OTD…Oct. 20 and STS-73

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.

Shuttle Endeavour launched for the 25th and final time on this date in 2011.

#OTD in 1994, the launch of Columbia on the STS-62 microgravity research mission