Somewhere in space at this moment, hurtling toward Earth at roughly 16 miles per second, is the doomsday rock.
The question of growing interest to scientists and engineers is exactly when it will approach the planet and whether anything can be done to avoid a catastrophic collision, such as nudging the rock off course with a nuclear blast or two.
The doomsday rock is an asteroid large enough to severely disrupt life on Earth upon impact, lofting pulverized rock and dust that would block most sunlight.
Agriculture would virtually end, and civilization could wither and die, just as the dinosaurs and many other forms of life are thought by some to have been wiped out by a massive object from outer space 65 million years ago.
So far, no astronomer has located such a killer asteroid, which by definition would be a mile wide or larger, would have an orbit that crossed Earth's and would do so at exactly the wrong moment.
But, given enough time, it is inevitable that one will appear. And the odds are that the moment could be relatively soon, in celestial terms.
Experts, extrapolating from craters observed on the Moon and from a partial survey of Earth-crossing asteroids, calculate that a "big one" slams into the planet once every 300,000 to 1 million years.
More graphically, that means there is between one chance in 6,000 and one chance in 20,000 of a cataclysmic impact in the next 50 years.
"Eventually it will hit and be catastrophic," said Dr. Tom Gehrels, a professor of lunar and planetary science at the University of Arizona who heads a team that searches the sky for killer asteroids.
"The largest near-Earth one we know of is 10 kilometers in diameter [about 6.2 miles]. If a thing like that hit, the explosion would be a billion times bigger than Hiroshima. That's a whopper," he said.
The field of asteroid detection and avoidance, once pooh-poohed as laughably paranoid, has grown in size and respectability in the last decade.
Last year Congress called for a series of detailed studies after a half-mile-wide asteroid crossed the planet's path at an uncomfortably close distance in 1989.
"The Earth had been at that point only six hours earlier," a House report noted. "Had it struck the Earth it would have caused a disaster unprecedented in human history. The energy released would have been equivalent to more than 1,000 one-megaton bombs."
The National Aeronautics and Space Administration is now spending somewhat less than $1 million a year to search for Earth-crossing asteroids.
To that end, Dr. Gehrels's team at the University of Arizona uses a 36-inch telescope on Kitt Peak equipped with an advanced electronic detector.
NASA is also studying the feasibility of nudging the asteroids aside and helped sponsor the first International Conference on Near-Earth Asteroids, in San Juan Capistrano, Calif.
"There's a good deal of interest," said Dr. Clark R. Chapman, chairman of the conference and an astronomer at the Planetary Science Institute in Tucson, Ariz., a private non-profit group.
"For a long time the problem was known only theoretically," he said. "Now we have hard data," referring to lengthening lists of Earth-crossing asteroids.
"The Earth is bound to be hit," he said. "Statistically, it's certain. It's unlikely that a really large asteroid will hit in our lifetime, but it's not beyond the pale."
"The risk of death," Dr. Chapman said, "is higher per person than a jet airplane crash.
"It's more likely than lots of things people worry about, like botulism or fireworks or many carcinogens," he said.
The risk is high enough, he added, to suggest the desirability of action.
The American Institute of Aeronautics and Astronautics, a society of professional engineers based in Washington, strongly agrees.
"We would be derelict if we did nothing," the group said in a position paper last year.
Asteroids are craggy remnants from the creation of the solar system that revolve around the sun, mostly in orbits between those of Mars and Jupiter. Some, however, follow a more eccentric course that takes them across the path of the Earth.
So far, 184 Earth-crossing asteroids have been observed and their orbits mapped. New ones are being added at the rate of about two a month. None found so far are expected to hit the Earth soon. On the other hand, it is estimated that only about 10 percent of the big ones have been found.
Experts say there are probably 500 Earth-crossing asteroids with diameters of roughly a mile, and perhaps a dozen that are three or more miles wide, making them the size of large mountains. The bigger ones would truly be doomsday rocks.
The father of the field is Dr. Eugene M. Shoemaker, a 63-year-old geologist-turned-astronomer with the U.S. Geological Survey in Flagstaff, Ariz. In the 1950s he closely studied a three-quarter-mile-wide crater in northern Arizona, which many geologists believed was volcanic in origin.
Instead, he proved it was created by a 150-foot-wide asteroid that slammed into the Earth 50,000 years ago.