[gvb]

It is extremely likely it is the hypergolic fuel used for the on-orbit manoeuvring rockets. Hypergolic fuel is really nasty stuff.

It is likely the same rocket fuel (maybe the same rockets) as used by the space shuttle (OMS).

http://en.wikipedia.org/wiki/Space_Shuttle_Orbital_Maneuveri...

See also:

* http://en.wikipedia.org/wiki/Space_Shuttle_Discovery#Decommi...

* http://en.wikipedia.org/wiki/Hypergolic_propellant#Disadvant...