Active experiments in space generally take the form of introducing contaminants into the space environment in a controlled manner, and then observing the response of the environment, the platform and the contaminants. The contaminants fall into three major categories: deployment of solid objects, release of material, and exposure of locally generated electromagnetic fields to the space environment. Solid object deployment is often the orbiting platform itself, although recently experiments have been performed with a specific object deployed from the space shuttle. Material releases may be either neutral gases or, occasionally, liquids. Alternatively, the material may consist of charged particles usually in the form of beams of ions or electrons in varying proportions. Electromagnetic fields may be either steady fields or varying fields emitted as electromagnetic waves.