OTD…Jan. 24 and STS-51C

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.

#OTD in 1998…The launch of Discovery on final shuttle-Mir docking mission

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