April 24, 2024

Building telescopes on the Moon could transform astronomy – and it’s becoming an achievable goal

Lunar exploration is experiencing a renaissance. Dozens of missions, organized by multiple space agencies – and increasingly by commercial companies – are expected to visit the Moon by the end of this decade. Most of them will involve small robotic spacecraft, but NASA’s ambitious Artemis program aims to return humans to the lunar surface by mid-decade.

There are several 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 will also certainly be a major beneficiary.

The moon still has a lot 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 of astronomy on Earth’s natural satellite was discussed at a meeting of the Royal Society earlier this year. The meeting itself was, in part, triggered by the improved access to the lunar surface now in prospect.

Benefits on the other side

Various types of astronomy would benefit. The most obvious is radio astronomy, which can be conducted on the side of the Moon that always faces the opposite side of the Earth – the far side.

The far side of the moon is permanently shielded from 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 silent” place in the entire solar system, since no other planet or moon has a side permanently facing away from Earth. It is therefore ideal 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 on the electromagnetic spectrum.

Radio waves with wavelengths longer than about 15 meters are blocked by Earth’s ionosphere. 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 is best studied on the far side of the moon.

Observations of the cosmos at these wavelengths fall under the umbrella of “low-frequency radio astronomy.” These wavelengths are capable of probing 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 used this property to study hydrogen clouds in our own galaxy – the Milky Way – since the 1950s.

Because the Universe is constantly expanding, the 21-centimeter 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 longer than 10 meters. The far side of the moon may be the only place we can study this.

Astronomer Jack Burns provided a good summary of the relevant scientific background at the recent Royal Society meeting, calling the far side of the Moon “a pristine and silent platform for conducting low-frequency observations of the Dark Ages of the early Universe, as well as climate space and magnetospheres associated with habitable exoplanets.”

Signs from other stars

As Burns says, another potential application of radio astronomy from the other side is trying to detect radio waves from charged particles trapped by magnetic fields – magnetospheres – of planets orbiting other stars.

This would help assess the ability of these exoplanets to support life. Radio waves from exoplanets’ magnetospheres would likely have wavelengths longer than 100 meters, so they would require a quiet environment in space. Again, the far side 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.

Artist’s conception of the LuSEE-Night radio astronomy experiment on the moon. Image credit NASA / Tricia Talbert

We should get an indication of the potential of these observations when NASA’s LuSEE-Night mission lands on the far side of the Moon in 2025 or 2026.

Crater depths

The moon also offers opportunities for other types of astronomy. Astronomers have a lot of experience with optical and infrared telescopes that operate in free space, such as the Hubble Telescope and JWST. However, the stability of the lunar surface may confer advantages for this type of instruments.

Additionally, there are craters at the lunar poles that do not receive sunlight. Telescopes that observe the universe in infrared wavelengths are very sensitive to heat and therefore have to operate at low temperatures. JWST, for example, needs a huge sunscreen to protect it from the sun’s rays. On the Moon, the natural rim of a crater could provide this protection for free.

A permanently shadowed lunar crater
Permanently shadowed craters at the lunar poles could eventually host infrared telescopes. Image credit: LROC/ASU/NASA

The Moon’s low gravity could also allow for the construction of much larger telescopes than would be feasible for free-flying satellites. These considerations led astronomer Jean-Pierre Maillard to suggest that the Moon could be the future of infrared astronomy.

The cool, stable environment of permanently shadowed craters could also bring advantages to the next generation of instruments for detecting gravitational waves – “ripples” in space-time caused by processes such as exploding stars and colliding black holes.

Furthermore, for billions of years, the Moon has been bombarded by charged particles from the Sun – the 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 the Sun and the Milky Way.

For all these reasons, astronomy will benefit from the current renaissance of lunar exploration. In particular, astronomy will likely benefit from the infrastructure built on the Moon as lunar exploration progresses. This will include both transportation infrastructure – rockets, landers and other vehicles – to access the surface, as well as humans and robots on site to build and maintain astronomical instruments.

But there is also a tension here: Human activities on the far side of the Moon could create unwanted radio interference, and plans to extract water ice from shadowed craters could make it difficult to use those same craters for astronomy. As my colleagues and I have recently argued, we will need to ensure that lunar sites that are uniquely valuable to astronomy are protected in this new era of lunar exploration.The conversation

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Image credit: NASA / Ernie Wright

Leave a Reply

Your email address will not be published. Required fields are marked *