How will we colonize other planets?

The ISS, the closest thing we have to a home in the sky, passes over Miami. See more space exploration pictures.
Image courtesy NASA

In sci-fi books and movies, colonizing other planets seems easy. All you have to do is make a jump to "hyper-space" in your star cruiser and -- voilà -- you punch through folded space-time and arrive at your destination instantly. In reality, we won't colonize space in great big leaps, but in a series of small steps, like living successfully in low-Earth orbit.

It's hard to imagine now, but in the heady days after Sputnik, scientists didn't know if humans could survive for long periods in space. The first flights into orbit carried animals, not astronauts, and it wasn't until 1961 that Yuri Gagarin rode a flaming rocket into space. Gagarin's historic flight lasted a mere 108 minutes, but it set the stage for longer missions.

By the mid-1970s, astronauts were living successfully in orbiting space stations. First came Skylab and Salyut, then Mir. On Mir, cosmonauts continued to break space endurance records. Musa Manarov and Vladimir Titov spent a year aboard the Soviet station in the late 1980s, but their achievement was eclipsed in 1995, when Valeri Polyakov completed a 438-day tour of duty.

Today, the International Space Station (ISS) stands as clear evidence that humans can live indefinitely in low-Earth orbit. Since the first crew arrived in 2000, the ISS has been continuously manned and has, through a variety of experiments, produced a vast body of knowledge about how to achieve self-sufficiency in space. In the coming decades, NASA and other international space programs hope to use that knowledge as a springboard to a destination beyond Earth's atmosphere.

From low-Earth orbit, it's just a hop, skip and jump to the moon (relatively speaking). We'll head there next.

Lunar Living

Artist's sketch of what a manned lunar observatory might look like. Note the radio telescope built into the lunar surface.
Artist's sketch of what a manned lunar observatory might look like. Note the radio telescope built into the lunar surface.
Image courtesy NASA

Ever since the Apollo program put the moon within our reach, establishing a lunar outpost has seemed a logical next step. Earth's natural satellite offers several advantages over more exotic moons, such as Saturn's Titan. First, it's relatively close, which means crews could get back and forth in just a few days. It also means communications between colonists and mission commanders on Earth wouldn't experience any significant delays. The moon would also make an ideal spaceport because rockets could escape its low gravity without expending so much energy. Finally, a lunar-based observatory would make it easier to study the universe and learn more about where our future travels should take us.

But living on the moon won't be any picnic. With no atmosphere, it experiences huge temperature extremes, swinging from 273 degrees Fahrenheit (134 degrees Celsius) at noon to minus 274 degrees Fahrenheit (minus 170 degrees Celsius) at night. Its surface is also peppered constantly by micrometeorites and cosmic rays. To survive this blitzkrieg, colonists will likely have to place their habitats under the lunar soil or at the base of a lava tube.

Then there's the issue of food and water. Scientists believe water is buried in the soil at the moon's south pole, but facilities will need to be built to extract it. And growing plants in the long lunar nights, with no insects for pollination, could prove to be difficult.

Despite these challenges, several countries are trying to put humans back on the moon. NASA's program, known as Constellation, seeks to put humans on the moon using a new generation of spacecraft -- the Ares launch rockets, the Orion crew vehicle and the Altair lunar lander. The target launch date had been set at 2020 until President Obama halted the program in early 2010. Meanwhile, some members of U.S. Congress have argued against dismantling Constellation, which would, in theory, revitalize the dream of building a successful lunar outpost.

Keep reading to find out what the plan is for Mars.

Successful Settlement of Mars

Artist's sketch of what the first Martian outpost might look like. What if that were your daily landscape?
Artist's sketch of what the first Martian outpost might look like. What if that were your daily landscape?
Image courtesy NASA

Some scientists think we should skip the moon and head straight for Mars. One of the most outspoken supporters of this strategy is Robert Zubrin, founder and president of the Mars Society. In 1996, he laid out the details of a Mars Direct mission, which could serve as a template for manned travel to the red planet.

Here's how it would play out: The first launch would carry an unmanned Earth Return Vehicle, or ERV, to Mars. The ERV would contain a nuclear reactor, which provided the power for a chemical-processing unit capable of manufacturing propellant using compounds found in the Martian atmosphere. Two years later, another unmanned ERV would launch and head for a second landing site. At the same time, a manned spacecraft would make the journey and touch down near the first ERV. The crew would stay on Mars for 18 months, exploring the planet and conducting experiments until it was time to return to Earth using fuel manufactured on-site. As the crew members departed, another team would arrive, and the process would repeat itself so that a string of bases was established.

