Hubble Space Telescope

The Hubble Space Telescope (HST, or the Hubble) 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 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 Next Generation
Space Telescope (NGST), in 2009.

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

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. It has various spectrometers and three cameras: one for
faint objects in a small field, one wide field camera for planetary
pictures, and one infrared camera.

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.

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 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.

   * 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:
        o 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.
        o 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.
        o 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.
        o Installation of a mechanical cryocooler unit into the
          nonfunctioning "Near Infrared Camera and Multi-Object Spectrometer
          (NICMOS)".
        o 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".
   * Servicing Mission 4, planned for February 2005, is due to be the last
     servicing mission, as Hubble reaches the end of its life expectancy.
     However in the aftermath of Space Shuttle Columbia disaster, the timing
     of future shuttle missions can only be stated tentatively.

The future beyond Hubble

NASA intends to shut down the Hubble in 2010 and to fly the Next Generation
Space Telescope (NGST) in 2009. Hubble was designed for 15 years of
operation, and it will end up serving for 20.

Now the space agency and the astronomy community have to sit down and figure
out what, if anything, should follow the Hubble. The NGST might seem to be
the answer to that question, but the NGST will be strictly 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.