One requirement for all Starfleet vessels is the ability to manipulate objects in space. Typically this will involve towing other vessels or holding sensor probes in place, but modifying the course of asteroids or space debris is also an option.
To accomplish this, Starfleet vessels are equipped with one or more tractor beam emitters.1 These employ superimposed subspace/graviton force beams which focuses interference patterns on a remote object. This results in significant spatial stress being imposed on the object; by controlling the focal point and interference patterns of the beam it is possible to use this stress pattern to place either a repelling or attracting force on the object.
Power for the tractor beam emitters is provided by variable phase graviton polarity sources feeding sub-cochrane subspace field amplifiers. Phase accuracy of less than 2.7 arc-seconds per millisecond is required for precise control of a tractor beam. Use of a tractor beam can involve placing significant stress on both emitter and target, and vessels with low structural strength can be damaged by a tractor beam. Because of the potential damage to a vessel using a tractor beam, all Starfleet tractor emitters are attached directly to the skeletal frame, and are protected by the structural integrity field system of the craft.2
All Starfleet vessels are equipped with at least one tractor beam emitter, generally placed so it can tow an object which is situated behind and below. Vessels of Runabout size or above are fitted with more than one emitter - large starships having enough to ensure all-round coverage.