Comets and asteroids according to scientists are small planets, ranging in size from baseball-sized meteors to 1/3 the size of the moon.

They are the rocky and icy bodies believed to have been left over from the formation of the solar system. Originally, their population was much larger.

With time, however, most of these bodies have either collided together to form the major planets or were ejected out of the solar system altogether. The comets and asteroids we see today are the residual population.

What is the known history of comets and asteroids impacting on Earth in the past?

The Earth, and all the other planets in the solar system, has continuously been hit by asteroids and comets ever since their formation. When scientists look at the moon through a small telescopes or a good pair of binoculars, the surface is covered by craters that were created by impacting asteroids and comets.

Similar craters have been observed on the surface of the Earth. One of the most spectacular is a 1/2-mile wide crater near Flagstaff, Arizona USA. Its speed by the time of hitting the Earth was estimated to be 26,000 miles per hour and it is estimated to have happened 50,000 years ago. The area is now a tourist attraction.

If it weren’t for the effects of erosion, and the cover of the oceans, the surface of the Earth would look much like the Moon or Mars. It is now known that an asteroid impact caused at least one mass extinction of life on Earth (the one that wiped out the dinosaurs).

The remnant of the resulting crater lies off the coast of the Yucatan Peninsula in Mexico, mostly buried by ocean sediments.

Any object striking the Earth can be dangerous if it makes its way through the atmosphere and you happen to be standing in the way. Most of the time, of course, this doesn’t happen. This is because most of the bodies striking the Earth are too small to make it through the atmosphere.

They usually burn up, leaving a beautiful glowing trail, popularly referred to by scientists as a shooting star. Some slow down in the atmosphere and land without making much of an impact. These are called meteorites.

Fortunately for us, bodies that are large enough to make it through without slowing down or burning up are very rare. The threshold size is roughly 100 meters. A body of about this size exploded over the Tunguska forest in Siberia in 1908 and exploded in about 6 miles above the ground.

It flattened about 700 square miles of trees. Scientists believe that such events occur once a century on average, but this estimate is uncertain.

Military sensors of the developed nations in orbit above the Earth today watch for explosions that might indicate a violation of nuclear proliferation treaties, have detected dozens of high- altitude explosions each year.

These are caused by meter-sized asteroids that have impact energies equivalent to tens of kilotons of TNT, comparable to the bomb dropped on Hiroshima.

The most dangerous asteroids, capable of a global disaster, are extremely rare. The threshold size is believed to be 1/2 to 1 km. These bodies impact the Earth only once every 1,000 centuries on average. Comets in this size range are thought to impact even less frequently, perhaps once every 5,000 centuries or so.

Near-Earth Asteroid Tracking Program (NEAT) is an effort to develop a system capable of detecting, in 10 years, most the Earth-approaching asteroids larger than 1 km. There are believed to be 1,000 to 2,000 such bodies, but only 100 or so have been discovered so far.

What the effects would be from a large-scale impact are very uncertain. Most scientists believe that an object in the 100 metres to 500 metres diameter range could penetrate the atmosphere and cause local damage if it either exploded in the atmosphere or impacted the land or ocean.

If the impact took place near a major city, or if it occurred in the ocean near a major city and caused a tidal wave, it could be very deadly. Fortunately, however, most of the Earth is still unpopulated.

The chance of this type of impact is probably lower than once every 1,000 to 10,000 years. The bodies larger than 500 m are of much more concern.

A single impact could cause global devastation. Calculations that were used to predict the effect of a global nuclear war have been applied to this problem.

It is likely that such a large impact would raise enough dust into the atmosphere to change Earth’s climate in effect, a "nuclear winter”. Agricultural production around the world would be severely disrupted.

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