In 1946, eleven years before the Soviet Union launched Sputnik into orbit, Princeton astronomer Lyman S. Spitzer authored a paper titled "Astronomical advantages of an extra-terrestrial observatory." According to NASA, Spitzer laid out the detailed technical case for placing a large optical telescope high above the obscuring and distorting effects of Earth's atmosphere. Decades before orbital spaceflight was possible, Spitzer recognized that ground-based astronomy would always be constrained by atmospheric blurring. His early vision set in motion a multi-decade effort to construct a major space-based observatory designed for long-term orbital operation.
An Idea Eleven Years Ahead of Orbit
NASA formally proposed a Large Space Telescope (LST) project in 1972, and Congress approved funding five years later in 1977. As originally conceived, the observatory featured a 2.4-meter (94-inch) primary mirror with an expected 15-year operational lifetime. NASA initially planned to use the Space Shuttle to return the telescope to Earth every few years for ground refurbishment and relaunch.
In 1983, agency managers made a pivotal strategic shift: they abandoned the plan to bring the telescope back to Earth, deciding instead to perform all routine maintenance, instrument replacements, and hardware upgrades directly in orbit using spacewalking astronauts working inside the Shuttle's payload bay. That same year, NASA renamed the observatory after astronomer Edwin P. Hubble and targeted launch for October 1986.
Ground Delays and the Three-Micrometer Flaw
The tragic loss of Space Shuttle Challenger in January 1986 grounded the Shuttle fleet and delayed Hubble's launch by three and a half years. On April 24, 1990, Space Shuttle Discovery finally lifted off on mission STS-31, carrying the 24,000-pound telescope into an unusually high 380-mile orbit to ensure clear operations above Earth's dense atmospheric layers.
Shortly after deployment, checkout images returned to ground controllers blurred instead of sharp point-sources of light. An official investigation revealed that the telescope's primary mirror suffered from spherical aberration caused by a manufacturing defect: technicians had ground the outer edge of the mirror too flat by just 0.003 millimeters—a fraction of the width of a human hair. Because physically replacing the primary mirror in orbit was impossible, engineers realized they had to correct the light path inside subsequent replacement instruments.
Optical Rescues and Servicing Missions
In December 1993, Space Shuttle Endeavour launched on STS-61 to attempt the optical repair. Across five consecutive days of spacewalks, astronauts removed the High-Speed Photometer instrument to install the Corrective Optics Space Telescope Axial Replacement (COSTAR) module, acting like contact lenses for the flawed primary mirror. They also installed the Wide Field/Planetary Camera 2 (WF/PC-2) and replaced vibration-prone solar arrays. Within weeks, restored images confirmed that Hubble had reached its intended optical sharpness.
Subsequent servicing flights expanded the observatory's scientific capabilities far beyond its original 1990 configuration. On STS-82 in February 1997, astronauts installed the Space Telescope Imaging Spectrograph (STIS) and the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) while repairing damaged external thermal insulation. In December 1999, STS-103—designated Servicing Mission 3A—was dispatched after three of Hubble's six stabilizing gyroscopes failed; spacewalkers successfully replaced all six gyroscopes and updated the main flight computer.
Rebuilding an Observatory in Space
Servicing Mission 3B followed in March 2002 aboard STS-109. Astronauts swapped out the Faint Object Camera for the Advanced Camera for Surveys (ACS), installed rigid solar panels, and added a mechanical cryocooler to revive the NICMOS infrared sensor. Following the 2003 Columbia accident, NASA leadership initially canceled all future dedicated Hubble visits due to safety rules requiring flights to reach the International Space Station. However, following recommendations from the National Academy of Sciences, NASA reversed the decision and authorized a final repair flight.
In May 2009, STS-125 delivered Wide Field Camera 3 (WFC-3) and the Cosmic Origins Spectrograph (COS), repaired damaged circuitry in ACS and STIS, and replaced the telescope's main nickel-hydrogen batteries. Across all five servicing missions, 16 astronauts completed 23 spacewalks totaling more than 165 hours in orbital airlocks. By designing the observatory around routine human servicing, NASA demonstrated that space-based infrastructure could be continuously repaired and modernized in orbit, extending Hubble's working lifetime decades past its original design limit.
Cover: AI-generated editorial illustration; not a documentary photograph.
Source: 30 Years Ago: Hubble Launched to Unlock the Secrets of the Universe
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