OTD…April 28 and STS-39

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 1999…Columbia launched with the first woman commander and the Chandra X-ray Observatory

#OTD in 1993…Endeavour launched a new NASA Tracking and Data Relay Satellite.