How to Colonize an Asteroid
The external tank carries the majority of the fuel (liquid hydrogen) and oxidizer (liquid oxygen) for the space shuttle main engines. The shuttle does carry an additional amount of these materials on board to provide for orbital maneuvering and de-orbit burn, but the vast majority of energy expended is carried in the external tank.
When the external tank is emptied, just before the shuttle gets to orbit, it is jettisoned and burns up in the atmosphere.
This tank offers some interesting possibilities for space craft construction, as it would not take much more fuel for the shuttle to carry these tanks into orbit. This is a large metal structure - very similar to a module of a space station or asteroid colony ship. Since the tank currently just burns up on re-entry to the atmosphere, it would be a relatively cheap way to get lots of processed metal into earth orbit for use in the construction of repair bays, cargo craft, or colony ships. Some individuals at NASA are currently assessing the costs and risks of external tank salvage in orbit.
On June 2, 1998, the shuttle made it's first flight with a redesigned external tank which is several thousand pounds lighter than the original tak. This means that the shuttle can now lift several thousand pounds more payload, which will be very important for launching the parts if the International Space Station (and our colony ship!).