Doomsday from above is unlikely. But just in case, the U.N. should develop a warning network to detect, track and deter potentially planet-destroying asteroids and comets, a group of astronauts said today.

The Association of Space Explorers (ASE) says in a proposal that it's set to deliver to the U.N. early next year that the agency should develop a network of ground- or space-based telescopes to detect and keep tabs on near-Earth objects (NEOs). But the organization doesn't specify how approaching asteroids should be deflected or destroyed, nor how Earthlings should be alerted to the threat.

ASE is a member of Action Team 14, part of the U.N.'s Committee on the Peaceful Uses of Outer Space, which was created to deal with the threat of near-Earth objects. It is set to deliver the recommendations during the committee's 2009 sessions in Vienna, Austria.

"We're identifying what needs to be done, not how to do it. When you have an asteroid threatening Earth, it's uncertain where it's going to hit until the last minute; the decision to take action has to be coordinated by the international community," Rusty Schweickart, the group's NEO committee chair, tells us. "We've developed a program to get the international community prepared to make timely decisions so the technology we know we have can be used to protect life."

In addition to telescopes to detect an incoming rock, that technology could include flying a spacecraft alongside an asteroid that is on course to impact our world. Schweickart says the gravitational attraction between the vessel and the space rock would tug on the latter just enough to alter its course and miss Earth. Another, less appealing option would be to shatter or blow up an approaching asteroid.

The U.N.'s Office for Outer Space Affairs did not immediately respond to phone or e-mail requests for comment on the proposal. The ASE presented its plan at the Google Foundation in San Francisco today after wrapping up a four-day meeting to develop the recommendations.

Improved telescopes would identify an estimated one million near-Earth objects over the next decade to 15 years, and 8,000 to 10,000 of them will have some probability of hitting the planet, Schweickart says. A hit by even one of the smaller rocks, say the size of a convenience store, would have the impact of 400,000 Hiroshima nuclear bombs exploding at once, he says. The larger varieties (a mile or more in diameter) could hit with as much force as millions of Hiroshima bombs, with devastating planet-wide effects, such as tsunamis, damage to the atmosphere, and radical climate change, with the magnitude of the damage depending on how big it the object is, its composition and if it hits land or water.

The current tracking, detection and warning system is "not very well organized" between countries, says Crispin Tickell, a former British ambassador to the U.N. who is a member of the ASE's Panel on Asteroid Threat Mitigation. The U.S., Britain and Italy all have tracking programs of various degrees, but there's little coordination, he says.

"If something, by God, did come our way, it would be a major global emergency," Tickell tells us.

But there's reason to believe the U.N. will develop a plan, he adds.

"If you feel it's a lost cause, it's important to remember the problem of the ozone layer and climate change," Tickell says. "In each case, you needed an impulse from the science to the political world to take it seriously."

(Image mosaic of 59 x 47 kilometers (36 x 29 miles) section of asteroid 253 Mathilde/NASA.)