NASA said: “Potentially hazardous asteroids are about 150 meters – almost 500ft – or larger, roughly twice as big as the Statue of Liberty is tall.”
The space agency added: “Potentially hazardous comets also get unusually close to Earth.
“Knowing the size, shape, mass, composition and structure of these objects helps determine the best way to divert one, should it have an Earth-threatening path.”
Asteroids and comets rarely strike Earth but the gravitational effects of other bodies in the solar system could nudge them on Earth-bound trajectories.
NASA said “this allows the possibility of a future collision” even if an initial flyby is harmless.
source: express.co.uk