European scientists are developing an inflatable radio telescope concept that could do groundbreaking science on the moon.
The idea, based on a recent feasibility study by engineers at the European Space Agency (ESA), proposes an array of radio antennas printed on the superlight space-blanket material kapton that would travel to the moon folded inside the planned European Large Logistics Lander, also known as Argonaut.
"You fold up the system, then you push gas into it and you inflate it. It's like an inflatable mattress on the moon," Marc Klein Wolt, a radio astronomer at Radboud University in Nijmegen, the Netherlands, who leads the ESA Astrophysical Lunar Observatory science team, said at the Astronomy from the Moon conference in London earlier this year.
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Astronomers are interested in building a radio telescope on the far side of the moon, as the region is free of the radio noise generated by technology on Earth. Unlike Earth, the moon has no atmosphere that would absorb radio waves, which makes it a convenient place for their detection. In addition to that, during the two-week lunar night, the far side of the moon is also protected from the radio clatter of the sun. All these factors combined make the moon "the most radio-quiet place in the solar system," Klein Wolt said.
A telescope placed in this region would therefore be able to detect signals that cannot be studied from Earth's surface. And some of these signals are rather precious, as they could allow astronomers to peek into the first few hundred million years in the life of the universe before the first stars and galaxies began to form.
The type of signal the lunar telescope would look for is what astronomers call the 21 centimeter emission line produced by atomic hydrogen, which permeated the universe in the first hundreds of millions of years after the Big Bang. The radiation that hydrogen atoms emit is initially in the microwave range of the electromagnetic spectrum, but since astronomers would be searching for it in the farthest reaches of the universe, they would be looking for hard-to-observe long radio waves. That's because the redshift effect caused by the accelerating expansion of the universe has stretched those microwaves before they reach us.
"To look for this type of radiation, you need to go to the far side of the moon so that you block the radiation that we produce ourselves and you have no atmosphere," Klein Wolt said. "Then you can map the hydrogen and trace it as it forms the first structures in the universe. You can look at different redshifts and make maps of the universe at certain moments in time and see the whole thing evolve."
Astronomers call this mysterious period in the history of the universe the Dark Ages and the Cosmic Dawn. Not even the famed James Webb Space Telescope, which has proved its powers in observing the most ancient galaxies, can see that far back.
A telescope on the far side of the moon is therefore on the list of the potential next big things in astronomy that the world's space agencies are currently considering. But such a project poses many challenges. First of all, technology doesn't exist yet that would enable humans to efficiently transport tens or hundreds of radio antennas to the moon.
The ESA feasibility study therefore analyzed how the Argonaut lunar lander, currently expected to make its debut moon visit in about 2030, could be used to deploy such an array.
With a carrying capacity of 1.5 metric tonnes (1.67 tons), Argonaut is designed to fit into the fairing of Europe's delayed heavy-lift rocket Ariane 6, which might perform its debut flight later this year. The study found that the Argonaut could comfortably deliver an array of 16 standard radio antennas to the moon in one go, but Klein Wolt said that would not be enough to provide the sensitivity and resolution needed to study the Dark Ages in sufficient detail. Hence the attempt to develop an inflatable array that could comprise many more antennas at a much lower mass.
"We could think about having the electronics on the other side [of the antenna] printed as well; we can even think about having solar panels printed in between the large open spaces," Klein Wolt said. "So then we have one deployment, and it becomes rather big. We have 32 by 32 elements, maybe even bigger, and we use the lander as the central unit, so everything is connected there to the lander."
The European team is currently working on antenna prototypes that could be tested directly on the moon in the future to help scientists understand how the dusty lunar environment affects their performance.
"We have several options of building prototypes and testing them," Klein Wolt said. "That's what we are working on today. It doesn't have to be [tested] on the far side; it could be a nearside mission, like the Apollo missions. There are plenty of options that we can explore."
There are other, more modest, nearer-term plans for astronomical observations from the moon's surface. Chinese scientists, for example, envision a mini-constellation of satellites orbiting the moon's equator, making radio measurements on the far side and sending data to Earth from the planet-facing side. They say they might be able to launch that mission in 2026. The NASA-led Lunar Surface Electromagnetics Experiment-Night (LuSEE-Night) pathfinder mission might make the first crude measurements of the Dark Ages signal from the lunar surface even a little earlier, in 2025.
