Hubble Space Telescope
The Hubble Space Telescope (HST) is a space-based optical and ultraviolet observatory launched by NASA on April 24, 1990, aboard the Space Shuttle Discovery (STS-31) into low Earth orbit at an altitude of approximately 547 kilometers, where it captures images free from atmospheric distortion across ultraviolet, visible, and near-infrared wavelengths.[1][2][3] Named after astronomer Edwin Hubble, who established the existence of galaxies beyond the Milky Way, the telescope's design incorporated modular components for on-orbit servicing, enabling five Space Shuttle missions that upgraded instruments and prolonged its lifespan well beyond initial projections.[1][4] Despite an initial setback from spherical aberration in its primary mirror—caused by a manufacturing error that blurred early observations—the flaw was rectified in 1993 via the installation of corrective optics (COSTAR), restoring and enhancing its imaging capabilities.[5][6] Hubble's observations have yielded transformative insights, including precise measurements of the universe's expansion rate (Hubble constant), the first direct evidence of planetary formation disks, exoplanet atmospheres, and the acceleration driven by dark energy, fundamentally reshaping models of cosmic evolution.[7][8] Continuing operations as of 2025, often in tandem with successors like the James Webb Space Telescope, Hubble exemplifies the value of maintainable space infrastructure in advancing empirical astronomy.[8][9]Development History
Proposals and Precursors
The idea of placing a telescope in orbit to escape Earth's atmospheric interference was proposed as early as 1923 by German rocket pioneer Hermann Oberth, who envisioned the potential for clearer observations beyond air turbulence and absorption.[10] In 1946, American astrophysicist Lyman Spitzer Jr. advanced the concept significantly with a report prepared for the RAND Corporation titled "Astronomical Advantages of an Extra-Terrestrial Observatory." This document detailed how a space telescope could achieve resolutions up to ten times sharper than ground-based instruments, enable ultraviolet spectroscopy unobstructed by ozone, and facilitate long-exposure imaging without atmospheric scintillation. Spitzer emphasized the causal benefits of vacuum operations, such as eliminating wavefront distortion from air density variations, and projected a 6-meter aperture as feasible with future launch capabilities.[10][11] Spitzer's advocacy persisted through the 1950s and 1960s, influencing NASA amid post-Sputnik space race priorities. Precursors included the Orbiting Astronomical Observatory (OAO) series, with OAO-1 launched unsuccessfully in 1966 and OAO-2 (Copernicus) operational from 1972 to 1981, demonstrating ultraviolet detectors and pointing stability for modest 0.38-meter telescopes. These missions validated key technologies like solar panels for power and attitude control, providing empirical data on orbital thermal management and instrument calibration in space.[1][12] Nancy Grace Roman, NASA's first chief of astronomy from 1959, formalized space-based optical astronomy by establishing programs for small observatories and convening panels in 1965 to scope a Large Space Telescope (LST). Her efforts bridged early proposals to engineering studies, securing initial funding commitments by 1972 for what evolved into the Hubble Space Telescope, named after Edwin Hubble for his empirical contributions to extragalactic distance measurements.[13][14]Funding Challenges and Political Battles
The development of the Large Space Telescope (LST), later renamed the Hubble Space Telescope, faced persistent funding obstacles from its inception, stemming from post-Apollo budget austerity and skepticism toward large-scale space projects amid competing national priorities like the Vietnam War and economic pressures. Lyman Spitzer first proposed a space-based observatory in 1946, advocating for it over three decades through reports and testimonies, but initial efforts yielded no dedicated appropriations as NASA prioritized crewed missions.[15][1] By 1969, NASA formally endorsed the concept following a National Academy of Sciences recommendation, yet fiscal constraints post-Apollo program limited progress, with congressional appropriations committees viewing the project as extravagant compared to ground-based alternatives.