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Hubble Space Telescope

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The Hubble Space Telescope is a space-based telescope that was launched in 1990 from the space shuttle Discovery. From its position above Earth's atmosphere, Hubble has expanded our understanding of the universe — and of star birth, star death, galaxy evolution, and black holes in particular.

The telescope's science instruments are the astronomer's eyes to the universe. During Servicing Mission 4, in May 2009, astronauts boosted Hubble’s scientific power by installing two state-of-the-art science instruments: the Cosmic Origins Spectrograph (COS) and the Wide Field Camera 3 (WFC3). To make room for the new instruments, astronauts removed the Wide Field and Planetary Camera 2, which was installed aboard Hubble in 1993. Other Hubble science instruments include the Space Telescope Imaging Spectrograph, Near Infrared Camera and Multi-Object Spectrometer, and Advanced Camera for Surveys.

When first launched, Hubble's primary mirror had a minor flaw that made it difficult for the telescope to resolve faint objects. Because the telescope is in low Earth orbit, it can be serviced by a space shuttle; thus, the defect was corrected during the first on-orbit servicing mission.


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NASA decided early in the telescope's development to design the observatory for on-orbit servicing. Instruments were designed as modular units, comparable to dresser drawers that could be easily removed and replaced. In addition, designers equipped the telescope with handholds and other special features to make servicing tasks less difficult for astronauts wearing bulky spacesuits.

By periodically upgrading the science instruments, NASA also reasoned that it could provide the science community worldwide with state-of-the-art technology that takes advantage of Hubble's unique position high above Earth's obscuring atmosphere.


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This mission boosted Hubble's scientific power and ensured the continued effective operation of the telescope. During Servicing Mission 4, astronauts installed two state-of-the-art science instruments: the Cosmic Origins Spectrograph (COS) and the Wide Field Camera 3 (WFC3). They also added six batteries, six gyroscopes, and a Fine Guidance Sensor.


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WFC3 replaced Hubble’s “workhorse” camera, the Wide Field Planetary Camera 2 (WFPC2), which observed celestial objects for 15 years. The new camera greatly improves Hubble’s ability to image large and distant objects, such as galaxies and clusters of galaxies, as well as planets in our solar system.

Astronomers use WFC3 to make wide-field surveys of the distant universe to study how galaxies formed and evolved from about 2 billion years after the Big Bang to the present. WFC3 allows Hubble to see objects that are about five times fainter than Hubble’s landmark deep view of the cosmos, called the Hubble Deep Field, taken in 1996 with WFPC2.


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COS examines the ultraviolet light streaming from distant objects. The new instrument allows Hubble to study galaxy formation and the births of stars and planetary systems. COS completes observations much faster than Hubble’s previous spectrograph.


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Normally, Hubble’s science instruments run on sunlight collected by its twin solar panels, which make the observatory look like it has wings. The batteries power Hubble’s science instruments when the telescope is in Earth’s shadow. The gyroscopes help keep Hubble steady as it orbits Earth and allow scientists to point the telescope at celestial targets. Hubble has three Fine Guidance Sensors, which also help in pointing the telescope at objects.


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When your car needs a tune-up, you take it to your mechanic for servicing. Engineers and scientists designed the Hubble Space Telescope with the same principle in mind. When a component breaks or a more technologically advanced instrument becomes available, astronauts install the new equipment using wrenches, screwdrivers, and power tools during periodic servicing missions.

Astronauts have serviced Hubble five times — in 1993, 1997, 1999, 2002, and 2009 — and have installed many new instruments featuring more advanced technology.


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Servicing Mission 1

The first servicing mission, launched in December 1993, provided the first opportunity to conduct planned maintenance on the telescope. In addition, new instruments were installed and the flaw of the optics in Hubble's primary mirror was corrected. The astronauts' task list appears below.

  • Change out the Wide Field Planetary Camera. The original instrument was replaced with an updated model called the Wide Field and Planetary Camera 2, or WFPC2. WFPC2 was equipped with built-in corrective optics that compensated for the flaw on the telescope's primary mirror.


