(covers information from several alternate timelines)
The Xindi weapon was a massive, mobile particle beam weapon designed to destroy entire planets. There were actually three weapons built while the design was refined. The weapons were constructed by the five Xindi species in cooperation, at the instigation of the Sphere Builders, who had informed the Xindi that Humans would be responsible for the destruction of the future Xindi homeworld in the 26th century. The final weapon's primary goal was the destruction of Earth.
Certain steps were taken to prevent any one species from controlling the weapon. The final version required activation codes from at least three of the five species in order to function. However, it could be fired instantly if all five codes were used. It could not be armed while the propulsion system was online. (ENT: "The Council", "Countdown")
Building the weapons
The weapon was designed primarily by Degra, a Xindi-Primate scientist and member of the Xindi Council, and constructed by the Aquatics. Most of the weapon's components were built by the Xindi, but at least one component came from the year 2573, supplied to the Xindi by the Sphere Builders. (ENT: "The Expanse") Another key component of the weapon was the mineral kemocite, which was provided by Gralik Durr's refining facility. (ENT: "The Shipment")
Research on the weapon was very dangerous. In September of 2153, there was an accident in Degra's laboratory. Nearly all of his laboratory was destroyed, and three of his researchers were killed. The loss of the data caused the weapon's development to be set back by months, which induced the Council to consider developing a bioweapon. (ENT: "Rajiin")
The first prototype of the weapon was a small, one-man probe launched against Earth in 2153. It was constructed with the assistance of the Sphere Builders, who provided at least one component from the future. The pilot was a Xindi-Reptilian personally selected by Commander Dolim, a Reptilian representative on the Xindi Council. Arriving at Earth through a subspace vortex, the probe fired a particle beam which cut a destructive swath from Florida to Venezuela, killing seven million people. The probe subsequently self-destructed, the debris landing in Central Asia. The wreckage and body of the pilot were recovered by a Vulcan transport and brought to Starfleet Command for analysis. Acting on intelligence provided by the Suliban Cabal's mysterious benefactor, Jonathan Archer – captain of the Enterprise NX-01 – quantum dated the fragment of the probe provided to the Xindi by the Sphere Builders, proving that it came from 420 years in the future. (ENT: "The Expanse", "Azati Prime")
The second weapon prototype was unmanned and test-fired in a "proving ground" in the Calindra system inside the Delphic Expanse, under the supervision of Degra himself in December 2153. The test did not go as planned, as the weapon's materials had been sabotaged by Gralik on behalf of Jonathan Archer. The weapon was subsequently stolen by the Andorian warship Kumari. The Andorians had hoped to take it home to use as a deterrent against the Vulcans. It was destroyed when T'Pol entered the activation codes and issued an overload command. The Kumari jettisoned the prototype before it exploded, but the shock wave caused significant damage to their power and engine systems. (ENT: "Proving Ground")
The final version of the weapon was substantially larger than its two predecessors and was designed to be manned by a skeleton crew. The interior of the weapon was mainly hollow. The exact core, however, contained the control area, where the weapon could be activated, fired or repaired. The only known way to deactivate the weapon was to invert a series of conduits in the proper sequence to cut power. Explosives could then destroy the core of the weapon. (ENT: "Twilight", "Proving Ground", "Countdown", "Zero Hour")
The eventual weapon was constructed in secret on Azati Prime, though when Enterprise discovered it there, it was moved to the Xindi Council planet. (ENT: "Azati Prime") When it became apparent that Captain Archer was becoming successful in convincing the Council to delay the attack on Earth, Commander Dolim's forces, working in concert with the Insectoids, hijacked the weapon. They also kidnapped Enterprise linguist Hoshi Sato and brainwashed her into decrypting the third activation code. (ENT: "The Council", "Countdown")
Enterprise, along with forces of the Primates, Arboreals, and Aquatics, attempted to stop the weapon from entering the vortex to Earth, but were repelled by a Reptilian and Insectoid fleet. The Reptilians and Insectoids were assisted by the creation of numerous anomalies by the Sphere Builders, and one ship of each race escaped into the vortex with the weapon. The Insectoid ship was very shortly destroyed by the Reptilian one, however, when its commander questioned Dolim. (ENT: "Countdown")
The weapon emerged in Earth orbit ten hours later, but very close behind it was the vessel of the late Degra, manned by Enterprise personnel including Jonathan Archer, Malcolm Reed, and Hoshi Sato, as well as a MACO squad. The Kumari ran interference for Degra's lightly-armed vessel, enabling the Enterprise team to board the weapon and destroy it by overloading its reactor. Captain Archer was initially thought killed in the attempt. (ENT: "Zero Hour")
In an alternate timeline where Archer was infected by interspatial parasites, Enterprise arrived at Azati Prime after the weapon had already been launched. The Xindi destroyed Earth with it before proceeding to wipe out most of the rest of Humanity. (ENT: "Twilight")
In the script for ENT: "Twilight", the final Xindi weapon is initially referred to as "massive" and the script goes on to say, "The weapon is far larger and more menacing than the device seen in 'The Expanse'... it’s covered with ablative armor and is bristling with weapon platforms." In other scripts written for Star Trek: Enterprise (such as the final draft scripts for both "Countdown" and "Zero Hour"), this weapon was commonly referred to as the "Primary Weapon".
