Build a radio telescope on the Moon by 2040!
Judging by the way the news has been commented on in the media, it might seem that the loss
of the Arecibo radio telescope has put the very future of radio astronomy in serious jeopardy.
But this is obviously not the case. Already for quite some time, the scientific
production of the great Puerto Rican dish had been waning. In addition, it must be borne in
mind that there are other instruments in the world that do not have much to envy to the old giant,
who, it must be said, was the son of a technology dating back to the late fifties. In fact, let us
remember FAST, the 500-meter Chinese telescope that China has recently made available to the
entire international astronomical community, the always valid Green Banks, a 100-meter steerable
dish located in West Virginia, the Qitai Radio Telescope, also Chinese, which will be ready in
2023 and with its 110 meters will take the record away from Green Banks, the Square Kilometer Array,
the largest and most sensitive ever, under construction in Australia and South Africa.
And then the Atacama Large Millimeter Array, the old Very Large Array in New Mexico,
the Allen Telescope Array in California, also dedicated to SETI... In short, we can say that
the future of radio astronomy can be defined as assured even in the absence of Arecibo.
The feeling is that, rightly, mourning beyond measure the disappearance of Arecibo,
radio astronomers want to force the hand to obtain in exchange something extraordinarily
innovative: a radio telescope on the Moon! Is it possible? Let's try to see more clearly.
After Arecibo collapsed, radio astronomers all over the world were debating what to do to fill
the great void left by that loss. Should the instrument be rebuilt? If so, with what money?
To remake it "as it was and where it was" requires quite a bit, at least $400 million. And so,
more than a few experts began to blow on the fire of "better to focus on extreme renovation".
And, interestingly, NASA did not back down ... Also because it was already some time
that the American space agency had in the drawer a project already nice and ready,
the Lunar Crater Radio Telescope, already funded in April 2020 to verify the feasibility under a
selection called Innovative Advanced Concepts. In short, it was already a while that NASA had
put in the crosshairs this possibility, and the disaster of the Arecibo radio
telescope has only accelerated this propensity to fund extreme technologies
rather than the preservation of the status quo. An audacious decision-making encouraged in part
by the enormous technological advances in materials, robotics, and communications
that have given rise to the sort of "space renaissance" we are currently experiencing.
The need for a new radio telescope and the advantages of building on the lunar surface also
coincides with the desire of the United States and its international and commercial partners
to return to the Moon as early as 2024. Joseph Silk, professor of astronomy at The Johns Hopkins
University, noted in an article in the journal Nature that the return of humans to the Moon,
this time to stay, presents a rare opportunity to advance the science of radio astronomy.
Back in the 1960s, astronomers wanted to build a radio telescope on the far side of the Moon. That,
in fact, is the best place in the inner Solar System to monitor low-frequency radio waves,
which is the only way to detect certain faint "footprints" that the Big Bang left in space. In
fact, terrestrial radio telescopes are disturbed by too much interference to get reliable signals.
The Earth, moreover, has never been a particularly suitable place to do any kind of radio astronomy
because of the dense atmosphere, which blocks a large portion of the radio spectrum.
The moon, by comparison, has no atmosphere and its nights last for weeks, which limits
the Sun's radio noise. And, as NASA discovered through a series of lunar orbiting missions in
the late 1960s, the Moon also acts as a natural shield for radio signals from Earth. As planetary
astronomer Phillipe Zarka put it, "the far side of the Moon during lunar night is the quietest
place in our local universe." That's exactly the kind of peace and quiet you want if you're looking
for faint radio signals from solar systems that might be hundreds of light-years away.
An Arecibo on the Moon, however, would be more than just a substitute. A bit like
when NASA decided to replace the Hubble space telescope. Rather than replicate the original,
the astronomical community preferred to embrace the very expensive idea of building something
completely different and innovative: the James Webb Space Telescope, two and a half times larger,
and designed to study the universe in infrared light. A risky choice, but that soon (the
telescope will be launched at the end of next October) could lead to extraordinary results.
The same is true for the construction of a large radio telescope on the Moon. While Arecibo has
devoted more than half a century to studying radio waves at centimeter and millimeter scales,
a lunar radio telescope could monitor wavelengths greater than one meter,
something astronomers cannot do from Earth. If an Arecibo-like observatory were built on
the Moon, it could potentially detect many exotic cosmic phenomena, such as auroras around distant
Earth-like exoplanets; but most fascinatingly, it could enable enormous scientific discoveries
in the field of cosmology by observing the early universe in the 10- to 50-meter wavelength range
(i.e., in the 6- to 30-MHz frequency band), which has not been explored by humans until now.
