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Deflector Shields or screens - generally referred to simply as shields - are a type of force field that surrounds a starship, space station, or planet to protect against enemy attack or natural hazard.

Sovereign Class Shield display

Operation and Use

Deflector shields operate by creating a layer of energetic distortion containing a high concentration of Gravitons around the object (ship, city, etc.) to be protected. Shield energies can be emitted from a localized antenna or "dish" (such as a ship's navigational deflector), or from a network of "grid" emitters laid out on the object's surface (such as a ship's hull). On modern starships, deflector shields are essential equipment. They are raised to full power in anticipation of environmental hazards or in combat situations.

Neither matter nor highly-concentrated energy (i.e., weapons fire) can normally penetrate a shield. When shields are "up," or energized at a high level, most matter or energy that comes into contact with the shields will be harmlessly deflected away. In contemporary starship combat, shields are essential for hull protection. When shields are up, only minor hull damage can be expected during combat. Without deflector shields, modern weapons are capable of causing catastrophic damage to starship hulls almost immediately. (Star Trek II: The Wrath of Khan, Star Trek VI: The Undiscovered Country, Star Trek Generations)

Continuous or extremely powerful energy discharges can progressively dissipate the integrity of a shield to the point of failure. Shield capacities vary according to many variables, from the power available to environmental concerns (such as nebulae), making definitive and universal calculations of how much damage they can take difficult to estimate (there is no way to know exactly how many phaser hits will cause a failure, for instance). Therefore, during combat, tactical officers continually report on shield strength. Usually, the officer reports shield strength as a percentage of total effectiveness, with 100% meaning that the shields are at full capacity, and lower percentage scores indicating weaker shield conditions. Often, specific sections of the shield grid (e.g. aft or starboard) will take more damage than other sections, so tactical officers will report on the health of any section that needs reinforcement with additional power reserves. Shields are said to be "holding" if damage is not sufficient enough to allow a compromise, while if the shields are "buckling" or "failing," then a total loss of shield protection is imminent. (Star Trek VI: The Undiscovered Country)

Shields operate within a range of shield frequencies to allow certain specific types of energy and matter to pass through, or to make them more effective at blocking them. The frequencies of shields are not usually discernible without examining the controls on board the ship deploying them, meaning that it is very difficult to tune weapons to the exact frequency of an opponent's shields to bypass them. (DS9: "The Jem'Hadar"; Star Trek Generations)

In combat situations, starships match their own shield and weapon frequencies so their shielding does not interfere with their own weapons. Some weapons technologies, including those commonly used by the Borg, have rapidly adjustable frequencies, meaning that they will more rapidly penetrate shields with static frequencies. An effective counter is to repeatedly and randomly alter the shield nutations to minimize the effectiveness of the weapon's retuned frequencies. (Star Trek Generations; TNG: "The Best of Both Worlds")

It takes time to activate a deflector shield. A refit Constitution Class starship needs exactly 13.5 seconds to lower and raise its shields when taking a shuttlecraft onboard via its tractor beam, though this includes the time required to tractor in the shuttle during an automated docking; flying the shuttle in manually reduces this time significantly. (Star Trek V: The Final Frontier)

Shields on a Nova Class starship can be fully recharged after charging the shield emitters for 45 seconds. This task requires the shields to be dropped. (VOY: "Equinox")

Normally Transporters are not capable of penetrating shields. (TOS: "Arena") Older Federation starships, such as the Constitution-class USS Enterprise, could not even transport through their own shields, but later starships such as the Intrepid Class and Sovereign Class vessels could transport personnel and objects freely to and fro without having to lower their shields.

There are at least two different shield configurations commonly used by starships. One type projects an ellipsoid shield bubble around the ship in addition to a relatively large region directly adjacent to the ship. In the other shield configuration, a contour-conforming shield layer is projected a few meters outside the main hull.

The ellipsoid shield configuration isn't shown until the 24th century. Though shield harmonics can be adjusted to change the shape of shields of the 24th century, it is unclear if ships could produce both shield configurations.

Certain starships have a shield system advanced enough to allow protection of only specific areas of the ship while leaving other areas unprotected. This is useful in times when power reserves are low. Lowering portions of the shields also allows usage of transporters without completely sacrificing the protection that shields provide.

