As a planet rotates on its axis, it drags its magnetosphere around with it. In the enormous magnetosphere of a rapidly rotating planet like Jupiter, charged particles are swept around at high speeds. These fast-moving charged particles slam into neutral atoms (which do not share the motion of material in the magnetosphere), and the energy released in the resulting high-speed collisions heats the plasma to extreme temperatures. (Recall from Chapter 9 that a plasma is a gas consisting of electrically charged particles.) In 1979, while passing through Jupiter’s magnetosphere, Voyager 1 encountered a region of tenuous plasma with a temperature of over 300 million K, 20 times the temperature at the center of the Sun. (The density of the plasma—about 10,000 atoms per cubic meter—was much lower than that of the best vacuum that can be produced on Earth, however, so the spacecraft was in no danger.)