Most of the visible matter in the universe exists as a fluid composed of electrically charged particles rather than as a gas made of neutral atoms or molecules. Gas mixtures of electrically charged particles, such as electrons and ions, are called plasmas. Plasmas are found in the following solar system environments: the solar atmosphere, the interplanetary medium, planetary magnetospheres, and planetary ionospheres. Most of the interstellar medium is also plasma, as are most other regions of our galaxy.
Most of the plasma found in our own solar system is accessible to in situ measurements made by instruments onboard spacecraft. Since the advent of the space age in the late 1950s, space probes have visited Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune, and comets Giacobini–Zinner, Halley, and Grigg–Skjellerup. The space environment surrounding the Earth has also been extensively studied by experiments onboard rockets and satellites. The Sun and astrophysical plasma environments outside our own solar system are not subject to direct measurements but must be observed remotely with sophisticated instruments located either at ground-based observatories or on orbiting observatories. An exception to this are the very energetic particles called cosmic rays, which can be observed using Earthbased or balloon-borne experiments. Solar cosmic rays have energies up to about 100 million electron volts (100 MeV) and originate in the solar corona.