Hubble Space Telescope

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, and the first large optical observatory in space.^[1] Launched aboard the Space Shuttle Discovery on mission STS-31 on April 24, 1990,^[1] it has made more than 1.7 million observations feeding roughly 23,000 peer-reviewed papers written by nearly 29,000 astronomers.^[9]

Observing from above the atmosphere in ultraviolet, visible, and near-infrared light, Hubble has reshaped most branches of astronomy. It is also the only space telescope designed to be serviced by astronauts: five shuttle crews visited between 1993 and 2009 to repair it and swap in new instruments.^[1][13] 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 and 1.3 million citations, with an archive past 400 terabytes.^[3] The telescope continues science operations in 2026, though it has pointed with a single gyroscope since June 2024.^[4][10]

Launch and a flawed mirror#

Hubble grew out of astronomer Lyman Spitzer's 1946 proposal for a large space telescope, and Congress approved funding for the Large Space Telescope project in 1977. After delays that included the 1986 Challenger accident, Discovery climbed to a record altitude of about 600 kilometers and released the telescope on April 25, 1990.^[2][11]

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: it was too flat near its outer edge by about 2.2 micrometers, roughly ten times the tolerance the optical design allowed. The cause was traced to the null corrector, the test instrument the contractor Perkin-Elmer used to check the polishing, which had been assembled with one of its lenses 1.3 millimeters out of position. NASA announced the fault on June 27, 1990, and the resulting spherical aberration blurred every image for the next three and a half years, making the observatory an early symbol of NASA mismanagement.^[12][2]

Servicing missions#

Because Hubble was built with handholds and replaceable modules, astronauts could fix it. Engineers measured the aberration precisely enough to build smaller optics carrying the opposite error, and the first servicing crew installed them in December 1993: COSTAR, a package that put corrective mirrors in front of the older instruments, and WFPC2, a new camera with the correction built into its own optics. Sharpness was restored, and four more missions went on to upgrade nearly every system.^{[12][14][15][18][19]}

SM3 was originally a single flight. Gyroscope failures made it urgent, and NASA split it in two so the repairs could go up sooner. When a fourth of the six gyros failed on November 13, 1999, Hubble dropped into safe mode, unable to do science with fewer than three working units, and SM3A launched the following month; the instrument work waited until SM3B in 2002.^[16][17][2]

SM4 was cancelled in January 2004 by administrator Sean O'Keefe, who judged a flight to Hubble too risky after the Columbia accident because the crew could not reach the International Space Station as a safe haven. His successor Michael Griffin reversed the decision in October 2006, approving the mission on condition that a second shuttle stand by for rescue, and it flew as STS-125 in 2009.^[34]

That last crew removed COSTAR, which was no longer needed once every remaining instrument carried its own correction; it now sits in the Smithsonian National Air and Space Museum.^[12] They also bolted a Soft Capture Mechanism to Hubble's aft bulkhead, a ring that lets a future vehicle dock with the telescope or attach a propulsion module for a controlled deorbit.^[20] The shuttle retired in 2011, and no crewed vehicle has visited Hubble since.^[13]

Instruments#

Hubble carries four science instruments plus three fine guidance sensors, which hold the telescope steady and also do astrometry. All four science instruments were working as of 2026, and the Space Telescope Science Institute puts the odds of all of them still operating in 2030 at 90 percent.^[27]

Observing time is awarded in annual cycles. Cycle 34 runs from November 1, 2026 to October 31, 2027 and takes proposals for all four science instruments and the fine guidance sensors, with roughly 2,700 orbits on offer.^[33]

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, give or take 8, settling a decades-long factor-of-two dispute.^[21] Later precision measurements fed the still-unresolved "Hubble tension" between the nearby and early-universe expansion values. In 2024 the James Webb Space Telescope re-measured Cepheids in the same galaxies and matched Hubble's numbers, ruling out a measurement error in Hubble's data as the explanation and leaving the discrepancy intact.^[23]

