Unlike a mission to the moon, which can be done in three or four days, a journey to Mars -- some 140 million miles away -- could take more than six months each way. For a trip that long, astronauts would need to set up a network of supply depots in deep space to refuel their spacecraft.
But how? As crazy as it sounds, one answer is -- asteroid mining.
Asteroids, those rocky fragments circling the sun, can contain water, oxygen, precious metals and other elements that could be used to produce fuel and life-support systems in space -- all at a much lower cost than ferrying them from Earth. There are hundreds of thousands of asteroids, ranging in size from large boulders to miniature planets hundreds of miles across.
Extracting these resources will be expensive and hugely difficult -- imagine trying to drill into a large rock hurtling through space at high speed -- but several private companies believe they're up to the task.
In fact, former NASA Mars mission manager Chris Lewicki thinks his company can do it in five to 10 years.
"We will go from finding the first asteroid mine, setting up the first asteroid mine, proving the technology ... and then delivering that first liter, then that first barrel, then that first ton of water, which is going to power the future economy in space," Lewicki told CNN.
Lewicki is chief engineer at Planetary Resources
, a Seattle-area company that wants to extract water, platinum and other resources from near-Earth asteroids. It's backed by such bold-name investors as tycoon Richard Branson, filmmaker James Cameron and Google execs Larry Page and Eric Schmidt.
A newer company, California-based Deep Space Industries
, also seeks to harvest natural resources from asteroids. Both companies would do this with robotic spacecraft.
Some questions remain over whether space mining is legal. The U.N.'s 1966 treaty on outer space
, signed by the United States and more than 100 other countries, states that nations can't own territory in space, although it's less clear about the operations of private space companies.
But mining of asteroids could prove to be hugely lucrative. Planetary Resources co-founder Eric Anderson estimates a resource-rich, 80-meter asteroid could contain more than $100 billion worth of materials
for use in space and on Earth.
Not surprisingly, NASA wants in as well. The space agency, which hopes to land a human on Mars by the mid-2030s, is also looking to asteroids as a way to help sustain life millions of miles beyond Earth.
Two years ago, NASA announced ambitious plans to "lasso" an asteroid
and tow it to a stable orbit around the moon, where astronauts could safely study it. The agency has since scaled back and now seeks instead to just capture a boulder from the surface of an asteroid
, using a robotic spacecraft, in 2020.
Once the asteroid boulder has been towed into lunar orbit, NASA hopes by the mid-2020s to launch its Orion spacecraft -- the same one built to go to Mars -- with two astronauts who will explore the rock and collect samples.
"Asteroids are a hot topic," said Jim Green, director of NASA planetary science, who called NASA's planned asteroid mission "a stepping stone to Mars
NASA has identified more than 12,000 near-Earth asteroids and is still searching for the one it wants to target.
In the meantime, Deep Space Industries and Planetary Resources are launching or preparing exploratory space probes.
Any actual space mining is still years away. But it's now possible that asteroids, long feared as potential dangers to Earth, may one day help us reach the Red Planet and even colonize space.