China has taken its first major step in a groundbreaking lunar mission. On 21 May, a probe launched from Xichang Space 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 Chang’e-4 lander will, later this year, become the first ever probe 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, before galaxies and stars began to form, are all but impossible to study from the Earth. But the spectrum of radiation from this age — when matter was nearly uniformly distributed across space as a thin, cold haze — could reveal information about the relative distribution of ordinary matter versus dark matter in the Universe .

One experiment is the Netherlands-China Low-Frequency Explorer (NCLE), which will linger with Queqiao at a gravitational resting point called Earth–Moon L2 beyond the Moon that tracks the Moon’s orbit around Earth (see ‘Far-side satellite’). The Dutch-built experiment will try to exploit the relative quiet there to measure radio waves 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 lie the ‘cosmic dawn’ signals from the first stars that lit up around 200 million years after the Big Bang, apparently detected for the first time by an experiment in Australia earlier this year . Other experiments are trying to replicate those results — but the NCLE is mainly looking for the lower-frequency signatures from the dark ages.)

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 radio noise that constantly leaks from our planet. Still, observing time and the bandwidth for sending data back to Earth will be limited. And because Queqiao is primarily designed as a data-relay station (its name is 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, to detect the dark-ages signal with this demonstrator mission, says Heino Falcke, a radio astronomer at Radbound University in Nijmegen 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.

Mission control will deploy the NCLE’s antennas only after the Chang’e-4 lander’s mission is completed, to avoid any risk of destabilizing the Queqiao probe, 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.

Satellite break-off

The second experiment that launched with Queqiao consists of two smaller satellites called Longjiang-1 and Longjiang-2, which will detach from the mothership and orbit the Moon. Built by researchers at the Harbin Institute of Technology in China, the instruments will test technology for a radio astronomy technique called very long baseline interferometry (VLBI). This approach combines data from multiple radio antennas to get images of much higher resolution than would be possible with a single dish.

Falcke and others have long studied the possibility of doing VLBI with a large array of lunar orbiters — or on the lunar surface — to map variations across the sky in signals from the dark ages and cosmic dawn. Klein Wolt says that his team might experiment with combining data from NCLE with those from the two lunar orbiters and even from a radio antenna on the Chang’e-4 lander itself.

The Chang’e-4 mission is another step in China’s ambitious lunar-exploration programme, which aims to establish a Moon base in the next decade and to begin human exploration in the 2030s. The lunar lander will carry a rover and was originally designed as a back-up probe for Chang’e-3, which in 2013 became the first craft to soft-land (rather than crash-land) on the Moon since 1976. Chang’e-4 has now been repurposed, and the mission’s main scientific goal is to study the geology of the hidden side of the Moon, which is pockmarked with many more small craters than the familiar near side .

The lander carries experiments including a sealed ecosystem, built by Chongqing University, which will test whether potato and thale-cress ( Arabidopsis ) seeds sprout and photosynthesize while silkworm eggs hatch and worms produce carbon dioxide. Another experiment will measure the radiation that future astronauts who visit the lunar surface will be exposed to. The rover, which will separate from the lander to move around the surface, will carry instruments including a solar-wind detector built by a Swedish team.