A 1.3 mm flaw in Hubble's mirror nearly doomed the telescope, but ingenuity saved it with corrective optics. The error was so small, yet its impact was cosmic.
Hubble's flawed mirror
The Hubble Space Telescope was launched with a mirror that was technically perfect but had a critical flaw. The mirror was polished using a device called a null corrector, which itself had a 1.3-millimeter error. This led to the mirror's outer edge being too flat by 2.2 micrometers, enough to cause blurry images.Early signs of trouble
A few weeks after Hubble's April 1990 launch, the telescope returned images showing stars surrounded by blurry halos. Engineers realized the issue was not a simple adjustment but a fundamental problem in the mirror's shape. The discrepancy was so small—about one-fiftieth of a human hair—that it still caused significant optical errors.Root of the problem
The Allen commission report, submitted in November 1990, found that Perkin-Elmer, the contractor, had not checked the null corrector's dimensions after assembly. Auxiliary tests had already hinted at an issue, but no cross-checking was performed. The faulty null corrector misled the team, leading them to polish the mirror to an incorrect shape.Correcting the error
NASA could not replace Hubble's main mirror once it was in orbit. Instead, engineers designed a corrective device called COSTAR. This system used five small mirrors to redirect light properly before it reached scientific instruments. COSTAR was installed during the STS-61 mission in December 1993, marking one of the most complex spacewalks ever conducted.Hubble's future
Despite its early problems, Hubble has continued to operate for over 30 years. However, its performance has declined over time. As of June 2024, it runs with only one functioning gyroscope and orbits at less than 500 kilometers above Earth. Scientists expect it to reenter Earth's atmosphere around 2033, but its legacy of human ingenuity and problem-solving remains intact.
