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(This question is related to How did the AT-ATs get to the surface of Hoth?, and was originally posted as an answer there, but it's not the same)

A group of AT-ATs approach Echo Base

In The Empire Strikes Back, the Empire attacks Rebel Echo Base located in Hoth. The Rebels are alerted before the attack, and raise some sort of shield to prevent orbital bombardment of their base. The Empire is then forced to deploy land forces to first disable the shield generator, before destroying the base. Presumably, the Imperial forces are deployed using dropships.

My question is about how the Rebel shield generator works, and how the Imperials manage to bypass it.

I think the shield generator is something of a plot hole. At first I incorrectly thought it was a planetary shield (i.e. a shield around all of Hoth) preventing orbital bombardment, and also preventing any dropships (with their AT-AT payload) from landing, but I think Wookiepedia got it wrong: the script states it is an energy field around just an area of Hoth, presumably Echo Base:

VEERS: My lord, the fleet has moved out of light-speed. Com-Scan has detected an energy field protecting an area around the sixth planet of the Hoth system. The field is strong enough to deflect any bombardment.

So my initial objection ("how did the Imperial dropships bypass the shield?") has been answered: they didn't. They landed outside the energy field to deliver the AT-ATs. But now I have another question: does an AT-AT have to walk inside the field to fire at the generator, or can it fire into the field from outside?

Maybe the field simply cannot stop matter, only energy weapons (therefore preventing orbital bombardment), in which case the AT-ATs would be able to just walk inside. However, the script also states:

LEIA: All troop carriers will assemble at the north entrance. The heavy transport ships will leave as soon as they're loaded. Only two fighter escorts per ship. The energy shield can only be opened for a short time, so you'll have to stay very close to your transports.

So it's clear that the energy field is also capable of stopping matter, in this case troop transports and fighters. It follows that it should also prevent AT-ATs from walking through it. So the only way I can see the walkers disabling the generator is by firing into the energy field!

This would mean the field is strong enough to stop matter such as ships and fighters, and energy weapons used for orbital bombardment (and also the Ion Cannon the Rebels use as an anti-orbit battery: Leia also states they must lower the shield to use it), but not able to stop whatever energy weapons the AT-ATs use!

So how on Earth did Imperial Walkers manage to disable the shield generator?

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Perhaps not canonical, but I swear I remember one of the Star Wars games (perhaps the Gamecube one?) depicted the AT ATs disabling the shield generator. –  Ben Brocka Jan 21 '12 at 15:44
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I don't know about the Star Wars shields but in various universes shields aren't able to stop slow objects from going through them--the blocking is related to the speed of the item being blocked. Thus something that stops a blaster bolt cold merely offers some resistance to a walker. –  Loren Pechtel Jan 21 '12 at 18:34
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@LorenPechtel: you mean like the shields in Dune, right? It makes sense. –  Andres F. Jan 21 '12 at 20:14
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It is conceivable that the shield poses a threat to the ships because of their energy output being disrupted by the field. So while it wouldn't stop the matter, it would probably shut down the ship's computer systems, leaving a useless, broken hunk of metal passing through the other side. –  corsiKa Jan 22 '12 at 0:56
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@AndresF. : also like Goa'uld shields. –  HNL Jan 22 '12 at 15:19

8 Answers 8

up vote 31 down vote accepted

I'll posit an alternative hypothesis to zephyr's.

I recall some mention that the shield did actually stretch all the way to the ground, but it was of a type that didn't impede the AT-ATs because they're relatively slow. Such shields do exist within the SW universe:

The Gungan shields used during the Battle of Naboo blocked lasers, but droids could pass through because of their relatively slow movement in comparison to the laser bolts. Some shields only held back anything with a high-energy output, allowing sentients to pass through unharmed.

The relevant wookiepedia entry doesn't specify which type of shield it is, unless you take "umbrella" to mean not reaching the ground. Possible, but I don't think it's a certainty.