[1] In 1974, amid public spending reductions under President Gerald Ford, Congress eliminated all funding for the LST, with the House Appropriations Committee recommending a zero allocation, reflecting broader post-Watergate distrust of federal expenditures and NASA's ambitious requests.[15] This cut exacerbated design compromises already underway, including a reduction in the primary mirror diameter from 120 inches to 94 inches (2.4 meters) to align with Space Shuttle payload limits and cost targets.[1] The following year, 1975, NASA Administrator Noel Hinners further rejected the budget, prompting a "firestorm of protests" from the astronomical community.[15] Opposition arose from high projected costs—initial estimates exceeding capabilities in an era of fiscal conservatism—and debates over scientific priority, with critics arguing that atmospheric corrections on Earth could suffice, though advocates emphasized ultraviolet observations unobtainable from the ground.[15] Political battles intensified through 1976–1977, as astronomers led by John Bahcall and Robert O'Dell organized nationwide lobbying, securing letters from scientific societies and testimonies to counter NASA's perceived overreach in initial funding asks, which had alienated lawmakers.[15] To mitigate costs and build international support, NASA partnered with the European Space Agency in 1975, granting ESA 15% of observing time in exchange for 15% of funding, including contributions to the Faint Object Camera and solar arrays.[1] These efforts culminated in congressional approval in 1977, allocating $36 million for fiscal year 1978 to commence construction, though at roughly half the originally sought amount, necessitating further efficiencies.[16] This victory, attributed to Spitzer's persistent advocacy and community mobilization, averted cancellation but underscored the project's vulnerability to annual budget cycles and partisan scrutiny over non-military space spending.[15][17]Engineering and Construction Process
The engineering and construction of the Hubble Space Telescope involved collaboration between NASA and major contractors, with primary responsibility for the optical components assigned to Perkin-Elmer Corporation and the spacecraft bus to Lockheed Missile and Space Company. Following congressional approval of full funding in 1977, detailed design and fabrication commenced, focusing on a Ritchey-Chrétien optical system with a 2.4-meter primary mirror to achieve diffraction-limited performance above Earth's atmosphere. The project emphasized lightweight materials, thermal stability, and vibration isolation to ensure precise pointing and data collection in orbit.[1] Fabrication of the primary mirror began in 1979 at Perkin-Elmer's facility in Danbury, Connecticut, where the ULE glass blank—measuring 2.4 meters in diameter and weighing approximately 828 kilograms—was ground and polished to a prescribed hyperbolic aspheric figure. This process required iterative figuring over two years, achieving a surface accuracy of better than 1/20th of a wavelength at visible light, as verified through interferometric testing using a custom null corrector lens. The secondary mirror, 0.12 meters in diameter, underwent similar precision polishing to maintain the f/24 focal ratio of the optical telescope assembly (OTA). Completion of the mirrors occurred in 1981, after which they were coated with aluminum and magnesium fluoride for enhanced reflectivity.[5][18] Integration of the OTA proceeded at Perkin-Elmer, where the primary and secondary mirrors were mounted within a graphite-epoxy metering truss structure designed to maintain optical alignment under thermal variations from -150°C to +120°C. The OTA, weighing about 828 kilograms, incorporated baffles to suppress stray light and a fine guidance system for pointing stability to 0.007 arcseconds. Meanwhile, Lockheed in Sunnyvale, California, constructed the support systems module (SSM), a cylindrical bus 4.2 meters long and 3.6 meters in diameter, housing propulsion, power, communications, and computers using aluminum honeycomb panels for rigidity and low mass, totaling 11,110 kilograms for the fully integrated observatory.