While by most standards space qualifies as 'empty', in fact there is a significant amount of material of one sort or another within the interplanetary and interstellar medium. Most of this material is of atomic size, but a small fraction is comprised of micrometeorites and other such debris. All of this represents a potential danger to the unprotected vessel travelling at high relativistic or any warp speed; the kinetic energy of even a one gram object at 0.25 c is in excess of 2.8 TeraJoules, sufficient to vaporize over a cubic metre of tritanium.
The navigational deflector is designed to protect a ship from this hazard. An offshoot of tractor beam technology, the navigational deflector uses graviton polarity source generator to feed a subspace field distortion amplifier in order to project a 'force beam' ahead of the ship. This is sufficient to deflect most material out of the path of the vessel well before impact would occur.
The navigation deflector is usually one of the largest components of a vessel, most especially so as it is usually combined with a large sensor cluster. On Federation starships the deflector is usually a large flat or concave area located to give it a clear line of sight ahead of the vessel - usually at the front of the engineering section. The deflectors are typically steerable to some 7o
from the normal line of sight.
Subspace field coils are used to split the deflector output into two components. First, a series of five nested parabolic shields extend nearly 2 km ahead of the ship. Very low power, these shields are sufficient to deflect stray hydrogen atoms of the interstellar medium as well as any sub-micron particles than may have escaped the deflector beam. While the vessel is at warp speed any particles hitting these fields will travel rapidly across the surface of the shields, passing through the ships warp field. This causes them to fluoresce, creating a 'rainbow streak' effect as they pass the ship. These streaks are bright enough that the uninformed often mistake them for passing stars!
The navigation shields also posses another intriguing property; they are immune to attack from laser weaponry. This is due to the trans-static flux effect which occurs as a by-product of the deflection process; when laser light impinges on a deflector field, the effect creates a small portal into subspace, causing the laser beam to pass harmlessly into this domain. As the beam is not subspace encased, it will re-emerge into normal space within a few milliseconds, putting it several hundred light seconds away. Since the beam never actually impacts on either the deflector shield or hull the power of the attack is irrelevant to the effect. This process is not regarded as a serious defensive measure, since laser weapons are considered obsolete by most major powers.
The second part of the navigational deflector system is a powerful tractor / deflector beam that sweeps thousands of kilometres ahead of the ship. This pushes aside larger objects that may present a collision hazard.