When most people imagine outer space, they imagine it as a perfect vacuum. This impression is incorrect, because the vacuum is filled with charged particles. The density of charged particles in deep space is so low that they rarely collide with each other. Instead of collisions, the motion of charged particles is controlled by forces related to the electric and magnetic fields that fill space. Except for very close celestial objects such as stars, moons, or planets, this lack of collisions occurs throughout space. In these cases, the charged particles are no longer travelling through space, but rather through a medium in which they can collide with other particles.