Death Asteroids are a class of ship created by The Others that serve as living space, Gamma Ray weapons, and hubs for drones to be released from. They are a standard part of expeditions sent by The Others to other star systems, and are featured in every major confrontation the Bobs have with the Others. They are described as spheres with a flat surface cut off inward to around 1/8 of the diameter. The flat surface is covered in a grid pattern, and is the source of the Gamma Ray beam weapon. They are propelled by Casimir drives, but do not have SCUT capabilities. Their name comes from their resemblance to the Death Star from Star Wars, but are significantly smaller, hence Death Asteroid.
Artist's impression of a Death Asteroid as described in Book 2, by M.W.F. Baily
In addition to severely damaging electronics, Death Asteroids are used to sterilize the surfaces of planets to allow The Others to more easily mine the planet's resources and collect the dead animal life for food.