OTD…Aug. 18 and STS-68 abort

#OTD in 2001…Discovery launched on an International Space Station crew exchange mission

#OTD in 1994…Columbia launched on a Spacelab microgravity flight

Today marks the 40th anniversary — July 20, 1976 — of the first U.S. robotic landing on another planet by the Viking 1 spacecraft on Mars.