The Hubble Space Telescope (HST) is possibly the most famous telescope in orbit around the Earth. It is named after astronomer Edwin Hubble. The telescope’s position outside the Earth’s atmosphere provides significant advantages over ground-based telescopes. This is because its images are not blurred by the atmosphere and there is no background from light scattered by the air. The Hubble can observe ultraviolet light that is normally absorbed by the ozone layer in observations made from Earth. Though it is not the first space telescope, since its launch in 1990, the Hubble has become one of the most important instruments in the history of astronomy. Since it has been in orbit astronomers have made many observations leading to breakthroughs in astrophysics. One of the most remarkable aspects of the telescope is that Hubble’s Ultra Deep Field allows for the most detailed visible light image of the most distant objects ever taken.
The history of the construction and launch of the Hubble is one that was beset by delays and budget problems. To further make matters worse soon after its 1990 launch, it was found that the main mirror suffered from a spherical aberration due to faulty quality control during its manufacturing. This severely compromised the telescope’s capabilities. After a servicing mission that was completed in 1993, the telescope was restored to its intended quality and became a vital research tool as well as a public relations boon for astronomy. The Hubble is part of NASA’s Great Observatories series in connections with the Compton Gamma Ray Observatory, the Chandra X-ray Observatory, and the Spitzer Space Telescope. Hubble is also a collaboration between NASA and the European Space Agency. This has become a worldwide project with astronauts of many nationalities taking part on the servicing missions of the Hubble.
The Hubble also has the distinction of being the only telescope ever designed to be serviced in space by astronauts. As of this year there have been four servicing missions, with a fifth and final mission planned for September 2008. Servicing Mission 1 took place in December 1993 when Hubble’s original imaging flaw was corrected. Servicing Mission 2 occurred in February 1997 when two new instruments were installed, tested and set up to run.
Servicing Mission 3 had to be split into two distinct missions: SM3A occurred in December 1999 when urgently needed repairs were made to Hubble; and then in March 2002 SM3B occurred when the Advanced Camera for Surveys (ACS) was installed.
Since the completion of SM3B, the Hubble has lost use of two major science instruments and is operating with viewing restrictions. This is due mainly because of rate-sensing gyroscope failures. There are six gyroscopes onboard the Hubble and three are normally used for observing. However, after experiencing further failures, and in order to conserve the lifetime of the gyroscopes, a decision was made in August 2005 to switch off one of the functioning gyroscopes and operate Hubble using only two gyros in combination with the Fine Guidance Sensors. This mode would retain the excellent image quality of Hubble, and provides a redundancy should it be needed. Further redundancy has also been put in place now that an operational mode requiring only one gyro has been developed and tested. Servicing Mission 4 will install six new gyroscopes.
Ironically the initial concept of the Hubble Space Telescope was that of a short-term project. But with the advancement of technology and design astronomers and other scientists are hopeful that Hubble can be left in space for much longer than was originally planned giving us all a unique and truly amazing window on the universe.