A Telescope Made of Moondust
July 9, 2008: A gigantic telescope on the Moon has been a dream of astronomers since the dawn of the space age. A lunar telescope the same size as Hubble (2.4 meters across) would be a major astronomical research tool. One as big as the largest telescope on Earth—10.4 meters across—would see far more than any Earth-based telescope because the Moon has no atmosphere. But why stop there? In the Moon's weak gravity, it might be possible to build a telescope with a mirror as large as 50 meters across, half the length of a football field—big enough to analyze the chemistry on planets around other stars for signs of life.
"If we lift all materials from Earth, we're limited by what a rocket can carry to the Moon," Chen explains. "But on the Moon, you're absolutely surrounded by lunar dust"—a prized natural resource in the eyes of Chen, an expert in composite materials.
Right: Astronauts erect a telescope on the Moon, an artist's concept. [more]
Composite materials are synthetic materials made by mixing fibers or granules of various materials into epoxy and letting the mixture harden. Composites combine two valuable properties: ultralight weight and extraordinary strength. On Earth, for example, bicycle frames made of a composite of carbon fibers and epoxy are favorites of racing cyclists.
Excited, Chen made a small telescope mirror using a long-known technique called spin-casting. First he formed a 12-inch (30-cm) diameter disk of lunar-simulant/epoxy composite. Then he poured a thin layer of straight epoxy on top, and spun the mirror at a constant speed while the epoxy hardened. The top surface of the epoxy assumed a parabolic shape—just the shape needed to focus an image. When the epoxy hardened, Chen inserted it into a vacuum chamber to deposit a thin layer of reflective aluminum onto the parabolic surface to create a 12-inch telescope mirror.
Above: A 12-inch parabolic moondust mirror made by spincasting. The mirror consists of a bottom layer of lunar soil simulant JSC-1A Coarse mixed with a small quantity of carbon nanotubes and bonded with thinned epoxy. Photo credit: Peter C. Chen, NASA/GSFC
The carbon nanotubes make the composite a conductor. Conductivity would allow a large lunar telescope mirror to reach thermal equilibrium quickly with the monthly cycle of lunar night and day. Conductivity would also allow astronomers to apply an electric current as needed through electrodes attached to the back of the mirror, to maintain the mirror's parabolic shape against the pull of lunar gravity as the large telescope was tilted from one part of the sky to another.
To make a Hubble-sized moondust mirror, Chen calculates that astronauts would need to transport only 130 pounds (60 kg) of epoxy to the Moon along with 3 pounds (1.3 kg) of carbon nanotubes and less than 1 gram of aluminum. The bulk of the composite material—some 1,300 pounds (600 kilograms) of lunar dust—would be lying around on the Moon for free.
Right: A moondust parabolic mirror. Sisters Sandra (left) and Sunry (right) Yen holding a 12 inch spincast 'moondust mirror.' The mirror reflects camera flashlight into a light plume above Sunry's head. Photo credit: P. C. Chen, NASA/GSFC.
"I think we've discovered a simple method of making big astronomical telescopes on the Moon at 'non-astronomical' prices," Chen declares. "Building a large space-based astronomical observatory using locally available material is something that is possible only on the Moon. That capability can be a major scientific justification for a return to the Moon."
"It’s a great idea in principle, but nothing is simple on the Moon," cautions physicist James F. Spann, who leads the Space and Exploration Research Office at Marshall Space Flight Center. "Launching a big spinning table to the Moon would be a challenge. If we got the machine spinning in the Moon's dusty environment, how long would it take the dust to settle?" he asks.
Sputtering aluminum vapor onto a large mirror in the presence of ambient dust would be another challenge, because "coating mirrors on Earth is done in a clean environment. There are practical issues about manufacturability that must be resolved."
Despite his concerns, Spann sees real promise in Chen's work and he's enthusiastic about starting out to make simple composite structures on the Moon, such as casting basic blocks from epoxy and lunar dust. "The blocks could be useful for building igloos or habitats for the lunar astronauts," he points out. Then astronauts could work up to making rods, tubes, and other composite structures, to learn how epoxy cures in the Moon's vacuum, and how robust the composites are under solar ultraviolet light. In the end, telescopes might prove practical. "We have a lot of work to do to find out what's possible," he says.
One thing is clear: The sky's the limit, especially when you have so much moondust to work with.
The paper "Moon Dust Telescopes, Solar Concentrators, and Structures," which Peter C. Chen and three colleagues presented at a poster session at the American Astronomical Society meeting in June 2008, appears here. Chen is now in the process of preparing a technical paper for publication.
For background about lunar simulants, see "True Fakes: Scientists make simulated lunar soil," and "Development of Standardized Lunar Regolith Simulant Materials."
The fact that a spinning liquid naturally assumes a parabolic shape was known at least as early as the nineteenth century. Spin-casting of astronomical telescope mirrors was tried in the 1960s by General Electric (see "Electroforming of Large Mirrors," by F. J. Schmidt, Applied Optics, vol. 5 (5), pp. 719–725, May 1966). The modern pioneer of spin-casting glass mirrors is widely acknowledged to be Arizona physicist Roger Angel at the Steward Observatory Mirror Laboratory; SOML has spin-cast glass as large as 8.4 meters in diameter: more.
Examples of the kinds of astronomical science that could be done with large telescopes on the Moon are given in Chapter 4 of the report "Heliophysics Science and the Moon" (September 2007) available here.
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