ALBUQUERQUE, NEW MEXICO -- Earth is on a "hit list" with hazardous asteroids and comets being nature's own terrorism from the sky. More than 100,000 dangerous near-Earth objects exist - known in sky watching circles as NEOs.

Off-the-shelf technology to prevent future impacts from making a mess of things already exists, but some assembly is required. More worrisome is that little is being done to prepare for the day when a big one smacks into our planet, stirring up global consequences.

The threat from celestial wanderers is real. A homeland defense plan for planet Earth is now needed. And time is running out.

"In my view, it's the most important technical challenge in history," said Andy Smith, head of the International Planetary Protection Alliance (IPPA), based in Albuquerque.

"An object is out there and has our name on it. The name of the game is to find it before it finds us," Smith said.

The technology needed to guard Earth and we earthlings from bothersome NEOs is in hand. "It would take us probably two years to put a system together using off-the-shelf components," Smith said.

Already, such NASA projects as the Near Earth Asteroid Rendezvous (NEAR) probe of Eros in 2000-2001, and the Deep Impact strike mission to a comet in 2005, provide engineering insight on doing battle with NEO bullies.

Experts at the meeting noted the upcoming Deep Impact mission and that craft's slam-bang of an assignment to comet Tempel 1.

Deep Impact will make a spectacular football field-sized crater on the speeding comet. Images from both the flyby spacecraft and the impactor will be sent back to distant Earth as data in near-real time. While these first-ever views beneath a comet's surface and other instrument readings will provide clues to the formation of the solar system, this mission is also a forerunner to fending off dangerous objects.

Deep Impact's killer application is one of delivering a powerful punch. The kinetic energy to be delivered equals some 4.5 tons of TNT. That should do wonders in excavating a crater some seven stories deep on Tempel 1. Mass of the impactor is 770 pounds (350 kilograms).

But the real trick is to hit an area on the comet less than 3 miles (5 kilometers) in diameter from millions of miles away. To do this, the impactor uses a high-precision star tracker and special navigation algorithms developed for the Deep Space-1 mission that eyed comet Borrelly late last year. While en route, the impactor's attitude control system will keep the device on its crash course alignment.

Clearly, Deep Impact can be viewed as a trial run system for learning how to take out or divert objects intimidating to Earth. Building a ready-to-fly NEO defense system, however, is another matter.

There are several elements needed, IPPA's Smith said, to shape an Earth protection plan: The ability to conduct spaceflight activities; super-explosives and ways to transport enough energy into space to fend off a menacing object; and NEO-spotting telescopes rigged with digital gear, coupled to speedy computers and specialized software.

Statistics argue for a call-to-action.

All life forms on the planet are at risk, IPPA's Smith said. Odds are in our favor that the next hit suffered by Earth won't be of a magnitude that endangers humanity. "If we're lucky it will be one that we can deal with and, if prepared, we can deflect it," he said.

"It is not a one-in-a-million-year risk. It's not one-in-a-thousand-year risk. It is a one-in-a-hundred-year risk. An object that is Tunguska size or larger hits us about every 100 years. So we're due for another hit," Smith cautioned.

Keeping the Earth free of impactors means knowing their whereabouts in the first place. To this end, more work is urgently needed.

Smaller asteroids that measure 165 feet (50 meters) to 655 feet (200 meters) across are also troublemakers, Smith said. They are plentiful in number, but aren't detectable at present.

"We have to have larger telescopes," Smith said, to complete the critical inventory of NEOs. At the present discovery rate, well over a hundred years is required to get the data. Bigger scopes should reduce that time to one decade, he said.

Even with better sky-spying tools, just giving a NEO the once-over isn't enough.

Alan Hale, director of the Southwest Institute for Space Research in Alamogordo, New Mexico, said that even well tracked, gas-spitting comets can become heart-stopping objects as well. He was co-discoverer of the Hale-Bopp comet that paraded by Earth in early 1997.

A comet that's on a non-threatening orbit one day, Hale said, can huff and puff its way onto a route that makes Earth end-of-the trail stomping grounds.

"We understand the effects of planetary perturbations. We've got a good handle on that," Hale said. But the non-gravitational forces -- jets of gas belching from a comet that act like rocket engines -- is "a little tougher nut to crack," he added.