Although most people think of space as empty, in truth it’s full of a lot of debris—stray atoms of hydrogen, micrometeoroid particulates, and the like. To a vessel traveling at warp or impulse speeds, single, or repeated, collisions with such objects can cause wear and tear, or even significant damage. To avoid these objects, Starfleet vessels use a device called a navigational deflector or main deflector to ‘push’ them out of the ships flight path.
A navigational deflector uses three redundant high-powered graviton polarity source generators to generate two effects: a series of five nested parabolic shields projected two kilometers ahead of the ship, which sweep aside stray hydrogen atoms and submicron particulates; and a main deflector beam projected thousands of kilometers ahead of the ship to move larger objects out of the way. These two effects are focused and manipulated with the main deflector dish, a predominant substructure easily visible on the front of most ships.
The navigational deflector works in conjunction with the long-range sensors, which locate and track objects in the ship’s projected course. Since the deflector a great deal of electro-magnetic and subspace radiation, which could interfere with long-range sensors, the long-range sensor array is located behind the deflector dish so that the axis of its scans matches that of the deflector projections.
In additional to the two deflecting effects, a ship can use its navigational deflector to emit a wide variety of electro-magnetic and subspace radiation effects. For example, if the ship needs to emit old-style delta radiation to communicate with a strange vessel, or a polaron burst to disable masking circuitry, it uses its navigational deflector to do that.
At impulse speeds or up to warp 8, only one of the deflector’s generators has to be operational. From warp 8.1 to 9.1, at least two must function. At warp 9.2 or higher (including specially calibrated Navigational deflectors to work in conjunction with FTW drives) all three generators must operate to ensure the safety of the ship. If the deflector does not have enough functioning generators, or is not supplied with sufficient power, the ship may suffer wear and tear *or worse, damage). Secondary deflectors are also commonly installed on vessels, such as those with separating sections, to provide a back up in case the main deflector fails.
Additionally, Starfleet now constructs some ships, such as the Intrepid-class Light Explorer, with a small auxiliary deflector.