China's lunar probe Chang'e I completed its fourth orbital transfer on Wednesday afternoon, a critical move to push it to fly to the moon "in a real sense".

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 km.

The main engine of Chang'e I started operation and helped raise the speed of the probe to 10.916 km 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 (BACC).

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 I is limited," said Zhu. "If the orbiter misses the entrance and it will keep moving on the earth orbit instead of flying to the moon."

The probe is estimated to fly another 114 hours before it reaches the moon orbit on November 5.

The probe will brake for the first time on November 5 when it arrives at a position 200 km away from the moon, which is considered another crucial moment, since the satellite will crash into the moon if the braking is too late or it may float elsewhere in space if the braking is too early.

"Before it enters the moon orbit, the probe will be subject to two or three orbit corrections, which will help it adjust the moving direction and speed to ensure that it will approach the perilune as designed," said Sun Zezhou, deputy chief designer of the satellite.

In moon explorations of other countries, lunar probes often lost in space due to unprecise flying postures and speeds, Sun explained.

Chang'e I was previously moving on a 48-hour orbit with an apogee of more than 120,000 km, which was raised from the former 70,000 km through a third orbital transfer on October 29.

The probe completed its first orbital change on October 25, which transferred the satellite to a 16-hour orbit with its perigee up from 200km to 600 km.

It completed its second orbital transfer on October 26, which made it move on a 24-hour orbit with an apogee of 70,000 km, up from the former 50,000 km.

The ultraviolet image sensors installed on the orbiter began working on the morning of October 30 to collect information on the earth and the moon.

It's the first time that an ultraviolet image sensor is put into actual use on a satellite, though a few countries had tested them on the ground, said Wang Yejun, chief engineer with the BACC.

The images Chang'e I collects will be transmitted back to the earth when it enters the lunar orbit, Wang said.