China's lunar probe Chang'e-1 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-1 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-1 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 Nov. 5.

The probe will brake for the first time on Nov. 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 imprecise flying postures and speeds, Sun explained.

Chang'e-1 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 Oct. 29.

The probe completed its first orbital change on Oct. 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 Oct. 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 Oct. 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-1 collects will be transmitted back to the earth when it enters the lunar orbit, Wang said.

Chang'e-1, named after a mythical Chinese goddess who, according to legend, flew to the moon, blasted off on a Long March 3A carrier rocket at 6:05 pm. on Oct. 24 from the Xichang Satellite Launch Center in the southwestern province of Sichuan.

After the probe enters the moon's orbit, it will brake several times to slow down its speeds so that it can be captured by the lunar gravity to become a real circumlunar satellite.

The 2,300-kg satellite carried eight probing facilities, including a stereo camera and interferometer, an imager and gamma/x-ray spectrometer, a laser altimeter, a microwave detector, a high energy solar particle detector and a low energy ion detector.

It will fulfil four scientific objectives, including a three-dimensional survey of the Moon's surface, analysis of the abundance and distribution of elements on lunar surface, an investigation of the characteristics of lunar regolith and the powdery soil layer on the surface, and an exploration of the circumstance between the earth and the moon.

The satellite will relay the first picture of the moon in late November and will then continue scientific explorations of the moon for a year.

China's lunar orbiter project has cost 1.4 billion yuan (187 million U.S. dollars) since research and development of the project was approved at the beginning of 2004.

The launch of the orbiter marks the first step of China's three-stage moon mission, which will lead to a moon landing and launch of a moon rover at around 2012. In the third phase, another rover will land on the moon and return to earth with lunar soil and stone samples for scientific research at around 2017.

China carried out its maiden piloted space flight in October 2003, making it only the third country in the world after the former Soviet Union and the United States to have sent men into space. In October 2005, China completed its second manned space flight, with two astronauts on board.

The launch of Chang'e-1 came shortly after Japan launched its first lunar probe, Kaguya, in mid-September, while India is planning to send its own lunar probe into space next April, sparking off concerns of a space race in Asia.

But Luan Enjie, chief commander of China's lunar orbiter project, said that "China will not be involved in moon race with any other country and in any form."

"China will, in pursuing its policy of peaceful use of airspace, share the achievements of the lunar exploration with the whole world," he told Xinhua.

(Xinhua News Agency October 31, 2007)