Hubble Space Telescope

"Hubble" redirects here. For the astronomer, see Edwin Hubble. For other uses, see Hubble (disambiguation).

The Hubble Space Telescope (often referred to as 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 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.^[8]

Hubble Space Telescope

Seen in orbit from the departing Space Shuttle Atlantis in 2009, flying Servicing Mission 4 (STS-125), the fifth and final Hubble mission.
Names	HST Hubble
Mission type	Astronomy
Operator	STScI
COSPAR ID	1990-037B
SATCAT no.	20580
Website	nasa.gov/hubble hubblesite.org spacetelescope.org
Mission duration	34 years, 4 months, 13 days (ongoing)[1]
Spacecraft properties
Manufacturer	Lockheed Martin (spacecraft) Perkin-Elmer (optics)
Launch mass	11,110 kg (24,490 lb)[2]
Dimensions	13.2 m × 4.2 m (43 ft × 14 ft)[2]
Power	2,800 watts
Start of mission
Launch date	April 24, 1990, 12:33:51 UTC[3]
Rocket	Space Shuttle Discovery (STS-31)
Launch site	Kennedy, LC-39B
Contractor	Rockwell International
Deployment date	April 25, 1990[2]
Entered service	May 20, 1990; 34 years ago (May 20, 1990)[2]
End of mission
Decay date	2030–2040 (estimated)[4]
Orbital parameters
Reference system	Geocentric orbit[5]
Regime	Low Earth orbit
Periapsis altitude	537.0 km (333.7 mi)
Apoapsis altitude	540.9 km (336.1 mi)
Inclination	28.47°
Period	95.42 minutes
Main telescope
Type	Ritchey–Chrétien reflector
Diameter	2.4 m (7 ft 10 in)[6]
Focal length	57.6 m (189 ft)[6]
Focal ratio	f/24
Collecting area	4.0 m2 (43 sq ft)[7]
Wavelengths	Near-infrared, visible light, ultraviolet
Instruments NICMOS Near Infrared Camera and Multi-Object Spectrometer ACS Advanced Camera for Surveys WFC3 Wide Field Camera 3 COS Cosmic Origins Spectrograph STIS Space Telescope Imaging Spectrograph FGS Fine Guidance Sensor
NASA Great Observatories Compton Gamma Ray Observatory → Large Strategic Science Missions Astrophysics Division

Hubble features a 2.4 m (7 ft 10 in) 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.

Space telescopes were proposed as early as 1923, and the Hubble telescope was funded and built in the 1970s by the United States space agency 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 finally launched 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 in 1993.

Hubble is the only telescope designed to be maintained in space by astronauts. Five Space Shuttle missions have 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^[1] and is predicted to last until 2030–2040.^[4]

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).^[9] The mid-IR-to-visible band successor to the Hubble telescope is the James Webb Space Telescope (JWST), which was launched on December 25, 2021, with the Nancy Grace Roman Space Telescope due to follow in 2027.^[10][11][12]

Proposals and precursors

Astronaut Owen Garriott working next to Skylab's crewed solar space observatory, 1973

In 1923, Hermann Oberth—considered a father of modern rocketry, along with Robert H. Goddard and Konstantin Tsiolkovsky—published Die Rakete zu den Planetenräumen ("The Rocket into Planetary Space"), which mentioned how a telescope could be propelled into Earth orbit by a rocket.^[13]

Lyman Spitzer played a major role in the birth of the Hubble Space Telescope project.

The history of the Hubble Space Telescope can be traced to 1946, to astronomer Lyman Spitzer's paper "Astronomical advantages of an extraterrestrial observatory".^[14] 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 2.5 m (8 ft 2 in) in diameter. Second, a space-based telescope could observe infrared and ultraviolet light, which are strongly absorbed by the atmosphere of Earth.^[14]

Spitzer devoted much of his career to pushing for the development of a space telescope.^[15] 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.^[16]

Nancy Grace Roman with a model of the Large Space Telescope that was eventually developed as the Hubble Space Telescope. While listed as a 1966 photo, this design was not the standard until the mid-1970s.

Also crucial was the work of Nancy Grace Roman, the "Mother of Hubble".^[17] 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^[18] and writing testimony to Congress throughout the 1970s to advocate continued funding of the telescope.^[19] Her work as project scientist helped set the standards for NASA's operation of large scientific projects.^[20]

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,^[21] and NASA launched the Orbiting Solar Observatory (OSO) to obtain UV, X-ray, and gamma-ray spectra in 1962.^[22] 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.^[23]

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 3 m (9.8 ft) 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.^[24]

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.^[25]

In 1977, then NASA Administrator James C. Fletcher proposed a token $5 million 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:^[26]

It was clear that year that we weren't going to be able to get a full-up start. There was some opposition on [Capitol] Hill to getting a new start on [Hubble]. It was driven, in large part as I recall, by the budget situation. Jim Fletcher proposed that we put in $5 million as a placeholder. I didn't like that idea. It was, in today's vernacular, a "sop" to the astronomy community. "There's something in there, so all is well".

