Most asteroids are located in orbit between Mars and Jupiter, but more than 200 intersect Earth’s orbit and there may be as many as 1,500 that could cause a global catastrophe if they collided with Earth. However the chances of such a collision average out to only one every 300,000 years. Most asteroids, regardless of size, rotate on their axes every 5 to 20 hours. Some asteroids are binary ( possessing smaller satellites of their own). A peculiar asteroid about 60 meters in diameter and dubbed 2002 AA29 was discovered during 2002 in an orbit roughly the same as that of the Earth. Over a period of centuries it overtakes Earth, but then slows down causing it to drop behind again.
On October 6 2008, an asteroid about four meters in diameter named 2008 TC3 was sighted by NASA's automated Catalina Sky Survey telescope at Mount Lemmon, Arizona. Computations predicted it would enter Earth's atmosphere at 2:45:28 UTC on October 7, 2008 over the Nubian Desert of northern Sudan, Africa. At 02:45:47 UTC it entered the Earth's atmosphere at a velocity of 12.8 kilometers per second and exploded about 37 kilometers above the surface releasing an energy between 1 and 2 kilotons. The flash lit up an area about 900 square kilometers and was captured by Meteosat 8, a geosynchronous weather sattelite. Meteors of this size occur about 3 times a year. However this was the first time in history that an asteroid was detected in time for an impact alert to be issued.
About a year earlier, on Oct. 11, 2007 another asteroid ( 2007 TU24 ) was discovered by Catalina and estimated to be up to 2,000 feet ( 610 meters ) in diameter. Fortunately It missed Earth by 334,000 miles ( 537,500 kilometers ). This is a very near miss in terms of astronomical distances. If it had hit the Earth, it would have released about 1,500 megatons of energy and forged a crater nearly 5 kilometers ( 3 miles ) wide. This would be larger than than the Barringer Meteorite Crater near Winslow Arizona. Or if it had hit in the ocean, it would have created a devastating tsunami of theatrical proportions. Detection of hazardous near Earth objects is part of a cooperative effort generically referred to as Spaceguard.