Particle deflector shields protect a vessel from impacts by high-speed projectiles. The precise mechanisms accomplishing this are presently unknown, but the shields resist or deflect the motion of high-speed particles much more effectively than slow-moving masses. For instance a space station may maintain its protection whilst slow-moving freighter craft are free to approach and dock and enemy fighters fly through with little impedence. This velocity-dependent behaviour of deflectors seems similar to the action of a magnetic field on charged particles, which explains why some spacers to colloquially refer to deflectors as "magnetic shields". The existence of any real connection between deflectors and genuine magnetic phenomena is doubtful, because deflectors have the distinction of acting on uncharged objects. The most powerful deflectors, such as the security shield at Endor or the defense shields of major civilised planets, are able to block the passage of objects moving at almost any appreciable speed. This has disasterous results for any craft which unwittingly collides with the field. Starfighters entering the deflectors of a Death Star might experience slight resistence whereas high-velocity missiles would be seriously perturbed.
Deflectors are not effective against indefinitely violent impacts. The energy absorbed by the field when the projectile is deccelerated must be dissipated by the shield generator sustaining it. These generators have limits to their maximum rate of power dissipation and total absorptive capacity for any one impact. The quality rating of a shield system determines its effectiveness and the magnitude of this shield failure point. The Death Stars possessed deflector shields of immense power by comparison to those of capital starships, though much weaker than many ground-based deflector systems. The huge power demands of the superlaser and other key functions of the Death Stars prevented the implementation of comparably impressive deflectors.
The particle deflector shields of the first Death Star were easily able to withstand the impact of the debris from the explosion of Alderaan. According to the A New Hope novel the Death Star was at a distance of approximately six planetary diameters when the explosion took place, which is approximately 75000km. As discussed above, the amount of energy deliverd to the planet was very much greater than its gravitational binding energy. Consequently most of the injected energy would have come out again in the form of kinetic energy of the debris. Assuming that the explosion was even in all directions, the fraction of this energy incident on the Death Star is the ratio of the battle station's crossectional area to the surface area of the 75000km-radius sphere. If the Death Star was only 120km in diameter the fraction is about 1.6 x 10-7. Thus the absorptive capacity of the particle deflector shields is comfortably greater than a number which is itself very much in excess of:
S > (I-U)/U 1026 Joules
where I is the amount of energy injected by the superlaser, U is the magnitude of the gravitational binding energy of Alderaan. The actual value of I is probably greater than U by a factor of tens of thousands.
Holographic schematic showing the circulations of the security deflector shield projected from the sanctuary moon. Note that it only encloses a small geographical region of the moon, which is nonetheless wider than the prominent dish seen on the ground later in the movie.
Atmosphere containment shields.
The apertures of deep-space docking bays, such as those of the Death Stars, are sealed against the vacuum by a specialised adaption of deflector technology. A weak deflector field produced by a ring generator around the opening is tuned to cause back-scattering of impacts with velocities and energies typical of the gas molecules of a warm and habitable atmosphere. In this way large objects are able to fly through the bay entrance whilst most of the air is retained inside.
The shields are obviously too weak to appreciably affect the passage of a starship. In A New Hope and Return of the Jedi we witness the Millennium Falcon and several shuttles pass through the atmosphere containment shields of docking bays. The rebel fugitives in the freighter met little resistence from the shield during their hasty takeoff several hours later.
It is possible that the passage of a ship temporarily undermines the integrity of the containment. In A New Hope the flight controllers cleared and evacuated the Millennium Falcon's docking bay before reeling it inside. When Lord Vader's shuttle arrived in Return of the Jeci its bay was clear of personnel until after landing. (The stormtroopers paraded in after landing but before the Dark Lord disembarked. We don't know whether the Emperor's arrival proceeded similarly because we never saw the actual moment of touchdown.) The clearance procedure may be a routine precaution to avoid danger to people within the docking bay.
On the other hand the clearance might simply be intended to allow the starship pilot to land without fear of squashing or irradiating any stray people. In this case the we don't need to postulate any loss of atmospheric containment integrity during the passage of large objects.
Lighting around the aperture of a hangar indicates but does not seem to be necessary for the operation of the atmospheric containment field. Although all of the Death Star hangars seen to date have a complete illuminated ring, at least two of the hangars of Lord Vader's Executor warship have light rings which are interrupted at some points. (See Executor for more information.) The lighting most probably is a warning to humans and droids that they should not walk near or beyond that point of the deck.
Sometimes termed "ray shielding", these devices have an absorptive effect on radiation and beam attacks of a massless or nearly-massless nature. Energy shields are used to dampen the potential damage of blaster hits. As with particle deflector shields, energy shield generators have maximum dissipative power ratings. The energy shields of the vulnerable exhaust port on Death Star I prevented rebel starfighters from making simple laser cannon attacks. The quality of energy shields are combined with the quality of deflector shields to give the overall shield rating which is used in roleplaying game statistics.