

HST is about 25,500 lbs (11,600 kg).  That doesn't include the cradle that
would have been in the cargo bay when it was deployed.  Spacelab-J on STS-47
was 21,861 lbs (according to the press-kit). 

As someone else pointed out if they had been unable to deploy it for some
reason that would have had to land with it still in the cargo bay and this
was a planned for contingency.  This is not a problem for the shuttle,
though it would eliminate KSC as a landing site, they still have to go to
Edwards when landing with something like Spacelab in the cargo bay. 

--GaryM