out of the trash they generate.
"The space trash compactor is not like the giant one on the Death Star that nearly squashed Luke Skywalker and the gang in the first "Star Wars" film. This one is smaller, producing circular tiles of trash 8 inches (20 centimeters) wide and a half-inch (1.3 cm) from a single day's worth of garbage. The discs then could be stowed away, or even used for radiation shielding to protect a spacecraft’s crew, NASA officials said."
This could solve two problems, what to do with the garbarge, and the fact that they can't carry enough shielding with them, as it costs too much to lift into space and is too bulky to fit in current launch vehicles. So we may see longer missions to other celestial bodies finally and again. :)