It turns out the rocket that is predicted to impact the Moon on March 4 is not the second stage of a SpaceX Falcon 9 rocket after all, but the upper stage of a Chinese Long March 3C used to launch the Chang'e 5-T1 lunar flyby mission in 2014.
Last month, astronomical software writer Bill Gray announced that he and a group of amateur astronomers had plotted the trajectory of an object that they believed to be the second stage of a Falcon 9 rocket used to launch NASA's Deep Space Climate Observatory (DSCOVR) probe in 2015. Based on observations, Gray was able to confirm that the rocket was in orbit around the Earth and had made a series of close encounters with the Earth and the Moon, which would cause it to crash into the Moon in the vicinity of Mare Orientalis.
This seemed cut and dried, and resulted in some comments on social media about whether the impact would push the Moon out of its orbit as well as chastising SpaceX for not disposing of the rocket properly and even demanding that the company clean up the mess it would cause on the Moon.
However, Jon Giorgini at the Jet Propulsion Laboratory in Pasadena, California emailed Gray to tell him that the trajectory of the rocket stage didn't match that of the DSCOVR spacecraft. Though the rocket and DSCOVR were not in identical orbits, they should have been in the same part of the sky. Instead, the rocket passed close to the Moon on February 13, 2015, while the probe remained far away.
According to Gray, this led him to revisit the trail of evidence that convinced him that the rocket was from the DSCOVR mission. The timing of its passing the Moon and the observed brightness seemed to be consistent with the launch of DSCOVR, and it isn't unknown for rockets to alter their trajectory after being discarded as fuel vents away. Not having the precise orbital figures for DSCOVR, this seemed a reasonable identification.
On reexamination, Gray concluded that the orbit was too far off to be credible. The point where it came closest to the Earth was too high to match the launch of DSCOVR, so he looked at earlier missions that might be a better match. It couldn't be much earlier because the object was too bright to be missed by Earthbound observers.
It turns out that the best candidate was Chang'e 5-T1, which lifted off on October 23, 2014 at 18:00 GMT. By running the trajectory of the mystery rocket backwards, Gray got a lunar flyby on October 28, or four days after the launch, and tracking back the trajectory put it closest to the Earth in the vicinity of the Chinese launch site.
In addition, this orbit was consistent with a lunar transfer orbit for Chang'e 5-T1, which was a rehearsal for a lunar sample return mission and was testing a reentry capsule by sending it on a cislunar trajectory so it would hit the Earth's atmosphere at the same speed as a returning lunar mission.
One final bit of evidence is that the orbital elements of the rocket matched those of an amateur radio CubeSat that shared a ride into space with the Chang'e 5-T1.
Source: Bill Gray
