Borg spheres were used by the Borg as scout ships or long-range tactical vessels. Borg sphere were also embedded into some Borg cubes, and were used as auxiliary craft. (Star Trek: First Contact; VOY: "Dark Frontier", "Drone") Borg spheres had a crew complement of 11,000 Borg drones. (VOY: "Unimatrix Zero, Part II") The spheres were approximately 600 meters in diameter, and had an interior bay large enough to hold an Intrepid-class starship. (ENT: "Regeneration"; VOY: "Endgame") Borg spheres had transwarp capability and ablative hull armor. They were also equipped with a tractor beam. (VOY: "Drone")
Provided its deflector shields were down, a Borg sphere could be destroyed by a Sovereign-class starship with relative ease using quantum torpedoes. (Star Trek: First Contact) In 2378, Starfleet attempted to destroy a sphere with a fleet of starships. The sphere was holding the USS Voyager, which was able to destroy the sphere from the inside using transphasic torpedoes. (VOY: "Endgame")
After the Battle of Sector 001, a Borg sphere was observed to have time travel capabilities, by generating a temporal vortex through the controlled emission of chronometric particles. The Borg sphere traveled back to the year 2063, in order to prevent the historical event on Earth known as First Contact. (Star Trek: First Contact) In 2153, a team of scientists found the remains of the Borg sphere from 2063 in the Arctic Circle. (ENT: "Regeneration")
In 2375, the crew of the USS Voyager successfully retrieved a Borg transwarp coil from a damaged sphere, as part of Operation Fort Knox. At the time, the sphere was only able to travel at warp 2 because it was regenerating from the damage. (VOY: "Dark Frontier")
A sphere had no living quarters, or a discernible engineering section for the propulsion systems. There was, however, a primary shield generator inside the shield matrix. The several transwarp coils of a sphere were contained inside specialized rooms called transwarp chambers that featured heavy shielding. (VOY: "Dark Frontier")
Ships of the Class
Following on from the sophisticated Borg Type 03, the sphere was the first simple shape devised as an alternative design of Borg spacecraft to the Borg cube and cubical Borg scout ship. Star Trek: First Contact co-writer Brannon Braga mused, "The idea that we would introduce that Borg ships had different shapes. They weren't all cubes. We have a sphere. It was like this great new idea, you know?" (audio commentary, Star Trek: First Contact (Special Edition) DVD/Blu-ray)
The Borg sphere was designed by illustrator John Eaves. During preproduction on First Contact, he submitted several concept sketches of the craft, at least two of which were dated January 1996. (Star Trek: The Next Generation Sketchbook: The Movies, pp. 204-206)
Designers of the Borg sphere made a conscious effort to make it look different from similarly shaped crafts in certain other science fiction movies, such as Star Wars and Starman. John Eaves later recalled, "Finding a new way to execute a spherical design that wasn't reminiscent of either film was difficult, but I did my darnedest to make it as different as possible." (Star Trek: The Next Generation Sketchbook: The Movies, p. 204) Production Designer Herman Zimmerman described the challenge of differentiating the Borg ship from the Death Star as "our biggest problem" and went on to say, "I think we managed that successfully. We just made it look completely different. The Borg, of course, have a unique look, and we Borgified that sphere." (The Making of Star Trek: First Contact, p. 71)
The minute detailing on the hull of the Borg sphere took its cue from that on the hull of the Borg cube from Star Trek: The Next Generation. Stated John Eaves, "The details are extremely faceted low relief, combined with high relief to cause a series of multiple planes. We also left cylindrical openings at the north and south poles where all the detail comes to an edge, then cascades down into a half sphere. Looking down on it, you can see a 'dome inside a dome.'" In one of the concept illustrations dated January 1996, Eaves hid the names of his wife Diane and his two daughters, Olivia and Alicia, among the Borg sphere's extremely intricate outer details. He added, "What's funny about this is that Playmates put out a toy Borg sphere–which just happens to contain the first initial of everyone's name." (Star Trek: The Next Generation Sketchbook: The Movies, pp. 204, 205 & 215)
The external complexities of the Borg sphere meant it was easier to depict realistically with a studio model than with CGI. Hence, the craft was built as a motion-control model by Industrial Light & Magic, an effort led by ILM veteran modelmaker John Goodson. (Cinefantastique, Vol. 28, No. 6, p. 23; Star Trek: The Next Generation Companion 3rd ed., p. 330) Despite appearing dark gray on screen, the model was actually painted mainly dark indigo (with silver and gold details). (Star Trek: The Next Generation Sketchbook: The Movies, p. 204)
That this craft's appearance is similar to the Death Star from the Star Wars films is not just coincidence, as ILM did all of the effects and modeling for Star Wars. The schematic of a Borg sphere shown in VOY: "Dark Frontier" depicts the craft's most striking resemblance to the Death Star.
The official size comparison chart from First Contact defined the size of the Borg sphere as being 1500 feet (457 meters) in diameter. However, according to the size of the debris field, as referenced in ENT: "Regeneration", the radius was calculated at "about 600 meters in diameter."