Ancient Technology in Stargate - Shielding

Shielding

Shields were used by the Ancients aboard their ships and outposts. Ancient shields prove to be almost impenetrable, as long as sufficient power is maintained to operate them. In contrast, the shields of other races, such as the Goa'uld and Asgard, are weakened directly by outside forces, like enemy fire and natural phenomena, causing them to fail or diminish in capacity rather quickly in cases of extreme stress. Various types of shields include:

• Prison cell shield: Ancient prison cells are known to employ shields between their bars to prevent occupants from escaping, as shown within Atlantis as well as their ships. Atlantis' sister ship was shown not to have this capability but this is most likely due to the severe lack of power available.
• Aurora-class shields: The shield of Aurora class warships contours around the ship, like an extra hull, instead of an ordinary bubble. These shields are incredibly powerful, and have a yellow gold-like color. The Orion's shields, operating at well-below full power, were able to withstand a supervolcano's eruption for 4.1 seconds, while using the eruption to propel it out of its hangar.
• Atlantis' shield: The massive dome-like force field protects the city from enemy attacks, extreme weather, and water pressure when submerged in the ocean. The power required to operate the shield depends on the load it is being subjected to. When interfaced with a Puddle Jumper's cloak generator, it acts as an invisibility shield and draws less power than it normally would, though it offers no protection other than invisibility. During normal operation the shield can be sustained by one ZPM. The shield draws a substantial amount of power when under strain. The shield could only be sustained by a single ZPM for just days when under heavy attack from a Wraith fleet, but for 10,000 years under the pressure of thousands of feet of water with only three ZPMs used in sequence.
• Personal shield: A palm-sized shield generator worn on one's person that projects a defensive barrier. It deflects any objects propelled at it, and has inertial dampeners. The shield is activated by the ATA gene, and controlled by the user's mind, though the user must be trained in mind-control to override the conscious desires in the use of the device; for example, the user may believe they do not need the shield, but their subconscious may still believe they require protection, causing the shield to remain active until the danger passes. Once a person activates the device, only that person can use it. However, on at least one occasion, a personal shield is worn by several users. The shield can prevent the user from eating or drinking while the device is active, but experienced users have been known to eat and drink while wearing it. The wearer still requires air to breathe, and thus is not protected from drowning or being buried alive. Three personal shields have so far been encountered. One was discovered in Atlantis and used by Rodney McKay, but its power source is depleted by an energy-absorbing creature. Another was used by a McKay from a Parallel universe to cross over to our universe. The third is used by Lucius Lavin, although its power source was later depleted.
• Stargate shield: Much like the Iris on the Earth Gate, Atlantis' Stargate is protected by a shield that can be activated at a touch of a button to prevent any matter from materializing beyond the event horizon. Unlike the iris, it is transparent and one can see the suppressed kawoosh and impact of objects prevented from rematerializing. The Shield has yet to be breached by anything, but a "back-door" is coded into the shield's control system by Dr. McKay, allowing him to deactivate it remotely from an off world location and preventing anyone else (including the inhabitants of Atlantis) from reactivating it.
• Puddle Jumper shield: The Atlantis Puddle Jumper cloaking devices can be modified to create a shield instead. The shield drains much more power than the cloak does, however.
• Destiny's Shield: Stargate Universe's Destiny appears to have a far more primitive Ancient shielding technology than Atlantis and other Ancient ships. Whereas Atlantis' shield appears to specialize in containing atmosphere, Destiny's shield is far inferior in this capability. Though Atlantis appears not to be airtight itself (stated several times throughout Atlantis that the shield is necessary for space travel), the shield retains the atmosphere with ease. The shield is battle tested in Season 1 Episode 11 and it is put under severe strain in the short fight with the aliens encountered, but Destiny is very damaged, with the power lines damaged enough for the ship being capable of storing only 40% of its maximum reserves, the shields also aren't at full power. The shield's obvious purpose is to withstand extreme temperatures when diving into a star's corona to recharge itself, though the recharging itself also powers the shields. However, with all available power devoted to shields during a battle, they can take a lot: during a battle simulation Destiny was running, all available power was devoted to the shields and they were said to be able to hold out for 44 minutes against nine alien ships and were only penetrated when six more ships showed up and joined in the firing.