Near-Earth Objects

Abstract

A near-Earth object (NEO) is an asteroid or comet orbiting the Sun with a perihelion distance of less than 1.3 AU (1 AU, an “astronomical unit”, is the mean distance between the Earth and the Sun, around 150 million km). If the orbit of an NEO can bring it to within 0.05 AU of the Earth’s orbit, and it is larger than about 120 m, it is termed a potentially hazardous object (PHO); an object of this size is likely to survive passage through the atmosphere and cause extensive damage on impact. (The acronyms NEA and PHA are used when referring specifically to asteroids.) The recognition that a giant asteroid or comet perhaps 5-10 km across most likely caused, or at least contributed to, the extinction of the dinosaurs in a geological episode known as the Cretaceous-Tertiary Event has highlighted the hazard to our civilization presented by NEOs. The energy involved in collisions of NEOs with the Earth can be much larger than that released in the detonation of nuclear weapons or naturally occurring phenomena on Earth (e.g. volcanoes, earthquakes, or tsunamis). Scientists cannot accurately predict what effects a major NEO impact would have on today’s technically sophisticated and highly networked world. Computer simulations of impacts provide some insight but natural phenomena elsewhere in the solar system provide real proof of the destructive potential of collisions between planets and small bodies. The collision of comet Shoemaker-Levy 9 with Jupiter, observed worldwide through telescopes in 1994, created scars in Jupiter’s atmosphere larger than the Earth. Even relatively small impactors can cause considerable damage on Earth. The object that exploded over the Russian city of Chelyabinsk in February 2013 had a diameter of only 17-20 m, yet it produced a blast wave that damaged buildings and injured some 1500 people. The potentially devastating effects on Earth of a collision with a large asteroid or comet are now well recognized by scientists and policy makers.