If there are Martians, they were probably laughing on Aug. 21, 1993. That's the day the Mars Observer went missing, vanishing into thin air (or rather, no air). The spacecraft had launched a year earlier, but contact was lost days before it was supposed to enter the Red Planet's orbit. At NASA's Jet Propulsion Laboratory, some comforted themselves by putting a milk carton with a picture of the spacecraft and the words "Have You Seen Me?" on the wall.