This video is sponsored by MagellanTV.
Welcome back to Launch Pad, I’m Christian Ready your friendly neighborhood astronomer.
In our last video on future space telescopes, a number of you commented wanting to know
why we don’t just put telescopes on the Moon instead of in space?
Well, it’s a fair question, and one that’s been
asked even before the Apollo program.
But now that NASA has a mandate to return humans
to the Moon under the Artemis program, the
question of lunar observatories has begun to return to the astronomical community.
There are some obvious advantages to the Moon, not the least of which is having a stable
surface to anchor a telescope.
And the Moon's far side makes for the perfect location for
some types of radio astronomy.
In fact, there are some really interesting radio
telescope proposals for the Moon's far side that I'm going to get into toward the end
of this video, so make sure you stick around.
However, it’s hard to overstate just how difficult it is to get to the Moon and how
unforgiving the lunar environment is.
In fact, I was reminded of this the other day when I watched "Moon Shots: Inside The
Lost Apollo Archives” on MagellanTV who, by the way, are kindly sponsoring today’s
video.
“Moon Shots” summarizes NASA’s Apollo lunar program, from Apollo 1 to 17.
The Apollo missions are, to this day, some of the greatest feats of human exploration
ever accomplished.
The documentary features rare archival footage and returns to moments when astronauts were
put in real jeopardy as they explored the Moon.
And it's available on MagellanTV!
MagellanTV is a new type of documentary streaming membership that was founded by filmmakers.
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The idea of a telescope on the Moon isn’t new.
German astronomers Wilhelm Beer and Johann Heinrich von Mädler drew the first accurate
maps of the Moon between 1834 and 1836.
They realized that the Moon’s lack of an atmosphere would allow a pristine view of
the sky and would be the ideal location for a telescope.
With the Apollo program more than a century later, the idea of a telescope on the Moon
finally became a reality.
An ultraviolet telescope designed by George Carruthers of the US Naval Research Laboratory
flew on Apollo 16 in 1972.
To block the Sun’s glare, astronauts John Young and Charles Duke set up the telescope
in the shadow of the lunar module.
During the mission, they would return to point the telescope at another target on Carruthers'
list.
At the end of the mission, the astronauts removed the film cartridge and left the telescope
behind.
The roll of film contained just 178 frames.
It wasn’t much, but it was enough for Carruthers to confirm the existence of interstellar molecular
hydrogen—and reveal the first-ever ultraviolet images of the solar wind, galaxy clusters,
and aurorae in Earth's ionosphere.
Carruthers’ telescope was a proof of concept, designed to demonstrate the capabilities of
a telescope on the Moon.
But the Moon wouldn’t host another observatory until 2013 when China landed another ultraviolet
telescope aboard its Chang’e 3 lander.
The Lunar Ultraviolet Telescope, or "LUT". was
only 15 centimeters or 6-inches in diameter,
but it operated for 18 months and collected thousands of hours worth of data.
So, why was there a 41-year gap between the first two lunar telescopes?
And why weren't there any others in the intervening years?
After all, it takes the Moon about a month to
make one rotation around Earth.
Thanks to tidal locking, it keeps the same side facing Earth the whole time.
That means a telescope on the Moon would experience roughly two weeks of daylight followed by
two weeks of darkness.
Since the Moon lacks an atmosphere, stars can still be imaged in the daytime as long
as the telescope doesn’t point too close to the Sun.
Sounds good, right?
Well, there’s a couple of problems.
A telescope on the near side of the Moon would have the Earth in its sky 100% of the time.
That's a good thing, because it allows for continuous communications back and forth.
But Earth reflects a lot more sunlight to the Moon than the other way around.
In fact, the full Earth as seen from the Moon is 50 times brighter than the full Moon as
seen from Earth.
That’s going to be much too bright for astronomical observations, so a nearside telescope would
have to have a permanent exclusion zone for Earth,
and an additional exclusion zone for the Sun
during the lunar daytime.