This was also useful to Captain Janeway on the USS Voyager when she wished to extract information from a renegade crewman concerning the whereabouts of Captain Ransom in 2375. In this instance, she lowered the shields around a cargo bay, with the crewman inside, to allow the nucleogenic lifeforms to attack him.

Shields may be "extended" to encompass another vessel, which is often accomplished by matching the ships' shield emitter frequencies. (TOS: "Mudd's Women"; TNG: "The Defector"; VOY: "Equinox, Part II"). While the extended shielding can provide some protection in the event of a shield failure on one ship in a convoy, this technique can put a strain on ship systems, and the overall strength of the extended shields is generally not as strong as two independent shield systems.

Shield strengths seem to vary from time to time, making an exact calculation of how much damage they can take impossible. However, there are some examples:

  • The type of deflector shield used by the Lysian Central Command had an output of 4.3 kilojoules and could have easily been taken out with a single photon torpedo. (TNG: "Conundrum")

To give further perspective to the Federation's 24th century encounter with the Lysians, the phase cannons found on the Enterprise NX-01 were rated at 500 gigajoules, suggesting that the 24th century Lysians were significantly behind even 22nd century Starfleet technology. (ENT: "Silent Enemy")

Shield types

Covariant Shield

A covariant shield is a type of deflector shield based on tetryons, used on the Norcadian starship where Tsunkatse matches are held. (VOY: "Tsunkatse")

Defense Field

Defense fields were a type of shielding that was employed by late 23rd century Federation starships. They were raised during yellow alerts, and protected the bridge area of the ship. (Star Trek II: The Wrath of Khan) 

Immersive Shielding

Immersion shielding is a deflector shield configuration designed for a fluid environment, such as air or water.

It was first used by Tom Paris, Seven of Nine, Harry Kim, and B'Elanna Torres during the first flight of the Delta Flyer in 2375, into the atmosphere of a gas giant to retrieve a multispatial probe. (VOY: "Extreme Risk")
Later that year, immersion shielding allowed the Delta Flyer to explore the Monean Waters. In conjunction with structural integrity field, the Delta Flyer could survive pressures at depths of 600 kilometers. (VOY: "Thirty Days")

Metaphasic Shield

Metaphasic shielding is a technology invented by Dr. Reyga, a Ferengi scientist, in 2369. It is capable of withstanding the pressure, radiation and energy of a star's corona. The technology was installed on the shuttlecraft Justman, but Reyga was murdered by Takaran scientist Jo'Bril shortly before the shields were proven. Subsequently, Dr. Beverly Crusher successfully demonstrated the shield's effectiveness by piloting the Justman into the corona of the star Veytan. (TNG: "Suspicions")

Lieutenant Commander Geordi La Forge developed the technology further for use on the USS Enterprise-D in 2370. These modifications allowed the Enterprise-D to lure a Rogue Borg ship into a star's corona and destroy it with a solar flare. (TNG: "Descent, Part II")
The Mokra prison facility in the Delta Quadrant was protected by metaphasic shields. (VOY: "Resistance")

Multi-adaptive Shield

Multi-adaptive shielding was developed by Erin and Magnus Hansen for use on the USS Raven (NAR-32450) during their three year mission studying the Borg. It is a form of shielding designed to hide a vessel from Borg sensors.

In 2375, the technology was incorporated into the systems of the Delta Flyer when the shuttle was used in a mission to rescue Seven of Nine after her capture by the Borg. However, as the Borg had assimilated Magnus Hansen, they were able to adapt and still detect the Delta Flyer. However, upon engagement with the Borg, the Delta Flyer was able to keep changing the shielding before the Borg could adapt, until the Borg managed to grab them with a blind tractor beam shot. (VOY: "Dark Frontier")

Multi-spatial Force Field

A multi-spatial force field is a type of advanced force field.

When The Doctor's mobile emitter and DNA from Ensign Mulcahey was assimilated by Seven's nanoprobes in 2375 following a transporter accident, One, a drone possessing 29th century technology was the result. Among the many advanced capabilities One possessed was a personal multi-spatial force field.
During a Borg attack on USS Voyager, One boarded the attacking Borg sphere, interfaced with the ship's navigation controls and steered the ship into a nearby stellar phenomena, destroying the ship. One escaped destruction himself aided by his personal multi-spatial force field. (VOY: "Drone")

Multiphasic Shield

Multiphasic shielding is a type of deflector shield, capable of withstanding close proximity to a star, much like metaphasic shielding.