I figured in my own little head that to get that community energized we'd be better off zeroing it out. Then they would say, "Whoa, we're in deep trouble", and it would marshal the troops. So I advocated that we not put anything in. I don't remember any of the detailed discussions or whether there were any, but Jim went along with that so we zeroed it out. It had, from my perspective, the desired impact of stimulating the astronomy community to renew their efforts on the lobbying front. While I like to think in hindsight it was a brilliant political move, I'm not sure I thought it through all that well. It was something that was spur of the moment.

[...] $5 million would let them think that all is well anyway, but it's not. So let's give them a message. My own thinking, get them stimulated to get into action. Zeroing it out would certainly give that message. I think it was as simple as that. Didn't talk to anybody else about doing it first, just, "Let's go do that". Voila, it worked. Don't know whether I'd do that again.

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.^[27]

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^[28] and to allow a more compact and effective configuration for the telescope hardware. A proposed precursor 1.5 m (4 ft 11 in) 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.^[29] Congress eventually approved funding of US$36 million for 1978, and the design of the LST began in earnest, aiming for a launch date of 1983.^[27] In 1983, the telescope was named after Edwin Hubble,^[30] who confirmed one of the greatest scientific discoveries of the 20th century, made by Georges Lemaître, that the universe is expanding.^[31]

Construction and engineering

Grinding of Hubble's primary mirror at Perkin-Elmer, March 1979

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.^[32] MSFC commissioned the optics company Perkin-Elmer to design and build the optical tube 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.^[33]

Optical tube 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.^[34] On the long wavelength end, the OTA was not designed with optimum infrared performance in mind—for example, the mirrors are kept at stable (and warm, about 15 °C) temperatures by heaters. This limits Hubble's performance as an infrared telescope.^[35]

The backup mirror, by Kodak; its inner support structure can be seen because it is not coated with a reflective surface.

Perkin-Elmer (PE) intended to use custom-built and extremely sophisticated computer-controlled polishing machines to grind the mirror to the required shape.^[33] 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.^[36] (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.)^[37] The Kodak mirror is now on permanent display at the National Air and Space Museum.^[38][39] An Itek mirror built as part of the effort is now used in the 2.4 m telescope at the Magdalena Ridge Observatory.^[40]

Construction of the Perkin-Elmer mirror began in 1979, starting with a blank manufactured by Corning from their ultra-low expansion glass. To keep the mirror's weight to a minimum it consisted of top and bottom plates, each 25 mm (0.98 in) thick, sandwiching a honeycomb lattice. Perkin-Elmer simulated microgravity by supporting the mirror from the back with 130 rods that exerted varying amounts of force.^[41] This ensured the mirror's final shape would be correct and to specification when deployed. Mirror polishing continued until May 1981. NASA reports at the time questioned Perkin-Elmer's managerial structure, and the polishing began to slip behind schedule and over budget. To save money, NASA halted work on the back-up mirror and moved the launch date of the telescope to October 1984.^[42] The mirror was completed by the end of 1981; it was washed using 9,100 L (2,000 imp gal; 2,400 US gal) of hot, deionized water and then received a reflective coating of 65 nm-thick aluminum and a protective coating of 25 nm-thick magnesium fluoride.^[35][43]

The OTA, metering truss, and secondary baffle are visible in this image of Hubble during early construction.

Doubts continued to be expressed about Perkin-Elmer's competence on a project of this importance, as their budget and timescale for producing the rest of the OTA continued to inflate. In response to a schedule described as "unsettled and changing daily", NASA postponed the launch date of the telescope until April 1985. Perkin-Elmer's schedules continued to slip at a rate of about one month per quarter, and at times delays reached one day for each day of work. NASA was forced to postpone the launch date until March and then September 1986. By this time, the total project budget had risen to US$1.175 billion.^[44]

Spacecraft systems

The spacecraft in which the telescope and instruments were to be housed was another major engineering challenge. It would have to withstand frequent passages from direct sunlight into the darkness of Earth's shadow, which would cause major changes in temperature, while being stable enough to allow extremely accurate pointing of the telescope. A shroud of multi-layer insulation keeps the temperature within the telescope stable and surrounds a light aluminum shell in which the telescope and instruments sit. Within the shell, a graphite-epoxy frame keeps the working parts of the telescope firmly aligned.^[45] Because graphite composites are hygroscopic, there was a risk that water vapor absorbed by the truss while in Lockheed's clean room would later be expressed in the vacuum of space; resulting in the telescope's instruments being covered by ice. To reduce that risk, a nitrogen gas purge was performed before launching the telescope into space.^[46]

While construction of the spacecraft in which the telescope and instruments would be housed proceeded somewhat more smoothly than the construction of the OTA, Lockheed experienced some budget and schedule slippage, and by the summer of 1985, construction of the spacecraft was 30% over budget and three months behind schedule. An MSFC report said Lockheed tended to rely on NASA directions rather than take their own initiative in the construction.^[47]

