There are now several dedicated search programmes looking for NEOs. The most successful in recent years has been the Catalina Sky Survey. In the first half of 2008, the Catalina Sky Survey accounted for 70% of newly discovered near-Earth asteroids.

When an NEO is first detected, it may only have been observed for a few minutes or hours of its orbit around the Sun. With such a small snapshot of a much longer orbit, there is inevitably a lot of uncertainty about the track the object will actually take around the Sun. This results in the calculation of many potential orbits that the object could be on. If any of these ‘virtual’ orbits bring an asteroid within 750,000 kilometres of the Earth’s orbit at some point in the future, it is classified as a Potentially Hazardous Asteroid (PHA).

This uncertainty makes it essential to make follow up observations of NEOs to better understand its ‘real’ orbit and rule out any ‘virtual’ orbits of concern. This follow up work is carried out by a world-wide network of professional and amateur astronomers collectively known as the NEO Community. The results of these observations are fed into the process of calculating the risks posed by NEOs.