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Star Trek: Federation Shields

Written: 1998-08-01
Last Revised: 2003-04-21

A Borg cutting-laser is dispersed by the Enterprise-E's shielding system
The Enterprise-E disperses a laser blast from a Borg warship. The spherical shield "bubble" is clearly visible.

The Enterprise-D fires phasers on a Klingon Bird of Prey
A Klingon BOP's shields disperse a phaser blast from the Enterprise-D.

Background Information

In any case, we know numerous things about Federation shield strength, mostly from the Shield-related entries in the Canon Database:

  1. Star Trek shields become luminescent when struck with weapons fire, consistent with an absorption, conduction, and subsequent retransmission mechanism (see characteristic images above).

  2. A shielded GCS cannot withstand a physical impact sufficient to accelerate its entire mass to a few kilometres per second1.

  3. Contrary to popular belief, electromagnetic weapons can be quite effective against Federation shields2.

  4. Electromagnetic disturbances such as X-ray bursts and fluctuating planetary magnetic fields can affect internal systems despite full shielding3.

  5. They are reluctant to approach a neutron star to less than 10 million km range4.

  6. A weapon carrying "four hundred gigawatts of particle energy" can knock down GCS shields, which must be "re-assembled" after being knocked down5.

  7. It seems to require more energy to briefly drop and then raise shields than it does to simply keep them up6.

  8. They can alter their shield geometry, extending the bubble at least 5 km ahead of the ship, at the expense of greatly reduced strength7.

  9. Small physical impactors will actually bounce off a Federation shield8.

  10. Heat-dissipation rates are the critical specification for shield strength9, consistent with the TM, and the Enterprise-D's heat dissipation rate is roughly twice that of the Enterprise-C.

  11. More important colonies and outposts (such as Khitomer) may have theatre shielding of sufficient strength to delay a Romulan attack long enough for reinforcements to be contacted10.

  12. There is a pulsar in Federation space whose radiation will knock down GCS shields after 18 minutes at 20 million km range11.

  13. Shields can interfere with sensors12.

  14. A terawatt-range "energy emitter" (albeit using an unspecified type of energy) can overload GCS shields in less than a minute13.

  15. Solar photosphere gas (plasma) appears to easily penetrate shields, even if its energy content is below the terawatt-range14.

  16. A fully-shielded GCS is noticeably affected by mere 2.1MJ disruptors15, with bridge quaking which indicates a brief disruption of propulsion or inertial control systems (that simply isn't enough energy to physically rock the ship).

  17. It is possible to circumvent Federation shields without having to disable them. Examples have included the Borg16 and a low-powered "nucleonic beam"17.

  18. Federation shield power requirements are in the gigawatt range as of late TOS18.

  19. Federation shields can withstand several hours of close-range solar radiation bombardment in the low terawatt range19.

  20. Shields are inoperative in the McAllister Nebula, as proven by the fact that nebula gases were contacting the Cardassian ships' hulls and corroding them, not to mention the fact that Riker was able to affix mines directly to their hulls20.

  21. Portable ground-based deflector units are inadequate to stop a natural firestorm21.

  22. Specially designed and optimized "metaphasic shields" exist which will allow a shuttle to survive for several minutes inside a star's photosphere22, even though a normal shield can survive in the star's corona for hours, where the bombardment is not much different (certainly within an order of magnitude, since it is defined by the inverse-square law). This reinforces the conclusion that there is some particular vulnerability to solar plasma.

  23. The shields on a GCS cannot withstand the windstorms inside a gas giant's atmosphere23.

  24. A "phase resonant pulse" at the "right frequency" will completely bypass GCS shields24.

  25. Telepathy and telekinesis are not blocked by GCS shields25.

  26. Certain types of particles (eg- "verteron particles") will bypass shields26.

  27. GCS shields cannot withstand low-velocity rock impacts27.

  28. GCS shields cannot be made strong enough to permit safe passage through a low-density asteroid field28 (never mind an extremely violent high-density field like the one at Hoth).

