The Queqiao spacecraft and two radio-astronomy experiments launched from the Xichang Space Centre in western China on 21 May. Credit: AFP/Getty

China has taken its first major step in a groundbreaking lunar mission. On 21 May, a probe launched from Xichang Satellite Launch Centre to head beyond the Moon, where it will lie ready to act as a communications station for the Chang’e-4 lunar lander. The nation hopes that the lander will, later this year, become the first craft to touch down on the far side of the Moon.

The relay probe, named Queqiao and designed by the Chinese Academy of Sciences, also carries two pioneering radio-astronomy experiments. Both are proof-of-principle missions designed to test technologies for exploring a period in cosmic history known as the dark ages. These first few hundred million years of the Universe’s existence, before galaxies and stars began to form, are all but impossible to study from Earth. But the spectrum of radiation from this age — when matter was distributed nearly uniformly across space as a thin, cold haze — could reveal information about the distribution of ordinary matter compared with dark matter in the Universe.

The first experiment is the Netherlands-China Low-Frequency Explorer (NCLE). It will remain attached to Queqiao, which will linger around ‘Earth-Moon L2’ — a gravitational resting point about 60,000 kilometres beyond the Moon that tracks the Moon’s orbit around Earth (see ‘Far-side satellite’). The Dutch-built NCLE experiment will try to exploit the relative quiet there to measure radio waves with frequencies between about 1 megahertz and 80 megahertz, coming from the Solar System, the Galaxy and beyond. Much of this frequency band is blocked by Earth’s atmosphere, but cosmologists expect it to contain information from the dark ages. Around the upper end of this band also fall the ‘cosmic- dawn’ signals from the first stars, which lit up around 200 million years after the Big Bang and were apparently detected for the first time earlier this year. Other experiments are trying to replicate those results — but the NCLE is testing technologies for identifying lower-frequency signatures from the dark ages.)

Credit: National Astronomical Observatory of China/Chinese Academy of Sciences

For at least part of its orbit, Queqiao will be eclipsed by the Moon, as seen from Earth, which could benefit the NCLE because its antennas will be further shielded from the radio noise that constantly leaks from our planet. Still, observation time and the bandwidth for sending data back to Earth will be limited. And because Queqiao is designed primarily as a data-relay station (its name comes from a folktale about magpies that form a bridge across the sky), it is not optimized for radio astronomy. That means it will be challenging, if not impossible, for this demonstrator mission to detect the dark-ages signal itself, says Heino Falcke, a radio astronomer at Radboud University Nijmegen in the Netherlands who is the experiment’s science leader. Nonetheless, the NCLE “is pioneering and an important first step toward investigating the dark ages and cosmic dawn”, says Jack Burns, an astrophysicist at the University of Colorado Boulder who is leading a proposal for a NASA mission with similar objectives.

To avoid jeopardizing the Queqiao probe, mission control will deploy the NCLE’s antennas only after the Chang’e-4 lander’s mission is completed, says Marc Klein Wolt, a Radboud astronomer who is NCLE’s manager. But the NCLE might go on collecting data for several years, he says.

For more details, please refer to https://www.nature.com/articles/d41586-018-05231-9.