[19][20] Final assembly occurred in Lockheed's cleanroom facility starting in the early 1980s, combining the OTA with the SSM and initial instruments such as the Wide Field and Planetary Camera, Faint Object Spectrograph, and High Speed Photometer. This phase included subsystem verifications, electromagnetic compatibility tests, and acoustic simulations to replicate launch vibrations. Thermal-vacuum testing in a 17-meter chamber simulated space conditions, confirming operational integrity across temperature extremes. By 1985, construction was complete, with the telescope shipped to NASA's Kennedy Space Center for storage and pre-launch preparations, marking the culmination of over a decade of engineering effort amid budget constraints and technical refinements.[21][22]Initial Instruments and Ground Support
The Hubble Space Telescope launched on April 24, 1990, equipped with five primary scientific instruments designed to exploit its ultraviolet and optical capabilities beyond Earth's atmospheric interference: the Wide Field and Planetary Camera (WF/PC), Faint Object Camera (FOC), Faint Object Spectrograph (FOS), Goddard High Resolution Spectrograph (GHRS), and High Speed Photometer (HSP).[23][24] The WF/PC, developed by NASA's Jet Propulsion Laboratory, served as the primary imaging system, capable of capturing wide-field views of extended objects and high-resolution planetary images across optical wavelengths using relay optics to sample different focal plane positions.[25] The FOC, built by ESA, specialized in ultraviolet imaging of faint, point-like sources with angular resolutions up to 0.05 arcseconds, utilizing redundant detector arrays for deep-space observations.[24] Complementing it, the FOS, a joint NASA-ESA project, performed ultraviolet spectroscopy on faint astronomical objects, resolving spectral features from quasars and galaxies with resolutions up to 1,000–4,000.[24] The GHRS, constructed at Goddard Space Flight Center, focused on high-resolution ultraviolet echelle spectroscopy, achieving resolutions exceeding 85,000 to study stellar atmospheres and interstellar medium absorption lines.[23] The HSP, developed by the University of Wisconsin, measured rapid photometric variations in bright sources, timing fluctuations on millisecond scales for phenomena like binary stars and gamma-ray burst counterparts.[23] Additionally, three Fine Guidance Sensors (FGS)—precision astrometric devices—provided pointing accuracy better than 0.007 arcseconds and supported scientific interferometry for relative position measurements of stars.[26] Ground support for Hubble's initial operations centered on the Space Telescope Operations Control Center (STOCC) at NASA's Goddard Space Flight Center in Greenbelt, Maryland, which managed real-time commanding, telemetry monitoring, and anomaly resolution through a 24/7 team of flight controllers.[27][28] The STOCC coordinated with the Tracking and Data Relay Satellite System (TDRS), a constellation of geosynchronous satellites enabling high-rate data downlink at up to 5.76 Mbps via S-band and Ku-band links, supplemented by direct contacts with five ground stations for redundancy.[27][29] Raw science and engineering data were processed at the Science Data Operations Center at the Space Telescope Science Institute in Baltimore, where calibration pipelines transformed observations into calibrated datasets for distribution to astronomers worldwide.[27] This infrastructure supported an initial observing schedule of approximately 1,000–1,500 hours annually, prioritizing Guest Observer proposals while allocating time for calibration and engineering tests.[30]Launch and Early Operations
Pre-Launch Delays Including Challenger Disaster
The Hubble Space Telescope (HST) faced multiple delays during its development and preparation phases, with an initial target launch date of 1986 slipping due to technical integration challenges, instrument testing, and spacecraft assembly issues at contractors like Lockheed Missiles and Space Company.[31] By 1985, construction of the observatory was largely complete, but final preparations were ongoing for a Space Shuttle deployment.[21] The most significant delay occurred following the Space Shuttle Challenger disaster on January 28, 1986, during mission STS-51-L, when the orbiter disintegrated 73 seconds after liftoff due to the failure of an O-ring seal in one of its solid rocket boosters, exacerbated by unusually cold launch temperatures and prior warnings about joint vulnerabilities that NASA and contractor Morton Thiokol had downplayed to adhere to a compressed flight manifest.