A telescope on the Moon’s far side avoids the problem of having an Earth occupying the
sky.
But it also means no direct communication would be possible with Earth, either.
Some additional mechanism would be required, such as a lunar orbiter or a direct link to
an antenna on the Moon’s near side.
But that would require a very long cable; at least several hundred if not thousands
of kilometers in length.
No matter which side of the Moon the telescope is placed, it would have to be insulated against
extreme temperature swings between day and night.
Daytime temperatures reach 127 °C.
When the Sun goes down 10 to 14 days later, the temperature plummets to minus 173 °C.
Earth’s atmosphere does a good job of redistributing heat around the planet.
This minimizes the difference between day and night temperatures and also slows down
the
transition.
That helps to ease the thermal stresses on telescope
mirrors and support structures.
But lacking an atmosphere, the day/night temperature changes are virtuously instantaneous.
That would permanently deform an ordinary telescope mirror and render it useless.
This is why space telescopes are protected by a surrounding enclosure or a sunshade.
A thermal shield on the Moon would have to withstand these extreme temperature changes
on a regular basis.
There’s another problem for telescopes on the Moon that telescopes in space don’t
have to deal with, and that’s Moon dust.
During the Apollo missions, astronauts and their equipment were covered in the stuff.
Moon dust is formed by micrometeorite impacts which pulverize local rocks into fine particles.
The energy from these collisions melts the dirt into a vapor that condenses on soil particles.
As the vapor cools, it crystallizes on the soil, turning the dust particles into tiny
shards of glass.
It gets worse.
Ultraviolet rays from the Sun strip electrons out of the dust, giving the dust an
electric charge.
The dust particles repel each other and billow up into fine clouds in the low lunar gravity.
This makes the dust cling statically to any surface it comes in contact with.
During Apollo 17, for example, “Jack” Schmitt and Gene Cernan reported
having trouble moving their arms during moonwalks because dust worked its way into the joints
of their spacesuits.
41 years later, that same dust would kill Chang’e-3’s rover, Yutu.
But the biggest challenge to putting a telescope on the Moon is…putting it on the Moon.
Not only does all of that mass have to be launched from Earth, it has to land safely
on the Moon.
That means launching a landing vehicle along with the telescope.
It’s no wonder then why it’s easier to launch a telescope into space and operate
it there.
In the coming decades, it will become possible to send large telescopes to the Sun-Earth
L2 point where the telescope can shield itself from the Sun, Earth, and Moon with a single
sunshade, all while keeping a line of sight communication
to Earth.
Oh yeah, and there’s no lunar surface blocking the telescope’s view, either.
But sure, there’s still the matter of actually pointing a telescope once in space.
This is done either with thrusters, reaction wheels, or a combination of both.
But thrusters run out of fuel and reaction wheels eventually break.
Which brings us back to the Moon.
With a surface to push against, operating the telescope becomes much easier.
But what about all that dust?
And what about all of that mass to land?
Well, what if there was a way to use that dust to help build the telescope and reduce
its launch mass?
In 2008, Peter Chen, then at NASA’s Goddard Spaceflight Center, demonstrated a simple
recipe for making a mirror out of moondust, carbon nanotubes, and epoxy.
The “moondust” was crushed Earth rocks NASA uses to simulate the lunar environment.
They poured the mixture into a 30 -cm rotating disc.
As the disk spun, the mixture formed into a parabola,
exactly the shape needed for a telescope mirror.
As more epoxy was added to the mixture, the disc hardened into shape and was coated with
aluminum.
The result is a moondust mirror!
Chen estimated a Hubble-sized mirror could be manufactured with 600 kg
of Moon dust, 60 kg of epoxy, 6 kg of carbon nanotubes and less
than a gram of aluminum.
However, telescope mirrors need to be polished to an accuracy less than the wavelength of
light itself.
