Near-Earth asteroids – Their physical nature and the impact hazard

Abstract

Collisions between planetesimals (the precursors of present-day asteroids and comets) and growing planets forged the Solar System as we know it today. After the epoch of planet formation around 4.5 billion years ago, collisions with the remnant population of small bodies continued and probably led to significant quantities of minerals, water and organic materials being deposited on the surfaces of the Earth and other planets. In later epochs impacts on the Earth may have abruptly altered the course of evolution and paved the way for mankind. However, mankind should now be aware that this natural process has not ceased. In particular the current population of so-called near-Earth asteroids contains many objects that are considered potentially hazardous. The origin of near-Earth asteroids, recent observational results relating to their physical properties, and the steps being taken to improve our understanding of the impact hazard and determine what defensive measures are appropriate, are discussed.