In 1996 there were approximately 20,000 known items of space junk derived from exploded satellites and discarded rockets. Most pieces are about the size of a medicinal tablet, but at they speed they travel could have the effect of a small bomb if they struck a new satellite. Because elliptical orbits are the norm, the probability of junk hitting from any direction becomes significant (despite the vastness of space); junk in the lower part of its orbit is the most dangerous.