19. ASTEROIDS and COMETS

Between MARS and JUPITER there is an immense belt of asteroids, still wider than the distance between earth and sun. The sizes of those asteroids differ from sandgrains to hundreds of meters. There are thousands of them and they create a great risk during space-missions beyond Mars: one single, small stone and it would be the end of the sonde because the speed of a spacecraft is of the order of 40.000 km/h and moreover the average speed of an asteroid is 5 km/s = 18000 km/h!
But nothing happened sofar, the probability is very low indeed.
There have been many impacts on earth during the past, where they are called: "meteorites". On Friday 9. October 1992 a meteorite destroyed the backbunker of a 1980 Chevrolet Malibu in New York, see picture. The stone was saled for $ 59.000 and the car inclusive the damage went for $ 10.000!
More serious are the larger ones: in 1908 a meteorite with a diameter of 60 m smashed into the Tunguska Valley in Siberia, an xplosion heard in London.
September 2002 a meteorite came down 1100 km north of the city Irkoetsk in East Siberia. At least 60 km^2 has been devastated, fortunately an inhabitated area, but Amsterdam would have been wiped out if that piece of stone has been turned up there, an effect corresponding to a strong atomic bomb. It is supposed that this cosmic object is exploded in the air.
Finally on 14. June 2002 a small asteroid, a hundred meter diameter, passed by the earth on 120.000 km, that is less than 1/3 of the distance earth - moon!
There are hundred thousands of such "near-earth asteroids" and it can happen once in a century, but three quarter of the earth is water and there are plenty of regions with little or no population, so the chance is large that no critical area is hit.
The graph in the center of the picture shows that the number of asteroids with large diameter is very low. The majority of the small-sized asteroids will evaporate in the atmosphere.