17.05.2018

On 21 May, China plans to launch a satellite with a vital but unglamorous mission. From a vantage point beyond the moon, Queqiao, as the satellite is called, will relay data from Chang'e 4, a lander and rover that is supposed to touch down on the lunar far side before the end of the year. But a Dutch-made radio receiver aboard Queqiao will attempt something more visionary. In the quiet lunar environment, it will listen to the cosmos at low frequencies that carry clues to the time a few hundred million years after the big bang, when clouds of hydrogen gas were spawning the universe's first stars.

The mission is a proof of principle for other efforts to take radio astronomy above the atmosphere, which blocks key radio frequencies, and far from earthly interference. "Putting the whole show into space is extremely appealing," says Michael Hecht of the Massachusetts Institute of Technology's Haystack Observatory in Westford, whose team is also developing small radio satellites that could be used to probe the cosmos. For Europe's astronomers, it is also a test of cooperation with China, something their U.S. counterparts at NASA are barred from doing.

The Netherlands-China Low-Frequency Explorer (NCLE) project stems from a 2015 Dutch trade mission to China, during which the two countries agreed to collaborate on space missions. The Netherlands is strong in radio astronomy: Its Low-Frequency Array (LOFAR) stretches across much of northern Europe. NCLE Principal Investigator Heino Falcke, of Radboud University in the Netherlands, has long advocated a "LOFAR on the moon." China has an ambitious program of moon missions, so he jumped at the chance to take a first step. "We put together a proposal in 2 weeks," he says. Once funded, the team had just 1.5 years to build the instrument. "Half of the experiment is how you work together" Falcke says. Jinsong Ping of the National Astronomical Observatories of China in Beijing, who leads the Chinese team working on the NCLE, agrees: "It is really challenging both sides. … Different culture, habit, language, working manner."

To see back into the dark age before the first stars, astronomers look for a signal emitted when electrons in the primordial neutral hydrogen gas spontaneously flipped their orientation. These photons started out with short radio wavelengths, but over their more than 13-billion-year journey to Earth, the universe's expansion stretched them out to long wavelengths, or low megahertz frequencies. After the gas clumped together to form the first stars, their radiation ionized the neutral gas and eventually snuffed out the faint signal.

Telescopes such as the LOFAR aim to detect the ancient signal and use it to map the distribution of primordial matter. But the signal is hard to discern in the maelstrom of radio noise from terrestrial sources and other objects across the universe. Only one detector, the Experiment to Detect the Global Epoch of Reionization Signature, a set of ground-based antennas in Australia, has so far claimed a detection.

Queqiao, orbiting a gravitational balance point beyond the moon called L2, will offer a quieter vantage. In order to relay signals from the moon to Earth, the satellite can't be completely in the moon's shadow, which means that Earth noise could still be a problem, says Jack Burns, an astronomer at the University of Colorado in Boulder who has long campaigned for a lunar radio observatory. Burns adds that the spacecraft itself will also be a source of interference. But by testing hardware in space, the NCLE "will set the stage for other missions."

Once Queqiao arrives at L2, the NCLE will wait its turn until after the Chang'e 4 lander has achieved its main mission: exploring the South Pole-Aitken Basin, a huge far side depression. Then, around March 2019, the instrument will unspool three 2-meter-long carbon-fiber antennas, each at right angles to the others.

Because Earth's atmosphere blocks all radio signals below 30 megahertz, the data will delight a range of astronomers. Falcke says the team will study solar flares, the aurora of Jupiter, and the galaxy's radio emissions. "There's nothing as good as having real data," he says. The dark age signal is a long shot, he admits. Realistically, the mission is about "gaining expertise to build a follow-up."

The Chinese NCLE team has its own plans. It has placed basic receivers on the Chang'e 4 lander and two microsatellites that Queqiao will release into lunar orbit to study solar radio bursts. Ping says his team will also try to combine signals received by the NCLE with those taken by earthbound detectors—a technique known as interferometry, which can improve resolution. "It is a demonstration," he says. It could show that, once detectors are sensitive enough, interferometry could help them map the newborn universe.

Burns and his colleagues are working on a proposal for a small satellite called the Dark Ages Polarimetry Pathfinder, which he says will be more sensitive to the dark age signal. But eventually, he wants to see an observatory on the lunar far side, deep with the moon's radio quiet shadow. He predicts a NASA-funded low-frequency telescope in the next 5 years. "There's great interest in the far side."

Quelle: Science