Hubble Space Telescope

The Hubble Space Telescope, also called HST or Hubble, is a space telescope. It was launched into low Earth orbit in 1990 and still works today. It was not the first space telescope, but it is one of the largest and most useful. Scientists use it for research, and it helps share astronomy with the public.

Hubble is named after the astronomer Edwin Hubble. It is one of NASA's Great Observatories. The Space Telescope Science Institute (STScI) decides what Hubble will look at and handles the information it collects. The Goddard Space Flight Center (GSFC) controls the spacecraft. Other meanings of the word Hubble are listed on [Hubble (disambiguation)](/wiki/Hubble_(disambiguation).

Hubble has a mirror that is 2.4 m (7 ft 10 in) wide. Its five main instruments can see ultraviolet, visible, and near-infrared light. These are types of electromagnetic spectrum light. Hubble orbits above Earth's atmosphere, which blurs the view for telescopes on the ground. Because of this, Hubble can take very clear pictures. It has taken some of the most detailed visible light images, showing us deep into space. Many of its discoveries have helped astrophysics, such as determining the rate of expansion of the universe.

NASA built and paid for the Hubble Space Telescope in the 1970s, with help from the European Space Agency. It was first planned to launch in 1983, but the project faced technical problems, delays, and budget problems. The Challenger disaster in 1986 also slowed things down.

Hubble launched on STS-31 in 1990. However, its main mirror had been shaped incorrectly. This caused spherical aberration, which made the telescope's pictures less clear. A repair mission, STS-61, fixed the mirror in 1993. Hubble is the only telescope designed to be fixed in space by astronauts. Five Space Shuttle missions repaired and upgraded the telescope, and they replaced all five of its main instruments. The fifth mission was first canceled for safety reasons after the Columbia disaster in 2003. Later, NASA administrator Michael D. Griffin approved it, and the mission was completed in 2009. Hubble celebrated 30 years in operation in April 2020. It is expected to keep working until about 2030 to 2040.

Hubble is the visible light telescope in NASA's Great Observatories program. Other telescopes in the program look at different kinds of light. These include the Compton Gamma Ray Observatory, the Chandra X-ray Observatory, and the Spitzer Space Telescope, which studies infrared light. Telescopes that followed Hubble for mid-infrared and visible light include the James Webb Space Telescope (JWST), which launched on December 25, 2021, and the Nancy Grace Roman Space Telescope, which launched on August 30, 2026.

Concept, design and aim

In 1923, Hermann Oberth wrote a book that said a telescope could be carried into Earth orbit by a rocket. The story of the Hubble Space Telescope began in 1946, when Lyman Spitzer wrote that a telescope in space could see more clearly than one on the ground. It could also see infrared and ultraviolet light. Spitzer worked for many years to push for a space telescope. Nancy Grace Roman, known as the "Mother of Hubble," helped by giving talks to the public about why it was valuable. Space astronomy started on a small scale after World War II. By 1968, NASA had firm plans for a large space telescope.

Money was a big problem. Congress cut the planning budget, and in 1974 all money for the telescope was taken away. In 1977, NASA's Noel Hinners cut all funding on purpose, hoping astronomers would fight for it. They did. The telescope's mirror was made smaller, and the European Space Agency agreed to help. In 1983, the telescope was named after Edwin Hubble. Many groups shared the work, including Marshall Space Flight Center, Goddard Space Flight Center, Perkin-Elmer, and Lockheed. Problems with the mirror and spacecraft caused long delays. The Challenger disaster then stopped shuttle flights. The telescope was finally launched by Space Shuttle Discovery on April 24, 1990.

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

Main article: Space Telescope Science Institute

List of Hubble instruments

Hubble can hold five science instruments at a time, plus the Fine Guidance Sensors. The sensors are mainly used to aim the telescope, but they are sometimes used for scientific astrometry measurements. Older instruments were replaced with newer, better ones during the Shuttle servicing missions. COSTAR was not a science instrument. It was a device that fixed the telescope's mirror problems, and it used one of the four main instrument bays.