Alice was equipped with multiphasic shielding. (VOY: "Alice")
DaiMon Nunk fantasized about buying a ship with multiphasic shielding and a gold-pressed latinum hull after selling Seven of Nine's nanoprobes. (VOY: "Inside Man")
The Omega Directive mandates multiphasic shielding be installed around the warp cores of any responding ships. (VOY: "The Omega Directive")

Paratrinic Shield

Paratrinic shielding is a type of advanced deflector shield technology. The shield creates a warpign field of space time which effectively transports any energy or matter with an atomic mass away from the field at a random angle from it's approach. This type of shiled is extremely power-hungry and requires a great deal of power to function. For this reason it can not be installed on a starship.

However in 2408, the crew of the ISS Rosenante stole a protoype Paratrinic Shield from a Bynar research facility and using a stabilized Omega Molecule to power it, used it to protect the Omega Device. The crew of the USS Furious managed to bypass the shields computer system and shut down the shield, allowing them to destroy the device before it could be detonated.

The Druoda series 5 long-range tactical armor unit was equipped with paratrinic shielding. (VOY: "Warhead")

Regenerative Shield

Regenerative shields were an advanced shield system used by the Borg and other species.

The basic principle of regenerative shielding was to quickly analyze an attacking weapon's frequency and then modulate shield freqency to increase damage mitigation without depleting shield energy.
Regenerative shields were used on Borg vessels and Borg drones. When in combat, Borg drones could be killed by phasers but only for the first several shots. The phaser became ineffective if used at the same setting after multiple shots. The phaser's effectiveness could be extended though by adding a frequency modulation chip. (TNG: "Q Who", "The Best of Both Worlds", "The Best of Both Worlds, Part II") However, Borg drone shields appeared to be less resilient to projectile weapons; when the Borg attempted to assimilate the USS Enterprise-E in 2063, Jean-Luc Picard lured two drones into the holodeck and used a holographic Thompson submachine gun to kill the two drones. (Star Trek: First Contact)
Starships from other races equipped with this type of shielding included sensors integrated within the shield systems' own energy that alters the frequency to better protect the ship.
The battle systems of Starfleet's Prometheus-class starships included regenerative shielding, ablative armor, and multi-vector assault mode. (VOY: "Message in a Bottle")
The warships of the Numiri were protected by regenerative shields. (VOY: "Ex Post Facto")

Temporal Shield

Temporal shields are a theoretical technology that protects against weapons utilizing aspects of temporal science. Temporal shields also protect a vessel against changes in the timeline.

In an alternate timeline, temporal shielding was designed by the crew of the USS Voyager in 2374. In this timeline, Voyager came under attack by the Krenim, whose chroniton torpedoes were able to pass right through the starship's shields. It was discovered that these torpedoes were in a state of temporal flux, and thus their shields were useless.
Lieutenant Commander Tuvok and Seven of Nine worked on a way to modify the shields, eventually succeeding after they determined the temporal variance of the torpedoes by scanning an undetonated warhead lodged in a Jefferies tube. They changed the shield frequency to match it, and set the deflector to the inverse of the pulse. These modifications were proven effective in a later Krenim attack.
However, Voyager's temporal shields had the additional effect of producing a level-9 temporal disruption, throwing off the timeline calculations being made aboard the Krenim weapon ship nearby. As a result, Annorax accidentally reverted the Krenim Imperium to a pre-warp state. He subsequently attempted to erase Voyager from history, but Voyager escaped.
Voyager later provided temporal shield technology to the Nihydron and the Mawasi, their allies in a joint attack on the weapon ship. Although their temporal shields protected them against changes in the timeline, they proved ineffective against a direct hit by the temporal weapon and two Nihydron warships were erased. The temporal shields around the fleet were ultimately taken offline, so that when the temporal weapon ship erased itself from history as a result of Voyager's kamikaze run, everything would be restored to its original state. (VOY: "Year of Hell", "Year of Hell, Part II")

Unimatrix Shield

The unimatrix shield is a type of shielding developed by Lieutenant Tuvok of the USS Voyager in 2375, capable of withstanding extremes of temperature and pressure. It was first tested on the multispatial probe, and later implemented on the Delta Flyer. (VOY: "Extreme Risk")