While some in the Mariner series went to Venus, others went to Mars. The NASA/JPL team sent six Mariners toward Mars. Four reached the red planet, the other two failed to do so. On November 5, 1964, Mariner 3 was the first in the series to be launched toward Mars but the shroud encasing the spacecraft atop its rocket failed to open properly.

First Mission to Mars

Three weeks later, on November 28, 1964, Mariner 4 was launched, giving scientists great hopes of finding an Earth-like Mars. The spacecraft arrived eight months later, on July 14, 1965. The flyby returned the first images of another planet's surface ever taken by a spacecraft. The content of the pictures, however, was tremendously disappointing. Rather than showing an Earth-like planet, Mariner 4 revealed a cold, cratered, moon-like surface on Mars.

In 1969, two identical Mariners performed flybys in February and March. These spacecraft returned images that showed Mars was not entirely moon-like. While parts of the planet were heavily cratered, other parts were relatively smooth. They also confirmed that the surface pressure of Mars was about one percent that found on Earth, and the surface temperature was far below the freezing point of water. The final Mariner to Mars, however, was the lab's greatest planetary success to date.