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.^[2] The 1995 Hubble Deep Field, 342 exposures gathered over ten consecutive days in December on a patch of sky chosen because it looked empty, uncovered almost 3,000 galaxies.^[36] It invented an observing style continued by the 2004 Ultra Deep Field, which reached about 10,000.^[37] Those stares pushed galaxy studies to within a few hundred million years of the Big Bang, including GN-z11 in 2016, seen as it was 400 million years after the Big Bang and the most distant galaxy known at the time.^[2][22]

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 July 1994 and discovered four small moons of Pluto between 2005 and 2012, work that fed into planning for the New Horizons flyby.^[2]

Recent observations#

Hubble tracked the interstellar comet 3I/ATLAS through its passage across the inner solar system. Images taken on August 20, 2025 set the first tight bounds on the nucleus, between about 440 meters and 5.6 kilometers across, since the solid body stays hidden inside the coma even at Hubble's resolution.^[24] A later analysis of observations from November 30, 2025 to January 22, 2026 extracted the nucleus signal directly and put its effective radius at 1.3 plus or minus 0.2 kilometers, assuming a comet-like reflectivity.^[25]

In July 2026 astronomers reported the first stellar-mass black hole found in the globular cluster Omega Centauri, designated oMEGACat BH-2 and weighing 4.46 solar masses. It was detected from more than 20 years of archived Hubble astrometry, which showed a star wobbling around an unseen companion on a 94-year orbit, backed by newer Webb observations.^[26]

For its 36th anniversary in April 2026, Hubble reimaged part of the Trifid Nebula, a star-forming region about 5,000 light-years away that it had last photographed in 1997, showing how jets and dust structures there shift over a span of 29 years.^[9]

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. On June 14, 2024 NASA shifted Hubble to single-gyroscope operations, keeping one healthy gyro in reserve. Gyro 6 now runs in the pointing control loop and gyro 4 is powered on as a backup.^[10][27]

The mode is a planned contingency, not an emergency. Magnetometers, sun sensors, and the surviving gyro get the telescope to within about 10 degrees of a target, fixed-head star trackers narrow that to tens of arcseconds, and the fine guidance sensors lock on to guide stars for the final precision. Once settled, the telescope is nearly as steady as it was on three gyros.^[4] The costs come before that point: slewing and locking take longer, which NASA estimated would cost roughly 12 percent in efficiency, only part of the sky is reachable at any moment, and Hubble can no longer track moving objects closer to Earth than the orbit of Mars.^[4][5]

In practice the loss has been smaller than forecast. The Space Telescope Science Institute reports an average of 83 orbits scheduled per week in reduced gyro mode against 84 in three-gyro mode, and no safe-mode anomalies at all since the switch in June 2024.^[27] NASA's Hubble project manager put the odds of at least one gyroscope still working in 2035 at better than 70 percent. The institute's current figures are a 90 percent chance of having at least one gyro and at least two fine guidance sensors in 2030, and 95 percent for at least three reaction wheels.^[8][27]

Orbit decay and reboost proposals#

Atmospheric drag has pulled Hubble down from its deployment altitude to about 480 kilometers, and the telescope carries no propulsion of its own to climb back.^[27] The decay has quickened as solar activity rose and the upper atmosphere expanded.^[29] How long that leaves is disputed: the Space Telescope Science Institute's operating assumption is reentry around mid-2033, NASA's astrophysics director said in 2024 that the orbit was fine into the mid-2030s, and an independent analysis by astronomer Jonathan McDowell in early 2026 warned that reentry could come as early as 2028 if solar activity stays high.^[27][28][29]

In 2022 NASA and SpaceX studied whether a Crew Dragon flying under the private Polaris Program could dock with the Soft Capture Mechanism 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 Jared Isaacman, citing the risk of losing science prematurely and the possibility of Dragon thruster exhaust contaminating the mirror.^[7][28] Isaacman was confirmed as NASA administrator on December 17, 2025 and sworn in the next day.^[35]

Cost, not appetite, is now the obstacle. NASA spent $98.8 million on Hubble in fiscal 2025, more than any astrophysics mission except Webb, and the Space Telescope Science Institute has been restructuring operations against budget guidance that would cut grants for research using Hubble data from about $30 million a year to $10 million.^[6][32] In June 2026 NASA's astrophysics division director, Shawn Domagal-Goldman, said the agency was open to a Hubble reboost but had to bring operating costs down first.^[6]