Since the last servicing mission in 2009, four instruments have been working: ACS, COS, STIS and WFC3. NICMOS is kept in a resting state and could be turned back on if WFC3 stops working. Some older instruments are on display. COSTAR, FOS and WFPC2 are shown at the Smithsonian National Air and Space Museum. The FOC is in the Dornier museum in Germany, and the HSP is at the Space Place at the University of Wisconsin–Madison.

• Advanced Camera for Surveys (ACS; 2002–present)
• Cosmic Origins Spectrograph (COS; 2009–present)
• Corrective Optics Space Telescope Axial Replacement (COSTAR; 1993–2009)
• Faint Object Camera (FOC; 1990–2002)
• Faint Object Spectrograph (FOS; 1990–1997)
• Fine Guidance Sensor (FGS; 1990–present)
• Goddard High Resolution Spectrograph (GHRS/HRS; 1990–1997)
• High Speed Photometer (HSP; 1990–1993)
• Near Infrared Camera and Multi-Object Spectrometer (NICMOS; 1997–present, hibernating since 2008)
• Space Telescope Imaging Spectrograph (STIS; 1997–present (non-operative 2004–2009))
• Wide Field and Planetary Camera (WFPC; 1990–1993)
• Wide Field and Planetary Camera 2 (WFPC2; 1993–2009)
• Wide Field Camera 3 (WFC3; 2009–present)

Flawed mirror

Within weeks of the telescope's launch, the pictures it sent back showed a serious problem with its optical system. Its first images looked sharper than those from telescopes on the ground, but Hubble never reached a sharp focus. Images of point sources spread out over more than one arcsecond. They were supposed to stay in a small circle. The point spread function (PSF) was meant to be much tighter. Analysis showed that the main mirror had been figured to the wrong shape. Its outer edge was too flat. This caused spherical aberration, a flaw where light from the edge of the mirror focuses on a different point than light from the center. This made the telescope much less useful for faint objects, and people began to question NASA. Comedians even joked about it. In the 1991 comedy The Naked Gun 2½: The Smell of Fear, Hubble was shown alongside the Titanic and the Hindenburg. Still, the telescope made many useful observations before its optical fixes. Astronomers also partly made up for the flaw using image processing methods such as deconvolution.

A commission led by Lew Allen, director of the Jet Propulsion Laboratory, found the cause. A null corrector, a testing device used to shape the mirror, had been put together incorrectly. Perkin-Elmer, the company that made the mirror, had also not checked its work properly. Earlier tests had shown the problem, but those results were ignored. Many people feared Hubble would be abandoned. However, Dr. Charles Pellerin gathered a team of experts to find a solution. The mirror could not be replaced in orbit, so engineers designed new optical parts with the opposite error. These worked like "spectacles" that corrected the flaw. Astronomers first measured the mirror's error carefully, using its conic constant and interferograms. The Hubble instruments needed two kinds of correction. The Wide Field and Planetary Camera 2 had its own relay mirrors that could cancel the error. The other instruments needed an outside device called the Corrective Optics Space Telescope Axial Replacement (COSTAR) system. To make room for COSTAR, astronomers chose to remove the High Speed Photometer. By 2002, the instruments that had needed COSTAR had been replaced with new ones that had their own corrective optics. COSTAR was brought back to Earth in 2009. Its former space is now used by the Cosmic Origins Spectrograph.

Servicing missions and new instruments

Main article: STS-61

Main article: STS-82

Main article: STS-103

Main article: STS-109

Main article: STS-125

Hubble was built so that astronauts could repair and upgrade it in space. NASA sent five servicing missions using Space Shuttles, from December 1993 to May 2009. On each mission, astronauts used the shuttle's mechanical arm to catch the telescope. They then made repairs during several spacewalks over four or five days. Afterward, the telescope was let go, often after being moved to a higher orbit. This was needed because drag from the air slowly pulled it down, a problem called orbital decay. The first mission, flown by Endeavour in December 1993, replaced several instruments and other equipment. This included the High Speed Photometer, which was swapped for the COSTAR corrective optics package. NASA called the mission a complete success in January 1994, and the first clearer images soon followed.

