And now for an update. It turns out that the piece of space junk that’s gonna hit the Moon next month is not, in fact, a Falcon 9 upper stage, but is actually the third stage from the Long March 3B that launched the Chang’e 5T1 spacecraft towards the Moon in 2014.

The story behind the revised identity of the stage goes as follows:

Back in late 2014, an object was spotted near the Moon days after the launch of the DSCOVR spacecraft, at the right time and brightness to be expected for the Falcon 9 upper stage. At the time, that was good enough to label it as the second stage from that launch.

Flash forward to this month. Bill Gray, the person who identified the object as the DSCOVR stage in 2015, received an email from a scientist at JPL. The object expected to hit the Moon couldn’t be the DSCOVR stage, because the DSCOVR spacecraft itself was in a completely different part of the sky at the time, and the upper stage wouldn’t be in a different location.

Bill went back to launches in 2015 to see if any of them could be the mystery object. It needed to be bright and launched just after March 2015. Chang’e 5T1’s third stage fit the bill. He ran the orbit back to just after launch and saw that it started in China and flew very close to the Moon only a few days after launch, closer than DSCOVR. As well, one of the secondary payloads had its own set of tracking data, and it was very close, too.

One of the interesting things about the stage, and one that even amateurs can try, was to receive radio signals from a payload on the derelict upper stage. It was called the Manfred Memorial Moon Mission, and it was a 14-kilogram payload with a radio beacon and a radiation sensor. The radiation sensor broke 215 hours into the flight, but the beacon worked. Amateur radio operators tracked the stage by using the Doppler shift of its signal until fourteen days after launch, 100 hours longer than it was expected to last. A total of 75 operators received signals from the spacecraft.

The mistaken identity of the stage would have been noticed earlier, but debris in lunar crossing orbits doesn’t bother anyone except asteroid surveys, so no one bothered to calculate its orbit. Also, they’re harder to track than objects closer to Earth even though they’re bigger than the ten-centimeter objects the Space Force usually tracks. In this case, the object literally slipped beneath the radar.

So where is the second stage that launched DSCOVR? It’s probably in solar orbit. Objects can’t stay in Lagrange 1 (L1) where DSCOVR is without propulsion, and the stage was drained of fuel and power only hours after launch. So it probably sailed right through L1 and now goes directly around the Sun.

More Information

The University of Arizona press release

Corrected identification of object about to hit the moon (Project Pluto)

PDF: Beyond Earth: A Chronicle of Deep Space Exploration, 1958-2016 (NASA)

PDF: Manfred Memorial Moon Mission (4M): development, operations and results of a privately funded low cast lunar flyby (Digital Commons)