This requires sophisticated polishing machines that grind away at a mirror for months at
a time.
So on the one hand, you wouldn’t have to launch the moondust, but you would still have
to launch the rest of the telescope, its instruments, and a mirror lab that could fabricate the
mirror
and install it when ready.
However, a liquid mirror makes forming a telescope as easy as spinning a pan.
A reflective liquid such as mercury is poured into a cylinder which is then spun up.
Gravity and the centripetal force acting on the liquid
forms it into a parabola.
Boom, instant telescope mirror, no polishing required!
However, such a mirror can only point straight up to the zenith.
Tilting it would change the mirror's shape and render
it useless.
The largest liquid mirror telescope ever was the Large Zenith Telescope at the University
of British Columbia.
LZT used a 6-meter liquid mercury mirror until it was decommissioned in 2019.
Mercury would be useless on the Moon.
Not only is it very dense and heavy to launch,
but it would evaporate quickly when exposed to the lunar vacuum.
But ionic liquids - which are essentially molten salts - don’t evaporate in a vacuum
and can remain liquid at very low temperatures.
Ionic liquids aren’t reflective in and of themselves, but coating
it with 50-100 nanometers of silver turns
it into a telescope mirror.
In fact, the silver is so thin it actually solidifies.
In the vacuum of space, a silver-coated liquid mirror would neither evaporate nor tarnish.
Liquid mirror telescopes can only look straight up.
But then again, that means there wouldn't be
any need for heavy mounts and pointing mechanisms.
That would dramatically simplify the construction and reduce its launch mass.
The Moon’s low gravity have two major advantages for liquid mirrors.
First, the mirror can spin slower than it would need to on Earth.
And second, the lower gravity would permit much larger mirrors to be made, even up to,
say, up to
100 meters.
Imagine if the Overwhelmingly Large Telescope could be built on the Moon!
A liquid mirror telescope could also be established in one of the many craters near the Moon’s
poles.
There it would remain in permanent shadow at the bottom of the crater.
The telescope would reach cryogenic temperatures without the need for complex sunshades.
Solar panels mounted around the crater could generate the necessary power to keep the mirror
spinning.
Such a telescope would be ideal for cosmology, which doesn’t require any specific pointing.
But even then, the telescope wouldn’t always have to
look in the same spot in the sky.
As long as the telescope isn’t mounted exactly at the Moon’s rotational pole, its orbit
around the Earth would allow the telescope to scan a ring around the sky every month.
And the Moon’s orbit precesses every 18.6 years.
That would allow the telescope to scan an even
wider swath of the sky.
In principle, the Moon offers several advantages for astronomy, but in practice its many disadvantages
kept telescopes on Earth or in space.
But commercial heavy launch vehicles like SpaceX Starship and Blue Origins
New Glenn will lower the cost of launch.
And NASA just awarded contracts for commercial lunar landers as part of the Artemis program.
But we already know how to launch and operate telescopes in space, so it’s no surprise
that the proposed observatories for the 2020 Decadal Survey are all space-based.
However, there is one kind of astronomy that could be done on the Moon’s far side.
In fact, there’s nowhere else in the inner Solar
System to do it.
Ultra-long radio waves are distorted and blocked by Earth’s ionosphere.
That means detecting the Universe at these wavelengths must be done in space.
But Earth is also the loudest radio source in the
entire solar system.
There’s no “radio shade” a spacecraft can carry to keep Earth’s signals from overwhelming
a telescope.
However, a radio telescope on the Moon’s far side is shielded by the Moon itself.
This makes the Moon’s far side the only location
in the solar system where ultra-low frequency
radio astronomy is possible.
In fact, there is a proposal to create the
Farside Array for Radio Science Investigations of the Dark age and Exoplanets.
Or simply, “FARSIDE.”
To understand the benefits of such a radio telescope, let's talk about a unique science
case.