Long-term settlement of Mars, however, would require a transformation of the planet, a process known as terraforming. Terraforming involves warming Mars to more Earth-like conditions. The only way to do this realistically is to build soil-processing units that pump super-greenhouse gases, such as methane and ammonia, into the Martian atmosphere. These gases would absorb solar energy and warm the planet, triggering the release of carbon dioxide from the soil and polar ice caps. As carbon dioxide builds in the atmosphere, atmospheric pressure would increase, additional warming would occur and oceans would form. Eventually, colonists would be able to survive without spacesuits, although they would still need to wear oxygen tanks.

After several decades of terraforming, the red planet might look just as blue and watery as our home planet. After several more, it might be transformed completely into an Earth-like, oxygen-rich environment. If and when that occurs, it will be able to support a thriving colony of humans, some of whom will undoubtedly turn their faces toward the sky and dream of traveling to the remote corners of the solar system.

Establishing Colonies Beyond Mars

Does this rock feel like home to you?
Does this rock feel like home to you?
Riser/Getty Images

Asteroids -- those rocky objects that orbit the sun in a wide band between Mars and Jupiter -- could serve as stepping-stones to the outer planets. There are only 100 asteroids larger than 125 miles (200 kilometers) across, but a billion or more may exist, making them one of the solar system's greatest resources. Ceres reigns as the largest asteroid (or dwarf planet, depending on your point of view), and may be a promising option for colonization. For one thing, it may have a layer of water ice or even liquid water beneath its crust, a fact that may be confirmed when the Dawn mission arrives at the round protoplanet in 2015.

How would humans colonize an asteroid? One option would be to turn the asteroid itself into a city. This would require a massive mining effort to hollow out the object's interior. Another option is to build a "city in the sky" -- a space station that orbits around the asteroid. Such a concept has been around for years.

In 1975, a group of professors, technical directors and students gathered for 10 weeks at Stanford University and Ames Research Center to develop a design study of space settlements. The team's final recommendation was to place settlements in orbit around planets or moons, not on their surfaces. They proposed a wheel-like habitat 1 mile (1.6 kilometers) in diameter. Colonists would live in a tube at the perimeter of the wheel, which would be connected by six spokes to a docking bay at the hub. The whole structure would rotate to simulate Earth's gravity and would use mirrors to collect sunlight for use in power generation and agriculture.

Interestingly, President Obama's plan for NASA has the space agency using technology and components of the Constellation program to launch a mission to a nearby asteroid by 2025 and to Mars a decade later. Even if the plan fails, asteroids will likely play key roles in future exploration and colonization of outer space.

Are you ready to head beyond the solar system?

Heading for a Planet in Another Star System

The Dawn spacecraft begins its 1.7 billion-mile journey to study asteroids, thanks to a trio of solar electric ion propulsion engines.
The Dawn spacecraft begins its 1.7 billion-mile journey to study asteroids, thanks to a trio of solar electric ion propulsion engines.
Image courtesy NASA

If we're going to colonize a planet in another star system, we have to answer two questions. First, do any Earth-like planets even exist outside of our solar system? Thanks to NASA's Kepler telescope, the answer to this question is yes. Kepler has located hundreds of planets -- what astronomers call exoplanets -- orbiting stars anywhere from a few hundred to a few thousand light-years away. Most of these planets probably don't have the right conditions to support life, but a few might.

The second question is purely logistical: How do we get to a planet located trillions of miles from our own? To answer this question, scientists will have to rethink space travel. For example, the idea that a single crew will fly to a remote planet is highly unlikely. Instead, spacecraft will need to carry family groups capable of living in space for generations. Scientists will also have to come up with better propulsion systems to cut the travel time. Nuclear fission- and fusion-based engines might be feasible, but more likely candidates include light sails, ion-propulsion systems or antimatter rockets.

Light sails work by directing laser light onto a huge aluminum-foil sail. As photons strike the sail, they transfer momentum and push the sail forward. Ion-propulsion systems use solar panels to generate electric fields that accelerate charged atoms of xenon. Such an engine is currently powering the Dawn mission, which is hurling an unmanned spacecraft to two asteroids, Vesta and Ceres. Antimatter rockets are the most efficient and achieve the highest speeds, but the technology is relatively untested. Such a rocket mixes equal amounts of antihydrogen and hydrogen, which annihilate each other in a combustion chamber to release enormous amounts of energy.

In the end, a combination of technologies may be the solution, proving once again that conquering deep space will require cooperation and collaboration among scientists of different disciplines and nationalities.

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