Two reasons this could be the case:

The alliance was fairly limited on tech and scientists. The base took two years to construct, and was invaded about a year thereafter. It could well be that it's much easier (in time and resources) to create an energy shield, and the scientists they brought to create the shield set one of those up first (possibly planning to upgrade it later).

And I'm pretty certain there is a mention of another modern shield of this type somewhere in the EU (but I don't remember where and that's a lot of material to scour), indicating they were still in use well after Hoth.

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In the game Star Wars: Rebellion, you had to either sabotage planetary shields or overload them with a larger bombardment in order to land troops. –  surfasb Jan 21 '12 at 19:30
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The personal shields used during hand-to-hand combat in Dune had the same vulnerability: they stopped fast attacks but a slow knife could slip through. AT-ATs are nothing if not slow. –  Adam Wuerl Jan 21 '12 at 20:36
    
I'm going to accept this answer, even though zephyr's also makes sense. This explanation, while speculative, doesn't rely on anything but tech shown in the movies (the Battle of Naboo). The shields don't look the same, but the tech could be similar. The alternative explanation requires we believe "umbrella-shaped" means the shield doesn't touch the ground. –  Andres F. Jan 21 '12 at 20:54
    
There's an episode of the clone wars where Asoka is teaching some locals how to fight Droidekas. She shows them how to throw a thermal detonator slow enough that it makes it just through the shield, and then blows up inside. The point is that it has to be moving very slowly. –  Scott Whitlock Mar 8 at 17:55

The most obvious answer is that the energy shield does not extend all the way to the ground. This would make sense, since the terrain is mountainous and uneven. Instead it could form an umbrella above the base that prevents shots and ships from passing through, but which the AT-ATs could simply walk under the edge of.

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Supported by wookiepedia: starwars.wikia.com/wiki/Theater_shield –  so12311 Jan 21 '12 at 16:16
    
Damn. Too fast for me. +1 –  DVK Jan 21 '12 at 16:57
    
Excellent answer! Though it does seem like an afterthought from the Extended Universe ;) –  Andres F. Jan 21 '12 at 17:26
    
Yeah, I didn't like how apparently this is a brand new piece of technology recently developed by the rebels - it seems like someone would have thought to put a shield over a military base before. –  so12311 Jan 21 '12 at 18:32
    
Yeah, they probably can't make the shield perfectly conform to the terrain. If the shield can keep out objects you certainly don't want it touching the ground. –  Loren Pechtel Jan 21 '12 at 18:32

Perhaps I'm remembering incorrectly, but I believe the reason they dropped the shield is so that they could then shoot the ion cannon that they had on the surface and disable the Star Destroyer that was in the way. This way the transport and small X-wing complements could just fly past and enter hyperspace without having to deal with a drawn out space battle. I imagined Leia telling the pilots to stay close to the transport was meant in case they missed or something and therefore they needed to make sure they're not flying too far away from the transport because it would need immediate protection.

Leia's reminder is still a little weird in this case (if you're on escort duty, I'm not sure why you would be flying away) but I think it still fits.

In this way, the shield generator was meant to stop energy weapons, which would include the ion cannon. Hence they need to drop the shield in order to fire the cannon. Matter could bypass it.

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Your answer also makes sense. But then, couldn't the Imperials simply drop kinetic/explosive weapons on top of the Rebel scum? You know, like... good old-fashioned bombs :P If not from orbit, then at least from bombers. –  Andres F. Jan 22 '12 at 15:38
    
A good point. Perhaps a mix of this and Kevin's answer would make the most sense. –  The Jug Jan 23 '12 at 0:08
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@AndresF - As I recall, Darth Vader didn't want to simply pound the Rebel base into oblivion from orbit - he wanted to capture the base (presumably to interrogate prisoners & snoop thru their computers to find clues to the location of the Rebel Fleet). More importantly, he wanted Luke Skywalker alive. –  RobertF Jun 19 '14 at 18:18

Star Wars Battlefront II best explains this. The last mission of the campaign was Hoth. The shield was protecting the transports, which were settled in a small basin behind Echo Base. The shield generator was in front of Echo Base's hanger. The Rebel offensive line was between the shield generator and Echo Base's hanger, facing downrange.

