HUBBLE SPACE TELESCOPE

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.

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.