Death from space: what an asteroid could do

An artist's conception shows an asteroid crashing into Earth in an event that scientists believe occurred in the Caribbean region at the boundary of the Cretaceous and Tertiary periods in Earth's geologic history about 65 million years ago causing the extinction of dinosaurs.

(Reuters) - A report released on Friday finds the United States is doing little to defend the planet against potentially devastating asteroids.

Up to 90 percent of the really big and obvious threats are being found, but smaller objects can also wreak vast destruction. Here are some facts about things that can hit the planet:

* Near-Earth Objects are asteroids, comets or really big pieces of them that may wander close to Earth.

* Like the moon and Mars, which are covered in craters, the Earth is also frequently bombarded. But erosion, the movement of continents, the oceans and even forests obscure them over time.

* The National Academies report says it is highly probable that the next destructive impact will be something less than 50 meters (164 feet) across. This happens about once every 1,000 years.

* The last one hit over Tunguska, in Siberia, in 1908, flattening forests. If one hit a populated area today, it would wreak destruction akin to a severe hurricane or tornado.

* Objects one kilometer (just over half a mile) across hit the Earth about once in a million years. Something this big could damage a large region or cause enormous tsunamis if it hit an ocean.

* Objects 3 miles or larger could kick up enough dust to cause global damage for years, perhaps decades of cold and dark conditions, and could cause mass extinctions. "Luckily such events are exceedingly rare, the last known being about 65 million years ago," the report reads.

* About 6,200 near-Earth asteroids are known, the largest being 1036 Ganymed, which is 20 miles across.

* About 500 to 1,000 asteroids are 1 km or wider.

* Smaller objects, five to 10 meters (16 to 32 feet) across hit the atmosphere about once a year, usually exploding with the force of a nuclear bomb in the upper atmosphere and causing no damage on Earth.

* Your own chance of dying in a large cosmic impact is 1 in 40,000, not because it is likely but because such an impact would kill so many people that it raises the odds. In comparison, your own risk of dying in a flood is 1 in 30,000, while the risk of death from a motor vehicle accident is 1 in 100.

(Editing by Paul Simao)