BEIJING - China's second lunar probe, Chang'e-2, was 100 million km into deep space in July, the longest journey of ant Chinese spacecraft, a senior engineer said Thursday.

The lunar probe, launched on Oct. 1, 2010, has extended its service by several years and remains in good condition, Zhou Jianliang, chief engineer of the Beijing Aerospace Control Center told Xinhua.

Chang'e-2 tested technology for Chang'e-3, the landing mission. It left lunar orbit for outer space in June 2011 after completing all of its tasks.

Chang'e-2 is expected to travel as far as 300 million km from the earth, after which it will return to about 7 million km from the earth around 2029.

Chinese space scientists are currently awaiting for the return of an experimental lunar orbiter, set for Nov. 1. The orbiter, launched Friday last week atop a Long March-3C rocket, has escaped lunar gravity and is approaching the Earth after 114 hours of space flight.

There are a few challenges on the way home, Zhou said. "The window for landing is very small and requires a highly sophisticated TT&C (telemetry, tracking and command) system," he said.

The test orbiter will maneuver on the edge of the Earth's atmosphere to slow from a speed of 11.2 kilometers per second before re-entry, a process that generates extremely high temperatures.

It is a test run for Chang'e-5, China's fourth lunar probe intended to fetch samples from the lunar surface.