Viking 1 was the first of two spacecraft (along with Viking 2) sent to Mars as part of NASA's Viking program. On July 20, 1976, it became the second spacecraft to soft-land on Mars, and the first to successfully perform its mission. (The first spacecraft to soft-land on Mars was the Soviet Union's Mars 3 on December 2, 1971, which stopped transmitting after 14. 5 seconds. ) Viking 1 held the record for the longest Mars surface mission of 2307 days (over 61⁄4 years) or 2245 Martian solar days, until that record was broken by the Opportunity rover on May 19, 2010.