Chang'e 5 robotic probe plants China's national flag on moon's surface. [Photo/CNSA]

The orbiter-reentry capsule combination of China's Chang'e 5 robotic probe carried out its second moon-Earth transfer injection maneuver on Sunday morning, starting its journey back to Earth, according to the China National Space Administration.

The combination activated four 150-newton-thrust engines at 9:51 am at an altitude of about 230 kilometers above the lunar surface. The engines worked about 22 minutes, moving the pair into a moon-Earth transfer trajectory.

The combination made the first injection operation on Saturday morning after traveling in a near-circular lunar orbit for nearly six days, the space administration said in a statement.

The agency noted that the combination will perform midcourse corrections during its flight back to Earth and will carry out the separation in due course.

There are about 2 kilograms of lunar rocks and soil contained inside the reentry capsule.

After arriving in an Earth orbit, the pair will break up, and the reentry capsule will conduct a series of complicated maneuvers to return to a preset landing site in North China's Inner Mongolia autonomous region in mid-December.

Chang'e 5, China's largest and most sophisticated lunar probe, has four main components－an orbiter, lander, ascender and reentry capsule. The spacecraft was launched by a Long March 5 heavy-lift carrier rocket early on Nov 24 at the Wenchang Space Launch Center in Hainan province, setting out on China's most challenging lunar adventure and the world's first mission since 1976 to bring lunar samples back to Earth.

The probe separated into two parts－the orbiter-reentry capsule combination and the lander-ascender combination－while in lunar orbit on the early morning of Nov 30.

Late on Dec 1, the lander-ascender combination landed on the moon, becoming the world's third spacecraft to touch down on the lunar surface this century after its predecessors – Chang'e 3 and 4. Shortly after the landing, the duo soon began to use a drill to obtain underground samples from 2 meters beneath the surface.

It finished the underground operation on early morning of Dec 2, and then started to use a mechanical arm to scoop up surface dirt.

All collection and packing processes finished on the night of that day, much sooner than expected. Samples were packed into a vacuum container inside the ascender.

The ascender activated an engine late on Dec 3 to lift itself into an elliptical lunar orbit to prepare for docking with the reentry capsule, marking the first time a Chinese spacecraft has blasted off from an extraterrestrial body.

It rendezvoused and docked with the orbiter-reentry capsule combination on the early morning of Dec 6 and then transferred lunar samples into the capsule.

The operation has become the first automated rendezvous and docking for any spacecraft in a lunar orbit.

The last time two components of a spacecraft rendezvoused and docked with each other in lunar orbit took place in December 1972 during the last Apollo mission, and that was monitored and controlled by astronauts.

The ascender then separated from the combination. It was commanded to impact on the moon on Tuesday morning.

If successful, the highly sophisticated Chang'e 5 mission would be the first in more than 40 years to bring lunar samples back to Earth, and will make China the third country to do so after the United States and the former Soviet Union.