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MAGNETOSPHERE
Pioneer
Pioneer 10 and 11 were the first to observe Jupiter's magnetosphere--the protective bubble created by its magnetic field. While scientists knew Jupiter had a magnetic field, only until the Pioneer spacecraft measured it for the first time did they discover that it was 10 times stronger than Earth's.
The magnetosphere turned out to be vast, consisting of distinct inner, middle, and outer regions. When the solar wind hits the magnetosphere, it creates a shockwave called a bow shock that acts as an outer boundary of the magnetosphere. The two Pioneer spacecraft flew in and out of the magnetosphere, crossing this boundary several times and finding that its location fluctuated over time--showing that instead of being a well-defined structure, the magnetosphere is malleable. Juno is the first mission sent to map Jupiter's magnetic field and study its magnetosphere up close.