Telescopes on the Moon - Script (Patreon)

It’s been said that the Apollo missions achieved all 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 Sichuan, carrying with it the Chang’e 3 lander. After five days of travel and another week of lunar orbit, it descended to the surface and became the first soft landing on the moon since the Soviet Luna 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 15 cm diameter. Yet it has one of the clearest views in the universe. The Moon has essentially no atmosphere. That means 1) the potential for extreme image sharpness, with no turbulence to blur light from above; 2) Earth’s atmosphere blocks all but a few narrow windows of the electromagnetic spectrum. From the Moon the entire spectrum is accessible; and 3) on Earth, sunlight is scattered to make our blue daytime 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 it's biggest advantage is that it can see into near-ultraviolet wavelengths - out of the visible range observable without our atmosphere. Here’s a UV picture of the pinwheel galaxy taken by LUT. OK, so it’s not quite the Hubble Space Telescope. But remember, LUT is only a 15cm 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 on Telescopes of Tomorrow: the diffraction limit defines the best resolution a telescope can achieve, and it’s 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 there. 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 g’s of acceleration during a rocket launch and you risk 7 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’s 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. But before I do, let’s talk a bit about why we’ve never sent anything larger than the cute little LUT. Two of these 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 the name of the rover sent with the C  hang’e-3 lander. Poor little Yutu was ravaged by the Moon’s harsh environment. It was gruesome. Let me tell you all about it.

An extremely thin atmosphere confers several advantages, but 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’s no such protection on the Moon. When exposed to the Sun, temperatures rise to around 125 Celsius, well over the boiling point of water. Put up some shade and temperature drops to -150 Celsius or lower. After a relatively short roll on the lunar surface, Yutu experienced a massive temperature differential between its sunny side and shaded side. That fried and frozen rabbit stopped moving pretty quickly. 

Even with no mobility, Yutu’s science instruments remained operational for a couple of years before succumbing to a worse problem: tiny shards of electrically charged glass. In other words, moon dust. See, without an atmosphere there’s no erosion of the lunar regolith. This is the upper surface of the Moon that has been pulverized by 4.5 billion years of meteor impacts. When hit by sunlight, tiny regolith particles build up electric charge and so repel each other into dust fountains in the low lunar gravity. They stick to electronics, impede solar panel function, and grind away at moving parts. Nasty stuff! It eventually killed Yutu. Fortunately the Chang’e telescope is better shielded, and so is still operational.

However the greatest obstacle to a telescope on the Moon, is, of course, getting there. But in that challenge the evil moon dust may actually be our friend. Perhaps we can build mirrors out of the stuff. Peter Chen at NASA’s Goddard Space Flight Center proposes shipping a relatively small amount of epoxy and carbon nanotubes, and combining it with a whole lot of Moon dust to make a sort of concrete. Grind it into shape and coat it with aluminium and it may be possible to build a 50 meter mirror on the Moon. They’ve already built a one-foot prototype and are planning a much larger test telescope on Earth. 

Just as ingenious is the idea of liquid-mirror telescopes. When a cylindrical container of liquid is rotated in a gravitational field, the liquid assumes a smooth parabolic shape – exactly the shape needed to bring light to a perfect focus. Liquid-mirror telescopes already exist on Earth. The biggest is British Columbia's Large Zenith Telescope. It spins a 6m dish of mercury at 8.5 revolutions per minute, creating the cheapest telescope of that size ever constructed. The caveat is that it only points straight up. Tip it and the parabola is ruined. Tip it too far and you spill mercury all over your grad student. But what you can’t do with hardware you fix with fast cameras and software to reconstruct a still image. It’s also possible to correct with a movable secondary mirror. Pointing options are still limited, but by taking advantage of Earth or the Moon’s rotation, it’s possible to scan a narrow arc across the sky. 

But mercury is expensive, heavy, and likely to evaporate in the Moon's low-pressure atmosphere. Pete Worden, Director of NASA’s Ames Research Center and Ermanno Borra of the University of Quebec are working on a plan to instead use nonevaporating ionic liquids - basically molten salts for the mirror base. They'll coat it with 50-100 nm of silver, which will solidify on the slowly flowing surface. Lower gravity could facilitate the formation of larger liquid mirrors, maybe up to a hundred meters across. Such a telescope could gaze on the universe when it was just 500 million years old. It's a cosmologist's dream, especially because cosmology tends not to require complex pointing. The whole liquid mirror configuration would weigh a few tons and could possibly launch in an Ares 5 mission in the 2020s.

Maybe I'm biased by the cool factor, but this all sounds like a great reason to get back to the Moon. Robotically and in person. Plans are underway. More Chang’e missions will launch over the next couple of years. The International Lunar Observatory Association and Moon Express are the first private enterprise to receive approval for lunar landing. They're hoping to set up a small optical telescope, just 7 cm in diameter. Still small, but momentum is building. And Moon Express has a manned based in the planning - useful if we really want a giant lunar telescope. The Moon with its thin atmosphere, stable surface, and long nights is an astronomer's dream. If we can meet its distant challenges with creative solutions, perhaps the Moon will become Earth’s lookout tower, granting clear views of all the rest of Space Time.