There are various treaties on Earth that prohibit the use of space as a battlefield. Although spy satellites have orbited for many years, they bear no weapons. These fictional battle stations mentioned in the book exist from the 2020's or the 2040's.
The spacecraft is designed to shoot down missles. It is shaped like a U. Some of them actually get thier power from sites on earth. There is a barrel at the top of the spacecraft that fires the laser. Adjacent to the barrel, there is a nuclear generator which supplies power for the beam. At the curve in the ship, there are two solar panels and an antenna for sending and recieving commands. There is an onboard computer that fires the beam -- humans are too slow to hit the moving objects that travel consistently.
There are two flaws that dictate the existence of this spacecraft. First of all, The station requires a massive amount of power to operate. Also, there are no areas visible where supply spacecraft can dock. Many scientists think that these stations will never successful.