Later missions added and improved instruments. In 1997, Discovery installed the Space Telescope Imaging Spectrograph and the Near Infrared Camera and Multi-Object Spectrometer. In 1999, a mission replaced all six gyroscopes, because several had broken. In 2002, the Advanced Camera for Surveys was installed. After the Columbia disaster in 2003, NASA Administrator Sean O'Keefe decided to cancel the last planned servicing mission. Many people, including members of Congress such as Barbara Mikulski, asked NASA to think again. NASA Administrator Michael D. Griffin later approved a repair flight with astronauts. The last servicing mission, flown by Atlantis in May 2009, replaced the telescope's main data-handling unit and installed new nickel–hydrogen batteries. It also added the Wide Field Camera 3 and the Cosmic Origins Spectrograph. The mission installed the Soft Capture and Rendezvous System, which will help Hubble be safely caught and disposed of in the future. Except for the High Resolution Channel, which could not be repaired, the telescope worked fully after this mission.

Major projects

Many research projects have been carried out since the program began. Some were done almost entirely with Hubble. Others worked together with facilities such as Chandra X-ray Observatory and ESO's Very Large Telescope. Hubble is nearing the end of its life, but major projects are still planned for it. The ULLYSES project began in 2022 and will last three years. It observes young stars of high and low mass to learn about star formation and what stars are made of. The OPAL project studies the atmospheres of outer planets such as Jupiter and Uranus by watching them over a long period.

Other projects have looked deeper into space. CANDELS was called "the largest project in the history of Hubble" in a 2013 press release. It aims to explore how galaxies changed in the early Universe. The Frontier Fields program studies early galaxy formation. It looks at distant galaxies in blank fields with the help of gravitational lensing. The Cosmic Evolution Survey (COSMOS) studies how galaxies formed and changed over a very long time. It is led by Caitlin Casey, Jeyhan Kartaltepe, and Vernesa Smolcic. The Cluster Lensing and Supernova survey with Hubble (CLASH) observed 25 massive galaxy clusters from 2010 to 2013. It helped scientists learn more about dark matter and dark energy.

Hubble also mapped nearby space. From 2010 to 2013, and again from 2021 to 2024, the Panchromatic Hubble Andromeda Treasury and Panchromatic Hubble Andromeda Southern Treasury (PHAT and PHAST) observed the Andromeda Galaxy. Andromeda is the closest large galaxy to the Milky Way. These projects created the most detailed photomosaic of it ever made. Studying Andromeda is a good alternative to studying the Milky Way. Earth is inside the Milky Way, and the galaxy itself blocks our view of much of it. The Universe holds many more galaxies than these projects could show in a single image.

Public use

Anyone can apply for time on the telescope, no matter what country they are from or what school they attend. However, only U.S. institutions can get money to study the results. Competition is strong. About one in five proposals gets time on the schedule. Calls for proposals come out about once a year.

Most proposals are "general observer" requests for regular observations. "Snapshot observations" take 45 minutes or less and fill gaps in the schedule. Astronomers can also make "Target of Opportunity" proposals. These are used when a short-lived event happens during a scheduling cycle.

Up to 10% of telescope time is called "director's discretionary" time. Astronomers can ask for this time at any time. It is usually used to study surprises, such as supernovae. This time also led to the Hubble Deep Field and Hubble Ultra Deep Field images. In the first four cycles, it also let amateur astronomers make observations. In 2012, the ESA held a contest for the public to process Hubble data and look for "hidden treasures" in the raw data.

In 1986, the first director of STScI, Riccardo Giacconi, said he would give some of his discretionary time to amateur astronomers. There were only a few hours per cycle, but many amateurs were excited. A committee of amateur astronomers carefully reviewed the proposals. They chose only those with real scientific value, that did not repeat professional work, and that needed the telescope's special abilities. Thirteen amateurs received time between 1990 and 1997. One of these studies was called "Transition Comets – UV Search for OH". Budget cuts at STScI later made this support impossible, so no more amateur programs have been carried out.

