It sounds like a science-fiction movie, but this surreal scene could, if President Obama has his way, become a reality. However, unlike Hollywood depictions in such movies as Armageddon, it's going to be a lot harder to pull off.

Almost 50 years after President Kennedy proposed sending a man to the moon "before this decade is out," Obama has set an equally improbable goal. He has proposed a 2025 date for NASA to land humans on an asteroid, a ball of rock hurtling around the sun.

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The moon is 240,000 miles away. A trip to an asteroid would be 5 million miles — at a minimum.

Why go?

If the mission ever gets launched, it would mark a milestone just as significant as Neil Armstrong's "small step" on the moon, experts say. To go to an asteroid, humans would have to venture for the first time into "deep space," where the sun, not the Earth, is the main player.

An asteroid trip "would really be our first step as a species outside the Earth-moon system," says planetary scientist Andy Rivkin of the Applied Physics Laboratory. "This would be taking off the training wheels."

Asteroids have always been passed over as a destination for human explorers. Then-president George H.W. Bush wanted NASA to go to Mars, while his son, George W. Bush, chose the moon. During the past six years, NASA spent $9 billion building a spaceship, rocket and other gear to help reach the second Bush's goal of returning humans to the lunar surface by 2020.

In February, Obama took steps toward killing Bush's moon program, which was beset by technical troubles and money woes. Two months later, in a speech at Cape Canaveral, Obama announced that the astronauts' next stop is an asteroid.

So far, the Obama administration has been quiet on the need for a major sum of money to accomplish his goal. And unlike Kennedy, who used Sputnik to promote the moon mission, Obama doesn't have a geopolitical imperative to justify the goal. Congress is resisting Obama's change of direction, which could delay investment in the program.

If Obama wants to bolster his cause, there's a rationale he could cite: An asteroid could wipe out as many human lives as a nuclear bomb. The dominant scientific theory posits that dinosaurs went extinct because of a direct hit from an asteroid as wide as San Francisco. A space rock big enough to kill thousands slams into Earth every 30,000 years, according to a January report from the National Research Council.

That scenario provided the rationale for asteroid missions in various Hollywood movies, including Armageddon. The 1998 film, which starred Bruce Willis, grossed more than $200 million at box office in the U.S. and more than $500 million worldwide. It went on to be a staple on cable television.

But if Americans think they have an understanding of the challenge of going to an asteroid, they're wrong. "I loved the movie," says Laurie Leshin, a top NASA official who is involved in the early planning stages of an asteroid mission, although "it was completely inaccurate."

Obama's plans for NASA have drawn many opponents, including Armstrong, but their criticism centers on the administration's reliance on private space companies to ferry astronauts to orbit. The goal of an asteroid hasn't been questioned as much.

That doesn't mean it would be easy. Although experts agree it could be done, here are four asteroid-size reasons why life won't imitate art.

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