The Hubble Space Telescope reference article from the English Wikipedia on 24-Jul-2004
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Hubble Space Telescope

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Hubble Space Telescope
Hubble Space TelescopeEnlarge

Hubble Space Telescope

OrganizationNASA, ESA
Wavelength regimeoptical
Orbit height600 km
Orbit period100 min
Launch date24 April 1990
Deorbit datecirca 2010
Mass11,000 kg
Other names(none)
Webpagehttp://hubble.nasa.gov
Physical Characteristics
Telescope Stylereflector
Diameter2.4 m
Collecting Areaapprox. 4.3 m2
Effective Focal Length57.6m (189 ft)
Instruments
NICMOScamera and spectrometer
ACSsurvey camera
WFPC2wide field camera
STISspectrometer and camera

The Hubble Space Telescope (HST) is a telescope located at the outer edges of Earth's atmosphere, about 600 kilometerss above the ground, orbiting the Earth every 100 minutes. It was placed into orbit in April 1990, as a joint project of NASA and the European Space Agency. The telescope can achieve optical resolutions greater than 0.1 arcseconds.

Table of contents
1 Overview
2 Technical description
3 Discoveries
4 Launch and initial disappointment
5 Servicing missions
6 Hubble's future and beyond
7 Further reading
8 See also
9 External links

Overview

The HST is named after Edwin Hubble. It is scheduled for replacement by the James Webb Space Telescope (JWST) in 2009. Every day, the Hubble Space Telescope archives 3 to 5 gigabytes of data and delivers between 10 and 15 gigabytes to astronomers.

Data from the telescope is collected and analyzed by astronomers and scientists at the Space Telescope Institute, located at Johns Hopkins University in Baltimore, Maryland.

Working outside the atmosphere has advantages because the atmosphere obscures images and filters out electromagnetic radiation at certain wavelengths, mainly in the infrared.

Technical description

Hubble weighs about 11,000 kilograms, is 13.2 meters long, has a maximum diameter of 4.2 meters and cost US 2 billion ($2,000,000,000). The telescope is a reflector with two mirrors; the main mirror has a diameter of about 2.4 meters.

The light collected and focused by the telescope ends up in one of several instruments. The instruments have been exchanged with different (and better) ones during the several maintenance missions.

The current (as of 2004) complement of instruments is:

Each of which also has some capability as a spectrometer. Additionally, the telescope's Fine Guidance Sensors (FGS) can and have been used for science.

Two solar panels provide electricity, which is mainly used to power the cameras and the four large flywheels used to orient and stabilize the telescope. The telescope's infrared camera and multi object spectrometer also need to be cooled down to minus 180 degrees Celsius for operation.

Montage of images taken by the Hubble Space telescope. Clockwise from the upper left: the Tadpole galaxy, the Cone Nebula, two colliding spiral galaxies dubbed Enlarge

Montage of images taken by the Hubble Space telescope. Clockwise from the upper left: the Tadpole galaxy, the Cone Nebula, two colliding spiral galaxies dubbed "The Mice", and stellar birth in the Omega Nebula

Discoveries

See also the external link: Additional Hubble Discoveries.

Launch and initial disappointment

The telescope was launched by Space Shuttle Discovery mission STS-31 on April 24, 1990. This had been postponed from a 1986 launch date by the Space Shuttle Challenger disaster in January that year.

The Space Shuttle and Hubble has always been closely connected dating back to the 1970's when a decision, later rescinded, was made to launch all new payloads with the Space Shuttle. While the Space Shuttle has allowed Hubble to be serviced in orbit, it has also caused Hubble's fate to be determined by delays in the shuttle schedule. A sizeable fraction of the astronomical community feels that the connection between the Shuttle and Hubble has been much too close and that it would have been better to launch Hubble on an unmanned launcher. Although this would have not allowed for servicing missions it has been argued that had Hubble been lauched using an expendable booster, that it would have been possible for the same price to launch several Hubbles.

The first images back from the telescope were generally regarded as a big disappointment for astronomers and all concerned in the project. They were blurred, and despite image processing could not match the predicted resolution. It was determined that the main mirror had been ground slightly too flat at the edges, a problem due to a miscalibrated measuring instrument, a mistake that most in the field consider a sign of gross negligence and incompetence.

Servicing missions

The telescope has been revisited several times by spacewalking astronauts in space shuttles in order to correct malfunctions, install new equipment, and boost the telescope back into a higher orbit (atmospheric drag causes Hubble to slowly fall out of orbit).

Hubble's future and beyond

Servicing Mission 4, planned for February 2005, was due to be the last servicing mission, as Hubble reached the end of its life expectancy. After the Space Shuttle Columbia disaster, all future shuttles must be inspected externally on orbit before reentry, a task which NASA has decided is too expensive to be done without the facilities of the International Space Station (ISS). The shuttle is incapable of reaching both HST and ISS during the same mission. Therefore, this and all future service missions have been cancelled. Among astronomers, there is also popular belief that this decision was motivated by the Bush administration's new manned space agenda. Lack of a future service mission can result in failure of the telescope in three ways: gyroscopes, batteries, and reentry.

