Hubble: keeping an observatory working across 200,000 orbits

A milestone during a repeat observation

Hubble completed its 200,000th Earth orbit on Sep 19, 2026, after more than 36 years, over 5 billion miles of orbital travel and 1.7 million observations. Designed for 15 years, the observatory had reached 100,000 orbits in 2008 and its millionth science observation in 2011. Its approximately 95-minute circuit, around 300 miles above Earth at 17,000 mph, supports ultraviolet, visible and near-infrared observations above atmospheric distortion.

The milestone coincided with monitoring galaxy cluster MACS J0417 for supernova Athena, discovered by Webb in 2025. Different gravitationally lensed light paths could reveal the same explosion again, potentially measuring the cluster’s mass and cosmic expansion. NASA predicted a reappearance by early March 2027. Following the final astronaut servicing in 2009, no further servicing mission was planned. Continued science into the next decade and re-entry no earlier than the mid-2030s remained expectations, not guarantees; atmospheric drag lowers the orbit.

An observatory designed for maintenance

Hubble’s optical system began with a 2.4-metre primary mirror. Its planned maintenance strategy changed before launch: in 1983, managers abandoned returning the telescope to Earth for refurbishment in favour of astronaut work in orbit. Discovery carried the observatory into space on Apr 24, 1990, during STS-31. Its initial instrument complement included cameras, spectrographs and a high-speed photometer. Steven Hawley used the shuttle’s robotic arm to deploy the telescope after checks of its solar arrays and antennas.

The first images exposed spherical aberration. The primary mirror’s outer regions were too flat by 0.003 mm, impairing image sharpness. Engineers designed corrective optics rather than replacing or physically reshaping that mirror in orbit. During STS-61 in December 1993, astronauts installed COSTAR in place of the High-Speed Photometer and replaced the original Wide Field/Planetary Camera with an improved version. New solar arrays also reduced unwanted vibration. Images obtained within a month showed the restored optical capability. The correction and subsequent upgrades were carried out through the telescope’s serviceable structure, while the original primary mirror remained aboard.

Five spacewalks renew instruments and supporting systems

Atlantis launched the final servicing crew on May 11, 2009. The shuttle captured Hubble on May 13, beginning a programme of five spacewalks. John Grunsfeld and Andrew Feustel installed Wide Field Camera 3 on May 14, replacing Wide Field and Planetary Camera 2. The work also included replacement of the Science Instrument Command and Data Handling system, which coordinated instruments and prepared data for transmission to Earth. The subsequent work covered both observing equipment and infrastructure:

• Michael Massimino and Michael Good replaced the three gyroscope rate-sensing units and a battery module.
• Grunsfeld and Feustel installed the Cosmic Origins Spectrograph and repaired the Advanced Camera for Surveys.
• Massimino and Good restored the Space Telescope Imaging Spectrograph by replacing an electronics card.
• The final spacewalk added the second battery module, a refurbished Fine Guidance Sensor and protective outer insulation.

Hubble was released on May 19. Atlantis landed on May 24, ending the mission.

A camera contains several different observing channels

The Advanced Camera for Surveys, installed on Mar 7, 2002, extended Hubble’s ability to map large areas with high sensitivity. By its twentieth anniversary in 2022, it had taken more than 125,000 pictures. Its wavelength coverage runs from ultraviolet through visible light into the near-infrared. A grism also allows spectroscopy. The instrument comprises three channels with different functions, rather than a single interchangeable camera mode.

The Wide Field Channel records optical and near-infrared images suited to distant galaxies and clusters. The Solar Blind Channel blocks visible light to reveal faint ultraviolet emission. The High Resolution Channel was designed for detailed views including galactic centres, but an electronics failure in January 2007 disabled it and the Wide Field Channel. The 2009 servicing repaired the latter; the High Resolution Channel remained unavailable. ACS supported the Ultra Deep Field and Frontier Fields. The latter combined imaging and gravitational lenses, observing six clusters and six parallel fields over 840 orbits.

Ultraviolet spectra become a shared stellar library

ULLYSES—the Ultraviolet Legacy Library of Young Stars as Essential Standards—completed its principal survey in December 2023. The programme obtained new observations of 220 stars and combined them with archived Hubble measurements of 275 others. The resulting library approached 500 stars and incorporated complementary observations from ground-based facilities and X-ray space telescopes. Its main products were spectra carrying information about temperature, composition and rotation.