[32] This tragedy killed all seven crew members and grounded the entire Shuttle fleet for 32 months while investigations revealed systemic flaws in NASA's safety culture, including pressure to maintain an ambitious launch cadence despite engineering concerns.[32] As HST was exclusively designed for Shuttle servicing and deployment, the halt in flights directly postponed its mission, which had been slated for late 1986.[23] Shuttle operations resumed on September 29, 1988, with STS-26, but a backlog of priority missions—including military payloads and delayed scientific flights—further deferred HST amid revised safety protocols and fleet recovery efforts.[33] Additional schedule adjustments in 1989, driven by Shuttle vehicle readiness issues such as refurbishments to orbiter Columbia, pushed HST's launch window by three to five months, ultimately to April 1990 aboard STS-31 on Discovery.[34] During the extended ground period exceeding four years from the original target, HST remained in powered storage within a cleanroom at NASA's Goddard Space Flight Center to preserve its systems and prevent contamination.[33] These delays, while frustrating for scientists anticipating ultraviolet observations unobtainable from ground-based telescopes, underscored the causal risks of relying on a reusable launch vehicle with unproven long-term reliability for irreplaceable payloads.[31]1990 Launch and Deployment
The Hubble Space Telescope was launched on April 24, 1990, at 8:33 a.m. EDT (12:33:51 UTC) from Launch Complex 39B at NASA's Kennedy Space Center in Florida, aboard the Space Shuttle Discovery as the primary payload of mission STS-31.[35][36] This marked the 35th Space Shuttle mission and Discovery's 10th flight, with the crew targeting a high-altitude orbit to accommodate the telescope's operational requirements.[35] The five-member crew included Commander Loren J. Shriver, Pilot Charles F. Bolden Jr., and Mission Specialists Steven A. Hawley, Bruce McCandless II, and Kathryn D. Sullivan.[35] The following day, on April 25, 1990, the crew executed the deployment sequence after Discovery reached its operational orbit.[37] Mission Specialist Steven A. Hawley operated the shuttle's Remote Manipulator System (RMS) to grasp and lift the 11-meter-long, 2.4-meter-diameter telescope from its cradle in the payload bay, suspending it above the orbiter for final checks.[38] Ground teams commanded the extension of the telescope's twin solar arrays and high-gain antennas, each process verified via telemetry to ensure structural integrity before proceeding.[38] At approximately 3:38 p.m. UTC, the HST was released into a nearly circular low Earth orbit at an altitude of 380 statute miles (612 km) and 28.5-degree inclination, the highest orbit achieved by a Space Shuttle up to that point.[35][39] Post-release, the shuttle crew performed separation burns to create safe distance, preventing potential recontact during the telescope's initial stabilization maneuvers.[40] Ground control at NASA's Goddard Space Flight Center established communications with the HST, initiating activation of its onboard systems, including the opening of the aperture door for future observations.[41] The deployment concluded successfully without anomalies, allowing the mission to shift focus to secondary experiments before Discovery's return to Earth on April 29, 1990, after 5 days, 1 hour, and 16 minutes in space.[35]Discovery of the Flawed Mirror
Following the Hubble Space Telescope's deployment on April 25, 1990, initial on-orbit checkout and calibration activities commenced, including tests with the Fine Guidance Sensors and early imaging from instruments such as the Wide Field and Planetary Camera (WFPC) and Faint Object Camera (FOC).[5] These observations, starting in late May 1990, produced images of star fields where point sources exhibited a characteristic diffuse halo surrounding a dim central core, rather than the expected diffraction-limited sharp Airy disks, indicating a systematic optical defect compromising resolution across all wavelengths.