Yes, it will happen
Catastrophic comet expected to hit Earth ... someday
(CNN) -- It happens once or twice every million years. A comet or asteroid tears through
Earth's atmosphere and smashes into the ground or ocean with enough force to destroy
civilization.
- The good news:
- Scientists will probably spot the object before it hits us.
- The bad news:
- Even if we get six months warning, there's nowhere to hide.
It sounds like the stuff of Michael Crichton (or for that matter, a rerun on
NBC), but in fact, the threat is very real, even if the odds are in our favor.
Right now, there are about 2000 large bodies -- also known as NEOs (Near-Earth
Objects) -- that cross the orbit of Earth and, in theory, could hit us.
There already have been a handful of close calls in recent years. On March
23, 1989, an asteroid about a half-mile wide crossed the Earth's
orbit about 400,000 miles from Earth. The Earth had been in that same spot
a mere six hours earlier. On January 17, 1991, an asteroid estimated
to be about 30 feet wide passed within 106,000 miles of Earth. It was the closest "near miss" ever recorded.
Smaller objects hit the Earth all the time. Most of them land in oceans
or uninhabitated areas, unnoticed. But some make headlines.
On October 9, 1992, a meteorite smashed through the rear end of a car in Peekskill, New York. No one was hurt, but the Chevy Malibu was totaled.
(6.3M/30 sec. QuickTime movie) 
The most vivid example yet of a comet's destructiveness occurred in the summer of 1994,
when Comet Shoemaker-Levy 9 collided with Jupiter, creating a spectacular plume that rose above the Jovian cloudtops. (235K/21 sec. AIFF or WAV sound) 
NASA scientists take the threat of an impact seriously, but they've had a hard time convincing Washington to fund a proposed early warning system.
In 1991, a NASA study, requested by Congress, recommended the development of an international
Spaceguard Survey -- a system of specially designed
ground-based telescopes that would
be used as surveillance cameras to detect Earth-threatening objects. The estimated cost was $10 million a year. Congress declined to fund the project.
NASA does spend about $1 million a year on smaller-scale asteroid and
comet search programs. But the programs are only able to monitor about 10 percent of the sky per month, according to one expert.
So what will we do if the unthinkable happens? Is Earth defenseless against a large
comet or asteroid?
Scientists and military planners say with enough warning we could intercept a cosmic cannonball.
Among the defensive manuvers under study ... and debate:
- Nuke It:
- A nuclear blast near the object couldblow it off course.
- Pulverize It:
- Rockets with heavy payloads smash the object into bits or divert it off course
- Melt It:
- Earth-based or Moon-based lasers would be used to boil off layers, redirecting it
-
All of these "solutions" would require months, maybe years of planning and development, not to mention the aim of a sharpshooter. In other words, the earlier the warning, the better. But given the modest surveillance of the stars now under way, some experts contend we're still more vulnerable than we need to be.
Even the ever-cautious NASA refuses to discount the danger. The following passage is included in a "Fact Sheet on Asteroid and Comet Impacts" found at NASA's Web site:
"At present no asteroid or comet is known to be on a collision course with the Earth. The chances of a collision within the next century with an object 1 km or more in diameter are very small (roughly 1 in ten thousand), but such a collision is possible and could happen at any time."
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