Building a radio telescope on the dark side of the Moon

Building a radio telescope on the dark side of the Moon

In the not-too-distant future, a spacecraft touches down on the rim of a massive crater on the far side of the Moon.

As stars spin through the sky above, 20 all-terrain robots roll out of the spacecraft and descend into the shadowy crater. Here, they meet a lander the spacecraft deployed earlier and unpack from it a huge net of mesh. Each taking hold of a guide wire, the robots set off in all directions and climb the crater’s steep walls to stretch the mesh out, until it’s suspended in a kilometre-wide, web-like dish.

In just 10 days, they construct the very first radio telescope on the Moon. The tension in the wires can be adjusted to create the right shape for the dish to receive long-wave radio signals from space – allowing the telescope to peer back in time to the universe’s earliest moments, before stars and galaxies blinked into existence, to ask one of humanity’s most profound questions: what are our cosmic origins?

This may sound like a scene from a sci-fi movie, but it’s actually a project that has just received US$500,000 from NASA.

The Lunar Crater Radio Telescope (LCRT) project is one of six developing technologies to recently win funding from the NASA Innovative Advanced Concepts Program (NIAC) program. Though this isn’t a guarantee that these futuristic projects will ever get off the ground, it does give them the research time to figure out what’s possible.

LCRT is headed by Saptarshi Bandyopadhyay, a robotics technologist at the NASA Jet Propulsion Laboratory (JPL) in California. It’s the second time the team has received NIAC funding, which is unsurprising – their idea is pretty cool.

“We cannot assume any existing infrastructure in the chosen lunar crater on the lunar far-side.”

Taking advantage of the Moon’s natural topography, the telescope will be constructed within a crater 3–5 kilometres wide. The dish itself will be a kilometre in diameter – much bigger than anything on Earth but quite similar in concept to the 305-metre-wide Arecibo Observatory, built in a natural sinkhole in Puerto Rico. This monstrous dish racked up some impressive discoveries over its 50+ years of operation, including spotting the first known planet beyond our solar system, but after being structurally weakened through hurricanes and earthquakes, Arecibo collapsed in 2020.

But the Moon doesn’t have our Earthly problems of earthquakes, wind or weather. The weaker gravity also makes it easier to support a massive structure – not only can it be built out of lighter materials, LCRT will have no need for towers that could collapse or anchored cables that could snap.

Renowned astronomer Frank Drake recognised these advantages and pitched the idea for a massive Arecibo-type telescope on the Moon at a 1986 NASA conference – it seems it took NASA some years to warm to the idea.

Bandyopadhyay’s inspiration for LCRT came from closer to home, while he was an undergraduate at university.

Image of the giant metrewave radio telescope in india
The Giant Metrewave Radio Telescope in India. Credit: GMRT

 “I did some radio astronomy research at the Giant Meterwave Radio Telescope (GMRT), India, where I came to know that humanity has no idea what the universe looks like at wavelengths longer than 10 metres,” he explains.

To be fair to humanity, we’ve only been able to look beyond the narrow visible portion of the electromagnetic spectrum for the past 90 years. Optical telescopes have been used in astronomy since Galileo first squinted up at the moons of Jupiter in 1610, but it was only in 1931 that radio astronomy was recognised, followed by an explosion of technological advances allowing us to study almost the entire electromagnetic spectrum, from radio waves and microwaves all the way up to highly energetic X rays and gamma rays.

Each different type of radiation allows us to see different astronomical objects and processes. Radio telescopes, for example, can pick up incredibly faint signals that have travelled for billions of years across the universe and been stretched out into long wavelengths over the journey.

But many wavelengths are only observable from space, outside of Earth’s protective atmosphere – including the longest radio waves.

This problem was in the back of Bandyopadhyay’s mind as he began working in robotics for space applications during his PhD. When he went to work at JPL, he saw an opportunity to bridge this gap in our scientific knowledge, using rock-climbing lunar robots.

NASA’s DuAxel robots look like standard 4WD rovers, but they can actually detach into two parts, with one acting as an anchor while the other navigates tricky terrain. This will allow them to climb and descend near-vertical walls of Moon craters, which is crucial to Bandyopadhyay’s radio telescope concept.

With this logistical problem solved, the new funding will help the team delve deeper into the project’s science goals and engineering challenges – particularly around the mesh dish.

“A major focus of this NIAC Phase 2 is to design the mesh, so that it satisfies interdisciplinary constraints from launch mass, deployment, radio frequency performance and operations on the Moon,” Bandyopadhyay explains. “We might try to build some small-scale prototypes to test out some ideas.”

Building a telescope out of mesh is less bizarre than it sounds. When most people think of a radio telescope – like Arecibo, or China’s Five-hundred-meter Aperture Spherical Telescope (FAST) – they picture a large, continuous dish made up of hundreds of reflective panels that focus radio waves to a detection device.

