Laser measurement to span record distance

XICHANG, Sichuan province — Scientists will conduct a laser-ranging test between the relay satellite for the Chang’e 4 lunar probe and an observatory on the ground, which may help lay the foundation for spacebased gravitational wave detection.

The relay satellite Queqiao, or Magpie Bridge, which was launched on Monday, will fly to a halo orbit in the Earth-Moon system.

It will be a communication link between controllers on Earth and the Chang’e 4 lunar probe, which is expected to make a soft landing on the far side of the moon at the end of this year.

Chinese scientists will conduct a laser-ranging test with the satellite at a distance of about 455,000 kilometers, a record distance for an experiment of this kind, said Zhang Lihua, the relay satellite project’s manager.

China reported it accomplished its first successful lunar laser-ranging, with a 1.2-meter telescope at southwest China’s Yunnan Observatories on Jan 22, when scientists measured the distance between the moon and the Earth, based on the signals of laser pulses reflected by the lunar retro-reflector planted by a US Apollo mission more than 40 years ago.

Scientists calculated the time a laser pulse takes to travel from a ground station to the retro-reflector on the moon and back again to get a measurement.

Just a few countries, including the United States, France and China, have successfully harnessed lunar laser-ranging technology.

The Queqiao satellite, carrying a reflector developed by Sun Yat-sen University, is expected to extend laser-ranging to a record distance.

As the satellite will fly at a constant high speed, it will be extremely difficult for the laser beam from the ground to target the reflector on the satellite about 455,000 km away.