The Viking Missions were a turning point in the history of space exploration. The initial plan had been to send two orbiters, each with a lander, to Mars in order to explore the planet and in particular to determine the presence or absence of life there.
As with most other space missions of the time, the crucial issue was enabling a successful mission with the minimum space and weight requirements. Whereas previous automatic probes had successfully functioned on relatively simple computing equipment, the Viking probes were relatively ambitious and required more complex and therefore bulkier and heavier hardware. To address this, redundancy in the hardware was reduced and the number of biology experiments on the Landers was reduced to two: the "Wolf Trap" and "Labeled Release" experiments. A number of other instruments were also carried, including cameras on board both the orbiters and the landers. In order to carry this payload, the rockets used to launch the Viking missions had to be more powerful than originally planned and the cameras were fairly low resolution due to bandwidth restrictions imposed by processing speeds of the onboard computers.
The Mission Itself
Viking I was launched on 20th August, 1975 and Viking II on 9th September. The Viking II Orbiter computer failed before reaching Mars. Viking I was more successful and was able to complete all experiments. In particular, the Wolf Trap and Labeled Release experiments both showed results consistent with the existence of life on the planet. This led to a great deal of public and scientific interest, excitement and support for the probes and future space exploration.
NASA's funding was increased considerably and the organisation was encouraged to send further missions to Mars in the near future. It was decided that the poor reliability of computers and their weight meant there was little financial difference between sending automatic probes to Mars and sending people there. The quantity of data was also reduced by the limited bandwidth, and the perceived probability of life on the planet along with those factors led to the decision that the human space programme needed to be revived and accelerated, and that there should be a human landing on Mars within a decade. This led to the human missions to Mars and the establishment of a Moon base and space colonies.