The topic of moon colonization has a decidedly retrofuturist flavor. Not so with Mars. Ridley Scott's 2015 film adaptation of Andy Weir's novel "The Martian" has helped stoke the general public's yearning for the Great Rusty Sphere. You can read frequent press releases about how space agencies like NASA and some private ventures are making preparations for the colonization of Mars even today. But articles about moon colonization are more likely to appear alongside illustrations that look like characters from "Mad Men" wearing glass helmets while they sip cocktails on mod furniture. Moon colonization feels like a dream of the 1960s rather than a dream of today. So why has the moon gone out of favor as a destination?
One answer would be that we've simply set our sights higher. Colonization of Mars is a more difficult project, but also a more rewarding one in terms of science outcomes. Not many scientists expect to find signs of past or present lunar life, but there's still plenty of hope for discovering what strange creatures may have once slimed across the lost oceans of Mars. Also, it's likely that Mars is ultimately a better candidate for long-term, self-sustaining human habitation and terraforming, due to features like somewhat stronger gravity and the presence of a (very thin) atmosphere.
Nevertheless, some scientists and pioneers still see lunar colonies as an important part of humanity's future space exploration efforts. For example, European Space Agency Director General Johann-Dietrich Wörner expressed support for the idea of “a permanent moon station as the successor of ISS” at a space symposium in April 2015, and the Russian Federal Space Agency has announced its intention of establishing and maintaining a lunar base, starting with a crewed landing in 2029.
In contrast to Mars, the moon actually does offer some unique opportunities. Here are several:
1. The moon as a source for fusion fuel
The soil covering the moon's surface is believed to contain helium-3, deposited by centuries of exposure to solar wind. Helium-3 is a potential fuel for safe, clean energy at scale through nuclear fusion. Now, the importance of helium-3 depends on developments in fusion research that may or may not be attained in the near future. If we can't get fusion to work, the question of fuel is irrelevant.
2. The moon as an off-world filling station
In science fiction, near-Earth space is often depicted as a teeming traffic jam of spacecraft taking passengers to and from space stations and other planets, returning raw materials mined from moons and asteroids, and conducting cosmic and planetary research. If real space ever becomes a high-traffic thoroughfare, we are going to need some filling stations to service outgoing traffic beyond the boundaries of Earth. The moon is an ideal location for such a station. One NASA project proposes to study the possibility of using machines called TransFormers to turn the Shackleton Crater at the southern pole of the moon into such a location. This would work by redirecting sunlight into the crater, which was previously frozen by eternal darkness, and enabling the creation of liquid hydrogen and liquid oxygen for rocket propellant on site.
3. The moon as a cosmic watchtower
Astronomers use radio telescopes for everything from the search for alien civilizations to the study of black holes. We have plenty of radio telescopes already on Earth, but when it comes to radio signals, Earth is a noisy place. We generate an astonishing amount of ambient radio chatter that clutters the frequencies we'd like to use to listen to the cosmos. For this reason, it would be wonderful if we could put a giant slab of rock between our radio telescopes and planet Earth. Well, several astronomers have already suggested this idea, drafting, in one example, a proposal for a radio telescope array built by astronaut-controlled robots on the lunar far side.
4. The moon as a safer proving ground
If we're going to test out methods and technologies for colonizing the solar system with human lives at stake, the moon is the safest place beyond Earth to run those tests.
The moon is only few hundred thousand miles away. A rocket fired from Earth's surface can enter lunar orbit within three or four days. Even in the best-case scenario, when the orbits of Earth and Mars are perfectly aligned to shorten the trip, a spacecraft launched from Earth will take more than six months to reach Mars.
This long trip is a problem for at least two reasons. One is that it makes the concept of a “rescue mission” or anything along those lines pretty much out of the question. Humans on the surface of Mars are essentially on their own. The other reason is that space travel takes a drastic toll on the human body. Astronauts in deep space are exposed to intense radiation that they're naturally protected from back home by Earth's atmosphere and magnetosphere. Researchers are working on developing special, shielded spacecraft to protect potential Mars colonists against that radiation, but this adds to the cost and difficulty of the mission.
On top of that, there are other risks associated with extended space travel, such as bone density and muscle loss due to microgravity. The longer you spend in space, the more the health risks pile up. After six or seven months in a capsule headed for Mars, even the most determined astronauts may wish they had opted for a three-day flight to the moon instead.
With the first three examples, one important question is whether such projects could be pursued by robots without the need for human crews and operators on site. But the same could be said about Mars — uncrewed missions like those carried out by the Curiosity rover and the Mars Reconnaissance Orbiter have been able to teach us a lot without putting human boots in the red soil. So in some sense, the motivation for human colonization is not just the furtherance of planetary science goals; human colonization is an end itself. If what we really want to learn is how humans can live, work and learn beyond Earth, why not start with the moon?