Potentially Hazardous Objects

describe the orbits of Potentially Hazardous Objects (PHOs)
demonstrate an appreciation of the need to monitor the motion of PHOs
demonstrate an appreciation of the potential consequences of a collision between an impactor and the Earth
describe how astronomers gather evidence of impacts between bodies within the Solar System and consider their effects.

About once every 10,000 years something very large hits the Earth.

There are many objects out there that could potentially hit the Earth and do a lot of damage. These are PHOs. They may be comets (PHCs) or asteroids (PHAs).

It is thought that to be potentially dangerous an object must be at least 150m in diameter and pass within 0.05 AU of Earth. NASA have, at present, identified just over 1000 objects like these and are keeping a close eye on them. Any movement in their trajectory, e.g. caused by the gravity of another large object, could have devastating results for us. If one of these objects hit land it would cause a huge amount of local devastation but also send up massive clouds of dust. These would block out the light from the Sun, stop photosynthesis and so effect all life on the planet.

If one of these objects landed in the sea it could cause a giant tsunami which could kill millions.

A PHO impact is thought to have caused the extinction of the dinosaurs 65 million years ago.	The Barringer crater in Arizona	In 1994 we observed the comet Shoemaker Levy disintegrate then hit Jupiter.

It is thought that such an object caused the extinction of the dinosaurs. They did not survive the cold temperatures caused by the blocking out of the Sun's radiation.