Visual effects supervisor Dan Curry was involved in designing the exterior appearance of the Xindi weapon. He later recalled, "The super weapon was really fun and one of the inspirations for that was... the Chinese carve these beautiful ivory balls that are layer upon layer upon layer, and can rotate in and out of themselves. So, we decided to give the super weapon a kinetic structure, so that it was constantly moving, so it looked always imminently threatening." Senior illustrator John Eaves created the final sketches of the weapon's exterior. ("Visual Effects Magic", ENT Season 4 DVD special features)
From the sketches, CGI artists at Eden FX built the computer models of the weapon. Pierre Drolet, Eden's lead 3D modeler, was instrumental in providing the CG version. As a time-saving measure, he built the digital model by first concentrating on only one quarter of the entire Xindi weapon, then multiplying that portion to become the whole exterior. "You don't really see it, when it's all together like that," he noted. "It saved a lot of time, actually." ("Visual Effects Magic", ENT Season 4 DVD special features)
The three rotating rings around the rotating sphere in the core of the Xindi weapon was a reuse of the three-ring rotating rings exercise device used by Commander Tucker in the episode "Vanishing Point". Even the purple color of the rings remained the same. However, Star Trek paint supervisor Chuck Clark used an unusual paint on both the frame and the inner ball; the paint seemed to change color depending on the perspective from which it was viewed.
The sphere itself was a reuse of the Romulan mine from "Minefield". The spinning gizmo was oddly not motorized to rotate but was instead firmly spun by hand, before each shot, by special effects technician Wil Thoms. ("Countdown" text commentary, ENT Season 3 DVD) Production designer Herman Zimmerman remarked, "The weapon core [sphere] is really a mechanical device–quite elaborate and quite effectively used on film. I will wager many people will look at it and say, 'Oh that must be a CGI effect'–when in reality it wasn't." Zimmerman also considered the entirely-mechanical sphere to have been "an amazing reuse of a Romulan mine from 'Minefield'." (Star Trek: Communicator issue 151, p. 55)
The set for the Xindi weapon's interior was built on Paramount Stage 9. ("Countdown" text commentary, ENT Season 3 DVD; Star Trek: Communicator issue 151, p. 55) Herman Zimmerman noted that the set "took up the whole of Stage 9." He further commented, "We built from scratch the last two episodes – really huge sets, both of which had to be destroyed because we don't have any place to keep them and we probably will never see them again." (Star Trek: Communicator issue 151, p. 55)
The set for the weapon's interior was constructed on platforms about eight feet above the floor of the stage. Although this allowed for the filming of live-action footage involving the weapon's catwalks, the platforming tended to slightly slow filming, as equipment such as cameras and lights had to be hauled up and down to be used on the set. ("Countdown" text commentary, ENT Season 3 DVD) Herman Zimmerman pointed out that, though the catwalks were brand new, "the arches behind them" were reused ship hulls, originally built as parts of the starship Enterprise's exterior in ENT: "The Forgotten" and turned vertical for their Xindi modification. (Star Trek: Communicator issue 151, pp. 55 & 59)
The weapon interior was additionally represented with some wide shots being completely computer-generated. The digital model of the interior was additionally used to extend the actual set. In the case of one such shot being required, a green-screen was used on the stage floor, to later be replaced by the visual effects crew in post-production. ("Countdown" text commentary, ENT Season 3 DVD)
To depict the destruction of the Xindi weapon from inside the craft, explosive charges were wired by special effects artist Dennis Drozdowski and spark cones, which directed showers of sparks, were rigged by electronics foreman John Peyser. (Star Trek: Communicator issue 152, p. 47) Footage of the weapon's destruction was not only included in "Zero Hour" but was also reused as archive footage in "Storm Front, Part II".
The cover for the novelization of "The Expanse" and "The Xindi" (entitled The Expanse) features an artist's rendering, illustrated by Doug Drexler, of the Xindi probe firing at the Earth. Though the probe itself looks much like it does in the episode "The Expanse", the beam it emits, in the picture, is colored blue.
The Expanse also states that, prior to the initial weapon's use against Earth, hundreds of simulations plotting the firing of the weapon were run. Commonly referring to this craft as "the destroyer/probe," the book indicates that the interior of this initial weapon consisted of two compartments, with an engine area and a command section. The latter portion of the craft had a deck for the pilot to sit or stand on, controls for navigational, tactical and communication functions, as well as a small monitor on which the craft's exterior could be viewed. To fire the weapon, the targeted area had to be programmed into the craft's sites. This was followed by the concentric spheres on the outside of the weapon automatically rotating into place, lining up the craft's emitters. The powering up of the weapon caused the deck beneath the pilot's feet to hum. The craft was also designed to relay information about the attack back to the Xindi and was programmed to self-destruct. The probe had multiple explosives that caused the weapon to auto-destruct, including one that was located in its engine and triggered separately from the rest.
Part of the video game Star Trek: Encounters involves Enterprise's search for the Xindi weapon. In the game's final level, a temporal anomaly makes the weapon, a Xindi fleet, and Enterprise appear in the 24th century, where it is destroyed thanks to both the Enterprise NX-01, and the USS Enterprise-E.
The path cut by the probe first sent to Earth was described as being "4,000 kilometers" long. Unless that was an exaggeration, such a path through Venezuela would extend into present-day Brazil. The images in the episode suggest the beam's path began just west of Lake Okeechobee, cutting south from there. With the direction of the beam initially shown, it would have to had changed direction to reach what is currently Venezuela.
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