Asteroids close-up: What we have learned from twenty years of space exploration

Asteroids are objects of fundamental scientific importance for several reasons. They are the remnant debris from the formation of the inner solar system. They offer therefore the unique opportunity to study the original materials and the mechanisms which formed the terrestrial planets. Moreover, they can be the objects of a strong interaction with Earth's biosphere. Asteroids may have played a role in bringing water and organic substances on the Earth, influencing the formation of life. With catastrophic impacts they may have changed in the past the evolutionary path of life forms and may still constitute a serious threat to the human presence on the planet. Finally, asteroids represent an extraordinary source of minerals which can be exploited for the increasing necessities of our civilization and for the future exploration and colonization of the solar system.