A test particle is an idealized body whose own fields and effect on its environment are negligible. Its motion probes an externally prescribed field. A gravitational test particle follows a timelike geodesic when nongravitational forces are absent; appreciable self-gravity requires a more complete treatment.
Treating a moving body as a test particle neglects its effect on the externally prescribed field. For a small meteorite moving near a planet, permits neglect of the planet's recoil and replacement of the relative two-body gravitational parameter by . This is separate from neglecting drag or assuming a Newtonian gravitational field.
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A test particle is a concept used in physics, particularly in fields such as classical mechanics, general relativity, and astrophysics. It refers to an idealized particle that is used to probe the effects of a force field or gravitational field without being affected by its own gravitational influence.