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.

The future

Equipment failure

Past servicing missions have exchanged old instruments for new ones, both avoiding failure and making possible new types of science. Without servicing missions, all of the instruments will eventually fail. On August 3, 2004, the power system of the Space Telescope Imaging Spectrograph (STIS) failed, rendering the instrument inoperable. The electronics had originally been fully redundant, but the first set of electronics failed in May 2001. It seems unlikely that any science functionality can be salvaged without a servicing mission.

Related Topics:
August 3 - 2004 - Space Telescope Imaging Spectrograph

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Hubble uses gyroscopes to stabilize itself in orbit and point accurately and steadily at astronomical targets. Normally, three gyroscopes are required for operation; observations are still possible with two gyros, but the area of sky that can be viewed would be somewhat restricted, and observations requiring very accurate pointing would be more difficult. In 2005, it was decided to switch to two-gyroscope mode for regular telescope operations as a means of extending the lifetime of the mission. The switch to this mode was made on August 31, 2005, leaving Hubble with two gyroscopes in use and two on backup. Estimates of the failure rate of the gyros indicate that Hubble may be down to one gyro by 2008, after which the telescope would be rendered unusable. {{ref|Sembach}}

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In addition to predicted gyroscope failure, Hubble will eventually require a change of batteries. A robotic servicing mission including this would be tricky, as it requires many operations, and a failure in any might result in irreparable damage to Hubble. However, the observatory was designed so that during Shuttle servicing missions it would receive power from a connection to the Space Shuttle, and this fact may be utilized by adding an external power source (an additional battery) rather than changing the internal ones http://news.bbc.co.uk/2/hi/science/nature/3652627.stm.

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Orbital decay

Hubble orbits the Earth in the extremely tenuous upper atmosphere, and over time its orbit decays due to drag. If it is not re-boosted by a shuttle or other means, it will re-enter the Earth's atmosphere sometime between 2010 and 2032, with the exact date depending on how active the Sun is and its impact on the upper atmosphere. The state of Hubble's gyros also impacts the re-entry date, as a controllable telescope can be made to minimize atmospheric drag. Not all of the telescope would burn up on re-entry. Parts of the main mirror and its support structure would probably survive, leaving the potential for damage or even human fatalities (estimated at up to a 1 in 700 chance of human fatality for a completely uncontrolled re-entry).

Related Topics:
Atmosphere - Drag - Sun

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Addition of an external propulsion module to allow controlled re-entry is currently being investigated by NASA. It would not have to be executed until the expected natural re-entry date, after Hubble has completed its operational lifetime. One potential model involves a Pac-Man shaped unit entirely enclosing the satellite. Alternatively, instead of being used to control re-entry, the propulsion module could boost the telescope into a much higher orbit, in which it could remain indefinitely.

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Another possibility for safely de-orbiting Hubble is retrieval by a space shuttle. The Hubble telescope would then most likely be displayed in the Smithsonian Institution. The problems with this method are the cost of a shuttle flight (about $500 million by some estimates) and risk to a shuttle's crew. In the wake of the Space Shuttle Columbia disaster, NASA's astronaut office is wary of risking a shuttle crew simply to retrieve a museum-bound telescope http://www.space.com/businesstechnology/technology/hubble_grunsfeld_0306731.html. Also, this mission would require a rebuild of the cargo space of the space shuttle sent to retrieve Hubble, since the only space shuttle unmodified since Hubble's launch (and therefore able to hold it in its cargo space) was the destroyed Columbia shuttle.

Related Topics:
Smithsonian Institution - Space Shuttle Columbia disaster

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Future space telescopes

The James Webb Space Telescope (JWST, formerly known as the Next Generation Space Telescope, NGST) may replace the HST in 2012. However, the JWST is an infrared telescope, while the Hubble covered the range from the near infrared through the visible into the near ultraviolet.

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One of the primary advantages of situating a telescope above the atmosphere is the enhanced optical resolution possible in the absence of atmospheric turbulence. However, in the time since Hubble was designed, there have been great advances in overcoming this at ground level. Imaging observations of bright sources using speckle interferometry or optical interferometry in the 1980s had higher resolution than Hubble could achieve, and in the 1990s microcomputer technology allowed the development of adaptive optics, in which telescope mirrors are distorted by actuators in real time to compensate for the turbulence of the atmosphere.

Related Topics:
Speckle interferometry - Optical interferometry - Adaptive optics - Actuator

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This means that there is less need for a space-based optical telescope. Ground-based telescopes are much less expensive than Hubble and can achieve resolutions approaching that of the space telescope. For example, the VLT was built at about one seventh of the cost of Hubble, and gave the astronomical community four 8.2 meter telescopes, each with an overall performance almost as high as the Hubble, and almost certainly representing better value for money in terms of the science returned.

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However, Hubble's detectors still have the highest sensitivity to visible light, and existing ground-based telescopes cannot compete with the Hubble deep fields in the visible due to airglow. A new space telescope employing new technologies and advances made in spaceflight over the last 20 years could revolutionize astronomy as Hubble did. The other major benefit of Hubble was the huge public relations boost for astronomy and for science in general. The media-friendly colour images produced by Hubble have appeared in countless newspapers and books, and Hubble has contributed greatly to the public interest and funding of astronomy, providing an overall science return much greater than the science actually done using the telescope.

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Space telescopes are still essential for observing wavelengths which are absorbed in the atmosphere. In particular, Hubble has been increasingly used for observations of the near-ultraviolet wavelength range, for which no new telescopes are currently planned.

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