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But then, what should this radio telescope look like? And in what way will it be built? The idea
of those who presented the project of the Lunar Crater Radio Telescope is to use a crater of 3-5
km in diameter, size class that includes craters whose profile is approximately spherical in shape,
just like the natural depressions of the land on which they were built Arecibo and FAST in China.
An idea, this one, first considered in the late 1970s, and which was abandoned due to the obvious
technological difficulties of the time. The project foresees then to cover the bottom of
the crater with a grid formed by metal cables, to compose a reflecting dish of one kilometer in
diameter, twice as large as the Chinese FAST. The reduced gravity on the Moon would in fact
allow a radio telescope much larger than any on Earth, which could significantly
improve its sensitivity and depth of research. The most immediate objection to such a project
would be the need for human labor. In fact, it would not only be a matter of building the dish,
but also a base to house the astronauts in charge of doing all the work...
But according to the project's authors, robotic systems have improved enough to make it possible
to remove humans from the equation. This was clearly demonstrated in 2019 when China's
Chang'e 4 rover landed autonomously on the far side of the Moon. These advances in autonomous
navigation have laid the groundwork for a lunar radio observatory built entirely by small rovers
like the ones NASA has already designed, rovers that would be tasked with laying out the wire mesh
and then, in a coordinated action at the crater rim, with energizing the cables holding the
receiver suspended above the dish. The Moon has so many craters that it should be relatively easy
to find one with an edge strong enough to serve as an anchor point for the telescope support cables.
This would avoid the construction of expensive towers like those that anchored Arecibo's cables.
By depending on robots rather than astronauts, the project could save a significant amount of money.
Indeed, any mission involving astronauts requires much deeper systems analysis
and much more expensive safety measures. NASA has also already begun testing a versatile model
of a rover called DuAxel, which could be used for many different lunar missions. Among other things,
DuAxel is capable of scaling steep crater walls. And as a bonus, it's relatively inexpensive.
There is no shortage of problems, however. Building an observatory on the lunar surface
obviously involves significant engineering and operational challenges.
The two-week-long nights mean, for example, that you'd have to find a way to power the observatory
with a form of energy that doesn't rely on electricity generated by solar panels...The energy
needs of an observatory of the size of Arecibo is, by the way, very, very high; and in this regard,
NASA has already developed small nuclear reactors that can keep a lunar base running, and that
could also be used to power all the activity of a radio telescope. Reactors that so far have never
been tested outside the terrestrial environment. An observatory located on the hidden side of the
Moon could also require the presence of an orbiter to transmit data back to Earth, which could create
radio interference, which is exactly what we would like to avoid. However, it may not be necessary
to place the radio telescope "exactly" in the furthest lunar hemisphere. In a separate paper
presented to the Artemis science team, which is NASA's project for a return to the Moon in 2024,
it was suggested that it might be possible to set up an observatory at the lunar south pole,
the presumed destination for NASA's first manned return to the Moon. Although the south pole is
not as quiet as the far side, if an observatory were located on the backside of Mount Malapert
near the pole, it would also be protected from much terrestrial radio interference.
But in either case, the observatory would be exposed to radio interference
from existing orbiters, such as the Chinese satellite Queqiao, which serves as a foothold
for its Chang'e 4 rover. And then there is the fact that, inevitably, the Moon
will also become a very "noisy" place over the years, increasingly frequented by satellites,
and invaded by electromagnetic pollution. For this reason, more than a few authoritative scientists
are advocating the need to make the "far side of the Moon" an oasis of "radio silence".
A concept similar to the protection zones created around terrestrial radio telescopes,
but complicated by the fact that establishing such a zone on the Moon would require the cooperation
of every country and private industry that has the technological means to get there.
So, will we ever see a radio observatory on the Moon? Proponents of the Lunar Crater
Radio Telescope are absolutely confident about that. "The future of astronomy will
be mostly in space. As access to space becomes cheaper and more democratized, astronomy will
move there, and I think much of that will be done from the lunar surface," said one of them.
There are a number of technical, political and financial challenges that will need to
be addressed before that happens, but the team is optimistic that a small orbital exploration
mission looking for the ideal site will be feasible within the next two years.
After that, we will have to rely on the synergy between politics, economics and the
entrepreneurial initiative of private companies. It will not be easy, as the costs are expected
to be around 5 billion dollars (if it seems an enormity, compare the figure to that of a B-2
Spirit fighter-bomber, a stealth that costs a billion dollars per unit!), but
the current space race, which is lowering costs and providing more and more extreme technologies,
allows us to say that if until a few months ago it could be reasonably assumed that the
first observatory on the Moon would be implanted not before 2100, that date has
now been brought forward to at least 2040. Guys, the future is running towards us
faster and faster, let's try to deserve it and be ready for the appointment!