  29. It is possible that GCS shields may be "destabilized" by the ship's own impulse "thrust", thus creating a weak point29.

  30. The ambient gases of the Mutara nebula can totally disable the shields of a TOS-era Federation starship30.

  31. A "low-level ionic pulse" can penetrate a Bird of Prey's shields to directly affect its warp coils, in order to take advantage of a warp coil design flaw31.

  32. GCS shields cannot deflect the heat of atmospheric re-entry, and the hull will heat up dramatically even before shields fail32.

From this data, we can deduce that GCS shields can handle energy bombardment in the terawatt range, and direct hits from torpedoes whose yield is in the kiloton range, or perhaps low megaton range. They allow some fraction of energy to pass through even when they are at full strength, they block both matter and energy simultaneously via the same system, they are frequency and phase coherent, and there are certain natural and artificial phenomena which they cannot block at all.

Analyses of Key Incidents


In "Relics", the Enterprise-D was pulled into a close orbit (150,000km) around a star enclosed in a Dyson sphere. This star was a G-class star which was most likely less than 45% of Sol's luminosity because the Dyson sphere was 2E11m in diameter (66% of Earth's orbital diameter) yet green vegetation and bodies of water could be clearly seen on its inner surface. If the star had been as luminous as Earth's sun, the bodies of water would have quickly boiled away and the vegetation would have been destroyed. The star is said to be releasing a lot of prominences, but we never see the ship actually get engulfed in one of these prominences so they aren't really relevant. At a 150,000km distance from that star's surface (perhaps 700,000km from its centre), the power intensity would be roughly 25-30 MW/m². This is based on the inverse-square law; divide the total luminosity by the surface area of a sphere of 700,000km radius to get intensity.

In order to determine the shield absorption rate at that intensity, we must multiply the intensity by rate of bombardment at that intensity, This figure is derived from calculating the ship's profile area. The stellar radiation which hits an object will be based on the distance from the star, the luminosity of the star, and the ship's profile area. Imagine taking a silhouette of the ship; the area of this silhouette is what the radiation from the star "sees". Now, a GCS is roughly 610m long, and 130m high when viewed from the side. If we assume that it is a rectangle, then its projected surface area is 78,000 m² (note that this is a very generous estimate- the ship is obviously not a solid rectangle when viewed from the side). Some would argue that the entire area of an ellipsoid "bubble" should be used instead of the profile of the ship, thus bumping up the estimate to 100,000 m², and while it is easy to get drawn into a debate over such minutae, it is ultimately a waste of time (it is an obvious nitpick to wrangle over a difference of only 30% in calculations based on low-accuracy source data in the first place). However, the 1E5 m² estimate does have the advantage that it simplifies the mathematics.

In any case, if we simply multiply the power intensity of 25 to 30 MW/m² by 1E5 m², we get 2.5 to 3.0 TW (let's say 3 TW for simplicity's sake). They said they could withstand roughly 3 hours of bombardment at this rate, and their shields were said to be down to 23%. Quarter-strength shields could mean one of two things:

1) a full-strength shield could handle 3 TW for 12 hours.
2) a full-strength shield could handle 12 TW for 3 hours.

Either way, it would appear that roughly 30 megatons of stellar radiation should overwhelm a Federation starship's shields over an extended period of time. A smaller amount would probably suffice if applied all at once.

Photon torpedo impacts

A single photon torpedo hit will generally fail to defeat a full-strength GCS shield, and at a minimum, a volley is required. Therefore, it stands to reason that GCS shields must be capable of handling the energy released by numerous photon torpedoes before failure.

