Publisher's Synopsis
An enormous number of asteroids and comets orbit the Sun, ranging in size from pebbles to mountains. Earth approaching asteroids and comets are collectively termed near-Earth objects (NEOs). Fortunately, only a tiny number of these objects cross the Earth's orbit, and our atmosphere protects us from small and structurally weak objects; however, smaller NEOs vastly outnumber the larger, thus resulting in a higher impact frequency. The goal of current search efforts is to catalog and characterize by 2008 the orbits of 90 percent of near-Earth asteroids (NEAs) larger than 1 km in diameter, currently estimated to number between 900 and 1300. Devastating impacts can also occur from smaller NEAs, short-period comets (SPCs) in asteroid-like orbits, and long-period comets (LPCs) that do not regularly enter near-Earth space because their orbital periods range from 200 years to millions of years. Impacts are extremely infrequent events relative to a human lifetime, but have the potential for massive loss of life and property. Impacts have occurred in the past and will occur in the future. Despite enormous destructive potential, there are many positive aspects of asteroids and comets. They represent a significant resource for commercial exploitation, space exploration, and scientific research. They may even provide clues regarding the origin of life on Earth. It is worthwhile to identify and understand these planetary neighbors both for what they can provide us and for what they are capable of taking away.
