One of the giant artificial spheres built by the Sphere-Builders to reshape the Delphic Expanse, whose destruction by Enterprise in Zero Hour brings down the whole network.
Sphere 41 is one of dozens of moon-sized metal spheres which generate the gravitational distortions and spatial anomalies of the Delphic Expanse. They were placed there about a thousand years earlier by the transdimensional Sphere-Builders, who are remaking the region so that they can live in it.
In the Enterprise episode Zero Hour, set in 2154, the crew establish that a handful of spheres hold the network together and that Sphere 41 is one of them. While Archer pursues the Xindi weapon to Earth, T'Pol takes Enterprise in to attack it.
The Sphere-Builders transform the surrounding space so severely that the crew's skin begins to break down, and Phlox can protect them for only a few minutes. A modified deflector pulse overloads the sphere, and a chain reaction then destroys every other one, returning the Expanse to normal space.