Queqiao, the relay satellite tasked with servicing the planned Chang'e 4 lunar mission, has entered its preset orbit and will carry out communication relay tests, China's space authority said on Thursday.

The China National Space Administration said in a statement that the Queqiao - named after a legendary bridge of stars in the Milky Way - entered a halo orbit around Lagrangian Point 2 (or L2), a gravitationally stable spot located 455,000 kilometers from Earth and 65,000 km from the moon, at 11:06 am after a journey of more than three weeks.

The satellite will begin in-orbit tests of its communication relay capabilities to prepare for the Chang'e 4 mission, which is scheduled to land a probe on the moon around year's end, the statement said, noting that it is the world's first communications satellite to operate near the Earth-moon L2 point.

Queqiao was lifted into space on a Long March 4C carrier rocket from the Xichang Satellite Launch Center in Sichuan province on May 21.

Developed by the China Academy of Space Technology of China Aerospace Science and Technology Corp, the satellite weighs more than 400 kilograms and has a designed life span of three years. It has an umbrella antenna with a diameter of 4.2 meters, enabling it to transfer data from the Chang'e 4 to ground control in real time.

China plans to launch the Chang'e 4 probe before the end of 2018 and land it on the far side of the moon, where no probe has successfully landed.

Tidal forces on Earth slow the moon's rotation so that the same side always faces Earth. The far side is virtually never visible.

According to Chinese scientists, the Chang'e 4 probe will be unable to directly communicate with ground controllers on Earth, as signals can't get around the mass of the moon, so a relay satellite hovering in orbit is required to relay signals.

The Chang'e 4 mission will enable scientists to study an area of the moon that has heretofore been out of reach. They also can take advantage of the far side's shield against Earth's interference to make clearer observations into deep space, said Bao Weimin, head of science and technology at China Aerospace Science and Technology Corp.

In another development, several images taken by a small lunar optical imaging device, developed by Saudi Arabia and carried by a Chinese mini satellite along with the relay satellite, were made public at a ceremony in Beijing co-hosted by the China National Space Administration and Saudi Arabia's King Abdulaziz City for Science and Technology, according to a news release.

zhaolei@chinadaily.com.cn