15 April

2002: On 15 April 2002, the European Space Agency reported that two asteroids had passed close by Earth, at distances of 1.2 and 3 times the distance to the Moon.

By monitoring known asteroids, astronomers can predict which may collide with Earth. ESA continues to study ways in which its missions can assist in monitoring unseen but potentially hazardous asteroids.

A discovery, made using data from ESA's Infrared Space Observatory (ISO), showed that there could be nearly two million asteroids larger than one kilometre in the main asteroid belt, between Mars and Jupiter.

When the asteroids remain in the main-belt, they pose no danger to Earth. However, they can be thrown into different orbits by collisions with other asteroids or by the influence of Jupiter's gravitational field. If their new orbits cross the Earth's orbit, they could one day collide with our planet.