Why does Chang'e-5 adopt a "relay" method for sample return?

By December 2021, the Chang'e-5 sample return mission of my country's lunar exploration project will have been successfully implemented for one year.

The Chang'e-5 probe consists of an orbiter, a lander, an ascender and a returner. This team is like a relay team, sending lunar samples back to Earth through a "relay" method.

Schematic diagram of the Chang'e 5 probe

So, why not send one member to complete the "treasure hunting" mission alone?

This is because the sampling return task is complex and cannot be completed by one member alone. Team members have clear division of labor and each has their own tasks:

Orbiter

The orbiter is responsible for "flying there", that is, after separating from the carrier rocket, it is responsible for sending the lander, ascender and returner to the lunar orbit. After completing the sampling mission, it will send the returner back to the vicinity of the earth.

Lander

The lander is responsible for "going down", that is, after reaching the lunar orbit and separating from the orbiter-returner combination (referred to as "orbiter-returner combination"), it is responsible for carrying the ascender to a soft landing on the lunar surface, and together with the ascender, complete lunar surface sampling.

Ascender

The ascender is responsible for "getting up", that is, after completing the sampling mission, it takes off from the lunar surface with the lunar samples, rendezvous and docks with the orbiter-returner combination flying in the lunar orbit, and transfers the lunar samples to the returner.

Returner

The returner is responsible for "landing", that is, when the orbiter-returner combination returns to the vicinity of the earth, it separates from the orbiter, re-enters the atmosphere, and sends the lunar samples back to Earth.

First of all, the role of the orbiter is equivalent to a space shuttle, which plays the role of round-trip transportation between the earth and the moon. Due to the long distance between the earth and the moon, it is necessary to change orbits many times during the flight. When it reaches the vicinity of the moon, it is necessary to brake and slow down so that it can be captured by the moon; after "getting" the lunar samples, it accelerates to get rid of the moon's gravity and fly back to the earth. These tasks require the orbiter's engine and control system to complete. The returner has no engine and does not rely on the orbiter for ferrying, so it cannot complete the round-trip mission between the earth and the moon.

Secondly, since the moon has no atmosphere, landing on the lunar surface from lunar orbit can only be achieved through engine reverse thrust, generating braking force, and reducing the speed of the lander to achieve a soft landing. The returner has no engine and cannot achieve a soft landing; although the orbiter has an engine, the orbiter-returner combination has a large mass of about 4.5 tons, and landing on the lunar surface and taking off from the lunar surface both consume a lot of propellant, and the orbiter also has the task of sending the returner back to Earth. Therefore, after reaching the lunar orbit, the orbiter-returner combination will fly around the moon without consuming propellant, while the lander and ascender (referred to as the "landing and ascending combination") will land on the lunar surface to complete the sampling mission.

Schematic diagram of the landing assembly on the lunar surface

Third, after completing the sampling mission, it is necessary to take off from the lunar surface. The weight of the lander is nearly 3 tons, and taking off from the lunar surface requires a lot of propellant. In order to reduce the takeoff weight and reduce the consumption of propellant, it is necessary to leave the lander that has completed the mission on the lunar surface to serve as the takeoff platform of the ascender, and the ascender weighing about 0.8 tons will take off from the lunar surface and transfer the lunar samples to the returner by rendezvous and docking with the orbiter-returner assembly remaining in the lunar orbit.

Schematic diagram of the ascender flying away from the lunar surface

Ascender and return assembly

Schematic diagram of rendezvous and docking in lunar orbit

Finally, in order to send lunar samples back to Earth, it is necessary to re-enter the atmosphere. Since the returner flies at a very high speed, friction with the atmosphere will generate a lot of heat. If heat protection measures are not taken in the shape design and material selection, the returner will burn up during the re-entry process. Only returners with heat protection design can safely re-enter the atmosphere. At the same time, due to their small mass (about 0.3 tons), they can be decelerated by parachutes and land smoothly on the ground. Orbiters and ascenders without heat protection design do not have the conditions to re-enter the atmosphere.

The returner returns to the ground with lunar samples

After 23 days of operation by the Chang'e-5 lunar sampling team, only the returner returned to the ground with lunar samples; the lander, which completed the mission, remained on the lunar surface; after the ascender handed over the lunar samples and separated from the orbiter-returner combination, it was controlled to leave orbit in accordance with ground commands and landed at the predetermined landing point on the moon; and after completing the round-trip transportation mission between the Earth and the Moon, the orbiter was given a new mission. It first explored the Sun-Earth Lagrange L1 point, and then returned to the moon from the Sun-Earth Lagrange L1 point. As of the first anniversary of the lunar sampling, it was still operating in the lunar orbit.

Source/"Space Exploration" 2022 Issue 1

Text/Zhang Tiejun

Editor/Yang Sishuang

Review/Yang Jie

Producer/Xu Chunmei

China Aerospace Science Popularization