Of course, this method of determining shield strength requires an accurate estimate of photon torpedo yield. The TM gives figures of 1.5kg antimatter charge, which works out to roughly 64 megatons, half of which would hit the shield (with the rest being radiated harmlessly out into space; there is no "shaped charge" effect, as demonstrated by their oft-stated fear that the blast from a torpedo detonation against an enemy vessel at close range will damage or destroy their own ship). However, not only is this figure reliant upon the TM (a book characterized as mere "speculation" by its own publisher), but it also assumes 100% efficiency, which is simply not reasonable; the violent release of energy in a matter/antimatter explosion would accelerate much of the reactant outwards so quickly that it would not react (if the particles are moving too quickly, they will pass by one another without reacting and shoot harmlessly into space). For this reason, the 32 megaton useful energy yield should be treated as an upper limit, not as a reliable estimate and certainly not as a lower limit. For all we know, the overall reaction efficiency is less than 10%, and it should also be noted that certain reaction products such as neutrinos are harmless.

Mind you, since the TM is characterized as mere "speculation" by its own publisher and there is no such status as "quasi-canon" in the ST universe, a canon figure would be more useful and reliable. Most canon photon torpedo incidents are based on hits against other ships, active systems or installations, etc. rather than inert objects. Sometimes the claims are based on semantics that are directly refuted by observation (see "The Die is Cast"). However, we do have one incident of damage limitations being clearly stated for photon torpedoes against an inert object, and that is from "Pegasus". In that episode, Riker stated that it would take virtually their entire payload of 275 photon torpedoes33 to destroy an asteroid which scales to roughly 5km width (see the Canon Database entry). It would take between 25 and 600 megatons to pulverize this asteroid (hard granite vs nickel-iron), disregarding the fact that it is hollow. This means that maximum photon torpedo yield would be somewhere between 0.1 megatons and 3 megatons.

If we combine these figures with the shield strength figures from "Relics", we find that it should take 10 or more photon torpedoes to bring down a GCS's shields, which seems reasonably consistent with what we see on the show.

Descent Part 2

In this incident, the Enterprise-D moved deep into the corona of a star using special metaphasic shields, and its chief engineer estimated that its shields would fail within 5 minutes under this bombardment. Based on the presence of a habitable planet in that star system as well as the colour spectrum of the star, we can conclude that its star was very similar to Earth's star in terms of general luminosity. We can estimate corona power intensity to be roughly 60 MW/m², since that is the approximate power intensity at the surface of our Sun (note that the corona is outside the star). If we use a 100,000 m² profile area estimate, total absorption is roughly 6 TW, which would add up to roughly 420 kilotons (let's just say ½-megaton).

The problem with this incident is that the same vessel has entered solar coronae in other episodes34 without using metaphasic shields, and it has demonstrated superior strength in these incidents as well, supposedly being able to stay there for hours in one case. Since multi-phasic shields were clearly portrayed as a strength improvement rather than a strength reduction, this is an obvious inconsistency. The most obvious explanation is that shields have some special vulnerability to plasma (as demonstrated in numerous nebulae, atmospheric re-entry incidents, etc), and they were in the densest part of the corona in this case, but not in the other cases. However, several other explanations have been floated by Trekkies:

1) They were actually in the photosphere, not the corona, since misuse of terminology is common in Federation starships. However, the ship was clearly visible as it moved into the corona, and it was nowhere near the photosphere. Furthermore, the Enterprise-D used a particle-beam to induce a solar prominence by aiming it downwards to hit the photosphere. This obviously indicates that the ship was well above the photosphere. Therefore, this explanation is unworkable.

2) The hot corona gases are extremely dangerous. However, while corona gases range up to a million K, they are extremely diffuse- from 1E10 to 1E14 ions/m³. This means that the average translational kinetic energy of individual ions would be less than 2E-17 J, so the thermal energy of a cubic metre of corona gas is less than 0.002 J (it's very hot, but there is very little of it per cubic metre). Therefore, a GCS would absorb less than 60 kJ for each km of forward movement, generously assuming 100% thermal energy transfer and assuming that it is moving through the densest, hottest part of the corona. Even if the ship is travelling at 0.9c, it would be absorbing thermal energy at a rate of only 16GW. Clearly, this explanation is also unworkable.

