You’re not even close to leaving the solar system when you pull into orbit around Jupiter, but you’d swear you’ve just entered a new one. Your ship’s scanners flash with readings of nearby worlds and faint rings. Jupiter, the largest planet in our solar system, rules a system of its own. Some astronomers consider it a failed star. Welcome to the realm of giants.
Jupiter is a gas giant, a ball of mostly hydrogen and helium large enough to hold more than 1,300 Earths. You won’t find any solid surface to explore here, and skydiving from your ship’s airlock would be a bad idea. You’d sink deeper into clouds of ammonia and water vapor until the intense atmospheric pressure and heat compressed the hydrogen around you into a molten liquid. Science fiction writers have proposed exploring Jupiter in hot-air balloons high above the crushing depths below, but you’re happy sipping cocoa aboard your ship in orbit. It’s a safer place to watch Jupiter’s spectacular cloud bands whiz by at more than 300 miles an hour (530 kilometers an hour).
Several of Jupiter’s nearly 70 moons grab your attention. Mega-moon Ganymede is larger than Mercury and has its own magnetic field. Volcanoes on Io, the most volcanic body in the solar system, spew clouds of yellow sulfur 300 miles (500 kilometers) high. Frozen Europa might hide a liquid ocean beneath its icy crust. Scientists believe Europa might hide something else inside that ocean: life.