380,000 years after the Big Bang, the Universe was an ocean of hydrogen and dark matter.
The only radiation was the weak 21-centimeter emission occasionally radiated from hydrogen.
But thanks to the expansion of the Universe, this already-long wavelength emission has
been stretched to ultra-long wavelengths that cannot be detected from Earth.
To detect this primordial emission,
FARSIDE proposes an array of 128 antenna nodes deployed across a 10 square kilometer area.
The array is folded up onto a commercial lander, such as Blue Origin’s Blue Moon.
A rover would unfurl the antenna and trace out a four-petal
array that is connected to the lander which
would handle the central processing.
The lander would then communicate with the Lunar Gateway or another spacecraft, which
would relay data back to Earth.
The lander would be powered by a pair of enhanced Multi Mission Radioisotope Thermoelectric
Generators, allowing a mission lifetime of about 5 years.
The rover would be solar powered, and be able to deploy one petal over a single 10-14-day
sunlight period.
Then the rover would hibernate for the lunar night, and deploy the next petal over the
next daylight cycle.
After four months, the entire array would be deployed, and FARSIDE will be ready to
begin its science mission.
By the way, not only would FARSIDE be shielded from Earth’s radio noise, but also from
radio discharge from Earth’s aurorae and noise from plasma in the solar wind.
Once deployed, FARSIDE would be able to study the the Universe’s Dark
Ages, and allow astronomers to understand how hydrogen flowed with dark matter to create
the Universe’s first stars.
It would also be able to monitor coronal mass ejections from nearby stars, and even detect
the magnetospheres for the nearest habitable exoplanets.
FARSIDE could even characterize similar phenomena here in our own Solar System by monitoring
the magnetospheres of outer planets.
In fact, it could even detect the magnetosphere of the hypothetical Planet 9, should it exist.
An even more ambitious concept under study is
the Lunar Crater Radio Telescope, or LCRT.
LCRT works by landing a lander containing the telescope’s
hardware in the bottom of a 3-5 km diameter crater.
Another lander delivers several DuAxel rovers which surround the crater.
DuAxel is a new technology under development at JPL and Caltech University to allow rovers
to navigate the steep
terrain of Martian craters.
The rovers maneuver into position around the crater and anchor themselves into the soil.
The rear axels detach on
a tether.
The rear axels descend the crater wall until they reach the lander, and
retrieve guide wires leading out from the lander.
They simultaneously ascend
the crater wall, and
as they do so, they suspend the receiver and link it to the waiting rovers.
Then the axels return to the lander to retrieve the telescope mesh.
They’d coordinate their movement to lift the mesh and form the telescope's receiver
(correction:dish).
With the mesh anchored to the surrounding terrain, the axels return to their host rovers.
The DuAxels can now coordinate with each other to adjust the
receiver's position.
The result is a telescope that operates very much like the Arecibo radio observatory and
China’s 500-meter FAST telescope.
The idea is still in it’s early stages but as currently envisioned, LCRT would produce
a 1-kilometer diameter telescope.
This would make LCRT the
largest single-aperture telescope... not in the world, but in the entire solar
system!
The Moon’s far side is the only place in the solar system that sits in the permanent
radio shadow of Earth.
However, the same exploration and expansion of the Moon that would make this kind of radio
astronomy possible could also just as easily ruin
it.
As more infrastructure is added to the Moon, radio interference will build up as well.
If the radio-quiet environment around the Moon’s far side isn’t preserved, then
long-wavelength radio astronomy won't be possible from anywhere in the inner solar
system.
Still, the future of astronomy in space is, on the whole, very promising.
That’s why I made a video about four new space telescopes NASA wants to build.
These telescopes would come after the James Webb and Nancy Roman telescopes which are
set to launch this decade.
There’s some really cool stuff in this video so if you haven't seen it yet, head over there
when we’re
done here.
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Until next time, stay home, stay healthy and stay curious,
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