Computer systems and data processing

DF-224 in Hubble, before it was replaced in 1999

The two initial, primary computers on the HST were the 1.25 MHz DF-224 system, built by Rockwell Autonetics, which contained three redundant CPUs, and two redundant NSSC-1 (NASA Standard Spacecraft Computer, Model 1) systems, developed by Westinghouse and GSFC using diode–transistor logic (DTL). A co-processor for the DF-224 was added during Servicing Mission 1 in 1993, which consisted of two redundant strings of an Intel-based 80386 processor with an 80387 math co-processor.^[48] The DF-224 and its 386 co-processor were replaced by a 25 MHz Intel-based 80486 processor system during Servicing Mission 3A in 1999.^[49] The new computer is 20 times faster, with six times more memory, than the DF-224 it replaced. It increases throughput by moving some computing tasks from the ground to the spacecraft and saves money by allowing the use of modern programming languages.^[50]

Additionally, some of the science instruments and components had their own embedded microprocessor-based control systems. The MATs (Multiple Access Transponder) components, MAT-1 and MAT-2, use Hughes Aircraft CDP1802CD microprocessors.^[51] The Wide Field and Planetary Camera (WFPC) also used an RCA 1802 microprocessor (or possibly the older 1801 version).^[52] The WFPC-1 was replaced by the WFPC-2 during Servicing Mission 1 in 1993, which was then replaced by the Wide Field Camera 3 (WFC3) during Servicing Mission 4 in 2009. The upgrade extended Hubble's capability of seeing deeper into the universe and providing images in three broad regions of the spectrum.^[53][54]

Initial instruments

Main articles: Wide Field and Planetary Camera, Goddard High Resolution Spectrograph, High Speed Photometer, Faint Object Camera, and Faint Object Spectrograph

Exploded view of the Hubble Space Telescope

When launched, the HST carried five scientific instruments: the Wide Field and Planetary Camera (WF/PC), Goddard High Resolution Spectrograph (GHRS), High Speed Photometer (HSP), Faint Object Camera (FOC) and the Faint Object Spectrograph (FOS). WF/PC used a radial instrument bay, and the other four instruments were each installed in an axial instrument bay.^[55]

WF/PC was a high-resolution imaging device primarily intended for optical observations. It was built by NASA's Jet Propulsion Laboratory, and incorporated a set of 48 filters isolating spectral lines of particular astrophysical interest. The instrument contained eight charge-coupled device (CCD) chips divided between two cameras, each using four CCDs. Each CCD has a resolution of 0.64 megapixels.^[56] The wide field camera (WFC) covered a large angular field at the expense of resolution, while the planetary camera (PC) took images at a longer effective focal length than the WF chips, giving it a greater magnification.^[55]

The Goddard High Resolution Spectrograph (GHRS) was a spectrograph designed to operate in the ultraviolet. It was built by the Goddard Space Flight Center and could achieve a spectral resolution of 90,000.^[57] Also optimized for ultraviolet observations were the FOC and FOS, which were capable of the highest spatial resolution of any instruments on Hubble. Rather than CCDs, these three instruments used photon-counting digicons as their detectors. The FOC was constructed by ESA, while the University of California, San Diego, and Martin Marietta Corporation built the FOS.^[55]

The final instrument was the HSP, designed and built at the University of Wisconsin–Madison. It was optimized for visible and ultraviolet light observations of variable stars and other astronomical objects varying in brightness. It could take up to 100,000 measurements per second with a photometric accuracy of about 2% or better.^[58]

HST's guidance system can also be used as a scientific instrument. Its three Fine Guidance Sensors (FGS) are primarily used to keep the telescope accurately pointed during an observation, but can also be used to carry out extremely accurate astrometry; measurements accurate to within 0.0003 arcseconds have been achieved.^[59]

Ground support

Main article: Space Telescope Science Institute

Hubble Control Center at Goddard Space Flight Center, 1999

The Space Telescope Science Institute (STScI) is responsible for the scientific operation of the telescope and the delivery of data products to astronomers. STScI is operated by the Association of Universities for Research in Astronomy (AURA) and is physically located in Baltimore, Maryland on the Homewood campus of Johns Hopkins University, one of the 39 U.S. universities and seven international affiliates that make up the AURA consortium. STScI was established in 1981^[60][61] after something of a power struggle between NASA and the scientific community at large. NASA had wanted to keep this function in-house, but scientists wanted it to be based in an academic establishment.^[62][63] The Space Telescope European Coordinating Facility (ST-ECF), established at Garching bei München near Munich in 1984, provided similar support for European astronomers until 2011, when these activities were moved to the European Space Astronomy Centre.^[64]

One complex task that falls to STScI is scheduling observations for the telescope.^[65] Hubble is in a low-Earth orbit to enable servicing missions, which results in most astronomical targets being occulted by the Earth for slightly less than half of each orbit. Observations cannot take place when the telescope passes through the South Atlantic Anomaly due to elevated radiation levels, and there are also sizable exclusion zones around the Sun (precluding observations of