Hubble Space Telescope

The Hubble Space Telescope is a telescope in orbit around the Earth. Its position outside the Earth's atmosphere allows it to take extremely sharp images, and since its launch in 1990, it has become one of the most important telescopes in the history of astronomy. It has been responsible for many ground-breaking observations and has helped astronomers achieve a better understanding of many fundamental problems in astrophysics.

Servicing missions and new instruments

Servicing mission 1

The telescope had always been designed so that it could be regularly serviced, but after the problems with the mirror came to light, the first servicing mission assumed a much greater importance, as the astronauts would have to carry out extensive work on the telescope to install the corrective optics. The seven astronauts selected for the mission were trained intensively in the use of the 100 or so specialised tools which would need to be used. The mission (STS-61) took place in December 1993, and over a total of 10 days installed several instruments and other equipment.

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Most importantly, the High Speed Photometer was replaced with the COSTAR corrective optics package, and WFPC was replaced with the Wide Field and Planetary Camera 2 (WFPC2), with its internal optical correction system. In addition, the solar arrays and their drive electronics were replaced, as well as four of the gyroscopes used in the telescope pointing system, two electrical control units and other electrical components, and two magnetometers. The onboard computers were upgraded, and finally, the telescope's orbit was boosted, having been slowly decaying for three years due to drag in the tenuous upper atmosphere.

Related Topics:
High Speed Photometer - Wide Field and Planetary Camera 2 - Solar array - Gyroscope - Drag

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On January 13, 1994, NASA declared the mission a complete success and showed the first of many much sharper images {{ref|Trauger}}. The mission had been one of the most complex ever undertaken, involving five lengthy periods of extravehicular activity, and its resounding success was an enormous boon for NASA, and of course for the astronomers who now had a fully capable space telescope.

Related Topics:
January 13 - 1994 - NASA

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Subsequent servicing missions

Subsequent servicing missions were less dramatic, but each gave the space telescope new capabilities. Servicing Mission 2 (STS-82) in February 1997 replaced the GHRS and the FOS with the Space Telescope Imaging Spectrograph (STIS) and the Near Infrared Camera and Multi-Object Spectrometer (NICMOS), replaced an Engineering and Science Tape Recorder with a new Solid State Recorder, repaired thermal insulation and again boosted Hubble's orbit. NICMOS contained a heat sink of solid nitrogen to reduce the infrared 'noise' from the instrument, but shortly after it was installed, an unexpected thermal expansion resulted in part of the heat sink coming into contact with an optical baffle. This led to an increased warming rate for the instrument and reduced its original expected lifetime of 4.5 years to about 2 years.

Related Topics:
STS-82 - Space Telescope Imaging Spectrograph - Near Infrared Camera and Multi-Object Spectrometer - Heat sink - Nitrogen

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Servicing Mission 3A (STS-103) took place in December 1999, replaced all six gyroscopes (one had failed and rendered the telescope unusable just weeks before the mission), replaced a Fine Guidance Sensor and the computer, installed a Voltage/temperature Improvement Kit (VIK) to prevent battery overcharging, and replaced thermal insulation blankets. The new computer was based on a space-qualified Intel 486 and permits some computing tasks that were previously performed by computers on the ground to be handled on board the spacecraft.

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Servicing Mission 3B (STS-109) in March 2002 saw the installation of a new instrument, with the FOC being replaced with the Advanced Camera for Surveys (ACS), and also saw the revival of NICMOS, which had run out of coolant in 1999. A new cooling system was installed which reduced the instrument's temperature enough for it to be usable again, although it was not as cold as its original design called for.

Related Topics:
STS-109 - Advanced Camera for Surveys

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The mission replaced the solar arrays for a third time, with the new arrays being smaller but generating more power. The new arrays were derived from those built for the Iridium comsat system and were only two-thirds the size of the old arrays, resulting in less drag against the tenuous reaches of the upper atmosphere, while providing 30% more power. The additional power allowed all instruments on board the Hubble to be run simultaneously, and reduced a vibration problem that occurred when the old, less rigid arrays entered and left direct sunlight. Hubble's Power Distribution Unit was also replaced in order to correct a problem with sticky relays, a procedure that required the complete electrical power down of the spacecraft for the first time since it was launched.

Related Topics:
Solar array - Iridium

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The completion of this servicing mission considerably enhanced Hubble's capabilities. The two instruments primarily affected by the mission, ACS and NICMOS, together imaged the Hubble Ultra Deep Field in 2003 to 2004.

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