Which NASA spacecraft was the first to successfully orbit Mars?

Mariner 9 was the first NASA spacecraft to successfully orbit Mars. It entered Martian orbit on November 14, 1971, becoming the first spacecraft to orbit any planet beyond Earth.

The mission was launched on May 30, 1971, as part of NASA's Mariner program. Its arrival coincided with a planet-wide dust storm that initially obscured much of the surface. Mission controllers adapted by delaying detailed imaging until the storm began to clear, allowing Mariner 9 to map Mars from orbit.

During roughly 349 days of operations, Mariner 9 returned 7,329 images covering about 85% of the planet. Those pictures revealed enormous volcanoes, ancient channels, craters, weather systems, and the immense canyon system later named Valles Marineris.

Mariner 4, launched in 1964, was the first spacecraft to perform a successful Mars flyby, but it never entered orbit. Mars 2 and Mars 3 arrived soon after Mariner 9, while Viking 1 came later in 1976.