TheAsgard are an ancient humanoid race, and had once assumed the personae of beneficent protective gods on many worlds and assumed the role of protectors of the Milky Way galaxy. Their physical forms changed gradually when they began to use scientific means to artificially extend their lifespans.
They gifted all of their knowledge and technology to the Tau'ri before the Ida Asgard committed mass suicide due to the failed attempts to repair the genetic degradation caused by continuous re-cloning, and unwilling to allow their technology to be plundered by other races.
However, a rogue group known as the Vanir were still alive in the Pegasus galaxy, consumed with the desire to survive, believed that the ends justified the means. Their primary goal was survival, as they did not wish to end up like their distant cousins, which would result in the loss of a hundred thousand years of history.
Over 30,000 years ago, the Asgard were a tall (approximately 2m), generally humanoid species that reproduced sexually. Their physical forms changed gradually when they began to use scientific means to artificially extend their lifespans. The "modern" Asgard averaged about one meter in height, with grayish skin tones, small, skinny limbs, large heads and black eyes (bearing a striking resemblance to the "Roswell Grays" described in countless UFO stories). Their body, now weakened by several millennia of cloning, is more susceptible to the discharge of a zat'nik'tel than other races (Loki fell almost immediately while most human and Jaffa stand for a moment).
The Asgard, pursuing means of extending their lifespans, began to use cloning technology. The mental patterns of Asgard that became ill or fatally injured were preserved by "downloading" them into computer memory crystals. The patterns were later placed in a new cloned body. This made the Asgard effectively immortal at the cost of the ability to reproduce sexually. This prevented the natural process of evolution, making Ascension impossible without assistance. (SG1: "Unending")
The excessive use of the cloning process began to damage and degrade the Asgard genome. This would result in the eventual extinction of the race unless a cure could be found. The Asgard devoted considerable time and resources to the project, but their best efforts proved fruitless. (The Vanir had more luck in dealing with this issue than their Ida counterparts, primarily because they were willing to experiment on humans when the main faction would not. Even so, they were not able to eradicate the problem, only postpone it.) The final attempt to solve the problem resulted in a rapidly progressing, fatal disease. (SG1: "Descent", "Covenant", "Reckoning, Part 1", "Reckoning, Part 2", "Unending") (SGA: "The Lost Tribe")
The Asgard were believed to had evolved on the planet Hala, in the Othala galaxy. Their collected history and culture would span over 100,000 years. They began exploring outside their galaxy around 28,000 BC. At some point, the Asgard began a program of cloning their people and transferring their minds into these bodies as a means of vastly extending their lifespans. At some point in time, the Asgard were hit by a plague that they called Barion's syndrome, a disease that wiped out 60% of their population and within a generation over 75% of the survivors of the disease became sterile and suffered from cellular degeneration. The remaining 10% of the population did not have the necessary genetic diversity to reverse the trend away from genetic breakdown. (RPG: "Roleplaying Game")
Around this time, an Asgard ship (its crew placed in suspended animation) left Othala. Its navigational computers were either damaged or malfunctioned at some point, and the ship drifted across galaxies, ultimately ending up in the Milky Way (whether or not the Milky Way was its intended destination is unclear). (SG1: "Revelations")
The Asgard made peaceful contact with the Ancients, the Furlings, and the Nox. They also came into conflict with the Goa'uld. Liberating several groups of Humans from Goa'uld worlds, the Asgard declared themselves their protectors until their cultures had sufficiently evolved, and threatened the Goa'uld with violent retaliation if they attempted to re-enslave the Humans.
The Vanir, a group of Asgard who did not share their brethren's ethical concerns about experimenting on humans to solve their genetic degradation problems, left the Ida galaxy and traveled to the Pegasus galaxy where they could continue their experiments, which were able to slow the advance of, but not reverse their genetic problems.
After the Ancients discovered they could not win their war with the Wraith and left for the Milky Way, the Wraith attacked the Vanir. The Vanir's intergalactic ships were damaged beyond repair in the first battle, and, as they lacked the means to construct new ones, they were stranded in the Pegasus galaxy.
In order to survive, the Vanir moved to a Toxic planet where the Wraith could not endure. At first they only needed to use simple breathing apparatus to survive, but the planet's atmosphere continued to degrade, forcing them to develop Armored exoskeletons capable of withstanding the new conditions. (Eventually, conditions would become too harsh, and they would be forced to venture out into the galaxy again.) (SGA: "The Lost Tribe")
Despite the efforts of the Asgard, the Goa'uld became a great threat to the human population of the Milky Way. The Goa'uld began taking humans as hosts during this time, which greatly angered the Asgard, who realized humanity had great potential. The Asgard's vastly superior technology was more than a match for the Goa'uld, who were forced to sign a treaty with the Asgard in order to keep from being destroyed.
