To save the world from the real threat of a major asteroid impact, one engineer has imagined a scheme similar to George Bailey's wish to lasso the moon for his sweetheart in "It's a Wonderful Life."

The plan is to attach a gigantic weight to an Earth-bound asteroid using an enormous cord. This crazy-sounding contraption would change the asteroid’s center of mass and subsequently its trajectory, averting a potentially catastrophic scenario.

Aerospace engineer Major David French of the Air Force Research Labs mathematically modeled how different weights and lengths of tether would affect a killer asteroid's orbit over time. The results are in the December issue of Acta Astronautica.

He found that, in general, longer tethers and larger masses would more significantly change the asteroid's orbit. The alteration would occur slowly, taking anywhere from 10 to 50 years.

The technique would require no simple mission. The cosmic counterweight would weigh a few million pounds, about the mass of a Saturn V rocket. And even more impressively, the rope would range anywhere from six miles (about the height of Mount Everest), to 60,000 miles (long enough to wrap around Earth two and a half times).

This solution may sound unrealistic, but the threat is real. To date, NASA's Near Earth Object Program, which tracks asteroids and comets that could approach the planet, has cataloged more than 5,500 objects. About 1,000 of these are classified as "potentially hazardous," meaning they could wipe out a city, spawn giant tsunamis or, in the worst case, eradicate life with a planet-shrouding cloud of debris.

To guard against this, scientists have produced many dramatic proposals, each with its own merits. French thinks his technique stands out for its relative ease.

"What interested me was that there is no active control system needed," he said. Once the rope and weight were installed, the asteroid would get nudged through nothing but the laws of gravity.

However, the method is not lacking critics.

"This tether-deflection idea is an interesting intellectual exercise," said astronomer David Morrison of the Asteroid and Comet Impact Hazards Group at NASA's Ames Research Center. "But it is of no practical value."

Morrison points out that putting enormous objects, such as a heavy tether and ballast, in space is far beyond the entire human race's launch capability. Furthermore, the cost of designing and building a strong enough rope makes the solution intractable.

"From a practical point of view, the technique is a mess," agreed Russell Schweickart, former Apollo astronaut and co-founder of the B612 Foundation, a group dedicated to protecting the Earth from asteroid strikes. He is concerned that no one knows how to hook a tether to a spinning asteroid and, once attached, there is no guarantee the line won't get tangled up.

Schweickart and Morrison offer a much simpler idea that uses current technology: Change the asteroid's orbit by crashing something into it. Even a relatively small satellite would alter the orbit enough to stave off certain doom, if we did it far enough in advance.

French understands these criticisms and thinks they are well-founded. But, he said Earth will still need protection from asteroids in the next century, and the next millennium. If our technology and expertise isn't enough to lasso an asteroid right now, we have time.

"The last extinction-level asteroid strike was 65 million years ago," he said, "I think it's important to take the long view and maybe dig into technology that is not quite ready."

Editor's note: This story was updated Dec. 7 to say that a weight of a few million pounds would be enough for the counterbalance.

Image: Painting of the Chicxulub impact, 65 million years ago. Donald E. Davis/NASA/JPL.

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