  • Remove the High-Speed Photometer and install the Corrective Optics Space Telescope Axial Replacement (COSTAR), which included an assembly of coin-sized mirrors polished to a special prescription that compensate for the flaw on the telescope's primary mirror.


  • Replace the solar array panels, wing-like structures that produce electricity to power the observatory.


  • Install solar array drive electronics, modular hardware that controls the position of the solar arrays.


  • Install the magnetometer, a device that measures the telescope's orientation within Earth's magnetic field to help in attitude control.


  • Install the DF-224 Flight Computer Co-processor, computer memory that improved backup and computational power.

Servicing Mission 2

The second servicing mission, launched in February 1997, greatly improved Hubble's productivity. The installation of new instruments extended Hubble's wavelength range into the near infrared for imaging and spectroscopy, allowing Hubble to probe the most distant reaches of the universe. The replacement of failed or degraded spacecraft components increased the efficiency and performance of the telescope. The astronauts' task list appears below.

  • Install the Space Telescope Imaging Spectrograph (STIS), which provides Hubble with unique and powerful spectroscopic capabilities. A spectrograph separates the light gathered by the telescope into its spectral components so that the composition, temperature, motion, and other chemical and physical properties can be analyzed.


  • Install the Near Infrared Camera and Multi-Object Spectrometer (NICMOS), which has provided valuable new information on the dusty centers of galaxies and the formation of stars and planets. NICMOS consists of three cameras. It is capable of both infrared imaging and spectroscopic observations of astronomical targets.


  • Install a refurbished Fine Guidance Sensor, which was needed to point the telescope at celestial objects.


  • Install the Optical Control Electronics Enhancement Kit, which provides the electronic pathway for commanding the alignment mechanisms in the refurbished Fine Guidance Sensor.


  • Install a solid state recorder, which stores ten times more data and replaced one of Hubble's three reel-to-reel tape recorders.

Servicing Mission 3A

The third servicing mission, Servicing Mission 3A, was launched in December 1999. NASA decided to split the Servicing Mission 3 into two parts, SM3A and SM3B, after the third of Hubble's six gyroscopes failed. In accordance with NASA's flight rules, a "call-up" mission was quickly approved, and developed and executed in a record seven months.

What was originally conceived as a mission of preventive maintenance turned more urgent on Nov. 13, 1999 when the fourth of six gyros failed, and Hubble temporarily closed its eyes on the universe. The telescope needs a minimum of three gyros to function. The astronauts' task list appears below.

  • Install six new gyroscopes, which help keep Hubble steady as it orbits Earth and allow scientists to point the telescope at celestial targets.


  • Add a faster, more powerful, main computer.


  • Install a next-generation solid state data recorder.


  • Add another enhanced Fine Guidance Sensor, which helps in pointing the telescope at objects.


  • Install a new transmitter, new insulation, and six battery voltage/temperature improvement kits.

Servicing Mission 3B

Servicing Mission 3B was launched on March 1, 2002. The astronauts’ principal task was to install a new science instrument, the first new instrument to be added to Hubble since 1997. Astronauts also installed a new cooling system for the telescope’s infrared camera. The astronauts' task list appears below.

  • Install a new science instrument called the Advanced Camera for Surveys (ACS). With its wide field of view, sharp image quality, and enhanced sensitivity, ACS doubled Hubble's field of view and collects data ten times faster than the Wide Field and Planetary Camera 2, the telescope's earlier surveying instrument.


  • Install a new cooling system for the Near Infrared Camera and Multi-Object Spectrometer, or NICMOS, which became inactive in 1999 when it depleted the 230-pound block of nitrogen ice that had cooled it since 1997.


  • Replace Hubble’s four large flexible Solar Array Panels, which collect sunlight and help power the telescope. The new solar panels were smaller and more rigid, producing 30 percent more power.


