The Space Shuttle was a vehicle designed to do many things, and in a deal with the US Military it was redesigned to make it able to perform a very specific secret mission. The redesign radically changed the Shuttle from the early concepts to the actual design which we saw fly, but, before the shuttle even flew the secret mission had been abandoned.
Most of the details of this mission are in this document found by James Oberg (thanks!)
http://www.jamesoberg.com/sts-3A_B-DRM.PDF
The shirt is by Oaklandish:
https://www.oaklandish.com/products/oakland-to-the-universe