Telescopes on the Moon

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Matt O'Dowd (0:32)

This episode is supported by 23andMe. It's been said that the Apollo missions achieved all of the science goals that require live astronauts. That we no longer need manned missions to do science on the moon. Whether or not that's true, what about the science we can do from the moon? I'm talking about building a telescope there. Actually, China already has one.

In December 2013, a Long March 3B rocket blasted off in southwestern Szechuan carrying with it the Chang'e 3 lander. After five days of travel and another week of lunar orbit, it descended onto the surface and became the first soft landing on the moon since the Soviet Lunar 24 in 1976

It's been doing science ever since. That includes soil analysis, extreme ultraviolet observations of Earth's magnetosphere, exploration with an ill-fated rover, I'll come back to that, and of course, a telescope. The Lunar-Based Ultraviolet Telescope, LUT, isn't large at only around 15 centimeters diameter, yet it has one of the clearest views in the universe. See, the moon has essentially no atmosphere. That means, one, the potential for extreme image sharpness, with no turbulence to blur light from above. Two, Earth's atmosphere blocks all but a few narrow windows of the electromagnetic spectrum. From the moon, the entire spectrum is accessible. And three, on Earth, sunlight is scattered to make our blue sky.

On the moon, the stars are visible day and night. In addition, nights are especially great, because they last a month due to the moon being tidally locked with the Earth. The LUT takes advantage of all of these, but its biggest advantage is that it can see into near ultraviolet wavelengths, out of the visible range observable within our atmosphere. Here's a UV picture of the Pinwheel Galaxy taken by LUT.

Okay, so it's not quite the Hubble Space Telescope. But remember, LUT is only a 15 centimeter diameter scope. Some of you probably have bigger telescopes at home. I know I do.

And the diameter of a telescope's aperture is everything. Not only does it allow the scope to collect more light, increasing sensitivity, but when you're in space or on the moon, aperture size defines image sharpness. We talked about this in our recent episode, Telescopes of Tomorrow. The diffraction limit defines the smallest resolution a telescope can achieve, and is inversely proportional to diameter. The LUT is a small scope, so its diffraction limit isn't great. But we can think of it as a proof of concept for building a much larger telescope on the moon.

Normally, there's a pretty strict limit on how big a mirror you can get into space. It has to fit in a rocket. Even clever solutions like the upcoming James Webb Space Telescope's folding mirrors still have a limit. In addition, mirrors are notoriously fragile things, subject them to the few Gs of acceleration during a rocket launch, and you risk seven billion years of bad luck. But on a platform as stable as the moon, it may be possible to construct a telescope mirror right there. In that case, there is no fundamental limit to its size. And there are plans. Big plans. I'm going to come back to a couple of the most promising in a moment, but before I do, let's talk a bit about why we've never sent anything larger than the cute little LUT.

Two of the big challenges hit the Chang'e program pretty hard. So Chang'e was named after the Chinese goddess of the moon. Her mythological pet is Yutu, the jade rabbit. Appropriately, Yutu is also the name of the rover sent with the Chang'e lander.

Poor little Yutu was ravaged by the moon's harsh environments. It was gruesome. Let me tell you all about it. An extremely thin atmosphere confers several advantages, but it also has its issues. To start with, Earth's thick atmosphere is a powerful buffer against rapid changes in temperature. It warms up gradually during the day and cools gradually during the night. There is no such protection on the moon. When exposed to the sun, temperatures rise to around 125 Celsius, well above the boiling point of water. But put up some shade and temperatures drop to minus 150 Celsius or lower.

During its relatively short roll on the lunar surface, U2 was experiencing a massive temperature differential between its sunny side and shaded side. That fried and frozen rabbit stopped moving pretty quickly. Even with no mobility, U2 science instruments remained operational for a couple of years before succumbing to a worse problem.