When Neil Armstrong set foot on the Moon in 1969,
600 million people tuned in to watch.
By 1972, enthusiasm had waned a little,
but Apollo 16 carried a significant, if little
celebrated, first: a telescope to view the
Earth and heavens from the lunar surface.
The chance to put a telescope on the Moon
was a great opportunity for the young astronomer
George Carruthers.
With this telescope, he confirmed that gas
clouds between stars contain molecular
hydrogen, as astronomers had expected, and
took photos of Earth in UV light—displaying
the extent of our hydrogen atmosphere.
The Apollo program was wound up later in 1972,
and Carruthers’ telescope has sat lonely on
the Descartes highlands ever since.
But that didn’t keep scientists from dreaming
and scheming of using the moon as a base for
observing the universe.
Now, exploring—and exploiting—the Moon
is back in fashion, and astronomers hope to
seize this opportunity to turn the moon into
a grand observatory.
The lonely telescope may soon have some new friends.
Radio astronomers are particularly eager to
build a telescope there to peer back into
the deep, dark past of the universe.
For several hundred million years after the
big bang had cooled, the universe was dark
and featureless—filled with neutral hydrogen
gas, but no stars.
Within those clouds, very occasionally a hydrogen
atom would spontaneously emit a photon with
a unique frequency of 1.4 gigahertz.
That light is referred to as the 21-cm line,
owing to its wavelength.
The Earth’s ionosphere blocks these faint
signals from space, and terrestrial transmitters
cause noisy interference.
The lunar far side is the perfect place to
detect these low-frequency signals, as it’s
shielded from interference from Earth
and the Moon has no ionosphere.
With an array of widely spaced antennas on
the far side, astronomers could map the density
of neutral hydrogen across space and time
during that dark age, identifying the clumps
of gas that would later form into galaxies.
Teams of astronomers are planning experiments
to see just how feasible this is, starting
with moon-orbiting satellites with extending antennas
designed to detect this dark age signal.
Another plan could use autonomous rovers to
lay out antennas made of conducting film in
huge arrays to produce a lunar radio observatory.
The rovers could be controlled by crew on
board NASA’s planned moon-orbiting space station.
Although radio astronomy has a compelling
case for a lunar observatory, optical and
infrared telescopes on the moon would also
be hugely valuable.
Telescopes on Earth can be made very large
to capture the faint light of distant objects,
but they're always at the mercy of shifting
fluctuations in the density of the atmosphere,
the process that makes stars twinkle.
Modern telescopes can compensate for this effect
to a degree with deformable mirrors,
but telescopes in space are immune to the problem.
This is why the Hubble Space Telescope is
able to peer further than any ground-based
instrument, despite its relatively small mirror.
The size of orbiting telescopes like Hubble
is limited by the capacity of the rockets
that launch them, but if a way could be made
to make mirrors on the Moon, it might be possible
to build a telescope with clearer vision than Hubble.
One approach that astronomers have been studying
would be to create a mirror from a spinning liquid.
This process seems straightforward—take a
large circular dish, fill it with a liquid,
gently spin it, and centripetal force naturally
creates the perfect parabolic shape for a mirror.
On Earth, this has been done with liquid mercury,
which is naturally reflective, but on the
moon mercury would evaporate.
Instead, researchers believe they could use
a molten salt that won’t evaporate; topped
with a thin layer of reflective silver,
less than 100-nm thick.
In the Moon’s low gravity, the necessary
support structure would be simple and cheap
to build, and since the moon is seismically
quiet, no tremors would disturb the spinning liquid.
But building a 20-meter telescope on the Moon,
10 times the width of Hubble, would still
require transporting over three tonnes of materials
from Earth, which doesn’t come cheap.
One solution to that problem is to use local materials.
One team has made a small test mirror by mixing
simulated moon dust with epoxy,
If such telescopes could be constructed cheaply,
then it would be possible to make arrays of
telescopes and combine their light to achieve
a resolution unimaginable on Earth.
Good enough to study the surface of planets
around nearby stars.
None of these plans have yet gotten the green light,
but with private space agencies and
companies planning expeditions in the near future,
opportunities may soon arise.
But renewed interest in the moon comes at a price:
If companies begin activities such
as mining for minerals on the lunar surface,
this pristine vantage point may be spoiled
before astronomers even get a chance to use it.