The 50th GPS (Global Positioning System) satellite was launched on March 21st. The launch was delayed a bit to prevent the 2.2 ton satellite from running into some space junk. Thousands of larger bits of space debris (from thousands of earlier satellite launches) are tracked by earth based telescopes and radars, and launches scheduled to avoid running into the stuff. The first GPS satellite was put into orbit in 1978, and the March 21 launch was to replace a satellite put up in 1993. There are 27 working GPS satellites in orbit to provide GPS navigation service world wide.