Off Station
In mid August 2021, the Chang’e 5 service module without any notice was noted to not be on station near the Sun-Earth L1 point where it had been since mid-March 2021. Observers were quick to reacquire the spacecraft’s X-band beacon and it was not long before it was clear from the data that Chang’e 5 was headed back to the Earth-Moon system where it would ultimately enter a Distant Retrograde Orbit (DRO) of the Moon.
Close Encounters with the Moon
As Chang’e 5 made it’s way closer to the Earth-Moon system radio positional data was used to develop a state vector and that state vector revealed that the spacecraft would have a close approach to the Moon on September 12, 2021.
Observers monitored Chang’e 5 as it passed the Moon and entered a very high orbit of the Earth.
Based on refined radio positional data and optical data shared by amateurs it became clear that Chang’e 5 would have another close encounter with the Moon on November 18, 2021.
Doppler data of the close encounter was collected and surprisingly there was no strong evidence of a burn at perilune. Observers continued to monitor Chang’e 5 for the next couple of weeks when the signal was lost on December 2nd some 19 degrees west of the Moon.
Angles and Signal to Noise Ratios
On January 6th, 2022, observers reacquired the signal from Chang’e 5 and noted it was angularly close to the Moon over the coming days then the probe swung out further east of the Moon to ~16 degrees elongation.
At this point a comparison of the spacecraft’s signal level was made to the signal level when it was at approximately a lunar distance from Earth and it closely matched. Using the angular distance from the Moon and the lunar distance it was calculated that Chang’e 5 was approximately 100000km from the Moon. Looking back at when observation was lost on December 2, 2021 revealed that Chang’e 5’s position was consistent with a DRO.
At this point the observers considered that Chang’e 5 trajectory could be consistent with a Distant Retrograde Orbit (DRO) of the Moon.
DROs are a family of orbits that can exist in a three body system around the smaller of the bodies. We reviewed published literature on NASA’s planned use of a DRO for the upcoming EM-1 test mission in Trajectory Design Considerations for Exploration Mission 1. The parameters looked very similar and an estimated state vector was generated that followed the general trajectory trends observed but refinement from a long arc of data was needed for a more precise estimate.
From Hypothesis to Truth…
Over the coming weeks the radio position of Chang’e 5 was monitored by a ground station with known stable aiming capability and at least daily reports of the position was obtained, thus a long arc of observations was obtained.
Another station collected Doppler data on a near daily basis. The goal was to develop a state vector from the radio positional observations and compare that to the Doppler data by using NASA’s General Mission Analysis Tool (GMAT) to create an orbital model.
After analysing the radio positional data the following state-vector was obtained:
Epoch = 22 Jan 2022 23:03:29
Coordinate System = Earth Equatorial
X = -303953.6243849
Y = -4703.2085043
Z = 20451.4288987
VX = 0.0334510387589
VY = -1.2032499575841
VZ = -0.6523284377768
The above state vector was then modelled in GMAT and the range rate estimates where then compared to the Doppler data obtained.
An archive of radio positional data, Doppler data and some of the analysis scripts are provided here.
Chang’e 5 in a Distant Retrograde Orbit of the Moon
Reviewing the GMAT models generated from real data in a lunar rotating reference frame clearly shows Chang’e 5 is in a DRO of the Moon.
The general parameters of the orbit are ~70000km in the x-axis (just beyond the Earth-Moon L1 and L2 points) and ~100000km from the Moon’s centre at it’s elongation points along the y-axis. The orbit is retrograde as it orbits in the direction opposite of the Moon’s orbital motion around Earth and the plane of the orbit is pretty much aligned with the ecliptic plane. As a DRO is somewhat chaotic in nature these parameters due vary over time, but this class of orbit is very stable in a general sense over time, so Chang’e 5 could stay here for a long time.
The parameters of the orbit are similar to that planned for the upcoming NASA SLS EM-1 mission presently scheduled to launch later in 2022. China has yet to make a public statement about the status of Chang’e 5’s service module since March of 2021 which seems unusual as this is the first time a spacecraft has used a DRO.
Riddles in the Sky 