Shields: Physical Impact Examples

Written: 2002-05-04
Last updated: 2003-04-12

Example #1: Hoth asteroid impact

Now that you've seen the pretty pictures, let's crunch some numbers. Let's suppose a 70 metre wide asteroid strikes a ship while moving at roughly 1 km/s (as all long-time Star Wars vs Star Trek people know, this is similar to the asteroid which hit an ISD bridge tower in TESB, as seen in this Divx5 video clip). This is considered a benchmark for an ISD's resistance to physical impacts, albeit somewhat over-conservative. Remember that all the ships had already sustained damage beforehand (from prior asteroid impacts, and perhaps also from the Battle of Hoth and a near-collision with two other Star Destroyers), the shields may have been down to permit the holo-transmission, and neither the bridge tower or its shield generators would be as robust as the main hull and its defense systems. As a slightly off-topic exercise, ask yourself what this incident tells us about the structural strength of the bridge tower (hint: does the asteroid pulverize against the tower's surface, penetrate deeply inside, or fly right through and out the other side?)

In any case, given nickel-iron composition and roughly 7000 kg/m³ density, this asteroid would have roughly 1.25 million tons mass, therefore its momentum would be 1.25E12 kg·m/s and its kinetic energy would be 6.25E14 J (roughly 150 kilotons). We might leap to the conclusion that a Star Destroyer's shields must be limited to 150 kilotons for any weapon regardless of whether it possesses mass or not (assuming its bridge shields were, in fact, up at the time despite the holo-transmission which normally requires shields to be lowered), but this conclusion is oversimplistic and wrong. Knowing what we know about collision physics, we know that the shields must apply enough reaction force to reduce the asteroid's velocity to zero before impact. From another scene in ROTJ where a stricken fighter explodes against an ISD bridge tower's shields, they appear to be less than 10 metres away from the hull. This would give them less than 0.02 seconds to stop the asteroid, and the reaction force would be at least 6.25E13 N (note that we are ignoring the fact that no shield interaction was visible in the asteroid impact, so we are humouring the common Trekkie belief that the shields were up). This defines the physical stress applied to the shield generator's mounts, and stress causes structural failure.

Now, let us consider an equivalent turbolaser blast (again, speaking from a structural perspective). We know that turbolaser bolts do not arc measurably downward in gravity even over distances where it should be obvious (eg- the ground battle at Hoth, the space battles in low orbit over Endor and Tatooine), so they appear to be massless (and the SW2ICS provides official confirmation of this interpretation). Moreover, we know (from the TESB asteroid vapourization scene) that a long turbolaser bolt takes roughly 2 frames at 24 fps (0.08 seconds) to impart its energy to the target. Since the time duration is 0.08 seconds instead of 0.02 seconds, it needs 4 times as much momentum as the asteroid (ie- 5E12 kg·m/s) in order to subject the bridge tower to the same stresses. Therefore, since the momentum of a massless particle is U/c, this means that its energy yield would have to be 1.5E21 J, or 350 gigatons. In short, you would need to hit an already-damaged ISD's bridge tower with a 350 gigaton laser or turbolaser blast in order to cause the same physical impact damage we saw in TESB (assuming its shields are up, otherwise the sheer energy transfer would overwhelm the ship's armour regardless of the physical impact).

Example #2: Jem'Hadar Ramming Attacks

The USS Odyssey just after impactIn the DS9 episode "The Jem'Hadar", the USS Odyssey was destroyed after an exchange of fire which lasted for less than 30 seconds, as seen in this Divx5 video clip. As it turned to flee, a Jem'Hadar fighter (which can be generously approximated as a 100 metre diameter, 25 metre high saucer) with a mass of perhaps 10,000 tons deliberately rammed the Odyssey at a velocity of roughly 600 m/s (it took 5 frames at 30 fps to cover its own length onscreen as it entered the frame). If we generously assume that it accelerated to 1 km/s by the time of impact, its kinetic energy would have been roughly 5E12 J (1.2 kilotons), and its momentum would have been roughly 1E10 kg·m/s (less than 1% of the TESB asteroid's momentum).

The Odyssey was so heavily damaged by the impact that the entire deflector array was destroyed and the plunging fighter smashed its way into the primary hull, thus destroying the entire forward area and leaving multiple decks exposed to space (it goes without saying that a warp core breach followed almost immediately afterwards). The duration of the impact was roughly 2 frames at 30 fps (0.07 seconds), so the reaction force would have been roughly 1.4E11 N (less than 1/400 of the TESB asteroid's impact force). Of course, the reflexive Trekkie response to this incident will be to find an excuse to dismiss it, so they might argue that the ship had suffered prior damage, so all onboard systems should be presumed non-functional (funny how they dismiss that possibility for the TESB asteroid, eh?). However, that would be ignoring the real point, which is that this incident conclusively demonstrated that the ship's physical structure cannot withstand that much force, and since its physical structure ultimately must absorb an impact regardless of whether it comes to its shields or its hull, this gives us an idea of the ship's general resistance to impact (a point hammered home by lethal 1 km/s ramming attacks against fully shielded Klingon cruisers in "Tears of the Prophets" and "All That You Leave Behind").