3) The star was not a main-sequence star, and was a highly anomalous, exceptionally luminous star. However, the star did not have an unusual colour, and there were naturally habitable planets in its system. Again, this theory is unworkable.

Ultimately, the simplest explanation remains that Star Trek shields do not handle gas contact very well. This was made even more clear by subsequent events, in which the Enterprise induced a solar prominence to destroy the Borg vessel which had chased them into the corona in the first place (note that it was sitting outside). The largest solar prominences are known as "coronal mass ejections", or CME's, and in such a prominence, the mass and velocity of ejecta are typically around 1E12 kg and 400 km/s. Anomalously large CME's have involved masses and speeds of as much as 1E13 kg and 2000 km/s in the past. However, the prominence in "Descent Part 2" was nowhere near this size- it was barely wide enough to engulf the Borg vessel which was, in turn, no wider than 5km. However, a large CME is more than 250,000km wide. If we assume that the Borg cube was hit with a large CME in spite of the 40 billion to 1 size discrepancy, then we can use the ~1E25 joule energy content of the largest observed solar CME's to estimate that roughly 1 forty billionth of such a vast CME would actually strike the Borg ship. In other words, the Borg ship was hit with a burst of energy equal to, at most, 4000 TJ.

This is much lower than the known capabilities of Federation vessels to resist EM radiation, and since Borg vessels are generally superior to Federation vessels (this one had recently crippled the Enterprise-D in battle), it is reasonable to assume that they should be able to handle much more than this. The Borg ship's immediate obliteration due to a <4000TJ plasma burst indicates that plasma is highly dangerous to Borg vessels as well as Federation vessels (note: flare information can be verified on NASA's solar flare page).

Based on these incidents and the general background information, we can conclude that a GCS can withstand as much as 30 megatons of radiation bombardment, with much lower figures for plasma and near-uselessness against some types of natural and artificial phenomena such as magnetic fields, X-ray bursts, certain nebula gases, and exotic imaginary particles. Certain incidents such as "Survivors" indicate a much lower threshold, but perhaps that can be attributed to some of those exotic particles.

Shield Mechanism Issues

Frequency and phase

See the related discussion on the Tech Myths page for details. To summarize briefly, it would appear that because the Federation can't open small holes in its own shields (hence the fact that they can't beam in or out with their shields up), they must use a shield system which cycles on and off at a preset frequency, so they can time their own weapons to shoot through with minimal losses in a manner analogous to the way machine guns on early fighter aircraft were synchronized to the propeller blade.

If they could synchronize their shield to an incoming phase-coherent weapon such as a laser or phaser, they would be able to achieve maximum efficiency. Conversely, if the enemy synchronizes but with a phase angle of 180 degrees, the shield would be virtually useless (please review trigonometry if you don't understand these terms; while I don't mind explaining some things, everyone should already know high-school trigonometry).

Against "mundane" radiation, such as light emitted from a star or heat flux from physical contact with hot gases, a system like this has the unfortunate characteristic of allowing a large fraction of incoming energy to pass through (the exact fraction would depend on the shape of the waveform). This would also be true of any weapon which does not produce frequency and phase coherent energy, such as a photon torpedo explosion. Since this is precisely what we see (ST6 was an excellent example, with the ship clearly being blackened and battered even though its shields were still up), the theory of an oscillating shield is consistent with observation.

Moreover, this would also explain why multi-phasic shields are particularly useful against stars but not used in battle, since a multi-phased shield would presumably overlay numerous offset waveforms on top of each other to produce a relatively flat overall shape. This would reduce leakage from incoherent energy, but it would also severely hamper their ability to fire their own weapons.

And finally, this theory would explain why the ship is visible, even though its shields must protect against full-spectrum radiation to be useful near a star. The human eye has trouble perceiving a flicker when the frequency is high enough, which is why you can be fooled into seeing 24 separate still pictures per second as a smooth movie. So if the shield blocks visible light frequencies but flickers on and off at, say, 1 million times per second, you would still be able to see the ship inside. It would look a little dark, but we can still suspend disbelief by assuming that the picture is artificially brightened for ease of viewing.


