New Frontier: Telescopes on the Moon

The recent race to the Moon would lead to a growth in astronomical sciences by building instruments and telescopes directly on the Moon

Lunar exploration is experiencing a renaissance. Dozens of missions, organized by multiple space agencies and commercial companies, will 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 the mid-2020s.

The primary reasons behind this surge in activity are the search for lunar resources. Water ice at the poles can be extracted and converted into hydrogen and oxygen, which can serve as rocket propellant. However, science will also benefit greatly from these missions. The Moon still holds many secrets about the origin and evolution of the Solar System, and it could serve as a platform for observational astronomy. Telescopes on the Moon were discussed at a meeting of the Royal Society earlier this year.

Advantages of the lunar far side

Among the sciences that would benefit from this endeavor is radio astronomy. Radio astronomical studies could be conducted with great success on the far side of the Moon. The far side is constantly shielded from the radio signals generated by humans on Earth. Additionally, the lunar night provides protection from solar radiation. All these characteristics make it the most “radio-quiet” place in the entire Solar System.

Radio waves are a form of electromagnetic energy, like infrared, ultraviolet, and visible light. They have different wavelengths in the electromagnetic spectrum, and those with wavelengths longer than about 15 meters are blocked by the Earth’s ionosphere. For astronomy, these longer wavelengths represent the last unexplored region of the spectrum and can reach the lunar surface without obstacles.

Observations of the cosmos at these wavelengths fall under the domain of “low-frequency radio astronomy”. They are the only wavelengths capable of probing the structure of the primordial universe, particularly the cosmic dark ages before the formation of the first galaxies. Astronomer Jack Burns provided relevant scientific justifications (ref.) at the recent Royal Society meeting. He sees the far side of the Moon as an “untouched and silent platform for conducting low-frequency radio observations of the dark ages of the early universe, as well as space weather and magnetospheres associated with habitable exoplanets”.

Signals from other stars

Another potential application of telescopes on the far side of the Moon would be to detect radio waves from charged particles trapped in the magnetospheres of exoplanets orbiting other stars. This could help assess their potential to host life. The radio waves coming from the exoplanet magnetospheres would have wavelengths longer than 100 meters.

Once again, the far side of the Moon would be the optimal location. A similar line of reasoning can be applied to the search for signals from intelligent aliens. Moreover, by opening up an unexplored portion of the radio spectrum, there is also the possibility of discovering new phenomena. We should gain an indication of the potential of these observations during NASA’s LuSEE-Night mission on the lunar far side between 2025 and 2026.

Craters and infrared telescopes

The lunar surface also offers opportunities for other types of astronomy. Astronomers have extensive experience with optical and infrared telescopes operating in free space, such as the Hubble Space Telescope and the James Webb Space Telescope (JWST). However, the polar craters on the Moon, which do not receive sunlight, would be excellent observation points.

Telescopes observing the universe at infrared wavelengths are highly sensitive to heat and thus need to operate at low temperatures. For instance, the James Webb Space Telescope requires an enormous structure to shield it from the Sun‘s rays. On the Moon, a natural rim of a crater could provide this shielding at no cost.

The low gravity of our satellite could also allow for the construction of much larger telescopes than what is feasible for free-flying spacecraft. These considerations have led astronomer Jean-Pierre Maillard to suggest that the Moon could be the future of infrared astronomy.

Craters and gravitational waves

The cold and stable environment of permanently shadowed craters could also offer advantages for the next generation of gravitational wave detection instruments, which detect “ripples” in spacetime caused by processes like star explosions and black hole collisions.

Furthermore, for billions of years, the Moon has been bombarded by charged particles from the Sun and galactic cosmic rays. The lunar surface may hold a rich record of these processes. Studying them could provide insights into the evolution of both the Sun and the Milky Way.

For all these reasons, astronomy will benefit from the current renaissance of lunar exploration. Specifically, astronomy is likely to gain from the infrastructure built on the Moon as lunar exploration progresses. This will include both transportation infrastructure and humans and robots on-site to construct and maintain telescopes on the Moon.

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