What are the chances of Earth being hit by a Comet?

Not much in our lifetimes, but evidence of ancient craters found all over Earth's surface prove that large objects have hit Earth in the past.

The probability depends on size: The bigger the comet or asteroid, the smaller the chance. Tons of meteoric debris — much of it smaller than grains of sand — strike Earth's atmosphere and burn up every day. Some larger rocks survive their fiery descent; the ones people find are displayed in museums. The truly dangerous objects may appear once every few hundred thousand years.
Many scientists believe an extremely large asteroid struck Earth 65 million years ago near the present-day Yucatan peninsula of Mexico. The impact caused catastrophic conditions across the entire planet, perhaps hastening the extinction of the dinosaurs and much of the rest of life on the planet.

The path Earth follows in its orbit around the Sun is littered with untold pieces of debris ranging in size from dust grains to objects astronomers call asteroids, or minor planets. In an effort to track the largest of these objects, whose impacts on Earth would severely alter life on our planet, NASA astronomers have developed a method of scanning the sky. A computer-controlled camera and a one-meter (39-inch) telescope operated by the U.S. Air Force atop Maui's Mount Haleakala automatically scan the sky.

As of March 1999, almost 14,000 previously undetected near-Earth asteroids and comets had been recorded in about three years of scanning. Knowing the orbits of these asteroids allows the astronomers to predict possible future collisions with Earth. They haven't found one on a collision course yet.

In March 1998, astronomers made headlines with news of an asteroid that would pass within 30,000 miles of Earth in 2028. The margin of error in their calculations left open the possibility of a cataclysmic direct hit by the huge space rock, dubbed 1997 XF11. In publishing their findings, the astronomers sought observations from their colleagues to refine their estimate of the asteroid’s orbit. A day later, Jet Propulsion Laboratory astronomers offered a revision. 1997 XF11 would pass no closer than 600,000 miles from Earth — and the world breathed a sigh of relief.

(end)