Shield

Shields are devices which use energy that prevents matter from passing through them. Shields are used by many races, so their strength and properties can vary.

Ancients
As the first race known to ever develop shield technology, Ancient shields are among the most powerful ever created, and have routinely proven capable of withstanding multiple hits from everything that has been thrown at them for a lengthy amount of time, ranging from extreme natural phenomena to both energy and projectile weapons. Shields upgraded with even the most rudimentary knowledge of Ancient shields have proven to be entirely resistant to the ion cannons used by the Tollan and even the some forms of Asgard weaponry, both of which most other shields are completely powerless against. In addition, the frequency of Ancient shields can be altered to allow the shield to take on a variety of properties, including cloaking, stunning, and Replicator disruption.

The most powerful shields ever known to be built by the Ancients were those utilized by city-ships. Powered by ZPMs, they are capable of withstanding Wraith energy weapons, the weight of the ocean on Lantea, and even coronal mass ejections. During a storm year, McKay and Zelenka channeled the energy of the storm's lightning strikes to power the shield of Atlantis, although when the Wraith attacked they were unable to activate the shield until a ZPM arrived. The shield also plays a critical role whenever a city-ship uses its stardrive, since it's the shield which prevents the air from leaving a city-ship and protects the occupants from the vacuum of space and the high temperatures caused by the friction of atmospheric entry, thus acting as an outer hull of sorts. Unlike the shield technology of other space-faring races, the effectiveness of the shield of a city-ship does not deplete under external forces, and will remain impenetrable (preventing any form of energy or object from passing through) as long as sufficient power exists to sustain it, which means that a city-ship will only suffer damage once the shield fails. The only downside to the shield of a city-ship is that the amount of power used up to sustain the shield is directly proportionate to the amount of stress the shield is under; if there is no pressure on the shield it remain activated for an indefinite amount of time.



Ancient outposts are known to possess shields, thout not much is known about them; however it can be assumed that they are almost as effective as those on city-ships. Unfortunately, they use up colossal amounts of energy even when under little to no strain, and were never designed to remain active at full strength for extended periods of time.



Aurora-class warships possess shields not seen on any other ship, being contoured around the hull instead of forming the 'bubble' commonly seen with other spacecraft. The shields on the Aurora-class are capable of resisting incredible amounts of heat and pressure, meaning they would have been more than capable of resisting the energy weapons of at least one hive-ship and/or several Wraith cruisers at full strength.

The Ancients have been known to place shields on Stargates, designed to prevent unwanted travelers from exiting a Stargate, and was possibly first developed to protect Lantean-controlled planets from the Wraith. Nevertheless, the only Stargate known to be protected by a Lantean shield is the one in Atlantis. The gate shield of Atlantis is placed so close to the event horizon that it prevents the integration of even sub-atomic particles, a flaw in the metal iris of Earth, thus preventing anything, even the unstable vortex of an activating wormhole, from penetrating the shield; indeed, it is the primary method of defense from hostile, non space-faring populations who would take Atlantis for themselves. It is unknown if the Ancients also placed shields on the Stargates of other city-ships, but it seems likely.

The Ancients also created a personal shield that, once activated by someone with the ATA gene, would generate an advanced dampening field that prevents penetration by objects such as bullets, as well as allowing the user to perfectly withstand a fall of almost any altitude, and even powerful explosions. The one weakness of the personal shield was that the user needed to breathe, so they were still susceptible to death via methods such as drowning or being buried alive. Also, the user needed to eat, which could only be done if the device was turned off.

Asgard
The shields on Asgard starships are as impressive as those on the Aurora-class, if not more so. They are capable of resisting most hits from the weapons of an entire fleet of Ha'taks, as well as several consecutive hits from the central energy weapons of multiple Ori motherships. Despite this remarkable protection, while traveling through and for a short period of time after exiting hyperspace, Asgard shields are inactive, leaving ships vulnerable to attack; a weakness Samantha Carter would exploit to develop a viable tactic against a Replicator fleet converging on the Asgard homeworld. Another weakness of Asgard shielding technology is the fact that, not only is their effectiveness directly proportional to the amount of power regulated to them, they will also gradually weaken under sustained fire, allowing energy from attacks to bleed through, and so they need time to recharge afterwards. Asgard shields are also one of the few known types of shield technology capable of preventing direct penetration from drone weapons, although they are placed under heavy strain doing so. Todd remarked that just one of Atlantis's drones could disable the Daedalus' hyperdrive, implying that the drone weapons of Atlantis are more able to penetrate Asgard shields than the drones used by the Asurans; he could have just been underestimating the shield's capabilities however. In gratitude to the Tau'ri for their help against the Replicators, the Asgard gave them access to much of their technology, including their shields. The shields on Tau'ri vessels are not as powerful as those on Asgard ships, but that is only due to power requirements. When the Odyssey was equipped with a Zero Point Module, its shield became at least as powerful as those of Asgard ships, and when the Odyssey was given the most advanced technology the Asgard possessed its shield likely became even more powerful than those on the O'Neill-class.

