What NEW SCIENCE Would We Discover with a Lunar Telescope? (Script) (Patreon)

In order to see the faint light from objects in deepest space, astronomers go to the darkest places on the planet. In order to listen to their quite radio signals, they head as far from any radio-noisy humans as possible. But there’s nowhere on the earth, or even orbiting the Earth, that’s far enough to hear to the faint radio hum from the time before stars. In fact, we may need to build a giant radio telescope in the quietest place in the solar system—the far side of the moon.

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Putting a giant telescope on the Moon sounds like a cool idea. After all, the moon has essentially no atmosphere so the sky is crystal clear. And that nice stable surface means would could potentially build a large telescope right there—perhaps a much larger one than we could put in orbit. People have been talking about doing this for decades, but the closest we’ve gotten is the ultraviolet scope on China’s Chang’e 3 lander - with its 2-inch diameter. So is the idea of a proper giant telescope just a sci-fi fantasy? Maybe not.

Meet the Lunar Crater Radio Telescope—a giant radio antenna filling a crater on the far side of the moon. If it’s built, it will be one of the largest telescopes ever built, and by far the largest outside our atmosphere. And it’ll see further than any existing telescope, peering into the time before the first stars and galaxies ever formed. And it may indeed get built. In 2020 the Lunar Crater Radio Telescope received a half million dollars in funding from NASA Innovative Advanced Concepts program to explore its feasibility. It’s going to take quite a bit more work to convince NASA to green-light the project, but first let’s see if we can even convince ourselves that this is a good idea.

First let’s remind ourselves real quick why it’s nice to have telescopes in space. It’s because of Earth’s pesky atmosphere. For most wavelengths of light, that thick blanket of breathable, transparent gas is actually a thick blanket of impenetrable fog. Gamma rays, X-rays, and most ultraviolet is stopped in the upper atmosphere, while only narrow bands of infrared light get past the water and CO2 molecules to reach the surface. This leaves only two windows on earth’s surface where nearly 100% of photons make it to the ground—visible light and a section of the radio spectrum.

To see most of the EM spectrum we need to put telescopes in space—JWST for infrared, Chandra for X-rays, Hubble for ultraviolet, etc. And Hubble is even great for visible light for a different reason—it avoids the blur introduced by the atmosphere.

But for a long time, radio astronomy has been left out of the space telescope game. Not only is the atmosphere transparent to a huge window of radio light, it also doesn’t introduce the same blur. For that reason, we’ve been pretty happy building our giant radio telescopes on the surface of the Earth. So why put one in space?

Well, radio photons with wavelengths from around 1cm to 10m can pass through the atmosphere just fine. But wavelengths longer than that tend to get reflected by the ionosphere.  This is where the last wisps of the atmosphere are fading into the near vacuum of space. The intense solar radiation up there separates electrons from their atoms, wreathing the planet in a shell of charged particles. When a radio photon reaches the ionosphere it will jiggle an electron, and that jiggling electron will send the photon right back where it came from.

So our planet is surrounded by a sort of radio mirror. This is actually handy—we can send radio signals extremely long distances across the surface by bouncing them off the ionosphere. But it also means that from the surface, space is completely dark to long-wavelength radio light.

So we’ve literally never looked at the universe across an enormous range of the electromagnetic spectrum. That’s what the lunar crater radio telescope will allow us to do. Now while I fully support peering at the universe in new ways just to see what we’ll see, remember that’s not going to be enough to convince NASA. They’re going to want to be sure that we learn something important if we expand our vision this way. Well, there’s always the fact that this huge blind spot in the EM spectrum corresponds to an entire cosmic epoch that we’ve never glimpsed.

