The United States Mariner 9 and the Soviet Mars 2 and 3 probes were circling the Red Planet, returning the first in-depth images and data about that world which showed that Mars was not the dead and merely cratered realm that earlier flyby missions with their limited coverage had led scientists to believe. There was also excitement about an upcoming mission named Viking to place two robot landers on Mars to search for life there.
Closer to the Sun, the Soviets had finally succeeded in landing intact and functioning on the hellish world of Venus with their Venera probes. America was preparing a probe named Mariner 10 that would not only flyby Venus and return the first close-up images of its thick and cloudy atmosphere, but proceed on to Mercury and reveal what that little world really looked like.
The outer solar system had not been neglected in NASA’s plans for deep space exploration. The agency was preparing a modified version of its original Grand Tour plan, which would have sent two nuclear-powered probes past every world from Jupiter to Pluto in the summer of 1977.
No human vessel had ever visited the celestial realm where the gas giant planets dominated, or even crossed the Planetoid Belt which lay between the small and rocky terrestrial worlds of the inner solar system and the Jovian behemoths. Space engineers and officials realized they needed a precursor mission to pave the way for the success of these later more sophisticated machines. Two aptly named craft called Pioneer – coming from a long line of automated explorers going back to the earliest days of the Space Age – were designed and built for this task.