Borg Sphere's are small long-range tactical vessels, transwarp capable with ablative hull armor. They were first observed in 2373 when a Sphere escaped a Borg Cube destroyed at Earth. It's primary objective is a scout vessel.
The Borg Sphere is built of a dark metallic alloy hull leaving partially open glowing sections. Using electromagnetic forcefields the Borg atmosphere of 39.1 degrees Celsius and 92% humidity is maintained. They measure from 500 to 700 meters in diameter, with an undetermined mass, and a crew of 275 drones. It's weapon systems include photonic missiles that fire from launchers along the circumference of its infrastructure. When Voyager tried firing phasers while in a Sphere's tractor beam it inverted the beam with a feedback pulse damaging Voyager instead.
Sphere's are also able to travel through time, one traveled back to Earth in 2373 to the year 2063 to stop first contact and assimilate the planet. When it's shields are down a Sphere can be easily destroyed, it took just four quantum torpedoes from the Enterprise-E. When a Sphere tried to bring Voyager into it's docking bay it was heavely damaged when a photon torpedo was beamed inside of it.
The USS Voyager first encountered a Sphere in 2375, the size of the Sphere dwarfed Voyager. The Sphere was destroyed by the advanced drone named One when he piloted it into a volatile nebula. The crew stole a transwarp coil from a Sphere that had been damaged by a ion storm in 2375.
It's unknown when the Sphere's came into use by the Borg, the earilest record of Sphere's was in 2365.