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.
Dr. Shoemaker now heads one of three teams in the United States that hunt for Earth-crossing asteroids.
"They're little things" and difficult to spot compared with the stars and planets usually studied by astronomers, he said.
The field has been buttressed by the discovery of numerous asteroid craters around the world that have not yet been totally eroded by the Earth's atmosphere and oceans. If it were not for these forces, the craters accumulated over the ages would be as prominent as those on the Moon.
Dr. Richard Grieve of the Department of Energy, Mines and Resources in Canada has compiled a list of confirmed asteroid craters, the total now standing at 131.
The list grows by five or six a year. A crater in south Australia is 100 miles across. The largest found so far, measuring 124 miles from rim to rim, is in Ontario.
Collisions with Earth have occurred as recently as 1908, when a celestial object exploded near the Stony Tunguska River in Siberia with the force of 12 megatons of TNT.
Instruments around the world recorded atmospheric shock waves from the blast. In Western Europe, the nights that followed were strangely bright, apparently because debris from the explosion had spread through the high atmosphere.
Then too, there are near-misses. In 1972, a large asteroid, estimated at up to 260 feet in diameter, or nearly the length of a football field, zipped through the upper atmosphere over the northern United States and Canada, blazing across the sky in a daylight fireball witnessed by thousands of people before it re-entered space.
And last January, a small asteroid, perhaps no larger than 30 feet wide, streaked by Earth, within less than half the distance to the Moon.
To better address the threat, scientists say they need more telescopes. These could be located on the Earth, in space, or eventually on the Moon.
"By launching a search program of relatively modest cost, millions of dollars, not billions, we could discover 95 percent of the large Earth-crossing asteroids in the next decade," said Dr. Chapman of the Planetary Science Institute. "Then we could chart the orbits and say, 'They're not going to hit,' or, 'Oh, my heavens.' Then at least we'd know."
DTC A number of ways have been proposed over the years to dodge the asteroid threat, with many of the suggestions coming from makers of nuclear arms. Their plan would be to fire nuclear-tipped interceptors.
"The warhead can be used either to fragment and disperse the object, or to deflect it en masse from its collision course with the Earth," Dr. Roderick A. Hyde wrote in a 1984 paper from the Lawrence Livermore National Laboratory in California, a federal center for warhead design.
"The human race should take out a cosmic collision insurance policy," Dr. Hyde concluded. "The premiums are small, and the benefits enormous."
So too, Dr. Edward Teller, the principal developer of the hydrogen bomb and Livermore's patriarch, gave a talk in Washington in 1989 on the occasion of his 81st birthday in which he echoed the nuclear-interceptor idea, saying that an asteroid shield would cost less than $100 million to build.
With congressional encouragement, NASA is now taking the lead in both detection and avoidance. A workshop on avoidance is planned for this fall. It will be headed by Dr. John D. G. Rather, NASA's associate director for space technology.
"It's a fascinating challenge," Dr. Rather said in an interview, noting that a host of non-nuclear solutions have been considered, including landing on a menacing asteroid and setting up an engine on its surface to gently nudge it off course.
"There have even been proposals to trap them in Earth orbit" and mine them for precious metals, he said. Not everyone is convinced that knocking asteroids off course with nuclear explosions is the best idea. "People are afraid of nuking them because they think the fragments would do as much damage," Dr. Rather said.