But, before you get too excited, the mystery object is NOT an alien spacecraft. (If it WERE an alien spacecraft, it wouldn't be an unidentified flying object, now, would it?)
Chances are good that the object is a piece of the Space Shuttle itself. Why? First, it appeared after the shuttle tested some propulsion rockets. Anything not tightly attached to the shuttle would then fly off on its own. But, unlike what you probably think, the rockets would not have "shaken" the object loose. Rather, when the rockets fire, the object will continue to follow the same orbit it had been on -- it is the shuttle that moves.
The other reason for thinking this is something from the shuttle is that the object is in almost exactly the same orbit as the space shuttle. The likelihood that this happened by chance is extraordinarily low. It would be like closing your eyes, spinning around, and throwing a rock as hard as you could, only to have it follow the same path as an airplane flying overhead. Both the rock and the airplane could be going in any direction; to have it be the same would be very rare. Plus, in the case of the UFO, it also has to match the height and speed of the space shuttle.
I think most likely this is some piece of a tool that the spacewalking astronauts didn't secure right, or maybe a piece of fabric from inside the shuttle's cargo bay that was ripped off during the installation of the space station's new solar panels.
To be on the safe side, NASA has delayed the shuttle's return for a day to make sure this object is not a piece of the shuttle's thermal tiles. I think that's a good idea.
Too bad it isn't an alien spacecraft, though. THAT would be exciting!