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?