NASA established the Planetary Defence Coordination Office (PDCO) in 2016 to provide early detection of threats from NEOs of a size large enough - starting at about 30 to 50 metres - to damage Earth's surface and to track their paths.
Astronomers are not currently aware of NEOs that could threaten Earth, but an asteroid impact some 66 million years ago that left a huge crater in Mexico's Yucatan Peninsula is believed by many to have led to the extinction of the dinosaurs.
Launched from California aboard a Falcon 9 rocket in November, the DART spacecraft has a mass of 610 kilograms and measures 1.8 by 1.9 by 2.6 metres.
Dimorphos has a diameter of around 160 metres and is a kind of moon orbiting around the larger asteroid Didymos.