Unwilling to completely give the galaxy over to the Goa'uld, the Asgard freed and transplanted humans on various worlds and posed as their protective gods as they felt they were not yet ready to have their belief system completely stripped away. Accordingly, they devised a series of challenges designed to determine when these people were advanced enough to learn (and presumably cope with) the truth about their protector's true nature.
To ensure the safety of many of these planets, the Asgard and the Goa'uld brokered the Protected Planets Treaty. The treaty stipulated that the Asgard would allow the Goa'uld various benefits (the precise terms are unknown), and in return the Goa'uld would leave the humans of the various worlds mentioned in the treaty alone. The treaty could be expanded to include other worlds if both parties agreed to negotiate.
For some time the Asgard protected these planets with their advanced technology and warships but the discovery and invasion of the Replicators in their home galaxy forced the Asgard to draw away resources they had been using to enforce the treaty, leaving the planets protected in name only.
The long lost Asgard ship from 30,000 years ago was rediscovered by Asgard scientists. Within it was one perfectly preserved Asgard ancestor. The Asgard scientist Heimdall began research on it since the clone was from a time when their cloning program was "not yet irreversible". (SG1: "Thor's Chariot", "Fair Game", "Revelations")
The Asgard's energy-based weapons were ineffective against the Replicators. Human weapons, on the other hand, used simple chemical reactions to drive metallic projectiles through kinetic force, something the mechanoids could not defend against.
With the aid of SG-1, the Asgard imprisoned all the Replicators in a time dilation field on the planet Hala. They then collapsed Hala's sun into a black hole in hopes of destroying the replicators but some managed to escape and attack the new Asgard home world of Orilla. Aegir attacked the ship that dropped out of hyperspace before it could raise its shields. Not all the Replicators were destroyed however, and the Replicator Fifth managed to land on the planet.
Colonel Jack O'Neill, with the knowledge gained from the Ancients' Repository of knowledge, invented a weapon that blocked communication between the Replicator cells permanently. Thor was able to construct a larger version capable of affecting an entire planet. The Asgard aided SG-1 when the Replicators invaded the Milky Way, but were unsuccessful. Eventually SG-1 and the Tok'ra Jacob Carter/Selmak destroyed the Replicators with the Dakara superweapon, eliminating them entirely from the Milky Way Galaxy. (SG1: "Nemesis", "Small Victories", "Unnatural Selection", "New Order, Part 1", "New Order, Part 2", "Reckoning, Part 1", "Reckoning, Part 2")
Odin, a scientist Asgard who studied on Valhalla for centuries, sought for a way to turn the Asgard into holographic beings. At an unknown date, he found himself threatened when the holographic test subjects rebelled, led by Surtr. However, thanks to SG-1's help, Odin and his test subjects combined, and together found a way to ascend. (SG1: "Valhalla")
Although the Goa'uld and the Replicators had been defeated, new threats arose, such as the Ori. In the face of this, the Asgard continued to provide aid to Earth by providing technologies for the development of the Daedalus-class, and "lending" them the Asgard engineer Hermiod. The Asgard also participated in critical events such as the Battle of P3Y-229. (SG1: "Reckoning, Part 1", "Reckoning, Part 2", "Camelot", "Flesh and Blood")
Their final attempts to repair the genetic degradation caused by continuous re-cloning failed, and unwilling to allow their technology to be plundered by other races, the Asgard decided to commit mass suicide and destroy their planet, but not without passing on their legacy. They gave Stargate Command an Asgard computer database containing all their knowledge, and made significant modifications to the Odyssey consisting of all their most recent technology.
As the upgrades were completed, three Ori warships arrived and attacked both Orilla and the Odyssey. The Odyssey fled with one Ori ship in pursuit, while the other two prepared to attack the planet. At that moment, the Asgard caused Orilla to explode, destroying the planet, the two ships, and the Asgard. The Asgard's upgrades to the Odyssey saved the ship and gave the Tau'ri an advantage in the war as the new plasma beam weapons installed on the Odyssey gave the Tau'ri weapons capable of destroying Ori motherships and almost any other ship they came across. (SG1: "Unending")
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