The Hubble Space Telescope (HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most versatile, renowned as a vital research tool and as a public relations boon for astronomy. The Hubble Space Telescope is named after astronomer Edwin Hubble and is one of NASA's Great Observatories. The Space Telescope Science Institute (STScI) selects Hubble's targets and processes the resulting data, while the Goddard Space Flight Center (GSFC) controls the spacecraft.
Hubble features a mirror, and its five main instruments observe in the ultraviolet, visible, and near-infrared regions of the electromagnetic spectrum. Hubble's orbit outside the distortion of Earth's atmosphere allows it to capture extremely high-resolution images with substantially lower background light than ground-based telescopes. It has recorded some of the most detailed visible light images, allowing a deep view into space. Many Hubble observations have led to breakthroughs in astrophysics, such as determining the rate of expansion of the universe. The Hubble Space Telescope was funded and built in the 1970s by NASA with contributions from the European Space Agency. Its intended launch was in 1983, but the project was beset by technical delays, budget problems, and the 1986 Challenger disaster.
Hubble was launched on STS-31 in 1990, but its main mirror had been ground incorrectly, resulting in spherical aberration that compromised the telescope's capabilities. The optics were corrected to their intended quality by a servicing mission, STS-61, in 1993. Hubble is the only telescope designed to be maintained in space by astronauts. Five Space Shuttle missions repaired, upgraded, and replaced systems on the telescope, including all five of the main instruments. The fifth mission was initially canceled on safety grounds following the Columbia disaster (2003), but after NASA administrator Michael D. Griffin approved it, the servicing mission was completed in 2009. Hubble completed 30 years of operation in April 2020 and is predicted to last until 2030 to 2040.
Hubble is the visible light telescope in NASA's Great Observatories program; other parts of the spectrum are covered by the Compton Gamma Ray Observatory, the Chandra X-ray Observatory, and the Spitzer Space Telescope (which covers the infrared bands). The mid-IR-to-visible band successors to the Hubble telescope are the James Webb Space Telescope (JWST), which was launched on December 25, 2021, and the Nancy Grace Roman Space Telescope which was launched on August 30, 2026.
Concept, design and aim
Proposals and precursors In 1923, Hermann Oberth, considered a father of modern rocketry along with Robert H. Goddard and Konstantin Tsiolkovsky, published ("The Rocket into Planetary Space"), which mentioned how a telescope could be propelled into Earth orbit by a rocket.
The history of the Hubble Space Telescope can be traced to 1946, to astronomer Lyman Spitzer's paper "Astronomical advantages of an extraterrestrial observatory".Spitzer, Lyman Jr., "Report to Project Rand: Astronomical Advantages of an Extra-Terrestrial Observatory", reprinted in NASA SP-2001-4407: Exploring the Unknown , Chapter 3, Document III-1, p. 546. In it, he discussed the two main advantages that a space-based observatory would have over ground-based telescopes. First, the angular resolution (the smallest separation at which objects can be clearly distinguished) would be limited only by diffraction, rather than by the turbulence in the atmosphere, which causes stars to twinkle, known to astronomers as seeing. At that time ground-based telescopes were limited to resolutions of 0.5–1.0 arcseconds, compared to a theoretical diffraction-limited resolution of about 0.05 arcsec for an optical telescope with a mirror in diameter. Second, a space-based telescope could observe infrared and ultraviolet light, which are strongly absorbed by the atmosphere of Earth.
Spitzer devoted much of his career to pushing for the development of a space telescope. In 1962, a report by the U.S. National Academy of Sciences recommended development of a space telescope as part of the space program, and in 1965, Spitzer was appointed as head of a committee given the task of defining scientific objectives for a large space telescope.
Also crucial was the work of Nancy Grace Roman, the "Mother of Hubble". Well before it became an official NASA project, she gave public lectures touting the scientific value of the telescope. After it was approved, she became the program scientist, setting up the steering committee in charge of making astronomer needs feasible to implement and writing testimony to Congress throughout the 1970s to advocate continued funding of the telescope. Her work as project scientist helped set the standards for NASA's operation of large scientific projects.
Space-based astronomy had begun on a very small scale following World War II, as scientists made use of developments that had taken place in rocket technology. The first ultraviolet spectrum of the Sun was obtained in 1946, and NASA launched the Orbiting Solar Observatory (OSO) to obtain UV, X-ray, and gamma-ray spectra in 1962. An orbiting solar telescope was launched in 1962 by the United Kingdom as part of the Ariel programme, and in 1966 NASA launched the first Orbiting Astronomical Observatory (OAO) mission. OAO-1's battery failed after three days, terminating the mission. It was followed by Orbiting Astronomical Observatory 2 (OAO-2), which carried out ultraviolet observations of stars and galaxies from its launch in 1968 until 1972, well beyond its original planned lifetime of one year.