But since radio waves are long, the surface of the dish doesn’t need to be smooth at all; chicken wire works surprisingly well at wavelengths of a metre or so. In fact, the GMRT that Bandyopadhyay worked on uses a mesh dish.

Diagram of the proposed moon telescope.
Credit: Saptarshi Bandyopadhyay

This is great news for LCRT, because launching thousands of reflective panels into space would be logistically tricky. A mesh dish would be lighter and less bulky – but what is it going to be made of?

“This hasn’t been decided yet, but we are leaning towards space-grade aluminium,” Bandyopadhyay says – as it would be flexible, light and durable enough to survive extreme temperature differences on the Moon.

This phase of the project will also try and figure out how to fold this giant web of mesh into a spacecraft. (Presumably, origami will be involved.)

“We will need to carry everything we need in our launch vehicle, as we cannot assume any existing infrastructure in the chosen lunar crater on the lunar far-side,” Bandyopadhyay explains.

They’ll also have to set up some kind of communication link with Earth, which he thinks could be done using a relay satellite.

The team are hoping to finish the design of the mesh within the next two years and take the complete product back to NASA.

If this telescope goes ahead, it’s going to help us see the universe in a whole new way, receiving radio signals from 10 to 100 metres in wavelength. These are impossible to observe from Earth because our ionosphere (the very upper atmosphere) blocks them out.

“Humanity has no idea about what the universe looks like at wavelengths longer like 10 metres.”

“There are phenomenal discoveries waiting to happen when we finally open this frontier for observations,” Bandyopadhyay says.

This would be revolutionary for astronomers – like Ben McKinley, from the Curtin Institute of Radio Astronomy in Western Australia.

“For me having a telescope like the Lunar Crater Radio Telescope would be incredible,” says McKinley, who isn’t involved in the project.

“One of the biggest problems for this type of science is getting away from human-generated radio emissions – that’s why the radio telescope I use, the Murchison Widefield Array, is located in a remote region of Western Australia.”

But even there, you can get interference, including from satellites.

“Really, the far side of the Moon is the only truly radio quiet place left that we can hope to build something,” McKinley says.

This telescope could measure radio signals from the very beginning of the universe, in the first few hundred million years after the Big Bang before stars and galaxies evolved. These cosmic Dark Ages are little understood – but studying long radio waves could help astronomers understand the processes that led to the very first stars.

Concept art for the lunar crater radio telescope
Credit: Vladimir Vustyansky / California Institute of Technology

LCRT could also be used for even more ambitious research, McKinley says.

“They probably don’t want to mention the search for extraterrestrial intelligence, since an enormous dish in a crater on the far side of the Moon already sounds crazy enough, but the LCRT could be ideal for SETI, if this were designed into the receiver electronics,” he points out.

“Searching for radio emissions from other civilisations would be an important and fun thing to do with LCRT.”

But while there are many advantages to building a telescope on the Moon, there are also some drawbacks.

“You wouldn’t be able to point it wherever you like on the sky,” McKinley says. “Where it is pointed depends on which crater you choose and then it is stuck there – you can only wait for the sky to drift across as the Moon rotates.”

Plus, large dishes can only see a small portion of the sky at a time, and they’re “immensely complicated, requiring all sorts of innovations in materials, construction, robotics, automation, making it extremely expensive and very risky”.

But don’t worry – McKinley has his own vision for a lunar radio telescope.

“I would start off with a single antenna – not a complicated dish, but something more like a TV antenna, tuned to perform best at the same very low frequencies of the LCRT,” he begins.

This antenna would be designed to detect and measure very faint radio signals without distorting them.

Now, where to put it?

“Where it is pointed depends on which crater you choose and then it is stuck there – you can only wait for the sky to drift across as the Moon rotates.”

McKinley says he would avoid craters altogether, instead planting his antenna into the lunar soil on a flat plain – “away from any discarded landers, rovers or golf clubs that may be lying around”.

“This could be built quicker, using existing technologies and would be less expensive – so who knows, maybe even Australia could afford it by piggy-backing on to one of Elon Musk’s spaceships,” he speculates.

“Once we got the single antenna working and received the Nobel prize, it would be easy to get the funds to send up many more of the same types of antenna, arranging them into an array, and combining their received signals together.”

Unlike the fixed view of the LCRT, this array would be steered electronically, able to gaze across wide swathes of the sky.

“I would call it First Radio Observatory on the Moon in honour of my favourite cartoonist, and my family and I would move to the Moon, where there would be freedom and peace for all,” McKinley concludes.

Tragically, he has not yet been funded for his visionary idea – NASA, are you paying attention?

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