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A look at a pair of gravitationally interacting galaxies called Arp 147. The image, published in October 2008, was the first snapshot taken by the Hubble Space Telescope since it failed a month earlier. ((NASA, ESA, M. Livio with the Space Telescope Science Institute))

Since its launch in 1990, the Hubble Space Telescope has provided unprecedented views of the universe, orbiting our planet every 97 minutes at an altitude of 569 kilometres.

Among its many scientific accomplishments, the telescope has helped change black holes from theory to fact, provided measurements that helped establish the size and age of the universe and offered evidence that the universe is expanding. It also helped popularize astronomy by transmitting more than 700,000 images during its years of operation.

This orbiting hardware requires regular maintenance. If it isn't repaired soon, the telescope may stop working by about 2009 or 2010.

The telescope, which was named after the astronomer Edwin Hubble, has been repaired four times since its launch, by astronauts who reached it by space shuttle. A fifth repair mission was cancelled after the 2003 space shuttle Columbia tragedy that killed seven astronauts.

Officials said it was a matter of shuttle safety. If a spacecraft heading to the telescope encounters a problem, there is no safety net since the astronauts would not be able to reach the International Space Station from Hubble's orbit.

On Oct. 31, 2006, NASA announced it would send a space shuttle to repair the Hubble Space Telescope.

But in the two and half years since the announcement, the mission has repeatedly been bumped. There have been delays with shuttle missions to the International Space Station and problems with the telescope itself that required NASA to refine what work needed to be done.

Hubble Quick Facts

Launch: April 24, 1990, from space shuttle Discovery

Deployment: April 25, 1990

Mission duration: Up to 20 years

Size: 13.2 metres long and maximum diameter is 4.2 metres. It is about the size of a large tractor-trailer truck.

Servicing mission 1: December 1993

Servicing mission 2: February 1997

Servicing mission 3A: December 1999

Servicing mission 3B: February 2002

Cost at launch: $1.5 billion U.S.

Hubble completes one orbit around the Earth every 97 minutes. Its speed is approximately 8 km per second.

At the height of its capability, Hubble transmitted about 120 gigabytes of science data every week. That's equal to about 1,097 metres of books on a shelf. The rapidly growing collection of pictures and data is stored on magneto-optical disks.

Source: NASA, Hubblesite.org

On May 11, 2009, Hubble is scheduled to finally get its fix, one that will keep the telescope running until 2014. On the list of Hubble repairs is the replacement of aging batteries, guidance sensors and gyroscopes.

Here we take a look at the ups and downs of the most famous of our eyes in the sky.

May 11, 2009

Hubble's long-awaited fifth repair mission is schedule to launch.

September 2008

Hubble's data handling system broke down, just weeks before the fifth repair mission was scheduled to launch. NASA ordered Atlantis back from the launch pad and scrapped the mission until 2009. The mission also added a replacement of the data handling system to the task list.

2006

NASA announces Hubble will get its fifth repair mission to extend the life of the orbiting telescope.

2002

The space shuttle Columbia launches March 1 with servicing mission 3B (SM3), where new solar panels are installed on Hubble.

1999

The shuttle Discovery takes off on Dec. 19 on the third servicing mission (SM3A). The RSU (Rate Sensing Units containing gyroscopes) are replaced, a new computer is installed and general maintenance takes place during the mission.

1998

The shuttle Discovery launches on Oct. 29 with the HST Orbital Systems Test (HOST) mission to test new technologies to be used in Hubble during the third servicing mission and beyond.

1997

The second servicing mission (SM2) begins with the launch of the shuttle Discovery. The Space Telescope Imaging Spectrograph and Near Infrared Camera and Multi-Object Spectrograph are put in place.

1993

The space shuttle Endeavour takes off on Dec. 2, with the first servicing mission (SM1) for Hubble. Corrective optics are installed and the Wide-Field Planetary Camera 2 (WFPC2) is put in to replace the WFPC.

1990

The shuttle Discovery launches on April 24. Hubble is deployed into orbit on April 25. A defect is discovered in Hubble's primary mirror on June 25. The creation of a complex package of five optical mirror pairs is approved to repair the problem.

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The Hubble Space Telescope is backdropped against black space as the Columbia space shuttle, with a crew of seven astronauts on board, approached in this March 3, 2002, file photo. ((NASA/Associated Press))

1981

Space Telescope Science Institute begins operations in Baltimore, Md. It takes over the science management of the Hubble Space Telescope from NASA.

1977

U.S. Congress approves funding for the Hubble Space Telescope.

1969

The National Academy of Sciences approves the Large Space Telescope (LST) project, allowing hearings and studies to continue.

1923

Rocket scientist Herman Oberth's article speculating on telescopes in orbit is published.

1918

The 2.5-metre Hooker Telescope, then the world's largest telescope, begins operations at Mt. Wilson Observatory in Pasadena, Calif. Astronomer Edwin Hubble uses this device to measure the distances of galaxies, work that led to his discovery of the expanding universe.