The target selection covered two contrasting groups. Massive, hot blue stars in nearby galaxies with relatively few heavy elements provided information about stellar winds and conditions relevant to early galaxies. Cooler young stars less massive than the Sun were still gathering material from surrounding discs. Their ultraviolet emission helped researchers investigate accretion and the energy reaching planet-forming material. Julia Roman-Duval of the Space Telescope Science Institute led implementation of the programme and anticipated uses beyond its original goals. The library’s completion was therefore the end of a coordinated acquisition programme, while research using its combined observations remained ongoing.

Ground teams recover the science-data system

On Jun 13, 2021, Hubble’s payload computer stopped communicating normally, placing the science instruments in safe mode. Tests of memory modules and the backup computer did not resolve the problem. Engineers then investigated other parts of the Science Instrument Command and Data Handling unit. Their analysis pointed to the Power Control Unit as a possible cause, rather than establishing that the initial suspected memory fault was responsible.

More than 50 people reviewed and tested procedures for switching to backup hardware. The preparation used a high-fidelity simulator and knowledge from former Hubble staff, including decades-old documentation. On Jul 15, the team transferred operations to the backup side of the unit, including its power-control hardware. Jim Jeletic, deputy project manager at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, described 15 hours of spacecraft commanding. Scientific observations resumed on Jul 17. The recovery involved coordinating several hardware changes; it was not simply a successful restart of the original payload computer. Most observations missed during the interruption were to be rescheduled.

One-gyro operation changes scheduling constraints

Hubble entered safe mode on May 24, 2024 after recurring faulty gyroscope readings. One unit increasingly indicated its maximum measurable rotation rate regardless of the telescope’s actual movement. Electronic resets restored normal readings only temporarily. NASA announced a transition on Jun 4 and returned Hubble to daily science operations on Jun 14. The new mode used one gyro while retaining another functional unit for future use; it did not mean that only one operational gyro existed.

Gyroscopes measure changes in pointing as the spacecraft turns. Using fewer of them affects the process of acquiring targets and the accessible sky at a particular time. NASA expected longer slews and target locking, less scheduling flexibility and an inability to track moving objects closer than Mars. Most science observations could continue. Hubble had operated with two gyros from 2005–2009, and one-gyro operation had been demonstrated briefly in 2008. The later transition required changes to spacecraft and ground systems as well as reviews of planned observations.

Many observing visits become one galactic mosaic

The Andromeda panorama released on Jan 16, 2025 combined observations accumulated over more than a decade and over 1,000 Hubble orbits. Approximately 600 overlapping fields covered the galaxy’s disc, producing a mosaic of at least 2.5 billion pixels. Andromeda lies about 2.5 million light-years away and appears tilted 77° to our view. The survey resolved around 200 million stars, a fraction of its estimated trillion-star population.

The work joined the Panchromatic Hubble Andromeda Treasury, or PHAT, with the later Southern Treasury survey, PHAST. PHAT used ACS and Wide Field Camera 3 to observe the northern portion at near-ultraviolet, visible and near-infrared wavelengths. PHAST added approximately 100 million stars in the southern portion. The resolved stars supplied measurements relevant to age, heavy-element abundance and stellar mass. Researchers used those properties to compare possible merger and disc-evolution histories. The stellar census has a sensitivity limit: many stars less massive than the Sun remain below the survey’s detection threshold.

Annual planetary observations build a climate record

The Outer Planet Atmospheres Legacy programme, OPAL, repeatedly observes Jupiter, Saturn, Uranus and Neptune. Its ten-year review in December 2024 described annual observations when the planets were closest to Earth. A consistent series allows atmospheric changes to be compared across years, rather than inferred from a single encounter. The observations sample wavelengths from ultraviolet to near-infrared, with different filters revealing cloud properties and atmospheric structure.

