Near-Earth Objects are comets and asteroids that have been pushed by the gravitational attraction of nearby planets into orbits that allow them to enter the Earth's neighborhood. They are about 4.5 billion years and can range in size from pebbles or lumps of ice, to rocky or icy worlds nearly 1,000 kilometers across. An asteroid or comet may become a Near Earth Object when its trajectory intersects the orbit of the Earth or is within 0.3 Astronomical Units (distance between the earth and the sun).
             A Comet is a body revolving around the sun. A comet is characterized by a long, luminous tail, but only in the segment of the comet's orbit when it passes closest to the sun. Composed mostly of water ice with rooted dust particles, comets originally formed in the cold outer planetary system while most of the rocky asteroids formed in the warmer inner solar system. A Meteor is a small solid body that enters a planet's atmosphere from outer space and raised to combustion by the friction resulting from its fast motion. An Asteroid is one of the many small or minor planets that are members of the solar system and that move in elliptical orbits primarily between the orbits of Mars and Jupiter.
             Some 65 million years ago dinosaurs and 75 percent of those species then on Earth seem to have been executed to extinction by the impact of a 10-kilometer-diameter (6-mile) NEO. The Tunguska event of 1908 in Siberia, warns the United States Air Force on small objects entering Earth's atmosphere, and particularly, the impact of some two dozen fragments of comet Shoemaker-Levy 9 with Jupiter in 1994 all underscore that dangerous impacts may occur today. The best estimate of the probability of a civilization-threatening collision in the next century is nearly one in 1,000.
             Estimates of the numbers of Near Earth Objects of different sizes can be made either by direct measurement using ground-based telescopes or inferred from the numbers and sizes of crat...