[5] [42] Engineers and scientists at NASA's Goddard Space Flight Center and the Space Telescope Science Institute analyzed the anomalous point spread functions (PSFs), which showed approximately 15-20% of incoming light focusing to the nominal focal plane while the remainder formed an extended halo, reducing effective resolution to about 1/7th of the design goal.[5] Diagnostic tests ruled out misalignment of the secondary mirror or instrument-specific issues, as the aberration persisted uniformly in data from multiple cameras and spectrographs.[43] Preliminary modeling attributed the symptoms to spherical aberration in the 2.4-meter primary mirror, with the outer zones polished excessively flat by roughly 2.2 micrometers relative to the intended aspheric figure.[44] [45] On June 27, 1990, NASA publicly announced the optical flaw, confirming spherical aberration as the primary cause of the telescope's degraded performance, which equated to a wavefront error of about 0.4 waves RMS at 632.8 nm.[5] [16] To investigate further, NASA established the Hubble Space Telescope Optical Systems Board of Investigation on July 2, 1990, comprising experts from NASA, contractors, and academia, who conducted ground-based simulations and traced the error to manufacturing tolerances during mirror figuring by Perkin-Elmer Corporation.[5] The board's analysis, completed in November 1990, verified that the primary mirror's conic constant deviated from specifications, rendering the telescope myopic but salvageable via corrective optics during a planned servicing mission.[43] Despite the flaw, limited science operations continued, yielding data that, while suboptimal, still advanced astrophysical research pending repairs.[46]Technical Design and Systems
Optical Telescope Assembly
The Optical Telescope Assembly (OTA) of the Hubble Space Telescope utilizes a Ritchey-Chrétien Cassegrain reflector design, which employs hyperbolic surfaces on both mirrors to correct for coma and provide a flat focal plane with reduced spherical aberration compared to parabolic designs. This configuration consists of a primary concave mirror 2.4 meters in diameter with an f/2.3 focal ratio and a secondary convex mirror 0.34 meters in diameter, separated by 4.9 meters along the optical axis.[5][47][48] Both mirrors are fabricated from Corning's Ultra-Low Expansion (ULE) glass, a titanium-silicate composite engineered for a coefficient of thermal expansion near zero (0 ± 30 ppb/°C), minimizing distortions due to orbital temperature fluctuations between -100°C and 100°C. The reflecting surfaces receive vacuum-deposited aluminum coatings overlaid with magnesium fluoride to achieve high reflectivity exceeding 80% in the ultraviolet and visible ranges, enabling observations from about 115 nm to 1 μm.[49][50][51] The assembled OTA yields a system focal length of 57.6 meters and an effective f/24 focal ratio, focusing incoming parallel rays onto the instrument focal plane after reflection from the secondary mirror. Internal baffles and aperture stops obscure approximately 15% of the primary mirror's area to block stray and off-axis light, while thermoelectric coolers and multilayer insulation maintain mirror alignment and figure stability to within micrometers. This design supports diffraction-limited angular resolution of 0.05 arcseconds at visible wavelengths, far surpassing ground-based telescopes limited by atmospheric seeing.[52][53][47]Spacecraft Structure and Propulsion
The Hubble Space Telescope's spacecraft structure integrates the forward Optical Telescope Assembly (OTA) with the aft Support Systems Module (SSM), forming a cylindrical configuration approximately 13.2 meters in length and 4.2 meters in diameter, with a mass of about 12,246 kg at launch.[19] The SSM consists of stacked interlocking cylindrical shells constructed from aluminum and magnesium alloys, topped by a 3-meter-diameter aperture door made of honeycombed aluminum and featuring an aft bulkhead for structural integrity.[54] This modular design encloses electronics in equipment bays, with the outer shroud protected by multi-layer insulation (MLI) and a lightweight aluminum shell over a graphite-epoxy composite frame to ensure rigidity and thermal control in orbit.[19][54] Hubble lacks a primary propulsion system for major orbital adjustments, depending on servicing spacecraft such as the Space Shuttle for altitude boosts to counteract atmospheric drag.