For nearly as long as there has been radio, there have been antennas trained on the sky, looking at the universe in a different light than traditional astronomy. Radio astronomers have used their sensitive equipment to study the Sun, the planets, distant galaxies, and strange objects from the very edge of the universe, like pulsars and quasars. Even the earliest moments of the universe have been explored, a portrait in microwave radiation of the remnants of the Big Bang.
And yet with all these observations, there’s a substantial slice of the radio spectrum that remains largely a mystery to radio astronomers. Thanks to our planet’s ionosphere, most of the signals below 30 MHz aren’t observable by ground-based radio telescopes. But now, thanks to an opportunity afforded by China’s ambitious lunar exploration program, humanity is now listening to more of what the universe is saying, and it’s doing so from a new vantage point: the far side of the moon.
Bouncing Both Ways
As any amateur radio operator can tell you, the key to direct global radio communication is the Earth’s ionosphere – those layers of charged particles that ebb and flow 50 to 600 miles (80 to 1000 km) above our heads. Produced by the constant stream of radiation flowing from the Sun and interacting with the Earth’s magnetic field, the ionosphere has long been known to refract radio waves. The degree to which radio waves are refracted depends on things like the structure of the ionosphere, which changes diurnally, as well as the angle at which the radio waves strike the charged particle layers. But refraction also depends heavily on the wavelength of the incident waves, with the 10-meter band, or 28 MHz, normally considered the upper limit for useful ionospheric bounce.
The refraction of radio waves below 30 MHz or so represents the core problem for ground-based low-frequency radio astronomy. (Nomenclature note: while commercial and amateur radio operators consider the space between 3 MHz and 30 MHz to be the “high frequency” (HF) band for historical reasons, the frequencies reflected by the ionosphere are very low for ground-based radio astronomy.) The ionosphere is nearly as efficient a reflector of radio waves coming from space as it is to those from terrestrial sources, and so acts as a blanket, isolating us from what the universe is telling us in those wavelengths. To be able to do any useful observations below 30 MHz, radio astronomers need to cast off that blanket, and the easiest way to do that is to build a space-based radio telescope.
The Magpie Bridge
The Chinese Chang’e-4 lunar mission presented a perfect opportunity to test what’s possible with low-frequency radio astronomy, and to potentially pave the way for larger-scale efforts in the future. Part of the ambitious, multi-decade Chinese Lunar Exploration Program (CLEP), which may culminate with a crewed mission in the 2030s, Chang’e-4 is a complicated mission made more so by the fact that it was designed to explore the far side of the Moon.
Thanks to tidal locking, the Moon rotates on its axis with exactly the same period as it rotates around Earth, meaning it only ever presents one face to us. So anything on the far side will be blocked off from radio contact with Earth. A practical far-side mission must therefore necessarily include some kind of relay system, to allow for communication between the Earth and Moon. While this could be accomplished with a satellite in lunar orbit, capable of buffering far-side signals until it’s back in sight of Earth ground stations, Chinese mission planners came up with a far more interesting idea: they’d place their relay satellite so that full-time communications would be possible.
To accomplish this, the Chang’e-4 mission planners aimed their relay satellite Queqiao at the Lagrangian point L2, a point in space that lies on the line between the Earth and the Moon, but 65,000 kilometers (40,000 miles) beyond the Moon. Queqiao orbits the L2 point in a halo orbit, an elliptical path around the point but with the orbital plane more or less perpendicular to the line between the Earth and the Moon. That gives the satellite, with its massive 4.2-meter dish antenna, full-time line-of-sight to both the Chang’e lander and the Yutu-2 rover on the lunar far side, as well as the ability to stay connected to ground stations on Earth. This also gives it full exposure to the Sun, allowing it to be powered by solar panels rather than RTGs.
(Queqiao literally means “magpie bridge”, and stems from a Chinese myth where magpies would flock to form a bridge once a year so that the daughter of the Goddess of Heaven could cross the Milky Way to be with her husband. Poetic, no?)
Queqiao has been on-station and fulfilling its primary mission as a communications relay since June of 2018. But Queqiao‘s unique position made it the perfect place to do some science too. The Chinese Academy of Science teamed up with astronomers from Radboud University in the Netherlands to design the Netherlands-China Low-Frequency Explorer, or NCLE. The ten kilogram package includes a sensitive broadband software-defined radio (SDR) receiver and digital signal processing capabilities, fed by a trio of monopole antennas that can be extended to a length of five meters each.
Deployment of the antennas was supposed to happen early in the mission, but Queqiao‘s primary mission had priority and the antennas remained stowed for most of the last 18 months. The command to unfurl the antennas was only recently sent, and while one antenna deployed to its full 5-meter length, the other two antennas appear to be stuck with only about 2.5 meters exposed. It’s possible that the extended stowage time led to lubrication problems like those that afflicted the high-gain antenna of the Galileo probe, but whatever the problem, the science that can be done by NCLE at this point is limited.
If the NCLE is able to fully deploy all the antennas, there’s a vast amount of science waiting to be done. The observatory is perfectly poised to listen in on drastically red-shifted emissions on the 21-cm hydrogen line. Normally in the L-band section of the UHF part of the spectrum, the H-line is the characteristic spectrum of the most abundant element in the universe, and thereby provide a map of its distribution. H-line emissions from the earliest parts of the Big Bang, the so-called “dark ages” that occurred when the universe was only 800 million years old, is extremely red-shifted, lowering its frequency to the point where the ionosphere and terrestrial interference make terrestrial observation impractical. NCLE’s quiet spot in space aboard Queqiao would let cosmologists listen in on the very earliest period of the Big Bang in ways never before possible.
The music of the universe is not the only thing NCLE will be listening to. Using beam-steering, the NCLE antennas will be able to observe the Sun, Jupiter – an extremely bright radio source – and the Earth. Astronomers will have a vantage point to study the interactions between the Sun and the Earth’s ionosphere at lower frequencies than ever before possible, and even to characteristic the “leakage” of man-made radio signals through the ionosphere. And all the planned observations will inform decisions on how to improve low frequency space-based radio astronomy, including possibly building a permanent observatory on the Moon’s far side, or orbiting more satellites to improve resolution through very-long baseline interferometry.
Queqiao was designed to last five years, so the NCLE team has a while to work out the antenna bugs and get the observatory up and running. Here’s wishing them the best of luck as they explore the low-frequency domain from the dark side of the Moon.