Jupiter’s images track cloud motions, storms and changes in the Great Red Spot. Saturn’s observations extended atmospheric monitoring beyond the Cassini mission and recorded changes in cloud colour, ring orientation and transient ring spokes. Uranus offered a different seasonal geometry because its axis lies almost in its orbital plane; Hubble followed the brightening northern polar cap. Neptune’s dark storms appeared and disappeared over time, and its cloud abundance showed a relationship with solar activity. These are distinct physical records, not observations of four planets undergoing an identical cycle. Amy Simon of NASA’s Goddard Space Flight Center noted that OPAL data had already supported more than 60 publications by the 2024 review.

A second observatory tests crowded stellar measurements

Hubble’s measurements of Cepheid variable stars form part of the cosmic distance ladder: nearby calibrations support measurements farther away, including those based on Type Ia supernovae. A potential complication is crowding. Light from neighbouring stars can blend with a Cepheid, altering its apparent brightness. Dust adds another uncertainty. Webb’s sharper near-infrared view provided a way to examine those effects in stars previously measured by Hubble.

Adam Riess’s SH0ES team compared observations covering five host galaxies of eight Type Ia supernovae and about 1,000 Cepheids. The sample extended to NGC 5468, approximately 130 million light-years away. Results described in March 2024 supported the reliability of the Hubble Cepheid measurements across that distance range. Riess argued that stellar crowding could not explain the discrepancy between the local expansion rate and predictions based on the early universe. The test addressed a proposed observational bias; it did not identify the physical cause of the Hubble tension or independently eliminate every uncertainty in cosmology. The combination used the different resolving capabilities of the two telescopes on the same distance indicators.

Testing a prediction with a delayed supernova image

Long before Athena, Hubble monitored supernova Refsdal behind galaxy cluster MACS J1149.5+2223. Four images seen in November 2014 formed an Einstein Cross around a foreground galaxy. The host galaxy was itself multiply imaged by the cluster. Unevenly distributed ordinary and dark matter created different paths for the light, allowing researchers to predict another appearance of the same explosion at a different position.

Seven models of the cluster, informed by Hubble, VLT-MUSE and Keck observations, predicted broadly similar timing. Hubble began regular monitoring at the end of October 2015 and detected the new appearance on Dec 11. The calculations also indicated an earlier appearance in 1998, which no telescope had observed. That distinction separates an actual detection from a retrospective model prediction. Tommaso Treu led the model comparison, while Patrick Kelly led discovery and reappearance studies. The observations tested the models’ mass distributions and time delays. Sjur Refsdal, for whom the supernova was named, had proposed using delayed supernova images to investigate cosmic expansion in 1964.

One exposure can contain several emission times

A separate result announced in November 2022 showed three stages of a distant supernova in a single Hubble exposure. The foreground cluster Abell 370 bent and magnified light from an explosion more than 11 billion years in the past. The images arrived together, but their different paths carried light emitted at different stages. They were views of one event, rather than three stars exploding simultaneously.

The colour differences traced cooling over approximately a week: the earliest appearance was bluer, and the later views were redder. Brightness and the cooling rate allowed researchers to infer a red-supergiant progenitor approximately 500 times the Sun’s size. Wenlei Chen and Patrick Kelly found the event while examining archived Hubble data for transients. Machine-learning searches helped identify candidates, but this was the only transient found in that investigation. The study illustrates a different use of time delays from waiting for a future reappearance: several past stages were already present within one stored image. The result depended on both lensing geometry and the ability to recover and analyse earlier telescope observations.

Curved trails reveal objects outside the original target

Hubble’s archive also contains foreground objects that were not the intended observing targets. An asteroid search reported in April 2024 examined 37,000 images spanning 19 years. Researchers identified 1,701 trails, including 1,031 associated with previously uncatalogued asteroids. About 400 of the uncatalogued objects were estimated to be smaller than a kilometre. Most belonged to the main belt between Mars and Jupiter.

The search combined work by 11,482 volunteers with automated classification. Their nearly two million identifications supplied training material for the algorithm. Hubble’s movement around Earth changes the viewing direction during an exposure, causing an asteroid’s trail to curve. Combining that curvature with the spacecraft’s known position allows a distance estimate; brightness and distance then constrain an estimated size. The method extracts additional information from observations made for other scientific purposes. It has a limitation: many trails were recorded years earlier, so the objects could not simply be revisited immediately to determine their full orbits. The archived detections therefore provided a population sample, rather than a guarantee of a precisely tracked orbit for each new object.