Goa'uld
The Goa'uld have shields which, while impressive, are nowhere near comparable to those of the Ancients, Asgard, and Ori. The shields carried on a Ha'tak vessel are capable of resisting naquadah-enhanced nuclear warheads, and if at full strength can remain in the coronosphere of a star for up to ten hours without danger of crew exposure to dangerous levels of radiation. The Goa'uld also use shields to cover the viewing post on the pel'tak, as they have no transparent substance capable of withstanding the stress of hyperspace travel. The Goa'uld also possess personal shields, however their effectiveness is directly proportional to the amount of kinetic energy used, meaning it will stop energy weapons and high speed projectiles, but not slower moving projectiles such as knives or arrows. This kinetic mechanism makes a great deal of sense, as an airtight personal shield would suffocate its user. This is not a limitation of the technology itself, as Aris Boch has made use of impermeable shields as traps. It is likely that the Prometheus originally used a reverse-engineered Goa'uld designed shield before the Asgard made their far superior shield technology available to the Tau'ri. After the design of Goa'uld shields was upgraded by Anubis using his limited ascended knowledge, Goa'uld shields became capable of resisting Tollan ion cannons and less advanced Asgard energy weapons, which they were previously ineffective against; however they remained powerless against drone weapons and the energy weapons mounted on the Asgard O'Neill-class.

Ori
Built with ascended knowledge, the Ori possess some of the most powerful shields ever encountered. They are capable of withstanding virtually anything, including a fleet of ha'taks and all but the most powerful Asgard weapons. Bra'tac once attempted to ram a fully functioning ha'tak into an Ori mothership at full speed, but it merely impacted against the shield. The only thing known to overwhelm the shields of an Ori mothership is the unstable vortex of an activating Supergate and the most advanced Asgard energy weapons; these shields are also incapable of blocking the Dakara superweapon. In an alternate reality, it was stated that an Ori mothership attacking Earth was driven off by drone weapons, but whether or not the vessel was destroyed is unknown. The Ori also possesses specially designed shields that collapse a planet into a micro-singularity for the purpose of providing power to a Supergate. These shields expand after absorbing enough energy, which can be accelerated by powerful weapons fire, although vast amounts of energy (typically provided by a black hole) is required to be transferred into them, often through an active Stargate on the planet being collapsed, for the expansion to be noticable.

Tau'ri
The Tau'ri do not possess shields of their own design, instead utilizing shields of Asgard origin, which were donated to them in thanks for the help the Tau'ri had provided the Asgard in their conflict against the Replicators. Asgard shielding was first equipped to the Prometheus and eventually the Daedalus-class, allowing the Tau'ri to better combat the warships of the Goa'uld and, eventually, the Wraith. However, with the invasion of the Ori, the shields of the Prometheus and even those on the more advanced 304-series of warships proved virtually ineffective against the powerful weapons of Ori motherships, and so a far more advanced version of Asgard shielding was later installed on the Daedalus-class series of warships after the Asgard donated their entire knowledge bank to the Tau'ri. When the Odyessey was equipped with a ZPM, the strength of its shield was increased even futher, allowing it to withstand a extended exposure of fire from the main weapon of an Ori mothership, which no shield apart from those on city-ships would likely be capable of. Due to the easy availability of Asgard shielding technology the Tau'ri have not yet seen fit to conduct their own research into this area, however when Rodney McKay had his intelligence increased he passed on an algorithm to Hermiod which increased the effectiveness of the shields equipped on Daedalus-class warships (and likely Asgard shields as a whole). Whether the algorithm was entirely McKay's design or if he was utilizing Ancient shield technology as a basis is still unknown, but is is likely that the majority of his knowledge on the subject came from the Atlantis database.

Tok'ra
The Tok'ra have one-way shields, usually arranged to completely enclose an area of space, that holds matter inside but allows external objects to enter. They have been used in an attempt to capture a Kull Warrior, and to keep Khalek imprisoned while allowing shots to be fired into the cell. What few ships the Tok'ra do possess almost always use Goa'uld shields for protection, which were typically obtained either through theft or were already integrated into the vessels when they were acquired.

Wraith
Hive-ships and Wraith cruisers do not have any energy shields per se, instead relying on an organic armored hull that has a limited form of regeneration to nullify damage. They do, however, utilize a program that prevents Asgard beaming technology from transporting objects inside a Wraith ship or close to it's exterior. The only form shield technology known to be utilized by the Wraith is in their Darts, which only use shields to serve as canopies for their cockpits. These shields are completely opaque, and send information to the pilot via written Wraith. Michael Kenmore managed to obtain a gateship and convert its cloak into a stun field; anyone who comes into contact with it suffers the same effect as a Wraith stunner. Despite their lack of traditional starship shields, they do use forcefields for a variety of purposes, such as protecting landed fighters from being hijacked, and for holding extremely dangerous prisonsers.