Stones in the Sky: From the Main Belt to Earth-Crossing Orbits

It is now well known that impacts play a great role in the formation of planetary bodies, and have some influence during their evolution -- particularly on the biosphere. The story begins officially in the early 19th century, with the discovery of the first asteroids and the recognition that stones can fall from the skies. Through the findings of the Kirkwood gaps, of Hirayama families and finally the importance of the 3:1 and other resonances, the processes that send Main Belt asteroids on trajectories crossing Earth’s orbit have been gradually understood. A little paradox remained: computations show that the resonances become completely empty in less than a billion years, but observations reveal that celestial bodies orbiting in such Earth-crossing trajectories still exist; this has been explained by collisions in the Main Belt, the products of which are sometimes injected into nearby resonances and then continue to supply the population of possible Earth-crossers; non-gravitational forces may also inject bodies into resonances. Besides, hazards due to collisions between such Earth-crossers and our planet have been recognized. The example of asteroid 4179 Toutatis, discovered in 1989 (at the Observatoire de la Côte d’Azur), very near the 3:1 resonance, is presented.