Building a giant "super telescope" on the moon? It's not easy

Recently, French scientists announced a plan to build a giant "super telescope" on the moon, which has attracted widespread attention. So why do humans want to place a telescope on the moon? What technical challenges will this face? Which countries are currently conducting similar explorations?

A good location for observing the universe

In order to explore the secrets hidden in the universe, humans have been working hard to improve their ability to observe extraterrestrial objects and cosmic phenomena while continuing to develop science and technology. In the long history of space exploration, humans have built a large number of space telescopes on Earth. At the same time, in order to observe space from a better angle, people also carry space telescopes on airplanes or directly launch space telescopes into Earth orbit. These different forms and types of space telescopes provide an effective way for humans to explore the mysteries of the universe, and scientists have also achieved a lot of scientific results.

As science and technology continue to develop, the way humans explore space is also constantly improving. At the end of 2021, the James Webb Space Telescope was successfully launched. After arriving in a halo orbit around the Sun-Earth Lagrange L2 point, it began to observe the distant space and achieved many scientific results.

In recent years, as humanity's exploration and understanding of the moon have deepened, scientists from various countries have begun to study whether it is possible to place or build a space telescope on the moon.

Massive crater on the moon reduces earthwork for building giant space telescope

There is no atmosphere or light pollution on the moon, so the telescope's resolution will not be damaged. The moon rotates every 28 days, and the long night allows the telescope to continuously observe the same target for nearly 14 days. If the telescope is placed on a crater on the back of the moon, according to the tidal locking principle between the moon and the earth, the telescope will never be affected by the earth's radio, satellite, solar radio noise, etc., which is a very good location for observing the universe.

These new lunar telescopes will provide new answers to key questions that have long puzzled humanity, such as where did the universe come from? Where did we come from? Are we alone in the vast universe?

In fact, placing and building a telescope on the moon is not a new idea. As early as the Apollo manned moon landing program carried out by NASA in the 1960s and 1970s, the astronauts who successfully landed on the moon placed retroreflectors on the moon so that astronomers could accurately measure the distance from the earth to the moon.

The Chang'e-3 lunar probe launched by my country on December 2, 2013, is equipped with a lunar-based astronomical telescope consisting of a swingable flat reflector and a large-aperture telescope system. It can achieve large-scale observation of the starry sky through the swing of the reflector. The lunar-based astronomical telescope makes full use of the advantages of the moon's lack of atmospheric interference and slow rotation to conduct long-term continuous monitoring of a number of celestial bodies of important scientific value in the near-ultraviolet band, completing observation tasks that cannot be achieved on the surface of the earth.

Many countries have begun to explore

Now, scientists have a more ambitious and bold plan to build a space telescope on the moon. In 2023, NASA's Jet Propulsion Laboratory announced a plan to build a giant radio telescope on the back of the moon - the Lunar Crater Radio Telescope. It plans to select a crater with a diameter of about 3 to 5 kilometers on the back of the moon and use the DuAxel robot to build a mesh reflector with an effective aperture of more than 1 kilometer. After completion, the Lunar Crater Radio Telescope will become the largest radio telescope in the solar system. Its main purpose is to measure long-wave radio waves generated during the dark period of the universe.

Currently, radio telescopes built on Earth cannot detect this mysterious period because the long-wave radio waves from that period will be reflected by the ion and electron layers at the top of the Earth's atmosphere, making them inaccessible to the telescopes.

At the same time, artificial signals from mobile phone communications, radars, satellites, etc. are millions of times stronger than signals from the universe, which has a great impact on the observation environment of ground-based radio telescopes. However, on the moon without an atmosphere, these signals will not be reflected. At the same time, the moon itself can shield the earth's radio communications, preventing the earth's electromagnetic waves from interfering with the work of space telescopes.

In addition, NASA and the U.S. Department of Energy are working together to develop a lunar surface electromagnetic experiment similar to the Lunar Craters Radio Telescope. Through this collaboration, the U.S. Department of Energy and NASA are creating conditions for successful exploration of dark age cosmology in the coming decades. The project's radio telescope is expected to be launched to the far side of the Moon in 2025, and for the first time in the nighttime lunar environment, it will use a deployable antenna and radio receiver to measure sensitive radio wave signals.

Scientists from various countries have also proposed projects such as the Lunar Pole Life Detection Telescope and the Lunar Optical Ultraviolet Detector. The former will focus on studying the atmosphere of exoplanets during transit, and the latter will focus on studying bright ultraviolet objects.

In the giant lunar super telescope project that was made public this time, French scientists proposed a more specific idea about "how to build it", that is, to use the special terrain of the lunar craters for construction.

The power of an optical telescope depends largely on the size of the primary mirror and the telescope's focal length, which can be increased by adding more reflecting surfaces. Scientists have proposed that a supertelescope could use an array of mirrors as the primary mirror, arranged along the terrain of the crater, with the telescope's detector cluster suspended above. The mirrors don't need to be very large, so they are easier to build and require less earthwork.

There are many challenges in engineering construction

At present, all these ideas are still in the early stages, and the biggest problem is how to build a telescope on the moon. Nowadays, all countries are vigorously promoting manned lunar landing plans. While seeking to successfully send astronauts to the moon, they are also planning to build space stations and lunar scientific bases in lunar orbit and on the lunar surface, but this still takes several years.

With the gradual implementation of the manned lunar landing program and the development of lunar scientific facilities, humans will have certain production and construction capabilities on the moon. However, manpower on the moon is always limited, and if you want to build a large space telescope on the moon, you will definitely need to use robots.

Transporting all the equipment, materials, and engineering machinery needed to build a lunar space telescope to the moon is a difficult task in itself. At present, humans are unable to transport enough materials to build a space telescope to the moon in one go. If these materials are transported to the moon one by one, the cost of launching related spacecraft will be very huge.

In addition, there are some serious challenges to overcome. Due to the influence of tidal locking, the temperature on the lunar surface is very extreme, with the highest temperature reaching over 100°C and the lowest temperature even as low as -173°C, which will have a great impact on the telescopes observing signals.

In addition, there are moonquakes on the moon, which will have a great impact on the construction of space telescopes and the observation alignment after they are built. In addition, as time goes by, dust will accumulate on the telescope, and how to clean the moon dust deposited on the surface of the telescope is also a big problem.

Building a super telescope on the moon is a major leap forward for astronomical research. The unique conditions on the moon provide opportunities for humans to explore the universe in depth. Although there are many challenges, the visionary pursuit of lunar astronomy by various countries demonstrates the collective determination to solve the mysteries of the universe that have puzzled mankind for thousands of years. If humans can successfully build a large number of astronomical telescopes with different functions on the back of the moon in the future, it will be a major breakthrough in human technology. (Author: Lin Yinzhi, Image source: NASA, Gatekeeper: Jiang Fan, Deputy Director of the Science and Technology Committee of China Aerospace Science and Technology Corporation)