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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.
 That's
the dream of Peter C. Chen, astrophysicist at NASA Goddard
Space Flight Center. And he wants to build it using lunar
dust—because that might just be the most economical approach.
"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.
"Why
not make a composite using lunar dust?" asks Chen, who
is also adjunct research professor at the Catholic University
of America in Washington, D.C. So in his laboratory, he mixed
NASA's simulated lunar dust called JSC-1A Coarse Lunar Regolith
Simulant with epoxy and a small quantity of carbon nanotubes,
a relatively recently discovered form of carbon that has many
unusual and useful properties. The result? "It came out
as hard, dense, and strong as concrete."
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.
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Author: Trudy E. Bell
| Editor:
Dr. Tony Phillips | Credit: Science@NASA
| more
information |
|
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.
NASA's
Future: US
Space Exploration Policy
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