An unprecedented low-frequency radio astronomy experiment is now underway, 18 months after entering an orbit beyond the far side of the moon.
The Netherlands-China Low Frequency Explorer (NCLE), aboard the Chinese Queqiao relay satellite, is set to begin observations at low frequencies that cannot be made on Earth because of the ionosphere, particularly between 1 to 30 megahertz (MHz).
The first targets will be the sun and Jupiter, which are expected to have strong emissions at low frequencies. But the team also hopes to pick up much weaker signals from the ‘Cosmic Dawn’—when the first stars lit up around 12 billion years ago—and even ultra-faint signals from the preceding Cosmic Dark Ages. Detections would give unprecedented insights into these formative periods of the universe.
The experiment is only now getting underway, with the deployment of NCLE’s three 5-meter-long antennas, as Queqiao (‘Magpie Bridge’) has played a crucial role in China’s Chang’e-4 lunar far side landing mission.
Queqiao was launched in May 2018 and is now in a halo orbit around the second Earth-Moon Lagrange point about 60,000 to 80,000 kilometers beyond the moon. There, it uses its huge 4.2-meter-diameter parabolic antenna to facilitate communications between terrestrial ground stations and the Chang’e-4 lander and rover on the far side of the moon, which never faces the Earth.
As the Chang’e-4 mission lander and rover have performed well—the duo completed their 12th lunar day of activities on Tuesday and returned valuable science data—secondary objectives can now move ahead.
With NCLE deployed, Queqiao can switch roles from comms relay to radio telescope when the Chang’e-4 spacecraft power down for the extreme cold of the lunar nights.
The deployment has not been smooth, however, possibly due to the instrument having already spent a year and a half in the vacuum of deep space. While one antenna has been deployed to close to its full length, two others remain stuck at about 2.5 meters. The team will continue to trouble-shoot the issue.
Even if the problem remains unresolved, there’s a silver lining. The shorter antennas will be more sensitive to the higher-frequency Cosmic Dawn signatures while the team will also be able to seek weaker signals that have longer wavelengths from the Cosmic Dark Ages with the one fully-deployed antenna.
This illustration shows the location of the Queqiao satellite carrying the Netherlands-China Low Frequency Explorer.
NCLE is just the first step for low-frequency space-based astronomy. “We need to have more antennas in space to map the Cosmic Dawn and the Dark Ages to see how the hydrogen in the early universe developed and formed structures,” says Wolt.
“We also need to develop interferometry techniques in space, so the next goal will be to have multiple antenna units on small satellites,” he adds.
Radboud could potentially become involved in China’s Chang’e-7, a mid-2020s landing mission which will target the lunar south pole and include an orbiter and relay satellite. “If the Chinese have a space for a payload and we can deliver on time, we’re going to try to do that,” Wolt says.
The Chang’e-4 lander also carries a similar instrument to NCLE, the Low Frequency Spectrometer. It has the extra benefit of operating in a pristine environment for astronomy on the lunar far side, where it’s shielded from electromagnetic interference from the Earth by the moon.
Together, the two instruments have opened a new window onto the universe.