The Empire had two goals during this battle:

  • Have Blizzard Team (the first wave, featuring AT-ATs, etc) destroy the shield generator.
  • Advance on Echo Base, clear out any rebel forces, and capture the base.
  • Darth Vader & co will then proceed through Echo Base to the Rebel evacuation point (the basin where the transports are) to place a targeting beacon for the Star Destroyer to home in on to bombard the immediate surrounding location.

In short, the shield generator was nowhere near the shield itself (I know, right?). If it wasn't for it being so theatrical, Gen. Veers and Blizzard Team could have just landed right at the rebels' door step (just outside EB hanger), but the calm before the storm builds tension and is much more dramatic in a war movie.

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Like in Dune, the slow knife walker penetrates the shield. The shield was designed to repel energy and high velocity particles. Wookiepedia states the AT-TE (a previous design) could walk right through it...

An AT-TE was impervious to ion damage due to built-in electromagnetic shielding. Its average speed was 60 kph (37.2 mph). This plodding pace allowed it to penetrate energy shields, a feat that was impossible for repulsorlift craft.

In addition, the Hoth shield was an umbrella shield only designed to defend against orbital bombardment. Presumably it came low enough to the ground to prevent low flying aircraft from entering (TIE Fighters aren't good enough in atmosphere to risk it), but it was generally oriented up. AT-ATs and ground troops could walk under it.

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This may also be a good explanation why an AT-AT is preferred over a repulsor lift tank in this recent question: scifi.stackexchange.com/questions/76087/… –  RobertF Dec 19 '14 at 19:31

I don't know what level of Canon the old Rogue Squadron games were, but I remember playing the Hoth level. If I'm not mistaken, the shield generators did not shield themselves. At the time (as a ~10 year old) it seemed like quite a major design flaw....

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do you mean they generator was outside the shield? Sounds like a huge flaw! Wouldn't the Imperials be able to attack the generator from orbit, then? –  Andres F. Jan 21 '12 at 20:13
    
Only if they were able to see it from space, if my memory serves the shield generator was quite small. –  ChrisFletcher Jan 21 '12 at 20:44
    
@AndresF. I have no idea....! –  Pureferret Jan 21 '12 at 21:26
    
The same design flaw was present in RotJ - the Imperial shield generator on Endor was unprotected. At least I'm assuming it was if Han, Luke, & Leia could approach on foot. So wouldn't it have been simpler to forgo the secret commando mission to blow up the shield generator, and just lob a bunch of missiles at the generator from orbit as soon as the Rebel fleet exits hyperspace? –  RobertF Jun 19 '14 at 18:01
    
I suppose the Empire could have had anti-missile batteries protecting the shield generator - but the generator would have still been vulnerable to ray weapons, right? –  RobertF Jun 19 '14 at 18:10

My theory is that you can't shoot or fly through a shield, but you can walk and probably roll through them as long as you remain in contact with the ground. Theater shields have been used twice in the movies (Battle of Hoth and Battle of Naboo). At Hoth it is established that the shield can withstand bombardment so they land AT-ATs which walk through the shield then destroy the power generators so the rest of the attack can land. During this entire sequence the much better equipped Imperials have no air support just heavily armored walkers. At Naboo, it is established that the shield can withstand bombardment from the tanks, the Trade Federation sends in its droids which destroy the generator so the hovering tanks can attack. If something solid can pass through why didn't the Gungans fire their catapults at anything outside of the shield. If something slow moving can pass through the shield, why didn't the tanks just slowly fly through them instead of waiting for the shield to get destroyed. This also explains a couple of weird things like why the Vulture droids (which are starfighters) are designed with the ability to walk and why the Empire uses walkers instead of hovering transports.

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The game, Star Wars Battlefront, specifies that the Empire deployed a special division of Stormtroopers to take the shields out. The walkers carried these troops safely into the barrier and the troopers secured the area around the shields before the walkers destroyed them.

The troopers were most likely acting as insurance in the off chance a rebel guarding the generator was carrying a heavy ordinance weapon.

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