China's lunar probe, Chang'e 1, completed its fourth orbital transfer late yesterday, a critical move to push it to fly to the moon.

The engine on the probe was started at 5:15pm. Thirteen minutes later, the probe was successfully shifted to the Earth-moon transfer orbit with an apogee of about 380,000 kilometers.

The main engine of Chang'e 1 started operation and helped raise the speed of the probe to 10.916km per second in a few minutes before the satellite reached the "entrance" of the Earth-moon transfer orbit, said Zhu Mincai, head of the Beijing Aerospace Control Center.

The probe will then start to fly to the moon "in a real sense," Zhu said.

"It's a success-or-failure point and we only have one shot as the fuel carried on the Chang'e 1 is limited," said Zhu. "If the orbiter misses the entrance it will keep moving on the Earth orbit instead of flying to the moon."

The probe is estimated to reach the moon orbit next Monday.

The probe will then brake for the first time when it arrives at a position 200km from the moon.

This is another critical moment for precision as the satellite will crash into the moon if it brakes too late or float elsewhere in space if it brakes too early.

"Before it enters the moon's orbit, the probe will be subject to two or three orbital corrections, which will help it approach the perilune (the point closest to the moon) as designed," said Sun Zezhou, deputy chief designer of the satellite.

In moon explorations of other countries, lunar probes have often been lost in space due to imprecise positioning and speed, Sun said.

The ultraviolet image sensors installed on the orbiter began working on Tuesday morning to collect information on the Earth and the moon.

It's the first time that an ultraviolet image sensor has been used on a satellite, though a few countries have tested them on the ground, said Wang Yejun, chief engineer with the BACC. The images Chang'e 1 collects will be transmitted back to the Earth when it enters the lunar orbit, Wang said.

Meanwhile, China is building a new range of carrier rockets designed to send heavyweight satellites into space, boosting the present carrying capacity nearly threefold, a space expert said.

The Long March 5 rockets will be able to carry payloads of up to 25 tons for low Earth-orbit satellites, up from the current limit of 9.2 tons, said Wu Yansheng, president of the China Academy of Launch Vehicle Technology, which is developing the new launch vehicles.