Keira Knightley: Is the science in 'Seeking a Friend For the End of the World' correct?

If the threat is more immediate, we could try to blow the asteroid up with a nuclear weapon. Many researchers are leery of this technique, however, saying that it would be tough to pull off politically. And the nuclear option may actually do more harm than good, sending a spray of asteroid chunks toward Earth instead of one intact rock.

So the key, scientists say, is to discover any potentially dangerous asteroid as early as possible so that all of our mitigation options remain on the table.

"If we do our jobs right, we'll know years in advance about a threat," Johnson said.

 The regional disaster movie?

The progress being made on the really big space rocks is not grounds for complacency. There are many thousands of smaller near-Earth asteroids out there that could inflict devastating damage if they hit us, albeit on a local scale.

"A 150-meter object could be a pretty bad day for a state-size area," Johnson said.

And many of these mid-size space rocks have eluded detection thus far. Scientists have found several thousand near-Earth asteroids at least 0.3 miles (0.5 km) wide, for example, but a roughly equal number probably still await discovery, Johnson said.

The percentages get worse as asteroid diameters go down, since small space rocks are both more numerous and harder to detect than their larger kin. Still, Johnson voiced optimism that we could get a complete catalogue of the truly dangerous near-Earth asteroids within the next few decades — especially if we launch a spacecraft that could search the region of space interior to Earth's orbit, an area that's tough for instruments on or near the planet to scan.

So in 20 or 30 years, movies that have asteroids wreaking havoc on even a regional scale may be unrealistic. And maybe that's just as well; a film about the destruction of Kansas or Illinois probably wouldn't fill the theaters.

But this is probably selling Hollywood short. If moviemakers want to rain death down on humanity, they don't necessarily need near-Earth asteroids. They could sling a space rock from the main asteroid belt toward us, for example, or use a comet. (Many comets orbit the sun at vast distances, and scientists think there may be billions of the icy bodies out there that we've never seen.)

And, of course, there's always planet Nibiru, the supposed stealth world trumped up by doomsaying conspiracy theorists. NASA has repeatedly assured us, however, that Nibiru doesn't exist.

Follow SPACE.com senior writer Mike Wall on Twitter @michaeldwall or SPACE.com @Spacedotcom. We're also onFacebook and Google+.

• Asteroid Basics: A Space Rock Quiz
• The 7 Strangest Asteroids in the Solar System
• When Space Attacks: The 6 Craziest Meteor Impacts

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