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The Hubble Space Telescope launched in April 1990 carrying enormous expectations . Its primary mirror was 2.4 metres wide and had taken years to grind and polish . When the first images arrived , they were unmistakably blurred . Engineers traced the problem to the mirror 's curvature , which was off by just 2.2 micrometres — roughly one - fiftieth the width of a human hair . That number sounds trivial . In optical terms , it was catastrophic . A mirror must focus all incoming light to a single precise point . Even a microscopic deviation scatters that light and ruins the image . The error came from a misaligned testing device used during polishing , and other tests that showed the flaw were dismissed . NASA 's response was to send astronauts to repair the telescope in orbit , a job Hubble had been designed for . In December 1993 , the crew of Space Shuttle Endeavour rendezvoused with Hubble . Over five spacewalks , they fitted a device called COSTAR — a set of small corrective mirrors designed to counteract the flaw exactly . The fix worked like a precisely prescribed pair of glasses . The images that followed were transformative . Hubble resolved individual stars inside distant galaxies and captured light that had travelled billions of light - years . Its data helped scientists calculate the universe 's expansion rate and showed that the expansion is speeding up . A flaw smaller than a strand of spider silk had nearly derailed all of it . The repair mission proved that science can recover from its own mistakes — provided someone is willing to do the work .
