Chang’e 5 Returns to the Moon – Riddles in the Sky

Chang’e 5 has quietly departed from the Sun-Earth L1 point to enter a distant retrograde orbit (DRO) of the Moon, a first.
Pre-empting NASA’s planned 1st use of the orbit during Exploration Mission 1 later this year.
Silence from China.

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.

Chang’e 5 is rapidly returning to the Earth-Moon system. Amateur observations of the radio signal have allowed it’s position in space to be precisely determined. Here’s an animation of the trajectory of CE5 until lunar periapsis. pic.twitter.com/8cF7Qt02cZ— Scott Tilley (@coastal8049) September 5, 2021

Observers monitored Chang’e 5 as it passed the Moon and entered a very high orbit of the Earth.

Amateurs have been quietly watching #Change5 in Earth orbit not only by radio but also optically. Here’s @mickeywzx’s observations from October 31st. Today I’m reducing over a months worth of observations to produce a new amateur state vector. THREAD pic.twitter.com/obl5kO4Vhj— Scott Tilley (@coastal8049) November 14, 2021

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.

#Change5 is now well within the Moon’s hill sphere and ‘perilunes’ tomorrow. It’s anticipated that the Chinese will use this close approach to redirect CE5 to a new unknown destination. Radio observations with RA/DEC and timings would be helpful to keep the world informed. pic.twitter.com/nFJy4UdFHS— Scott Tilley (@coastal8049) November 16, 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.

I kept the station running when I went to bed, so it captured the Doppler maximum at 03:19:10. Again no indication of a burn. A handful of amateur observes will keep on monitoring and tracking the thing to see what the next target could be. pic.twitter.com/qObTLR3mIS— Edgar Kaiser (@df2mz) November 18, 2021

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.

🟥 Chang’e-5 is back! It was missing since early December. Thanks to a hint from Bill Gray I checked around the moon today and found it. Currently about -2.5° from the moon in elevation. RA = 22.84h, Dec = -15.84°, t = 18:15. Frequency is 8486.26 MHz. pic.twitter.com/WaeqCnGBk4— Edgar Kaiser (@df2mz) January 6, 2022

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.

The last observation before #Change5 ‘disappeared’ after Dec 2nd, was 19 degrees from the Moon on the western side. Perhaps just before a burn!? This as you will see is significant to understand CE5’s present trajectory and why I’m pretty much convinced it is in a DRO now. (1/n) pic.twitter.com/DJcogeEwPs— Scott Tilley (@coastal8049) January 13, 2022

At this point the observers considered that Chang’e 5 trajectory could be consistent with a Distant Retrograde Orbit (DRO) of the Moon.

What in tarnation is a Distant Retrograde Orbit (DRO)?For those wondering what I’m waving my hands about these days about here’s a THREAD about what a DRO is and why it may be something you hear a lot more of soon… (1/n) pic.twitter.com/j1S2A8mYVP— Scott Tilley (@coastal8049) January 12, 2022

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.

#Change5 DRO state vector estimate with rationale. Amateurs lost signal ~2021-12-03. About 3 integer periods of ~13 days, which is around the optimal DRO period. We appear to be seeing spacecraft at eastern elongation now. So… (1/n) pic.twitter.com/ycsMR26okS— Scott Tilley (@coastal8049) January 9, 2022

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.

Jean-Luc @LucMilette operates a 2.3m dish and routinely tracks low Earth orbiting satellites on X-band which requires precise alignment. This makes his measurements accurate and consistent for radio observations and because the system is available 24/7 we can take good observations anytime.

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.

Radio Positional data obtained to determine a state-vector estimate for Chang’e 5.

After analysing the radio positional data the following state-vector was obtained:

Epoch = 22 Jan 2022 23:03:29Coordinate System = Earth EquatorialX = -303953.6243849Y = -4703.2085043Z = 20451.4288987VX = 0.0334510387589VY = -1.2032499575841VZ = -0.6523284377768

The state-vector ephemeris for Chang’e 5 plotted over the same time period as the radio positional data.

The above state vector was then modelled in GMAT and the range rate estimates where then compared to the Doppler data obtained.

Chang’e 5 Doppler data compared to the state-vector quoted above. Data that does not match modelled state-vector is due to the spacecraft being in lock to a ground station.

The difference (residuals) left over after subtracting obtained data from modelled data. The remaining error is low and could be accounted for by the known instability of the Chang’e 5 local oscillator, chaotic nature of a DRO and errors in the state-vector estimate.

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 Chang’e 5 DRO as seen from above the Moon.

The Chang’e 5 DRO as seen from the Earth.

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.

Approximately one complete rev of the Moon in a Lunar rotating reference frame. The period of the orbit is approximately 15.75 days and it’s elliptical shape is approximately 70000 x 100000km.

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.