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Hubble Space Telescope

The Hubble Space Telescope (HST, or Hubble) 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 ESA. The telescope can achieve optical resolutions greater than 0.1 arcseconds. 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.

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

Hubble Space Telescope as seen from the Space Shuttle Discovery on mission STS-82.

Pictures 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. (Images courtesy of NASA)

Table of contents
1 Technical description
2 Discoveries
3 Launch and initial disappointment
4 Servicing Missions
5 Hubble's future and beyond
6 External Links

Technical description

The unit weighs about 11,000 kilograms, is 13.2 meters long, has a maximum diameter of 4.2 meters and cost US 2 billion (2 × 109 dollars). 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 changed with different (and better) ones during maintenance missions.

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

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

It uses two solar panels to generate electricity, which is mainly needed 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.


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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 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 that could have been tested for on the ground if the funds had been available.

Servicing Missions

The telescope has been revisited several times by spacewalking astronauts in space shuttles in order to correct malfunctions and install new equipment. Because of atmospheric drag, the telescope slowly loses height (and gains speed) over time; the shuttle pulls it back to a higher orbit every time it visits.

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, future shuttle missions must include the International Space Station (ISS) on their route, from which it is not possible to reach the HST. Therefore, this and all future service missions have been cancelled. There is popular belief that this decision was also motivated by the Bush administration's new manned space agenda.

The gyroscopes are expected to fail in 2012, causing the satellite to enter the Earth's atmosphere. Not all of the telescope will burn up in reentry, leaving a 1 in 700 chance of human fatality upon impact. NASA originally intended to attach 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. 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. At that time, the conventional wisdom was that there was no way to make mirrors much bigger for ground-based telescopes, since they wouldn't be able to cool off and stabilize at night before the sun came up. Besides, continuously changing variations in atmospheric seeing would ensure that such bigger telescopes would return images no better than those obtained by smaller telescopes. Building a space telescope seemed to be the only way around these obstacles.

In fact, the obstacles fell more easily than anyone expected. All it required was a different mindset on how to make big telescopes. Instead of building one huge mirror that would take all night to stabilize, modern giant telescopes use smarter schemes. For example, the Keck telescope at Mauna Kea in Hawaii has a 10 meters segmented mirror, composed of a mosaic of separate mirrors arranged together and continuously adjusted by a bed of computer controlled actuators to ensure that they maintain their proper shape.

As far as the seeing problem goes, such a telescope can use an adaptive optics system, adjusting the mirrors continuously to compensate for changes in the atmosphere. Furthermore, astronomy organizations have been able to find and make very good use of high, dry sites with excellent seeing, such as Mauna Kea, and the high Atacama Desert in Chile.

This means that there may not really be any need to 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 naturally 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's one.

Space-based astronomy remains irreplaceable for those wavelengths that are blocked by the atmosphere, such as most of the infrared, and all the ultraviolet, X-ray and Gamma ray regions of the electromagnetic spectrum.

While NASA has long had a good relationship with the astronomy community, the agency's space-based astronomy programs have tended to operate on a parallel, independent track from ground-based astronomy efforts. Some observers believe that NASA and the National Science Foundation, which handles US government-funded ground-based astronomy, will soon be in discussions, and even that eventually both space and ground based astronomy will be directed under the same overall program.

External Links