The Mariner series of missions were set up NASA to study the red planet,  our closest neighbor in space (not counting the moon.) “Close” is a somewhat relative term – even television signals take four days to travel from Mars to Earth, so several passes at the planet would be needed to fully explore it. The Mariner 4 spacecraft was the first to reach Mars, beaming back the first visuals of the planet from orbit. The Mariner 6 and 7 twin spacecraft followed up, flying by Mars with better equipment for more detailed snapshots of the surface. The flagship of program, the Mariner 9 was next.

On this day, May 30, in 1971 the Mariner 9 launched towards Mars, arriving in orbit in November after a 372 million mile journey. The Mariner 9 was equipped with every sophisticated sensor and probe that could be fitted on, from cameras to atmosphere analyzers, and completely revolutionized our understanding of the planet.

Measured in terms of bits, the Mariner 9 mission contributed more than 27 times the amount of data brought back by all previous missions combined. It beamed back to earth over 7300 images, covering about 80% of the planet, which seemed to confirm there was once water on Mars. Images showed ancient river beds, layers of ice on the planet’s poles, and even wind and weather patterns. The evidence of water’s past on Mars brought back hopes that water could still be there, and revived calls to actually make a Mars landing.