Space Cover #18, Hubble Space Telescope ------------------
Launched on April 24, 1990 aboard Space Shuttle Discovery, the Hubble Space Telescope (HST) was designed to be the most powerful astronomical observatory ever built. HST far surpasses the capabilities of ground based optival telescopes. In addition, HST was the forst observatory designed for extensive on-orbit maintenance and refurbishment.
Two months after launch, Hubble produced a discovery not about space but about itself. Hubble was out of focus by 1/50th the width of a human hair due to the curvature of the mirror being too flat near the edge. The result was a focusing defect of spherical aberration. To compensate for the out of focus mirror, corrective optics had to be installed much the same as glasses of contact lenses correct human sight.
The first HST Servicing Mission (SM-1) launched December 2, 1993 aboard Space Shuttle Endeavour was an unqualified success restoring the optical capabilities of HST to its original specifications.
The above cover was cancelled on the launch dates of STS-31 and STS-61. I chose this cover to demonstrate what extra can be done to make a cover more interesting with, in this case, an appropiate cancel added for a subsequent and relevant mission.