Which spacecraft was involved in outer planet exploration aside from Cassini?

Galileo is recognized for its significant role in the exploration of the outer planets, specifically its mission to study Jupiter and its moons. Launched in 1989, Galileo provided a wealth of information about the planet's atmosphere, magnetic field, and its numerous moons, including Europa, Ganymede, and Callisto. The spacecraft made numerous discoveries, such as evidence of a subsurface ocean on Europa, which has implications for the potential for life beyond Earth.

While other spacecraft like Giotto and the Mars Global Surveyor have made important contributions to space exploration, their primary missions focused on different targets. Giotto was designed to study comets, notably Halley's Comet, and Mars Global Surveyor was tasked with mapping the surface of Mars and studying its climate and geology. The Pathfinders does not specifically refer to a spacecraft known for outer planet exploration and is more associated with lander missions, particularly those concerning Mars. Therefore, Galileo stands out as the correct answer for outer planet exploration aside from Cassini.