ESA SP-1310 The asteroid hazard
Since its formation more than four and a half billion years ago, Earth has been subject to impacts from space. Although the frequency of the encounters and the characteristics of the impactors have changed over time, Earth still encounters a significant flux of these objects as it sweeps through interplanetary space.They range from micron-sized dust particles, of which thousands of tonnes enter the upper atmosphere and burn up each year, to much larger bodies, with diameters measured in kilometres, which strike Earth with globally catastrophic consequences over much larger timescales.
A limited number of observing programmes are currently searching for hazardous objects. Five of them are in the US, one in Japan, and one in Italy. Globally, they discover about 40 NEOs per month. However, discovery is only the first step in a long process; follow-up observations of the objects must continue for an extended period of time in order to obtain sufficient positional data to compute reliable orbits. This book extensively covers all aspects of the problem posed by the flux of small bodies that collide with Earth and occasionally produce local or even global devastation that may have been responsible in the past for mass extinctions in the terrestrial biosphere.