Asteroids are small and often irregularly-shaped Solar System bodies. The known majority of them orbit the Sun in the so-called main asteroid belt, between the orbits of the planets Mars and Jupiter. However, due to gravitational perturbations caused by planets, as well as non-gravitational perturbations, a continuous migration brings main-belt asteroids closer to Sun. They therefore cross the orbits of Mars, Earth, Venus and Mercury. An asteroid is called a near-Earth asteroid (NEA) when its trajectory brings it within 1.3 au from the Sun and hence within 0.3 au of Earth's orbit.