A plasma is a partly or wholly ionised gas whose particles exhibit collective response to magnetic and electric fields. The collective motion is brought about by the electrostatic Coulomb force between charged particles. This causes the particles to rearrange themselves to counteract electric fields within a distance of the order of, l, the Debye length.

Spacecraft in Earth orbit experience a number of distinct plasma regimes. At the top of the atmosphere is the so-called ionosphere, above that the magnetosphere, and surrounding this the solar wind, originating on the Sun and blowing throughout interplanetary space.

Once outside the spacecraft, neutral atoms produced by outgassing and sputtering can be ionised by sunlight or charge-exchange with other ions, to create a low-energy (<10 eV) ion population. These ions can be drawn to negatively charged surfaces and can adhere. This coating may alter optical properties e.g. of mirrors or solar panel covers, or change the secondary and photoemission yields and the susceptibility to surface charging. Within the spacecraft, e.g. in electronics boxes, residual gasses can facilitate electrostatic discharges from high voltage components.