Asteroid close encounters with Earth: risk assessment

When an asteroid with an orbit close to the Earth is observed only over a short arc, there is no way to accurately predict the future close approaches. The first approach could change the orbital period in such a way that the asteroid is injected into a resonance, allowing an even closer approach a few years later. By using both a qualitative theory for these resonant returns, and a systematic numerical investigation of alternate orbital solutions, it is possible to identify future encounters and to assess their impact risk. This allows one to monitor the impact risk by all known near-Earth asteroids.