It is widely believed that shields are a gravitic phenomenon. However, the behaviour of gravity is highly inconsistent with tbe behaviour of shields for numerous reasons, starting with the fact that certain types of exotic particle can effortlessly pass through them (not to mention other weaknesses). The gravitic theory is popular because it is found in the TM, but even if the TM enjoyed quasi-canon status (which it doesn't), its explanations would still be subordinate to the behaviour of shields on the canon shows.

Another popular misconception is that shields produce a "cancellation effect", similar to the "quiet zones" produced by active noise-cancellation headsets. This idea is no doubt popular because it is related to acoustic frequencies, and we know that the word "frequency" is often heard on the show with respect to shields. However, the cancellation idea suffers from numerous problems, not least of which is the fact that cancellation is a localized effect, not a global one. If you "cancelled" an incoming energy beam, it would disappear at the edge of the cancellation zone and then reappear at the far side. Not only do we not see this, but we clearly see an absorption and retransmission effect.

Wear and tear

So what makes a shield "wear down" after repeated hits? This is a tougher question than you might think; people who propose a forcefield explanation cannot explain why a shield would lose, say, 30% of its strength after a few hits. A forcefield is either up or down, right? Why would each hit take some strength away from the shield? If it's getting through and causing damage to surface-mounted shield emitters as some might say, then shouldn't the effect of a hit be more random? And why would Worf be able to slowly "re-assemble" the shields from his control panel afterwards? If the root cause of the shield failure was physical damage to the shield emitter system, then how could he possibly get the shields back up without sending repair teams?

Of course, there is also the "wall of energy" idea, but that's a non-starter for several reasons (see the "Nature of Shields" page for more). One remaining theory is a cloud of material (perhaps an exotic form of matter) which is manipulated by some kind of containment forcefield. Such a system would obviously lose strength after repeated hits because bits of the matter would be thrown off with each impact. It would be interactive with both matter and energy, it would explain why it takes so long to rebuild a shield once it's down, and it would explain the fact that shields work poorly when in direct contact with certain gases (some gases may react with the material in the shield and remove the characteristics which allow the shield generator to contain it, so it gets carried away easily). Similarly, certain particles may not be very interactive with the material in the shield, so they might pass through, thus explaining the various "mystery penetrations" that we've seen.

Of course, every theory has a weakness, and this theory's weakness is the fact that it would not have a coherent frequency. However, even that may not be fatal; it is possible that, just as a magnetic field can be used to "flex" a group of atoms by forcing them to align themselves in certain ways, the shield generator may be able to manipulate this cloud of matter in such a way that a polarity reversal makes it more or less of an effective barrier to matter and energy, so it stiffens and softens cyclically.


Federation shield technology is clearly in the low megaton range for resistance. The evidence for this is simply overwhelming, regardless of whether you employ the TM or not, and since Imperial shields are rated at a much higher level, there is really no contest at all. The high-megaton and gigaton-class missile weapons of a patrol craft like Slave-1 could effortlessly destroy any Federation warship with a direct hit, to say nothing of the much heavier weapons on an Imperial capital ship.

As for mechanism, that is somewhat more interesting. Theories abound, often with massive flaws and contradictions with observation, but for many reasons, my preferred theory is that of a cloud of some sort of exotic matter in a containment forcefield.


1"The Naked Now"

2"The Last Outpost"




6"Booby Trap"

7"The Defector"

8"The Hunted"

9"Yesterday's Enterprise"

10"Sins of the Father"


12"The Wounded"

13"The Nth Degree"

14"Redemption Part 2", "Descent Part 2"


16"Q who"

17"The Inner Light"



20"Chain of Command Part 2"




24"Gambit Part 1"

25"Gambit Part 2"

26"Force of Nature"



29"Pre-emptive strike"

30Star Trek 2: The Wrath of Khan

31Star Trek: Generations

32"Arsenal of Freedom"


34"Relics" and "I, Borg"

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