COMETS THE NIGHT keen watchers of the night sky know that, from time, new objects appear against the familiar back of the stars. These newcomers are pieces of ro ranging in size from specks of dust to chunks a hundreds of wide. The most spectacular the comets balls of ice with dazzling tails that stre for millions of The pieces of sna debris are massive rocks called a Most asteroids stay within an area called the asteroid between the orbits of Jupiter and Mars Some b however, stray asteroids or comets end up on collision course with a planet, with potentiali catastrophic results.