This artist’s impression shows what Mars looked like about four billion years ago, when more life was possible.
ESO
In the year 2016, one might think that the world is going to hell. Across much of Europe and the United States, a troubled and angry working class has begun to wield nationalism as a blunt weapon against the disconnected ruling class. Islamic radicals have stepped up attacks against both the West and moderate practitioners of their own faith. Then there’s humanity’s rampant use of fossil fuels, increasing water shortages, and other environmental degradation of the planet — not to mention the ever-growing threat of nuclear, biological, and chemical weapons. Finally, we have also not yet got around to tracking every asteroid that could destroy humanity.
In short, Earth may need a backup plan.
Elon Musk thinks so, as he’s poured much of his fortune into SpaceX and its relentless pursuit of colonizing Mars. But Mars isn’t the only place humans can go. There are other worlds in the solar system where people can walk without spacesuits, find enough energy, or even swim in subsurface oceans. None of these places is remotely as habitable as Earth, not even at our planet’s cold poles. But they also don’t have the political baggage of Earth. So here’s our guide to the top five options for DIY settlers:
Mars
The good: Mars is the obvious choice. It’s close enough for people to travel to in about six to nine months with existing propulsion technologies. The planet also has soil, surface ice at the poles, and liquid water underground at lower latitudes. There is enough sunshine to provide solar energy. Mars’ thin atmosphere provides some degree of protection from cosmic and solar radiation. All of the above factors suggest that humans could build a somewhat sustainable colony on Mars over time, though it would largely be an indoor existence.
The bad: There is so little atmosphere that it might as well not exist. About 12 miles above Earth, pilots reach the Armstrong limit, where water boils at the temperature of the human body, and the atmosphere is about 6 percent that of the Earth’s surface. The atmosphere of Mars is about one-tenth the value of the Armstrong limit. An exposed human on the surface would die in a matter of tens of seconds. The recent discovery that the solar wind has wiped out the Martian atmosphere over billions of years has also dashed hopes that the world could be terraformed by releasing carbon dioxide trapped in Martian rocks.
The moon
NASA
The good: Close proximity. Humans can get to the moon in a matter of days, making travel and resupply by far the easiest of all off-world locations. Scientists also believe there is plenty of water ice at the lunar poles, providing a significant source of drinking water, radiation shielding, and rocket propellant. The surface of the moon contains valuable minerals, such as silicon, which can be used to make solar cells, and helium-3, which could be a good source of energy. It is also likely that large lava tubes exist just below the lunar surface, which could be large enough for human cities.
The bad: Of course, the moon has no atmosphere, which makes surface activities dangerous and makes astronauts vulnerable to radiation. The Apollo astronauts, too, found that the moon’s dusty surface was difficult to combat when they returned the dust to their spacecraft. Finally, if the earth really starts to rot, maybe the moon would be just a little bit at close to?
Venus
NASA
The good: An atmosphere! We are not talking about the surface of the planet, where the crushing atmospheric pressure is equivalent to about 900 meters below the surface of the Earth’s oceans, and the temperature averages 462 degrees Celsius. We are talking about 50 km above the surface, where the atmospheric pressure is equal to that on Earth and temperatures range from 0 degrees to 50 degrees Celsius. In addition, Venus’ remaining atmosphere provides radiation shielding. A human could sun himself on an open-air deck wearing only a breathing mask. NASA has already done some preliminary work to study a floating airship concept with its Project HAVOC.
The bad: When it rains, it rains sulfuric acid. This would damage the solar panels and the airships themselves, not to mention the people who would accidentally end up in them. The other problem is the supply of water or metals. Reaching Venus’s atmosphere would be more difficult than reaching the Moon or even Mars, and launching from the airships would require rockets nearly as powerful as those needed to lift off from Earth.
Titan
NASA
The good: Energy, and lots of it. Saturn’s moon Titan’s great lakes are largely filled with pure methane, providing an almost limitless supply of energy for human activities. Its surface pressure is also about 1.4 times that of Earth, meaning it’s one of the few places in the solar system where people don’t need a pressure suit (other than an air mask) to walk out. The nitrogen, methane and ammonia in Titan’s atmosphere can be used as fertilizer to grow crops in greenhouses, and there may also be underground water. The thick atmosphere would provide good radiation shielding.
The bad: Temperature and distance. Titan’s surface is about -180 degrees Celsius (so no pressure suit, but MANY layers of warm clothing). And since this moon orbits Saturn, it’s the farthest of all of these options, taking humans the better part of a decade to get to with current propulsion systems.
Callisto
NASA
The good: Callisto isn’t as famous as some of the Jovian moons, like Europa, but it would probably be the best to live on. Similar in size to Mercury, it is the farthest from Jupiter of the planet’s four major moons, meaning it receives less radiation from the gas giant. Spacecraft have observed bright patches of ice on the rocky surface of the moon, and deeper into the planet there is likely a salty ocean that is 50 to 200 km deep. With its Human Outer Planets Exploration study, NASA has looked at eventually sending humans to Callisto, because of its geological stability and potential to convert the surface ice into rocket propellants.
The bad: It’s not as far as Titan, but Callisto is still several years from Earth. The moon has only a tenuous atmosphere, so it wouldn’t be any different than life on the surface of Earth’s own moon. And while the radiation levels are lower than those of Jupiter’s other moons, they are still higher than those of Titan and some of the other alternatives. Solar panels would be of limited use to colonists, as Callisto receives only about 1/25th of the solar energy that the Moon receives from Earth.