Still, Hubble proposals often include discoveries made by amateurs and citizen scientists. These often happen through teamwork with professional astronomers. One early example is the Great White Spot on Saturn in 1990. It was found by amateur S. Wilber and observed under a proposal by J. Westphal (Caltech). Other examples come from the Galaxy Zoo project, including Voorwerpjes and Green Pea galaxies. Hubble has also observed minor planets found by amateurs, such as 2I/Borisov. It has also studied changes in the atmospheres of the gas giants Jupiter and Saturn and the ice giants Uranus and Neptune. In the backyard worlds project, Hubble helped study a planetary mass object called WISE J0830+2837.

Scientific results

Early on, NASA and STScI picked a few key projects. These projects were important to science and needed a lot of telescope time. They were planned to finish early in case the telescope stopped working sooner than expected. Hubble has helped solve some old problems in astronomy and has also raised new questions. Some results needed new theories to explain them.

Hubble's measurements of Cepheid variable stars helped constrain the value of the Hubble constant. This number relates to how fast the universe is growing and to its age. Two research teams, the High-z Supernova Search Team and the Supernova Cosmology Project, used Hubble to study distant supernovae. They found that the universe's growth is accelerating instead of slowing down because of gravity. Three members of these groups later received Nobel Prizes. The cause, called dark energy, is still not well understood. Hubble's images also suggest that black holes are probably common at the centers of all galaxies. Its Hubble Deep Field, Hubble Ultra-Deep Field, and Hubble Extreme Deep Field images show galaxies billions of light years away. In 2016, researchers announced the farthest confirmed galaxy so far, GN-z11. Hubble saw it as it looked roughly 400 million years after the Big Bang.

Hubble also studied objects in our Solar System. It watched Comet Shoemaker-Levy 9 crash into Jupiter in 1994. Its images of Ganymede helped show a large ocean under the moon's icy surface. Hubble studied the dwarf planets Pluto, Eris, and Sedna. It also discovered Styx, a tiny fifth moon orbiting Pluto. Hubble helped find a new target, 486958 Arrokoth, for the New Horizons spacecraft, which flew past it in 2019.

Hubble data helped measure the mass of the Milky Way Galaxy. The telescope also detected the farthest single star seen so far, WHL0137-LS, nicknamed Earendel. As of 2025, more than 22,000 papers based on Hubble data have been published in scientific journals. Hubble has greatly helped astronomy. It cost much more to build and maintain than a 4 m (13 ft) telescope like the William Herschel Telescope. Still, its papers receive about 15 times as many citations. Hubble also taught engineers useful lessons about graphite composite structures and multi-layer insulation in aerospace engineering.

Hubble data

Hubble's data is first stored on the spacecraft. At launch, it used old-fashioned reel-to-reel tape drives. These were later replaced by solid state data storage during servicing missions 2 and 3A. About twice a day, the telescope sends its data to a geosynchronous satellite in the Tracking and Data Relay Satellite System (TDRSS). That satellite passes the data down to large antennas at the White Sands Test Facility in White Sands, New Mexico. From there, the data goes to the Space Telescope Operations Control Center at Goddard Space Flight Center. Finally, it reaches the Space Telescope Science Institute, where it is stored. Each week, Hubble sends down about 140 gigabits of data.

Every Hubble image starts as a black-and-white monochromatic grayscale picture. Each picture is taken through a filter that lets in only certain colors of light. Color images are made by combining these separate pictures. Scientists can also make false-color images. In these, infrared light is shown as deep red and ultraviolet light is shown as deep blue.

All Hubble data is eventually shared through the Mikulski Archive for Space Telescopes at STScI, CADC, and ESA/ESAC. Usually, the data is kept private for twelve months. Only the principal investigator (PI) and the astronomers the PI chooses can see it during that time. Some observations, along with calibration data such as dark frames, are released to the public right away. The archive stores data in FITS format. This format is good for astronomy but is not easy to look at for fun. So the Hubble Heritage Project shares a small selection of striking images as JPEG and TIFF files. Scientists can analyze the data using software called Space Telescope Science Data Analysis System (STSDAS). This software runs inside IRAF, a popular astronomy program.

