Hubble Space Telescope

History

The Hubble Space Telescope is a NASA-led space observatory developed with major participation from the European Space Agency. It was designed to operate above Earth's atmosphere, allowing astronomical observations without the atmospheric distortion that limits ground-based telescopes and providing access to ultraviolet, visible, and near-infrared wavelengths.

The observatory uses a 2.4 m primary mirror and was designed from the beginning for servicing by astronauts in orbit. This approach allowed instruments and spacecraft systems to be repaired or replaced during five Space Shuttle servicing missions, extending Hubble's scientific life far beyond its original minimum mission goal.

Hubble was launched aboard Space Shuttle Discovery on April 24, 1990, and released into orbit the following day. According to NASA's Hubble history, the telescope has since accumulated more than three decades of observations and remains an active astronomical observatory.

Origins and development

The concept of placing an astronomical telescope above Earth's atmosphere predates the space age. In 1923, German scientist Hermann Oberth discussed launching a telescope into orbit. In 1946, Princeton astrophysicist Lyman Spitzer described the scientific advantages of a space telescope and later became a leading advocate for such an observatory.

After the early Orbital Astronomical Observatory program demonstrated the value of space-based ultraviolet astronomy, scientific and governmental groups began planning a larger observatory. The National Academy of Sciences approved the Large Space Telescope concept in 1969. In 1974, the Large Space Telescope Science Working Group recommended an observatory with interchangeable instruments, angular resolution of at least one-tenth of an arcsecond, and coverage extending from ultraviolet through visible light to infrared wavelengths.

The European Space Agency joined the project in 1975. ESA provided approximately 15 percent of the funding through contributions including the Faint Object Camera and solar arrays, while European astronomers were guaranteed at least 15 percent of the telescope's observing time. The United States Congress approved funding for construction in 1977.

NASA assigned Marshall Space Flight Center responsibility for design, development, and construction. Perkin-Elmer Corporation developed the Optical Telescope Assembly and Fine Guidance Sensors. Lockheed Missiles and Space Company built the spacecraft outer structure and Support Systems Module and assembled the telescope. Goddard Space Flight Center became responsible for scientific instrument development and ground control.

The original instrument complement consisted of the Faint Object Camera, Wide Field/Planetary Camera, Faint Object Spectrograph, High Resolution Spectrograph, and High Speed Photometer. The Space Telescope Science Institute began operations at Johns Hopkins University in 1983 to evaluate observing proposals and manage scientific observations.

The primary mirror was completed in 1981, and the assembled spacecraft was completed in 1985. The observatory had originally been expected to launch earlier, but technical and budget difficulties delayed the schedule. The 1986 Space Shuttle Challenger accident then grounded the Shuttle fleet. During the resulting delay, the Hubble team improved its solar arrays, modified the aft shroud to simplify instrument replacement, upgraded computers and communications equipment, and conducted additional testing.

Launch and optical problem

Space Shuttle Discovery carried Hubble into orbit during the STS-31 mission on April 24, 1990. Hubble was deployed on April 25. Soon after scientific observations began, astronomers discovered that the primary mirror suffered from spherical aberration. Its outer edge had been polished slightly too flat because of an error associated with the equipment used to test the mirror's curvature.

The mirror itself was not designed for orbital replacement, but the telescope's serviceable architecture provided another solution. Engineers incorporated corrective optics into the Wide Field and Planetary Camera 2 and developed the Corrective Optics Space Telescope Axial Replacement, or COSTAR, for other instruments.

Servicing missions

The first servicing mission took place in December 1993. Astronauts installed Wide Field and Planetary Camera 2 and COSTAR, correcting the effects of the primary mirror's optical error and restoring the intended image quality.

A second servicing mission followed in 1997 and installed two new instruments. Servicing Mission 3A in 1999 replaced several important technical systems, while Servicing Mission 3B in 2002 added further scientific instrumentation.

The fifth and final servicing mission was completed in May 2009. Astronauts installed Wide Field Camera 3 and the Cosmic Origins Spectrograph, repaired the Space Telescope Imaging Spectrograph and Advanced Camera for Surveys, and replaced or upgraded equipment including the Science Instrument Command and Data Handling Unit, a Fine Guidance Sensor, batteries, gyroscopes, and insulation.

Design

Hubble is a free-flying orbital observatory built around a 2.4 m-diameter primary mirror. Its optical system directs collected light to scientific instruments capable of observing ultraviolet, visible, and near-infrared wavelengths. Its position above Earth's atmosphere avoids the atmospheric turbulence that blurs visible-light observations from the ground and allows access to wavelengths that are partly or largely absorbed by the atmosphere.