The OSO and OAO missions demonstrated the important role space-based observations could play in astronomy. In 1968, NASA developed firm plans for a space-based reflecting telescope with a mirror in diameter, known provisionally as the Large Orbiting Telescope or Large Space Telescope (LST), with a launch slated for 1979. These plans emphasized the need for crewed maintenance missions to the telescope to ensure such a costly program had a lengthy working life, and the concurrent development of plans for the reusable Space Shuttle indicated that the technology to allow this was soon to become available.
Quest for funding The continuing success of the OAO program encouraged increasingly strong consensus within the astronomical community that the LST should be a major goal. In 1970, NASA established two committees, one to plan the engineering side of the space telescope project, and the other to determine the scientific goals of the mission. Once these had been established, the next hurdle for NASA was to obtain funding for the instrument, which would be far more costly than any Earth-based telescope. The U.S. Congress questioned many aspects of the proposed budget for the telescope and forced cuts in the budget for the planning stages, which at the time consisted of very detailed studies of potential instruments and hardware for the telescope. In 1974, public spending cuts led to Congress deleting all funding for the telescope project.
In 1977, then NASA Administrator James C. Fletcher proposed a token $5million for Hubble in NASA's budget. Then NASA Associate Administrator for Space Science, Noel Hinners, instead cut all funding for Hubble, gambling that this would galvanize the scientific community into fighting for full funding. As Hinners recalls:
The political ploy worked. In response to Hubble being zeroed out of NASA's budget, a nationwide lobbying effort was coordinated among astronomers. Many astronomers met congressmen and senators in person, and large-scale letter-writing campaigns were organized. The National Academy of Sciences published a report emphasizing the need for a space telescope, and eventually, the Senate agreed to half the budget that had originally been approved by Congress.
The funding issues led to a reduction in the scale of the project, with the proposed mirror diameter reduced from 3 m to 2.4 m, both to cut costs and to allow a more compact and effective configuration for the telescope hardware. A proposed precursor space telescope to test the systems to be used on the main satellite was dropped, and budgetary concerns also prompted collaboration with the European Space Agency (ESA). ESA agreed to provide funding and supply one of the first generation instruments for the telescope, as well as the solar cells that would power it, and staff to work on the telescope in the United States, in return for European astronomers being guaranteed at least 15% of the observing time on the telescope."Memorandum of Understanding Between The European Space Agency and The United States National Aeronautics and Space Administration", reprinted in NASA SP-2001-4407: Exploring the Unknown Chapter 3, Document III-29, p. 671. Congress eventually approved funding of $36million for 1978, and the design of the LST began in earnest, aiming for a launch date of 1983. In 1983, the telescope was named after Edwin Hubble, who confirmed one of the greatest scientific discoveries of the 20th century, made by Georges Lemaître, that the universe is expanding.
Construction and engineering
Once the Space Telescope project had been given the go-ahead, work on the program was divided among many institutions. Marshall Space Flight Center (MSFC) was given responsibility for the design, development, and construction of the telescope, while Goddard Space Flight Center was given overall control of the scientific instruments and ground-control center for the mission. MSFC commissioned the optics company Perkin-Elmer to design and build the optical telescope assembly (OTA) and Fine Guidance Sensors for the space telescope. Lockheed was commissioned to construct and integrate the spacecraft in which the telescope would be housed.
Optical telescope assembly Optically, the HST is a Cassegrain reflector of Ritchey–Chrétien design, as are most large professional telescopes. This design, with two hyperbolic mirrors, is known for good imaging performance over a wide field of view, with the disadvantage that the mirrors have shapes that are hard to fabricate and test. The mirror and optical systems of the telescope determine the final performance, and they were designed to exacting specifications. Optical telescopes typically have mirrors polished to an accuracy of about a tenth of the wavelength of visible light, but the Space Telescope was to be used for observations from the visible through the ultraviolet (shorter wavelengths) and was specified to be diffraction limited to take full advantage of the space environment. Therefore, its mirror needed to be polished to an accuracy of 10 nanometers, or about 1/65 of the wavelength of red light. On the long wavelength end, the OTA was not designed with optimum infrared performance in mind, e.g. the mirrors are kept at stable (and warm, about 15°C) temperatures by heaters. This limits Hubble's performance as an infrared telescope.
Perkin-Elmer (PE) intended to use custom-built and extremely sophisticated computer-controlled polishing machines to grind the mirror to the required shape. However, in case their cutting-edge technology ran into difficulties, NASA demanded that PE sub-contract to Kodak to construct a back-up mirror using traditional mirror-polishing techniques. (The team of Kodak and Itek also bid on the original mirror polishing work. Their bid called for the two companies to double-check each other's work, which would have almost certainly caught the polishing error that later caused problems.) The Kodak mirror is now on permanent display at the National Air and Space Museum. An Itek mirror built as part of the effort is now used in the 2.4 m telescope at the Magdalena Ridge Observatory.