Planetary defense is the term used to encompass all the capabilities needed to detect the possibility and warn of potential asteroid or comet impacts with Earth, and then either prevent them or mitigate their possible effects. Planetary defense involves:
As interesting as asteroids are, they also pose a threat. On February 15, 2013, a bolide (a meteor that explodes in the atmosphere), entered Earth's atmosphere near Chelyabinsk, Russia and created an airburst whose shockwave struck six cities across Russia. The impact may have been caused by an asteroid just 21 yds. (20 m.) in diameter. Prevalent dashboard cameras in Russia have offered many images of the meteor hurling across the sky, and visual evidence of the damage that would accompany an asteroid impact.
Despite the worldwide attention of the Chelyabinsk event, these bodies are continually colliding with Earth; however, the vast majority of these objects are very small and pose no threat to human activity. Although the chance of a major asteroid impact is low, the potential consequences to our society could be very severe.
Small asteroids – sizes of only about a meter – hit Earth's atmosphere and disintegrate with surprising frequency – around every other week.
Impact craters are visual evidence of the meteor strike. The 1908 Tunguska impact in Siberia, the largest impact in recorded history, is thought to have been triggered by an incoming object of 65-207 yards (60-190 meters) in diameter.
Researchers in the U.S. and Europe have been considering the use of space missions for asteroid risk assessment for almost two decades. We have several technologies available to mitigate such a threat, but none have been tested in realistic conditions. Moreover, the design of an efficient mitigation strategy relies on our understanding of the physical properties of threatening objects and their responses to mitigation techniques, which is still extremely poor.
Most of the techniques that have been proposed to avoid an Earth impact event are linked to altering the trajectory of an asteroid on a collision course with Earth. Among these proposals, the one that is currently being considered as more mature, because it is based on existing and affordable spacecraft technology, is the kinetic impactor, which changes the orbit of an asteroid by a direct hit of a spacecraft at a very high relative speed.
Adapted from ESA's Asteroid Impact Mission “Planetary Defence”