Lunar probe Chang'e I completed its fourth orbital transfer Wednesday afternoon, a critical step in the journey to the moon.

Thirteen minutes after the engine on the probe was started at 5:15 pm, the probe was 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 to 10.916 km per second in the 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).

"It's a success-or-failure point and we have only one shot as the fuel carried on the Chang'e I is limited," Zhu said earlier.

"If the orbiter misses the entrance, it will continue 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 next big moment in the fate of the country's first moon orbiter, said Hao Xifan, deputy head of the Lunar Exploration Project office.

Chang'e I will brake for the first time when it arrives at a position 200 km away from the moon - it will crash into the moon if the step is too late and may float elsewhere in space if it is too early.

"Once it is captured by lunar gravitation, I'll be at ease," Hao said.

"Before it enters the moon orbit, the probe will be subject to two or three orbit corrections," said Sun Zezhou, deputy chief designer of the satellite.

Chang'e I was previously moving on a 48-hour orbit with an apogee of more than 120,000 km, which was raised from 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 200 km to 600 km.

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

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

It is the first time that an ultraviolet image sensor has been used on a satellite, though a few countries had tested them on the ground, said Wang Yejun, chief engineer with the BACC.

Chang'e I, named after a legendary Chinese fairy who flew to the moon, was launched on a Long March 3A carrier rocket last Wednesday from the Xichang Satellite Launch Center in Sichuan Province.