Exploration of Mars
The Viking program launched "Viking 1" and "Viking 2" spacecraft to Mars in 1975; The program consisted of two orbiters and two landers – these were the first two spacecraft to successfully land and operate on Mars.
The primary scientific objectives of the lander mission were to search for biosignatures and observe meteorologic, seismic and magnetic properties of Mars. The results of the biological experiments on board the Viking landers remain inconclusive, with a reanalysis of the Viking data published in 2012 suggesting signs of microbial life on Mars.
The Viking orbiters revealed that large floods of water carved deep valleys, eroded grooves into bedrock, and traveled thousands of kilometers. Areas of branched streams, in the southern hemisphere, suggest that rain once fell.
NASA
Outside Mars, Jupiter was first visited by "Pioneer 10" in 1973. More than 20 years later "Galileo" sent a probe into the planet's atmosphere, and became the first spacecraft to orbit the planet. "Pioneer 11" became the first spacecraft to visit Saturn in 1979, with "Voyager 2" making the first (and so far only) visits to Uranus and Neptune in 1986 and 1989, respectively. The first spacecraft to leave the solar system was "Pioneer 10" in 1983. For a time it was the most distant spacecraft, but it has since been surpassed by both "Voyager 1" and "Voyager 2".