When was the first spacecraft launched to make close flybys of outer planets?

Pioneer 10 launched on 2 March 1972 and made the first close flyby of Jupiter on 4 December 1973. Its mission provided the first close-up data from the outer Solar System and showed that spacecraft could survive the intense radiation environment near Jupiter. After the flyby, Pioneer 10 continued outward and eventually became one of the first probes to head toward interstellar space. The mission helped open the path for later outer-planet spacecraft such as Voyager and Galileo.