Why do spaceships orbit at 17,500 miles per hour around the Earth?

After seeing the amazing pictures of NASA Astronaut Wheelock here: http://triggerpit.com/2010/11/22/incredible-pics-nasa-astronaut-wheelock/ , and looking at photo #8... I wondered how is it or why is it that it seems that 17,500 miles per hour (5 miles per second) is often the speed that space ships orbit around the Earth. Why is it at that particular speed? That is my question.