Hubble Space Telescope

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Hubble Space Telescope
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 kilometers 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.

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:

  • Near Infrared Camera and Multi-Object Spectrometer (NICMOS)
  • Advanced Camera for Surveys (ACS)
  • Wide Field and Planetary Camera 2 (WFPC2)
  • Space Telescope Imaging Spectrograph (STIS)

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 "The Mice", and stellar birth in the Omega Nebula.

Discoveries

See also the external link: Additional Hubble Discoveries.

  • Hubble provided dramatic pictures of the collision of comet Shoemaker-Levy 9 and Jupiter in 1994.
  • Evidence of planets surrounding stars other than the Sun was obtained for the first time with Hubble.
  • Observations with Hubble also showed that the missing dark matter in our galaxy cannot consist solely of faint small stars.
  • Some of the observations leading to the current model of an accelerating universe were performed using the Hubble space telescope.
  • The theory that most galaxies host a black hole in their nucleus has been partially confirmed by many observations.
  • In December 1995, Hubble photographed the Hubble Deep Field, a region covering one 30-millionth of the area of the sky and containing several thousand faint galaxies. A similar patch of southern sky was also imaged and looked remarkably similar, strengthening the position that the Universe is uniform over large scales, and that Earth occupies a typical place in the Universe.

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).

  • Servicing Mission 1, December 1993 (STS-61) installed several instruments and other equipment. The most important astronomically were: the Corrective Optics Space Telescope Axial Replacement (COSTAR), which was a set of five corrective mirrors; and the Wide Field/Planetary Camera (WF/PC-II), an upgraded version of the previous ultraviolet detector which also incorporated the corrective optics. On January 13, 1994, NASA declared the mission a complete success, and showed the first of many much sharper images.
  • Servicing Mission 2, February 1997 (STS-82) replaced High Resolution Spectrograph and Faint Object Spectrograph with Space Telescope Imaging Spectrograph and added Near Infrared Camera / Multi-Object Spectrograph.
  • Servicing Mission 3A, December 1999 (STS-103) replaced faulty gyroscopes and fine guidance sensors (reusing one returned by SM-1), installed new computer.
  • Servicing Mission 3B, March 2002 (STS-109) repaired and upgraded several items, requiring lengthy and delicate spacewalks. Fixes to the telescope included:
    • Update of its Power Converter Unit, which was particularly tricky as it was not designed for in-orbit replacement, and also required taking the satellite completely off-line for the first time since it was put into operation.
    • Replacement of its solar arrays. The new arrays were derived from those built for the Iridium comsat system. They are only two-thirds the size of the old tattered arrays, resulting in less drag against the tenuous reaches of the upper atmosphere, while providing 30% more power. The additional power will permit all instruments on board the Hubble to be run simultaneously. They also reduce a vibration problem that occurred when the old, more rigid arrays entered and left direct sunlight.
    • Replacement of the "Faint Object Camera (FOC)" with the "Advanced Camera for Surveys (ACS)". Both the FOC and the ACS are about the size of a telephone booth.
    • Installation of a mechanical cryocooler unit into the nonfunctioning "Near Infrared Camera and Multi-Object Spectrometer (NICMOS)".
    • Replacement of a reaction control wheel.
    The completion of this servicing mission, considerably enhanced Hubble's capabilities, some enthusiasts claiming that it is now effectively a "new instrument".

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.

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 (it may end earlier as, at the moment, three gyros are required. Work is underway to allow operation on 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 it always point perpendicular to the earth in what is known as a gravity-gradient position.

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 2022. 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. NASA currently calculates 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 options are:

  • Addition of an external propulsion module. This would allow either continued operation, controlled reentry, or both.
  • Addition of an one-time-use external propulsion module to boost the telescope into a ~2300km holding or disposal orbit. Hubble would likely remain in such an orbit for hundreds if not thousands of years.
  • Retrieval by a space shuttle. This was the original plan for Hubble disposal. It would then most likely be displayed in the Smithsonian. The problems with this method are the cost (~$500,000,000) and risk of a shuttle's crew.

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 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 (though, of course, not nearly as high as a Hubble replacement).

However, space telescopes are needed for wavelengths outside of visible wavelengths. In particular, Hubble 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]

Further reading

  • The Hubble Wars, Eric J. Chaisson, 1998, ISBN 0-674-41255-9, a book by a member of the science team at the Space Telescope Science Institute, about the launch of the HST and the endless struggles between the science and engineering teams over operational problems and inherent flaws in the telescope, as well as the attempts by publicists and politicians to spin the problems to the public.

See also: space observatory.

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