OTD…April 29 and STS-51B

#OTD in 1991, 1992 and 1993…flights by Discovery and Endeavour

#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.