China's Chang'e-6 is carrying a surprise rover to the moon

Chang'e-6 launched on a Long March 5 rocket from Wenchang early May 3. The mission will attempt to bring back to Earth the first ever samples from the lunar far side. 

China had disclosed objectives, a landing site and science payloads for the mission ahead of launch. However, following launch, the spacecraft's maker, the China Academy of Space Technology (CAST), revealed an image showing a rover attached to the mission lander. 

Chang'e-6 is a backup to the 2020 Chang'e-5 nearside sample return mission. With that mission successful, Chang'e-6 has been repurposed for a more ambitious lunar far side sampling mission. A similar process saw the backup Chang'e-4 mission land in Von Kármán crater on the lunar far side, following the success of the Chang'e-3 lander and rover mission.

The Chang'e-6 mission includes new, international payloads—from France, Sweden, Italy, and a Pakistani cubesat—and now an additional rover, something not carried by Chang'e-5.

Little is known about the rover, but a mention of a Chang'e-6 rover is made in a post from the Shanghai Institute of Ceramics (SIC) under  the Chinese Academy of Sciences (CAS). It suggests the small vehicle carries an infrared imaging spectrometer.

The payload would utilize how different minerals and compounds absorb and emit infrared radiation in characteristic ways to determine the composition of rocks, soil, and regolith on the lunar surface. This could be used for water detection. 

It is not clear how the rover, attached to the side of the lander, will descend to the surface. It could be released, or use a foldout ladder or other mechanism. The rover may communicate with the lander using WiFi. Given the diminutive size of the vehicle and short lifetime of the mission lander, it is likely the rover will have limited operational time and objectives.

Surprise packages  

It is not the first time China has included bonus spacecraft on missions. China's Tianwen-1 Mars provided a series of punctuated surprises. It first released a disposable spacecraft while on its transfer orbit, taking pictures of the Tianwen-1 orbiter in deep space on its way to Mars.

The Zhurong rover, which landed on the Red Planet as part of the mission, also dropped a camera. This imaged the posing rover and used WiFi to send images to Zhurong.

The Tianwen-1 orbiter then repeated the selfie trick with another detachable camera while in orbit around Mars.

Chang'e-6 mission timeline

Chang'e-6 is likely to reach the moon late Tuesday Eastern (early Wednesday UTC, May 8) and enter lunar orbit. China has yet to publish a clear, official timeline for the mission. It is however expected to last 53 days from launch till landing, according to information published by the Deep Space Exploration Laboratory (DSEL) under the China National Space Administration (CNSA).

Lighting conditions over the target landing area within Apollo crater will likely not be immediately suitable for a landing and surface operations. Sunset over Apollo crater will occur early UTC May 13, according to moon observation software. 

Sunrise will occur May 28, meaning Chang'e-6 will likely attempt to land early June. This means the spacecraft will spend more than three weeks in lunar orbit before attempting landing. Timing of the attempt will be dependent on the spacecraft's orbit and constraints regarding surface lighting conditions for generating power. 

Sampling operations will, as with Chang'e-5, likely be wrapped up within 48 hours of landing. Samples will be sent into lunar orbit via an ascent vehicle, which will then track the Chang'e-6 orbiter. Based on the earlier mission, the pair will likely rendezvous and dock around two days after launch, with the ascender to be discarded a further couple of days later. 

The orbiter would then prepare to leave lunar orbit at a calculated time. It would then release a reentry capsule just ahead of its return to Earth, around June 25. The orbiter will likely fire its engines to send the spacecraft onto a secondary mission, should it have remaining propellant. The Chang'e-5 orbiter visited Sun-Earth Lagrange point 1 and entered a lunar distant retrograde orbit as part of extended missions.

If successful, samples delivered by the Chang'e-6 mission could change our understanding of the Earth and moon. They could also provide new clues as to the history of the early solar system.