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MI0KQU > ASTRO 27.04.23 20:10l 149 Lines 8222 Bytes #999 (0) @ WW
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Subj: Building telescopes on the Moon could transform astronomy
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Building telescopes on the Moon could transform astronomy
The current race to the Moon is opening up opportunities for lunar astronomy.
Published: Wednesday, April 26, 2023
Lunar exploration is undergoing a renaissance. Dozens of missions, organized by
multiple space agencies - and increasingly by commercial companies - are set to
visit the Moon by the end of this decade. Most of these will involve small
robotic spacecraft, but NASA's ambitious Artemis program, aims to return humans
to the lunar surface by the middle of the decade.
There are various reasons for all this activity, including geopolitical
posturing and the search for lunar resources, such as water-ice at the lunar
poles, which can be extracted and turned into hydrogen and oxygen propellant
for rockets. However, science is also sure to be a major beneficiary.
The Moon still has much to tell us about the origin and evolution of the solar
system. It also has scientific value as a platform for observational astronomy.
The potential role for astronomy of Earth's natural satellite was discussed at
a Royal Society meeting earlier this year. The meeting itself had, in part,
been sparked by the enhanced access to the lunar surface now in prospect.
Farside benefits
Several types of astronomy would benefit. The most obvious is radio astronomy,
which can be conducted from the side of the Moon that always faces away from
Earth - the farside.
The lunar farside is permanently shielded from the radio signals generated by
humans on Earth. During the lunar night, it is also protected from the Sun.
These characteristics make it probably the most "radio-quiet" location in the
whole solar system as no other planet or Moon has a side that permanently faces
away from the Earth. It is therefore ideally suited for radio astronomy.
Radio waves are a form of electromagnetic energy - as are, for example,
infrared, ultraviolet and visible-light waves. They are defined by having
different wavelengths in the electromagnetic spectrum.
Radio waves with wavelengths longer than about 15 meters are blocked by Earth's
ionoshere. But radio waves at these wavelengths reach the Moon's surface
unimpeded. For astronomy, this is the last unexplored region of the
electromagnetic spectrum, and it is best studied from the lunar farside.
Observations of the cosmos at these wavelengths come under the umbrella of "low
frequency radio astronomy." These wavelengths are uniquely able to probe the
structure of the early universe, especially the cosmic "dark ages" - an era
before the first galaxies formed.
At that time, most of the matter in the universe, excluding the mysterious dark
matter, was in the form of neutral hydrogen atoms. These emit and absorb
radiation with a characteristic wavelength of 21 centimeters. Radio astronomers
have been using this property to study hydrogen clouds in our own galaxy - the
Milky Way - since the 1950s.
Because the universe is constantly expanding, the 21-cm signal generated by
hydrogen in the early universe has been shifted to much longer wavelengths. As
a result, hydrogen from the cosmic "dark ages" will appear to us with
wavelengths greater than 10 meters. The lunar farside may be the only place
where we can study this.
The astronomer Jack Burns provided a good summary of the relevant science
background at the recent Royal Society meeting, calling the farside of the Moon
a "pristine, quiet platform to conduct low radio frequency observations of the
early Universe's Dark Ages, as well as space weather and magnetospheres
associated with habitable exoplanets."
Signals from other stars
As Burns says, another potential application of farside radio astronomy is
trying to detect radio waves from charged particles trapped by magnetic fields
- magnetospheres - of planets orbiting other stars.
This would help to assess how capable these exoplanets are of hosting life.
Radio waves from exoplanet magnetospheres would probably have wavelengths
greater than 100 meters, so they would require a radio-quiet environment in
space. Again, the farside of the Moon will be the best location.
A similar argument can be made for attempts to detect signals from intelligent
aliens. And, by opening up an unexplored part of the radio spectrum, there is
also the possibility of making serendipitous discoveries of new phenomena.
We should get an indication of the potential of these observations when NASA's
LuSEE-Night mission lands on the lunar farside in 2025 or 2026.
Crater depths
The Moon also offers opportunities for other types of astronomy as well.
Astronomers have lots of experience with optical and infrared telescopes
operating in free space, such as the Hubble telescope and JWST. However, the
stability of the lunar surface may confer advantages for these types of
instruments.
Moreover, there are craters at the lunar poles that receive no sunlight.
Telescopes that observe the universe at infrared wavelengths are very sensitive
to heat and therefore have to operate at low temperatures. JWST, for example,
needs a huge sunshield to protect it from the Sun's rays. On the Moon, a
natural crater rim could provide this shielding for free.
The Moon's low gravity may also enable the construction of much larger
telescopes than is feasible for free-flying satellites. These considerations
have led the astronomer Jean-Pierre Maillard to suggest that the Moon may be
the future of infrared astronomy.
The cold, stable environment of permanently shadowed craters may also have
advantages for the next generation of instruments to detect gravitational waves
- "ripples" in space-time caused by processes such as exploding stars and
colliding black holes.
Moreover, for billions of years, the Moon has been bombarded by charged
particles from the Sun - solar wind - and galactic cosmic rays. The lunar
surface may contain a rich record of these processes. Studying them could yield
insights into the evolution of both the Sun and the Milky Way.
For all these reasons, astronomy stands to benefit from the current renaissance
in lunar exploration. In particular, astronomy is likely to benefit from the
infrastructure built up on the Moon as lunar exploration proceeds. This will
include both transportation infrastructure - rockets, landers, and other
vehicles - to access the surface, as well as humans and robots on-site to
construct and maintain astronomical instruments.
But there is also a tension here: Human activities on the lunar farside may
create unwanted radio interference, and plans to extract water-ice from
shadowed craters might make it difficult for those same craters to be used for
astronomy. As my colleagues and I recently argued, we will need to ensure that
lunar locations that are uniquely valuable for astronomy are protected in this
new age of lunar exploration.
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