[19] Attitude determination and control rely on a momentum-based system featuring four reaction wheel assemblies (RWAs), each 0.58 meters in diameter and weighing 45 kg, capable of accelerating to 3000 rpm for precise pointing with 0.01 arcsecond accuracy and 0.007 arcsecond stability over extended periods.[54] Wheel momentum desaturation is achieved via four magnetic torquer bars, each 2.5 meters long and 45 kg, which generate torque through interaction with Earth's magnetic field, eliminating the need for chemical propellants and enabling propellant-free fine guidance.[54] This design supports the telescope's stringent observational requirements without onboard thrusters.[19]Computer and Control Systems
The Hubble Space Telescope's onboard computer systems primarily consist of the flight computer and the Science Instrument Control and Data Handling (SI C&DH) module. The original flight computer, a DF-224 system developed by Rockwell Autonetics, operated at 1.25 MHz and served as the central processor for monitoring the observatory's health, executing commands, and managing attitude control.[55] This computer handled essential functions including the processing of telemetry data and coordination with subsystems like propulsion and pointing control.[55] During Servicing Mission 1 in 1993, a coprocessor based on an Intel 386 was added to enhance performance, addressing degradation in the aging DF-224 hardware.[56] A major upgrade occurred in Servicing Mission 3A in December 1999, replacing the DF-224 with the Advanced Computer (AC), which featured an Intel 80486 processor running at 25 MHz, providing 20 times the processing speed and six times the memory capacity of its predecessor.[55][57] This upgrade improved real-time data handling and responsiveness for complex observation sequences. The SI C&DH subsystem, distinct from the flight computer, interfaces directly with the science instruments, synchronizing operations, formatting data, and relaying it to the Data Management Unit (DMU) for transmission to Earth.[58] A new SI C&DH module, incorporating updated electronics for enhanced reliability, was installed during Servicing Mission 4 in May 2009.[59] The Pointing Control System (PCS) integrates with these computers to maintain precise attitude determination and slewing capabilities, using inputs from gyroscopes, fine guidance sensors, and reaction wheels to achieve pointing accuracy better than 0.007 arcseconds.[60] Flight software, which governs these operations, has undergone multiple updates to resolve anomalies, such as the 2021 safe mode entry due to a software error in the payload computer, with patches uploaded to restore functionality.[61] Ground control is managed from the Space Telescope Operations Control Center (STOCC) at NASA's Goddard Space Flight Center, established in 1984, where engineers monitor telemetry, generate command loads, and oversee safing procedures.[27] The STOCC processes signals including ground commands, onboard telemetry, and engineering data via the Tracking and Data Relay Satellite System (TDRSS) using S-band frequencies.[29] Redundant systems and automated fault protection ensure continuity, with the operations team capable of switching to backup hardware, as demonstrated in 2021 when the payload computer was transitioned to resolve synchronization issues.[62]Instrument Suite Evolution
The Hubble Space Telescope launched on April 24, 1990, with an initial instrument suite consisting of five primary scientific instruments: the Wide Field and Planetary Camera (WF/PC) for imaging, the Faint Object Camera (FOC) for ultraviolet imaging, the Faint Object Spectrograph (FOS) for spectroscopy, the Goddard High Resolution Spectrograph (GHRS) for high-resolution ultraviolet spectroscopy, and the High Speed Photometer (HSP) for rapid photometry.[23][63] These instruments operated from radial and axial bays, but the primary mirror's spherical aberration, discovered in June 1990, degraded their performance by spreading light and reducing resolution across the board.[64] During Servicing Mission 1 (SM1) in December 1993, astronauts replaced the WF/PC with the Wide Field and Planetary Camera 2 (WFPC2), which incorporated internal corrective optics to compensate for the aberration, enabling sharper wide-field imaging in visible and ultraviolet wavelengths.