  • Replace the outdated Power Control Unit, which distributes electricity from the solar arrays and batteries to other parts of the telescope.


  • Replace one of the four Reaction Wheel Assemblies that make up Hubble's Pointing Control System. Flight software commands the reaction wheels to "steer" the telescope by spinning in one direction, which causes Hubble to spin in the other direction.


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The payload team trains in NASA's deep water Neutral Buoyancy Lab at Johnson Space Center in Houston, Texas, which simulates the weightlessness of space. The astronauts practice procedures using tools specifically designed for the mission. In addition, the payload team travels to the NASA Goddard Space Flight Center's clean room, where they train on the actual equipment or mock-ups that will be used in space.

For example, training activities for Servicing Mission 3B included a demonstration of a specific alignment tool that would be used on the Advanced Camera for Surveys and the procedure for rolling up the flexible solar array panels that were to be removed and returned to Earth. The flight crew also uses simulators to practice the capture and deployment of the telescope. The robotic arm operator must practice transporting the astronauts and the new equipment from the cargo bay to the telescope. The robotic arm is a temporary work platform for one or both of the astronauts performing a spacewalk.


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Veteran astronaut Scott D. Altman commanded the final space shuttle mission to Hubble. Navy Reserve Capt. Gregory C. Johnson served as pilot. Mission specialists were veteran spacewalkers John M. Grunsfeld and Michael J. Massimino, and first-time space fliers Andrew J. Feustel, Michael T. Good, and K. Megan McArthur. Grunsfeld, Massimino, and Altman visited Hubble on previous servicing missions.


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Not since Galileo turned his telescope toward the sky in 1609 has any event so changed our understanding of the universe as the success of the Hubble Space Telescope. Hubble orbits above Earth’s atmosphere, working around the clock to unlock the secrets of the universe. It uses excellent pointing precision, powerful optics, and state-of-the-art instruments to provide stunning views of the universe that cannot be made using ground-based telescopes or other satellites.

Hubble was originally designed in the 1970s and launched in 1990. Thanks to on-orbit service calls by the space shuttle astronauts, Hubble continues to be a state-of-the-art space telescope.

Hubble's accomplishments are extraordinary. Before Hubble, distances to far-off galaxies were not well known. Questions such as how rapidly the universe is expanding, and for how long, created great controversy. Hubble discoveries have changed all of that.


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  • Pointing the Hubble Space Telescope and locking onto distant celestial targets is like holding a laser light steady on a dime that is 400 miles away.


  • The Hubble Space Telescope whirls around Earth at a speed of 5 miles per second. If cars moved that fast, a coast-to-coast trip across the continental United States would take only 10 minutes.


  • Each month the orbiting observatory collects enough information to fill the Library of Congress almost two times over.


  • Images and data collected by the telescope travel 90,000 miles over satellite and ground links before they reach the Space Telescope Science Institute in Baltimore, Maryland.


  • Engineers designed Hubble with servicing in mind. The telescope is equipped with 31 foot restraints and 225 feet of handrails.


  • The tool chest that astronauts use during servicing missions contains more than 100 tools, including common screwdrivers and wrenches.


"Q&A: Hubble Space Telescope" is a series of questions and answers about the Hubble Space Telescope written for teachers and students. The questions are ones that students might ask while studying telescopes. Teachers can use this Q&A to gain additional knowledge about the Hubble Space Telescope, or use it in the classroom as outlined below.

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Adaptable, at teacher's discretion
How to use in the classroom

• An engagement activity. Use selected questions to start a discussion.

• A source of information. Students can use the questions and answers as part of their research on telescopes.

• A form of review. Use the questions as a review at the end of a unit on telescopes.

• A follow-up. Have students read the questions and answers to gain additional information about telescopes following a related activity.

• A starting point for a debate. "The Hubble Space Telescope’s impact rivals that of Galileo’s telescope." The question "What impact has Hubble had on society?" addresses this issue (see question 11 in Q&A: Hubble Space Telescope).

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Online Exploration: Mission Mastermind