So, the oldest light we can detect is the cosmic microwave background radiation, which was emitted when the universe was only 370,000 years old. The hot plasma of hydrogen and helium that filled the early universe back then cooled to the point that electrons and nuclei could form the first atoms, rendering the universe transparent to light. The photons released at that moment were mostly infrared, but since then they’ve traveled for 13 and a half billion years through a universe that expanded by a factor of over 1000. We say that the light has been redshifted—from infrared to the millimeter wavelength photons of the cosmic microwave background.

But after the release of the CMB there’s a huge gap in the fossil record. At some point, all that hydrogen and helium gas collapsed to form the first stars and galaxies, lighting up the universe. The James Webb Space Telescope is seeing some of this very early starlight. But between the release of the CMB and the time when star formation really took off there’s a gap of at least a 100 million years.

We call these the cosmic dark ages, and they’re crucial for understanding everything that happened after. It was during this time that gravity began to pull matter together into the massive halos that would form superclusters of galaxies. At the moment, everything we know comes from our simulations. But with a giant moon telescope, we could actually peer into this time.

And see … what? If there were no stars or galaxies back then, what light would even look for? The matter that would go on to form stars would be in the form of hydrogen and helium gas that would quickly be losing the heat it had when it first formed. As such, it would emit very little light. But not NO light. When the electron in atomic hydrogen flips the direction of its spin, it emits a very low energy photon of radio light with a wavelength of 21cm. If you point a radio antenna at the space between the stars in the Milky Way and tune the receiver through the wavelengths, you’ll get a spike of signal at 21cm. And we’ve actually mapped the cold gas in the Milky Way by looking at this light with our big Earth-based radio telescopes.

Any 21 cm radiation emitted during the cosmic dark ages will have been redshifted by the expansion of the universe. For example, a 21 cm photon emitted when the universe was just 17 million years old will be stretched to 21 meters by the time it reaches us. More distant, earlier gas will be seen in longer wavelengths. While regular 21 cm radio reached Earth’s surface, the stuff coming from the dark ages has been redshifted to the point that it bounces right off our ionosphere. Perhaps you’re starting to see why a space radio telescope might be a good idea.

OK, sure, but why a giant one, and why on the moon? Well, the bigger the telescope the more sensitive it is, so that’s nice. But there’s a more critical reason for giant-ness. In order to detect light of a certain wavelength, you need a telescope that’s at least as big as that wavelength. And to cleanly bounce that light around in order to make an image, the telescope should be quite a bit larger than that wavelength. In fact the resolution of a telescope depends on the ratio of its aperture size to the wavelength it’s looking at. In order to get a good resolution looking at 10-100-meter-long radio photons, you’re going to need a telescope a hundred of meters

in diameter at least.

That sounds like a big scope. It was an enormous challenge to get JWST’s 6 and a half meter mirror safely into space. But mirrors for infrared, visible, and ultraviolet light are heavy, fragile glass. Radio telescope “mirrors” can be a simple mesh of conducting material. Remember I said that the ionosphere reflects radio waves because it contains electrons for those waves to move around. Well, metal wire has moveable electrons, and so a radio wave hitting a wire mesh will just bounce off as long as the wire grid size is sufficiently smaller than the wavelength of interest.

That reflecting surface still has to be the right shape—and that shape is a paraboloid—a circular parabola—just as with regular telescope mirrors. A paraboloid will focus incoming waves coming from a particular direction to a single point.

Most radio telescopes swivel to point at the object of interest. But above a certain size, swiveling becomes impracticable. For one thing, the shape of the dish warps as it moves. That’s why our largest radio dishes were built fixed in place in valleys, like China’s 500m diameter FAST telescope or Puerto Rico’s slightly smaller Arecibo …  before it collapsed. Rest in peace buddy. Just a side note—because it’s cool how these fixed dish telescopes work. They have a feed antenna—which is the detector—that can slide around to the focal points of light coming in from different directions. Arecibo used an elongated feed antenna to correct for its spherical dish shape, while FAST uses movable panels to warp the dish into a paraboloid pointing at the feed antenna location.