Outreach activities

NASA wanted the telescope to inspire the public, because taxpayers helped pay for its building and running costs. In its early years, a faulty mirror hurt its reputation. The first repair mission fixed the telescope, and it then took many amazing pictures.

In the United States, outreach work is led by the Space Telescope Science Institute (STScI) Office for Public Outreach. It was started in 2000 so that taxpayers could see what their money helped create. STScI runs the HubbleSite.org website. The Hubble Heritage Project shares high-quality pictures of amazing objects in space. Its team includes amateur and professional astronomers, and they focus on the aesthetic beauty of the images.

Since 1999, the main Hubble outreach group in Europe has been the Hubble European Space Agency Information Centre (HEIC). It is based at the Space Telescope European Coordinating Facility in Munich, Germany. The European Space Agency also makes learning materials, including a videocast series called Hubblecast. The telescope won two Space Achievement Awards from the Space Foundation for its outreach work, in 2001 and 2010. A copy of the telescope stands on the courthouse lawn in Marshfield, Missouri, the hometown of Edwin P. Hubble. Some anniversaries have been marked with special pictures, such as one of the Carina Nebula for the 20th anniversary in space. Images of Westerlund 2 and the Bubble Nebula were shared for later anniversaries.

Further information: List of Hubble anniversary images

Equipment failures

Hubble uses gyroscopes to sense how it is turning. This helps it stay steady and point at targets in space. It has six gyroscopes, and normally three are needed. After the Columbia disaster in 2003, engineers wrote new software so the telescope could work with fewer gyroscopes. In 2005, it switched to a mode that used two gyroscopes, which helped it last longer. By the final repair mission in May 2009, only three gyroscopes still worked, so all six were replaced. In October 2018, the last of the old gyroscopes stopped working. The telescope was placed into "safe" mode while scientists fixed the backup gyroscope.

Hubble's instruments have also had problems. The power system of the Space Telescope Imaging Spectrograph failed in August 2004 and was fixed in May 2009. In January 2019, the Wide Field Camera 3 went into safe mode, but it was working normally again by January 17. In June 2021, a computer problem stopped science work. NASA switched Hubble to a backup power control unit, and the telescope was working again on July 16.

Future

Hubble orbits Earth in the very thin upper atmosphere. Over time, its orbit slowly gets lower because of drag, which is a kind of pushing force from the air. This is called decays. If Hubble is not reboosted, it will fall back into Earth's atmosphere within a few decades. The exact date depends on how active the Sun is. If it came down without control, some parts of the main mirror and its support structure might not burn up, and they could cause damage on the ground. In 2013, deputy project manager James Jeletic said Hubble could keep working into the 2020s. A natural re-entry is expected between 2028 and 2040. In June 2016, NASA extended Hubble's service contract until June 2021, and it was later extended until June 2026. During Servicing Mission 4, NASA installed the Soft Capture Mechanism. This could allow either a crewed or robotic mission to bring Hubble down safely.

As of 2017, the Trump Administration was thinking about a plan from the Sierra Nevada Corporation. The plan would use a crewed version of its Dream Chaser spacecraft to visit and repair Hubble in the 2020s. In 2020, John Grunsfeld said that SpaceX Crew Dragon or Orion could make another repair trip within ten years. In September 2022, NASA and SpaceX agreed to study a private astronaut mission, led by Jared Isaacman. This mission could raise Hubble to a higher orbit. By June 2024, NASA had decided not to go ahead with a private repair mission, because it could damage the telescope.

Further information: James Webb Space Telescope

See also: List of proposed space observatories

There is no direct replacement for Hubble. The James Webb Space Telescope was launched on December 25, 2021, and is the official successor to Hubble. It is designed to work far from Earth, at the Lagrangian point L2. It is expected to see stars in the early Universe, which are about 280 million years older than the ones Hubble can see now. Another idea is the Large Ultraviolet Optical Infrared Surveyor, which could be a more direct successor to Hubble. Larger telescopes on the ground can gather more light, but adaptive optics only sharpen a very small view. Space telescopes can also study types of light that Earth's atmosphere blocks. Some examples of Hubble's pictures include a Deep Field of a galaxy cluster, called SMACS J0723.3-7327.