The Hubble Space Telescope is a 2.4-meter reflecting telescope in low Earth orbit, operated by NASA with participation from the European Space Agency. Carried aloft by the Space Shuttle Discovery on mission STS-31 on April 24, 1990, it was the first large optical observatory in space and the only one designed to be serviced by astronauts. Five shuttle crews visited between 1993 and 2009 to repair it and swap in new instruments.[1][2]
Observing from above the atmosphere in ultraviolet, visible, and near-infrared light, Hubble has reshaped most branches of astronomy. By its 35th anniversary in April 2025 it had made nearly 1.7 million observations of about 55,000 targets, feeding more than 22,000 scientific papers.[3] The telescope continues science operations in 2026, though it has pointed with a single gyroscope since mid-2024.[4]
Launch and a flawed mirror
Astronomer Lyman Spitzer proposed a large space telescope in 1946, and Congress approved the project in 1977. After delays that included the 1986 Challenger accident, Discovery released the telescope into an orbit roughly 600 kilometers high in April 1990.[2]
Within weeks, engineers found that stars would not come into sharp focus. The primary mirror had been ground with exquisite precision to a subtly wrong shape: its edge was too flat by about 2.2 micrometers, a consequence of a miscalibrated test instrument used by the contractor Perkin-Elmer. The resulting spherical aberration blurred every image and made the observatory an early symbol of NASA mismanagement.[2]
Servicing missions
Because Hubble was built with handholds and replaceable modules, astronauts could fix it. The first servicing crew installed corrective optics (COSTAR) and a new camera with built-in correction (WFPC2) in December 1993, restoring the intended sharpness. Four more missions upgraded nearly every system.[2]
| Mission | Shuttle flight | Date | Highlights |
|---|---|---|---|
| SM1 | Endeavour, STS-61 | December 1993 | COSTAR corrective optics, WFPC2 camera |
| SM2 | Discovery, STS-82 | February 1997 | STIS spectrograph, NICMOS infrared camera |
| SM3A | Discovery, STS-103 | December 1999 | Six new gyroscopes, new main computer |
| SM3B | Columbia, STS-109 | March 2002 | Advanced Camera for Surveys, new solar arrays |
| SM4 | Atlantis, STS-125 | May 2009 | WFC3 camera, COS spectrograph, six gyroscopes, instrument repairs |
The 2009 flight was the last; with the shuttle retired in 2011, no crewed vehicle currently in service can reach and grapple the telescope.[2]
Key discoveries
Hubble was named for astronomer Edwin Hubble, and its signature achievement matched its name: the Hubble Key Project used Cepheid variable stars to measure the universe's expansion rate at 72 kilometers per second per megaparsec, settling a decades-long factor-of-two dispute. Later precision measurements fed the still-unresolved "Hubble tension" between nearby and early-universe expansion values.[2]
Observations of distant supernovae by Hubble and ground-based telescopes in the late 1990s revealed that cosmic expansion is accelerating, the effect attributed to dark energy.[3] The 1995 Hubble Deep Field, a 10-day stare at a blank patch of sky that uncovered thousands of galaxies, invented an observing style continued by the 2004 Ultra Deep Field and pushed galaxy studies to within a few hundred million years of the Big Bang, including the record-holding galaxy GN-z11 in 2016.[2][3]
The telescope also opened exoplanet atmospheres as a field, detecting sodium in the atmosphere of the transiting planet HD 209458 b in 2001, the first atmospheric detection for any world outside the solar system.[2] Closer to home, Hubble watched comet Shoemaker-Levy 9 strike Jupiter in 1994 and discovered four small moons of Pluto. Its imagery now regularly pairs with infrared data from the James Webb Space Telescope in joint observing programs.[3]
Gyroscope trouble and one-gyro operations
Hubble points using gyroscopes, six of which were installed in 2009. By 2024 only three still worked, and one of those began returning faulty readings, repeatedly knocking the observatory into safe mode. In June 2024 NASA shifted Hubble to single-gyroscope operations, keeping one healthy gyro in reserve.[4]
The mode is a planned contingency, not an emergency, but it carries costs: scheduling efficiency drops by about 12 percent, only part of the sky is reachable at any moment, and the telescope cannot track fast-moving targets closer than Mars.[4][5] NASA estimated a better than 70 percent chance that at least one gyroscope will keep working through 2035.[8] The approach has held: Hubble marked its 35th anniversary in April 2025 with a new image release and continued full science operations into 2026.[3]
Reboost proposals and outlook
Atmospheric drag has pulled Hubble down from its deployment altitude to roughly 520 kilometers, and without intervention current projections put an uncontrolled reentry around the mid-2030s.[6] In 2022 NASA and SpaceX studied whether a Crew Dragon flying under the private Polaris Program could dock with and raise the telescope, and NASA later solicited broader commercial ideas, drawing a joint Astroscale and Momentus concept.[7]
In June 2024 NASA declined the private servicing offer championed by businessman Jared Isaacman, judging the risk to the observatory too high for the benefit.[7] Agency officials have since said they remain open to a reboost if costs come down, but no mission is funded as of mid-2026, and NASA expects eventually to attach a propulsion module to bring the telescope down safely over an ocean at the end of its life.[6]
References
- Hubble Space Telescope - NASA.
- Hubble Space Telescope - Wikipedia.
- Hubble's 35th Anniversary - NASA.
- Operating Hubble with Only One Gyroscope - NASA.
- Aging Hubble telescope moves to 'one-gyro' operations, reducing productivity and limiting science - Science.
- NASA interested in Hubble reboost if costs can be reduced - SpaceNews.
- A billionaire wanted to save the Hubble Telescope, here's why NASA politely declined - Space.com.
- Hubble Space Telescope faces setback, but should keep working for years, NASA says - CBS News.