A Klingon cruiser after being rammed by a Jem'Hadar fighterAnd now we come to the really interesting part: numerous observers have claimed that a Federation ship's navigational deflectors can ward off any laser of any power level because Riker once dismissed lasers as useless (yes, they actually think the Enterprise could shrug off the Death Star's planet-destroying superlaser). Leaving aside the inherent stupidity of this idea, we can use the destruction of the USS Odyssey to show that even if it is true, the ship would still be destroyed by a 700 megaton laser blast! Remember: a structural failure is a "weakest link" situation, and even if we accept the silly notion that the navigational deflector's forcefield is infinitely resistant to certain threats, its physical structure is most decidedly not. Therefore, the ship will still be destroyed by a sufficiently large impact, since the entire navigational deflector array would be either ripped off its moorings or crushed against the rest of the ship by the impact force, whereupon it would promptly explode and take the rest of the ship with it.

In this case, a 700 megaton laser burst would have 1E10 kg·m/s of momentum, and if it is applied over 0.08 seconds, the reaction force would be 1.25E11 N (sound familiar?). In other words, a 700 megaton laser burst at that point would have destroyed the Odyssey as easily as the Jem'Hadar fighter's ramming attack did, even if its navigational deflector completely blocked the shot! Of course, this is by no means the most damning physical impact incident (the nacelle tap in "Cause and Effect" or their stated inability to survive a zero-gravity asteroid cave-in upon their fully shielded vessel in "Pegasus" share dubious honour), but it gives you an idea of how to approach such incidents.


To summarize the physical impact incidents above:


Impactor Momentum

Impactor KE


TESB Asteroid

1.25E12 kg·m/s

6.25E14 J

Pulverized against surface. The impact damaged the bridge tower, although the ship's commander was still standing for at least 2 seconds before apparently being injured by secondary explosions resulting from the damage (notice bright flashes lighting his holo-transmission before he recoils from something and the transmission is cut off).

Jem'Hadar fighter against USS Odyssey

1E10 kg·m/s

5E12 J

Destroyed on impact (the Odyssey's navigational deflector may have exploded, thus inadvertently acting as reactive armour and keeping the fighter from shredding its way through the primary hull). Heavy damage to the Odyssey's primary hull caused a warp core breach in seconds, totally destroying the vessel.

Jem'Hadar fighter against Klingon cruisers

1E10 kg·m/s

5E12 J

Tore through the Klingon cruiser's hull, ripping it in half. The fighter was invariably damaged so heavily that it could be considered destroyed. The Klingon cruisers' shields and hull appeared incapable of stopping or even significantly slowing the attacking vessels.

Of course, all of the above examples lead to an obvious question: if the ships cannot withstand these impacts, then how can they withstand the force of their own sublight engines, which can accelerate them at many kilometres per second? This seems like a serious contradiction upon first glance (although it is mitigated by "mass-lightening fields" in the case of Star Trek), but as with any engineered structure, the ships were obviously designed to withstand the particular magnitudes, directions, and types of stresses imposed by their own accelerative force. Design for this load would have involved the placement, shape, and size of structural members, as well as (possibly) the inclusion of active force-field systems designed to spread the load more evenly throughout the ship's hull. However, it does not necessarily follow that they could therefore withstand similar forces on any arbitrary system in the ship, coming from any arbitrary direction. After all, in real engineered structures, mechanical anisotropy is unheard of, except in nearly solid objects.

In most cases, a starship's engines are an integral part of its spaceframe. In order to move the engines relative to the rest of the hull, one would have to literally twist, bend, or break the entire hull. However, other systems such as a shield generator or a bridge tower are not necessarily so integral or heavily reinforced. The bridge tower of a modern aircraft carrier is very lightly built compared to its main hull, and it could theoretically be ripped away without compromising the integrity of the ship as a whole (note: Stuart Slade writes that "There's no 'theoretically' about it. The island structure is actually designed to do just that in case of dire emergency (the usually quoted case is a nuclear blast where a shockwave would remove the island but leave the hull)". An ISD is presumably similar, with a very thickly armoured and reinforced main hull upon which a relatively flimsy bridge tower sits. Similarly, the navigational deflector of a Federation vessel is obviously a rather delicate system, and it can be easily destroyed by a weak impact. Its shield generators may be little better; if they are moderately sized systems buried in the hull, they could conceivably be ripped from their moorings by a sufficiently large impact. If they are an array of small surface-mounted devices (or "field emitters"), they might be smashed against the hull when subjected to excessive force.

