The first probe to reach the gas giant, Jupiter, was Pioneer 10 on 3rd December 1973 with principal instruments measuring the magnetosphere and the atmosphere. A year and a day later it was followed by Pioneer 11.
The next to arrive was Voyager 1 in March 1979 followed in July by Voyager 2. The Voyagers discovered active volcanism on the moon Io.
In February 1992 Ulysses turned its instruments on Jupiter as it made a swing by to throw it into it’s solar polar orbit.
From December 1995 until 2003 the Galileo mission undertook a detailed study of the planet and it’s moons, including the sending of a probe into the atmosphere. It made many scientific discoveries including finding what is thought to be a liquid sea below the ice of Europa.
During this period, in December 2000, Cassini did a flyby on it’s way to Saturn