.93, east longitude 233.20”. He admits he could be somewhat wrong about the location, given the uncertainties in the rocket's current position and the complexity of the forces it is experiencing. However, even if he is out by a few degrees, Gray thinks there is no chance the rocket will miss the Moon entirely and get to continue its journey.
The key significance of the location Gray predicts is that it places impact on the far side of the Moon. Telescopes on Earth will miss the explosion of lunar material released from the rocket's impact at the expected speed of 2.6 kilometers per second (1.6 miles per second).
On the other hand the Lunar Reconnaissance Orbiter and Chandrayaan-2 may be better positioned. With exceptional luck one or both will be able to witness the impact. Failing that, finding the crater afterward is more likely. Gray is doing his best to narrow down the landing site as much as possible to help.
The main obstacle is not the rocket's current position or the gravitational force on it, Gray writes, but uncertainties in the way sunlight is pushing on the rocket. The light “doesn't just push outward; some of it bounces "sideways". The object is a long cylinder, spinning slowly,” Gray writes. “These unpredictable effects are very small. But they will accumulate between now and 2022 March 4.”
Videos of the rocket's tumbling motion reveal the complexity of the situation, but Gray expects an observing window February 7-10 to improve precision.
If there have been any previous cases of human-made objects unintentionally hitting the Moon, no one has identified them.
Although the idea of fining SpaceX for littering, as the town of Esperance did to NASA when parts of Skylab landed on them has its attractions, the landing could be useful to science. The impact isn't expected to be in a particularly interesting area, unlike the Lunar Crater Observation and Sensing Satellite's hit near the Lunar South Pole, but the size of the crater, and spectral analysis of ejecta if we see it, could still tell us something about the site's composition.
[H/T: The Guardian]