The support mothership is an alien mothership and the final boss of the Fallout 3 add-on Mothership Zeta.
When the Lone Wanderer and the prisoners took control of the mothership Zeta, another mothership, seeming to be near to the Earth came after Zeta and began hostilities. After an exchange of shots with their respective death rays, the support mothership was destroyed and exploded. While some parts drifted off into space, the body of the mothership crashed in a region of North America on Earth. No other motherships arrived, stopping the alien menace for a while.
The support mothership appears in the Fallout 3 add-on Mothership Zeta.