Story highlights Several private companies want to mine asteroids for water, precious metals

Asteroids could help provide fuel for a NASA human mission to Mars

(CNN) If you've read or seen "The Martian," you know that getting humans safely to Mars and back will be a gargantuan task.

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.

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