 The first spacecraft to explore Jupiter was Pioneer 10 , which flew past the planet in December 1973 , followed by Pioneer 11 twelve months later . Pioneer 10 obtained the first @-@ ever close @-@ up images of Jupiter and its Galilean moons ; the spacecraft studied the planet 's atmosphere , detected its magnetic field , observed its radiation belts and determined that Jupiter is mainly fluid . Pioneer 11 made its closest approach , within some 34 @,@ 000 km of Jupiter 's cloud tops , on December 4 , 1974 . It obtained dramatic images of the Great Red Spot , made the first observation of Jupiter 's immense polar regions , and determined the mass of Jupiter 's moon Callisto . The information gathered by these two spacecraft helped astronomers and engineers improve the design of future probes to cope more effectively with the environment around the giant planet . 