Gyroscopes

Hubble uses gyroscopes to stabilize itself in orbit. Without them it will be unable to remain steady long enough to take meaningful pictures. The current gyroscopes are expected to have all failed by 2012, resulting in the end of Hubble's science mission. However, as three gyros are currently required for, the mission may end earlier; work is underway to allow operation on only two. The gyros also are what allow ground controllers to position the telescope. Without them, the difference in gravity between the top and the bottom of the telescope will cause always it to point perpendicular to the earth in what is known as a gravity-gradient position.

Batteries

In addition to predicted gyroscope failure, Hubble will require a change of batteries. A robotic servicing mission including this would be tricky, as it requires many operations, and a failure in any would likely result in loss of all research capabilities for Hubble. However, the observatory was designed so that during Shuttle servicing missions it would receive power from a connection to the Space Shuttle. This fact may be utilized by adding an external power source (an additional battery) rather than changing the internal ones. Plans for such a mission are in the works.[1]

Reentry

Hubble is currently (July 2003) in a 569 km orbit. If it is not reboosted by a shuttle or other means, it will reenter the Earth's atmosphere sometime between 2010 and 2032. The exact date is dependent on how active the Sun is and its impact on the upper atmosphere, though it is likely to be earlier rather than later. The state of Hubble's gyros also impact the reentry date, as a controllable telescope can be made to minimize atmospheric drag.

Not all of the telescope will burn up on reentry. Parts of the main mirror and its support structure are expected to survive, leaving the unfortunate potential for damage or even human fatalies (estimated at up to a 1 in 700 chance of human fatality for a completely uncontrolled reentry). The most recent deorbit plan involved attaching a propulsion module to the satellite in a future mission, which would provide a controlled reentry in 2010. At this time only Russia has the automatic docking capabilities to complete such a plan. Even this option is not simple, as Hubble has none of the active docking hardware required for the Russian automated docking systems to function.

Other Hubble mission options include:

Future

Hubble was designed for 15 years of operation, and it will end up serving for 22.

Now the space agency and the astronomy community have to sit down and figure out what, if anything, should follow the Hubble. 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.

What complicates the question are the breathtaking advances in Earth-based astronomy since the Hubble was conceived. During the 1970's when Hubble was designed, the conventional wisdom was that ground based telescopes would never have the resolution of space telescopes because the atmosphere seeing limited the resolution of ground telescopes. In fact, microcomputer technology starting in the 1990's allowed for adaptive optics which adjusts the mirrors continuously to compensate for changes in the atmosphere.

This means that there is not any need replace the Hubble to obtain better astronomical imagery in the visible range. The new ground-based telescopes can do the job, and even the most ambitious of them, like the Keck in Hawai'i and the Very Large Telescope (VLT) in Chile, are much less expensive than the Hubble and much more sensitive to light. This naturally is much easier to service and update. For example, the VLT cost was roughly 1/7 of the HST cost, and gave the astronomic community four 8.2 meters telescopes, with a resolution almost as high as the Hubble.

The other side of this, of course, is that ground based telescopes are only just now approaching the abilities of a 20 year old 2.4 meter space telescope. Much of the technology developed for these ground telescopes is applicable to new space telescopes. A new space telescope employing these new technologies and other general advancements made in spaceflight over the last 20 years could once again revolutionize astronomy.

Space telescopes are needed for wavelengths outside of visible wavelengths. In particular, Hubble is now used largely for observations of the near-ultraviolet, a frequency for which no new telescopes are currently planned.

On January 29, 2004, NASA's Administrator said that that he will review his decision to cancel the final servicing mission of the Hubble Space Telescope due to public outcry and requests from Congress for NASA to look for a way to save the Hubble Space Telescope. Adm. Hal Gehman, chairman of the NASA board that investigated the Space Shuttle Columbia incident will review and send his opinion to NASA according to a letter sent from NASA Administrator Sean O' Keefe to Maryland Senator Barbara Mikulski; Mikulski is the ranking Democrat on the Senate subcommitte that oversees NASA's budget. Websites have been set up dedicated to saving the Hubble Space Telescope. Federal lawmakers noted that NASA's next generation space telescope, named the James Webb Telescope is not scheduled for launch until 2010, many years after the Hubble Space Telescope is expected to cease functions. They also noted that about $200 million has already been spent on two new instruments designed for the Hubble Space Telescope and it might cost $300 million for a mission to return the Hubble Space Telescope safely to the Earth. Source: [1]

On July 13, 2004, an official panel from the National Academy of Sciences made the recommendation that the Hubble telescope be preserved despite the aparent risks. Their report urged "NASA should take no actions that would preclude a space shuttle servicing mission to the Hubble Space Telescope."

Further reading

See also

External links

Servicing cancellation links