Spacecraft and servicing architecture

The spacecraft was designed to fit inside the Space Shuttle cargo bay and to permit maintenance by astronauts. Replaceable instruments and subsystems allowed the observatory to be modernized during its service life instead of depending entirely on its original hardware.

The Support Systems Module contains spacecraft functions including computing, electrical power, communications, pointing, and control. The observatory uses reaction wheels to maneuver and gyroscopes to monitor attitude. Fine Guidance Sensors lock onto guide stars to provide the precise pointing required for astronomical observations.

Optics and instruments

At the center of the observatory is the 2.4 m primary mirror. According to the ESA Hubble overview, Hubble's scientific payload includes cameras and spectrographs covering wavelengths from infrared through visible light to ultraviolet, together with Fine Guidance Sensors used for accurate pointing and, in some cases, astrometric measurements.

Following the 2009 servicing mission, major scientific instruments included Wide Field Camera 3, the Cosmic Origins Spectrograph, the Space Telescope Imaging Spectrograph, and the Advanced Camera for Surveys. Earlier instruments were replaced or retired as the observatory was upgraded.

Power and attitude control

Two solar wings provide electrical power for the spacecraft computers and scientific instruments. They also charge six nickel-hydrogen batteries, which were designed to power Hubble for approximately 25 minutes during each orbit while the spacecraft passes through Earth's shadow.

Reaction wheels rotate the telescope toward selected targets without relying on continuous propulsion. Gyroscopes measure spacecraft attitude, while the Fine Guidance Sensors provide precise reference information by locking onto guide stars.

Operational history

Hubble was initially deployed into a circular low Earth orbit about 575 km above Earth with an inclination of 28.5 degrees. One orbit takes approximately 96 to 97 minutes. Its location above the atmosphere permits high-resolution observations over wavelength regions ranging from ultraviolet through visible light into the near-infrared.

The observatory has been used to investigate targets ranging from planets and nearby stars to nebulae and distant galaxies. Its observations contributed to research on galaxy formation and evolution, black holes, stellar life cycles, exoplanet atmospheres, and the expansion of the universe.

NASA reports that Hubble has made more than 1.7 million observations and that its data have contributed to more than 23,000 peer-reviewed scientific publications. Its archive has accumulated more than 477 terabytes of data spanning ultraviolet, visible, and near-infrared observations.

In 2026, Hubble observations produced a detailed view of the emission nebula LHA 120-N44, also called N44, in the Large Magellanic Cloud about 160,000 light-years away. The nebula contains a central cavity described as a superbubble measuring about 210 by 140 light-years. The observing program surveyed nearly half a million stars in and around the region, including about 30,000 stars in very early stages of development, supporting studies of star formation and stellar evolution.

No further astronaut servicing missions are scheduled. NASA engineers have continued adapting spacecraft operations to aging hardware, including development of an operating mode that can use one gyroscope together with other spacecraft sensors to support continued observations.

Specifications (Hubble Space Telescope)

General characteristics

• Type: Space-based astronomical observatory
• Primary mirror diameter: 2.4 m
• Observed wavelengths: Ultraviolet, visible, and near-infrared
• Orbit at deployment: Approximately 575 km circular low Earth orbit
• Orbital inclination: 28.5 degrees
• Orbital period: Approximately 96 to 97 minutes

Power and pointing

• Primary electrical source: Two solar wings
• Energy storage: Six nickel-hydrogen batteries
• Battery-powered eclipse period: Approximately 25 minutes per orbit
• Attitude maneuvering: Reaction wheels
• Attitude sensing: Gyroscopes
• Precision pointing: Fine Guidance Sensors using guide stars

Scientific equipment

• Wide Field Camera 3: Installed during the 2009 servicing mission
• Cosmic Origins Spectrograph: Installed during the 2009 servicing mission
• Space Telescope Imaging Spectrograph: Repaired during the 2009 servicing mission
• Advanced Camera for Surveys: Repaired during the 2009 servicing mission
• Fine Guidance Sensors: Provide precise target pointing and can support astrometric measurements

Program status

Hubble remains in operation after exceeding its original planned mission duration. Its hardware is aging and no further Space Shuttle servicing missions are planned, but NASA continues to operate the observatory and has developed alternative operating techniques intended to extend its useful scientific life.