[6] They also installed the Corrective Optics Space Telescope Axial Replacement (COSTAR), a set of five mirrors that provided optical correction to the axial instruments (FOC, FOS, and GHRS), restoring their functionality without modifying the instruments themselves.[6] The HSP remained installed but yielded limited science due to inadequate correction.[16] Servicing Mission 2 (SM2) in February 1997 marked further evolution by replacing the GHRS with the Space Telescope Imaging Spectrograph (STIS), a versatile instrument for imaging spectroscopy across ultraviolet, visible, and near-infrared wavelengths, and removing the FOS in favor of the Near Infrared Camera and Multi-Object Spectrometer (NICMOS), which extended observations into the infrared for the first time.[16] The HSP was decommissioned and removed during this mission, freeing a bay, while the FOC continued operations with COSTAR corrections until its later replacement.[4] NICMOS initially operated with a passive cooler but faced thermal challenges, leading to a temporary hiatus. Servicing Mission 3B (SM3B) in March 2002 introduced the Advanced Camera for Surveys (ACS), replacing the FOC and providing high-resolution imaging and spectroscopy in visible and ultraviolet light with a wider field of view, significantly boosting discovery rates for distant galaxies and clusters.[65] A mechanical cryocooler was also installed to revive NICMOS infrared capabilities.[16] The final Servicing Mission 4 (SM4) in May 2009 represented the pinnacle of instrument evolution: WFPC2 was replaced by the Wide Field Camera 3 (WFC3), offering panchromatic imaging from ultraviolet through near-infrared with improved sensitivity and field of view; the Cosmic Origins Spectrograph (COS) was installed in an axial bay, excelling in high-sensitivity ultraviolet spectroscopy for probing cosmic gases; COSTAR was removed as its corrections were obsolete with built-in optics in newer instruments; and repairs extended the life of STIS and ACS.[66][67] These upgrades, leveraging modular design, extended Hubble's scientific productivity into the 2020s, with WFC3 and COS remaining active as of 2025.[4]Servicing Missions
Servicing Mission Overview and Logistics
The Hubble Space Telescope was engineered from its inception for on-orbit servicing, incorporating modular Orbital Replacement Units (ORUs) such as instruments, gyroscopes, and solar arrays that could be swapped by astronauts during extravehicular activities (EVAs).[3] This design facilitated five dedicated servicing missions flown by NASA Space Shuttle crews between December 1993 and May 2009, extending the telescope's operational lifespan beyond its initial five-year projection and enabling hardware upgrades that enhanced its scientific capabilities.[4] Each mission required meticulous planning by the Goddard Space Flight Center's Hubble Operations Project, involving extensive ground simulations, crew training at the Neutral Buoyancy Laboratory, and coordination with the Space Telescope Science Institute for instrument integration.[68] Logistically, missions commenced with Space Shuttle launches from Kennedy Space Center, followed by orbital rendezvous maneuvers to match Hubble's 28.5-degree inclination low Earth orbit at approximately 540 kilometers altitude.[4] Upon arrival, the shuttle's Remote Manipulator System (RMS), or Canadarm, grappled the telescope via a dedicated fixture, berthing it securely in the payload bay for stability during EVAs.[68] Typical mission durations spanned 8 to 13 days, with crews of seven astronauts—comprising pilots, mission specialists trained in Hubble-specific procedures, and EVA experts—conducting 4 to 5 spacewalks per flight, each lasting 6 to 8 hours.[4] Tools and spare ORUs were stowed in the payload bay or on pallets, with contingency plans including onboard repair kits and, for later missions post-Columbia disaster, enhanced thermal protection inspections and potential safe-haven docking capability with the International Space Station.[68]| Servicing Mission | Launch Date | Shuttle Mission | Duration (Days) | Number of EVAs |
|---|---|---|---|---|
| SM1 | December 2, 1993 | STS-61 (Endeavour) | 10.8 | 5 |
| SM2 | February 11, 1997 | STS-82 (Discovery) | 9.9 | 4 |
| SM3A | December 22, 1999 | STS-103 (Discovery) | 7.9 | 3 |
| SM3B | March 1, 2002 | STS-109 (Columbia) | 10.8 | 4 |
| SM4 | May 11, 2009 | STS-125 (Atlantis) | 12.9 | 5 |