The Lunar Crater Radio Telescope will be a 350 meter diameter fixed-dish—so at bit bigger than Arecibo and a bit smaller than FAST. Now this isn’t a new idea. There have been proposals to build fixed-dish radio telescopes on the moon since the 60s. One of the reasons these never went anywhere is that it’s not really possible to build an Arecibo equivalent there. Both Arecibo and FAST heavy support structures to hold them in the correct shape. This requirement is one of the things that killed previous proposals. It was, and still is, too difficult to transport the heavy support structure required to maintain the shape of the telescope.

But the Lunar Crater Radio Telescope has a beautiful workaround—just hang the telescope like a giant space hammock. Normally when you hang a thread between two points you get a shape called a catenary, not a parabola. But that’s only if the rope has a constant thickness. If the wires are thicker at the edges and become thinner towards the center in just the right way, that thread will hang as a parabola.

So it’s possible to create a perfect parabolic dish with no supports besides whatever tethers it to the edge.  This design will also safely withstand the 300 Kelvin range of temperatures it will be exposed to—a fluctuation that could damage a more rigid architecture, although actual observations will only happen during lunar night when the radio-noisy sun is out of the way and the temperature fluctuation is more like 10 Kelvin.

The new LCRT design calls for the entire reflector mesh to weigh about half a ton, with a total launch mass around 3 times that. This is well within the capabilities of current launch technology.

But it’s one thing to get a giant net to the moon and entirely another to set it up. Let me talk you through the current plan, with this nice animation produced by the LCRT team. The spacecraft containing the folded-up telescope lands in the center of the chosen crater—and I’ll come back to how that’s selected. Then harpoons are fired beyond the crater rim. These are pulled tight and then the feed antenna is raised to what will be the focal point of the dish. Then the mesh is unfolded by pulling it up towards the rim. By adjusting the tethers, the shape of the reflector can be adjusted to a nice, round paraboloid.

OK, so we know how we might go about installing a lunar crater radio telescope. We just need to choose a crater. We want it to be on the far side, and preferably deep into the far side to minimize interference from the moon’s annoyingly radio-loud planetary neighbour.

Since the telescope won’t be steerable, it’ll point wherever that spot on the moon is pointing. That means it sweeps out a ring as the moon rotates once per month. That ring is as large as possible if the telescope is on the lunar equator. Unfortunately, that exact location sweeps through the Milky Way, which is loud with radio signals that we don’t care about. Positioned around 15 degrees north of the equator gives us a large circle above the galactic disk, which gives us a much clearer view of the beginning of the universe.

Add the restriction of a nice, round, boulder-clear, and relatively deep crater really cuts down the options. The current top candidate is this little guy. At 1.3km across and nearly 300 meters deep, it’s small enough that it doesn’t even have a name. However it’s large enough to comfortably fit our proposed new toy.

As nice as it’ll be to have the entire mass of the moon acting as a shield against human radio interference, it does make communication with the observatory challenging. Radio telescopes in particular collect an enormous amount of data. So how do we transmit it back to Earth? We’ll probably need a dedicated relay satellite, perhaps orbiting the Moon so it uploads new data on the far side and transmits it on the near side. Or perhaps a relay parked at the Earh-Moon L2 Lagrange point.

So, what’s NASA going to say? The proposal certainly seems plausible, and the science is very compelling. There are other options for looking into the cosmic dark ages besides specifically a radio telescope in a lunar crater. For example, there are plans to cover much larger regions of the moon’s far side in simple dipole antennae, or even build such a dipole array from a swarm of satellites in orbit around the moon or at the L2 point. There are advantages and disadvantages to all these approaches—or possibly a combination. Honestly, turning the far side of the moon into a giant radio telescope with crater dishes and dipole arrays sounds too cool not to do. But I’m not NASA, so we’ll have to wait and see what they decide is the smartest way to map peer into the time before stars at the beginning of space time.