The M100 galaxy photographed by the Hubble Space Telescope before and after the correction missionNASA/ESA
A mirror problem
From the study of the blurry images it was possible to trace the problem. The primary mirror of the telescope had a defect. The edge of the mirror was less curved than expected, by just a 450th of a millimeter. A tiny value, but enough to generate that optical catastrophe. It seems that this error was not in the design of the instrument, but in the tests that were supposed to verify that the mirror respected the expected optical quality. We didn’t give up. Hubble as it was, was little more than useless, and something had to be invented to solve or work around the problem.
The first service mission
Hubble’s program already included some service missions. Periodically some astronauts would visit the space telescope to replace and upgrade some parts such as gyroscopes or batteries or electronics. 5 were made, between 1993 and 2009, taking advantage of the transport capabilities of the Space Shuttle. The first would therefore have been in ’93. If there was a way to solve Hubble’s vision problem, it needed to be implemented on that mission. In December 1993, 7 astronauts led by Commander Richard O. Covey arrived aboard the Space Shuttle Endeavor to bring the solution to the Hubble problem. They would have mounted several instruments, but in particular two optical devices: the Wide Field and Planetary Camera 2 and the Corrective Optics Space Telescope Axial Replacement. Both of these tools were designed to compensate the error in the shape of the primary mirror. If the latter introduced a problem, the new devices introduced the opposite problem, so that they canceled themselves out. Thus, Hubble’s vision problem was solved, and thanks to that mission the future of the space telescope was saved.