Borg drones

Borg drones are a good example of this kind of "strongest link in the chain" thinking. The Trekkie "Cult of Borg" shrilly insists that Borg drones must be capable of blocking all physical impacts because "they can adapt" (does not follow; adaptation cannot be assumed to be infinitely flexible). They argue that drones must have particle shielding even though we've never seen drones adapt to kicks, punches, elbows, rifle butts, bullets, claws, etc. despite many dozens of fatalities (argument from ignorance; they claim it is true because it has not been proven false). Indeed, a common feature of most Trekkie Borg scenarios is unstoppable drones; some even describe conquering entire planets or even heavily armed, heavily manned space stations such as the Death Star, from nothing but a "seed force" of a few hundred drones, who will quickly "adapt" and become invincible to all weapons.

But of course, they don't realize that even if all of their painfully simple-minded claims were completely true, any drone particle shielding would merely transmit reaction forces to the Borg implants inside the drone's body, which would then be ripped loose, tearing organs, blood vessels, and surrounding tissues apart in the process (ouch). Sorry, but no one is going to conquer any planet (including present-day Earth) by simply landing a small group of Borg drones on the surface. A single artillery shell would smear a Borg landing party just as easily as it would a group of insurance salesmen.

Bizarre Objections

Within 24 hours of posting this article, I received numerous complaints that I was not accounting for the explosive energy released by weapons, warp cores, etc. cooking off. For example, Bothan spies sent me the following excerpt from

BLPhalanx: Starships are not inert objects. That Jem'Hadar vessel carried fuel, weaponry, other reactants, and M/AM reactor that would've gone off in the suicide attack. I mean, Warsies are quick to point this out for the suicidal A-Wing in ROTJ, so... don't try to apply a double-standard.

And the babies wonder why I think of them as ignoramuses ...

While it pains me to explain something which should be obvious to even the most dim-witted simpleton, it is also clear that there are some people in this world who actually need to have such things explained in excruciating detail for them (or who simply look reflexively for anything I don't mention and then hold it up as proof of a major issue which I've supposedly ignored, in order to play sophistry games).

Very well: for those of you who did not understand the dumbed-down physics explanation above (even with the picture!), the energy released after the instant of impact is a red herring. It does not disprove the central point about failure against physical impacts, because it would not be released until after the ship had already suffered structural failure. This extra energy released after failure is the reason why there is so much flame, heat, light, and unrelated damage (the Odyssey and Klingon vessels exploded after these impacts), but it does not explain the failure itself. In fact, I reiterate that it may have been beneficial in the case of the Odyssey, because the nav deflector explosion acted in a manner similar to modern reactive armour, thus halting the Jem'Hadar ship's forward progress (compare this to the Klingon warships, through which the Jem'Hadar ships crashed with little or no impedance to their forward movement). There is no "double standard" here; the ROTJ fighter impact's fuel and active systems are not used to justify its ability to penetrate the Executor's bridge windows; they are only used to explain the level of destruction afterwards.

If the vessels in question had sufficiently strong particle shields and and structures to survive impacts of that magnitude without damage, they would have harmlessly bounced off each other. At the very least, the Jem'Hadar fighter would have exploded against the Klingon shields rather than plunging straight through with no appreciable deceleration, and if the Klingon ship was damaged by the explosion, so be it. But what we saw in those incidents was a complete failure to generate sufficient reaction force to slow down the impactor.

Trekkies will continue to evade the point, of course. Already, I've been told that their new theory revolves around the Jem'Hadar deliberately self-destructing their vessel milliseconds before impact, and using its explosive output to destroy the ship. Yes, they're obviously going to neurotically obsess over the Odyssey and produce silly alternate explanations involving never-before-seen millisecond-timed instant self-destruct sequences, probably with the usual "creative" interpretations of screenshots. Yes, they're obviously going to contradict their usual policy of considering writer's intent above "visual FX", since the script described it as a ramming incident (their policy on proper analysis methods changes depending on what they want to "prove" today). Yes, they're obviously going to quietly ignore the Klingon incidents in which this explanation fails completely because their forward movement was unaffected and they did not explode at the moment of impact (moreover, an explosion would tend to randomize the fighter's velocity, rather than accelerating it through the Klingon ship's structure in its current direction). And yes, they're going to expect us to accept that even though the Odyssey was the only ship to stop a Jem'Hadar fighter's forward progress and coincidentally the only ship to have a volatile (according to Picard in STFC) nav deflector at the point of impact, one has nothing to do with the other.

The Trekkies' predictable red-herring "rebuttal" does not, as usual, even address the root problem, which is that Star Trek ships are mechanically flimsy, and that ultimately, mechanical limits dictate the limits of particle shielding. Hell, anyone who simply looks at Star Trek ships can see how flimsy they are, and that they were obviously designed by people with the mechanical aptitude of a squirrel. The ability of fanatical Trekkies to ignore simple and intuitively obvious concepts such as mechanical constraints, "cause must come before effect" or "you can't obsess over one piece of evidence to the exclusion of others" never ceases to amaze me :)


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