sentences:  The "Viking 1" Orbiter was inserted into Mars orbit on June 19, 1976, and trimmed to a 1513 x 33,000 km, 24.66 h site certification orbit on June 21.Landing on Mars was planned for July 4, 1976, the United States Bicentennial, but imaging of the primary landing site showed it was too rough for a safe landing.The landing was delayed until a safer site was found, and took place instead on July 20, the seventh anniversary of the Apollo 11 Moon landing.