Asteroid

Abstract

Planet embryos formed some 4.5 billion years ago from dust accretion and the collisional coalescence of primordial building blocks called planetesimals. Most asteroids are thought to be remnant collisional fragments of planetesimals and planet embryos from the inner Solar System that were not incorporated into planets; comets, on the other hand, are remnants from the cold, outer regions of the Solar System. Subsequent impacts of asteroids and comets have played a major role in the development of planets, especially the Earth. Asteroids and comets probably contributed significantly to the early Earth’s inventory of water and organic materials, facilitating the development of life. In contrast, impacts of asteroids and comets present a small but scientifically well-founded threat to the future of our civilization: There is no scientific reason why asteroids and comets should not continue to hit the Earth at irregular and unpredictable intervals in the future. As a result of modern observing techniques and directed efforts thousands of near-Earth asteroids have been discovered over the past 20 years and the reality of the impact hazard has been laid bare. Future observation programs and space missions will be crucial for a better understanding of the processes and conditions governing the development of planets and life the composition and physical nature of asteroids, and the techniques that would be most effective in preventing a collision of an asteroid with the Earth.