COMETS, METEORS AND ASTEROIDS

In 1962, two American policemen in the little town of Manitowoc on the shores of Lake Michigan found a 9 kg (20lb) chunk of red-hot metal embedded in the street. It was the remains of the Russian satellite, Sputnik 4. Quite apart from the man-made junk from old spacecraft, There are also various natural object, mostly lumps of rock and metal, that approach the Earth from outer space from time to time. Most are spectacular but harmless, burning a blazing trail through the sky, but others threaten the continued existence of life on Earth.

Impact Earth
The Earth is constantly bombarded by debris from space. Most is small enough to burn up as it enters the Earth's protective atmosphere. But from time to time, larger object get through. Some explode before they reach the Earth's surface. Others collide with the Earth and puncture is crust.