The nearer test is a smaller one. NASA contracted Katalyst Space Technologies in September 2025 to build Link, a robotic servicing spacecraft that will raise the orbit of the Neil Gehrels Swift Observatory; SpaceNews put the contract at $30 million.^[38][6] Link launched on July 3, 2026 and was about halfway through commissioning in mid-July, with the rendezvous and a months-long altitude boost to follow.^[30] Isaacman said in July 2026 that if it works, "this gives us options for Hubble and other scientific instruments."^[31] No Hubble reboost is funded as of August 2026, and the Soft Capture Mechanism remains the designed route for a future vehicle to dock or for a propulsion module to bring the telescope down under control.^[20]

References#

1. Hubble Space Telescope - NASA.
2. Hubble History Timeline: Non-Interactive, Full Text - NASA.
3. NASA Celebrates Hubble's 35th Year in Orbit - NASA, April 23, 2025.
4. Operating Hubble with Only One Gyroscope - NASA.
5. Aging Hubble telescope moves to 'one-gyro' operations, reducing productivity and limiting science - Science.
6. NASA interested in Hubble reboost if costs can be reduced - SpaceNews, June 6, 2026.
7. A billionaire wanted to save the Hubble Telescope, here's why NASA politely declined - Space.com.
8. Hubble Space Telescope faces setback, but should keep working for years, NASA says - CBS News, June 5, 2024.
9. NASA's Hubble Dazzles With Young Stars in Trifid Nebula - NASA, April 20, 2026.
10. NASA's Hubble Restarts Science in New Pointing Mode - Space Telescope Science Institute, June 2024.
11. April 25, 1990, Deployment of the Hubble Space Telescope - NASA.
12. Hubble's Mirror Flaw - NASA.
13. Missions to Hubble - NASA.
14. Servicing Mission 1 (SM1) - NASA.
15. Servicing Mission 2 (SM2) - NASA.
16. Servicing Mission 3A (SM3A) - NASA.
17. STS-103 - NASA.
18. Servicing Mission 3B (SM3B) - NASA.
19. Servicing Mission 4 (SM4) - NASA.
20. Servicing Mission 4 Engineering Components - NASA.
21. Final Results from the Hubble Space Telescope Key Project to Measure the Hubble Constant - Freedman et al., The Astrophysical Journal 553, 47, 2001.
22. Hubble Team Breaks Cosmic Distance Record - NASA, March 3, 2016.
23. NASA's Webb, Hubble Telescopes Affirm Universe's Expansion Rate, Puzzle Persists - Space Telescope Science Institute, March 2024.
24. 3I/ATLAS Facts and FAQs - NASA.
25. Nucleus and Postperihelion Activity of Interstellar Object 3I/ATLAS Observed by the Hubble Space Telescope - Hui et al., The Astrophysical Journal Letters, March 6, 2026.
26. NASA's Hubble Discovers First of Star Cluster's Missing Black Holes - NASA, July 13, 2026.
27. HST Overview and Status - Space Telescope Science Institute.
28. Hubble Good for Another Decade Despite Gyro Woes - SpacePolicyOnline, June 4, 2024.
29. Hubble could re-enter atmosphere as early as 2028 - The Register, February 25, 2026.
30. Spacecraft Commissioning On Track for Mission to Boost NASA's Swift - NASA, July 15, 2026.
31. Transcript: NASA administrator Jared Isaacman on "Face the Nation with Margaret Brennan" - CBS News, July 5, 2026.
32. Hubble budget cuts could impact science and mission operations - SpaceNews, January 15, 2025.
33. Hubble Space Telescope Call for Proposals for Cycle 34 - Space Telescope Science Institute.
34. 15 Years Ago: STS-125, the Final Hubble Servicing Mission - NASA.
35. Senate confirms Jared Isaacman as 15th NASA Administrator - Spaceflight Now, December 18, 2025.
36. The Hubble Deep Fields - ESA/Hubble.
37. Hubble Science Highlights - NASA.
38. NASA to Preview Katalyst Mission to Boost Swift Spacecraft's Orbit - NASA.