Satellites typically lead a lonely existence. Once launched, they orbit in isolation until falling back to earth. The Hubble Space Telescope is different—it was made to be fixed in space. It has a modular design so that space-walking astronauts can replace broken parts and upgrade outdated systems. The HST's designers even put 76 handholds around it for the convenience of the astronauts who would maintain it in space.

A view inside the clean room.

The original 1979 plans for an orbiting telescope called for it to be brought down, serviced, and relaunched every five years. Halfway between these major missions, astronauts would service the telescope while it was still in orbit. In 1985, however, concerns that the telescope might suffer damage during regular returns to earth led NASA to decide to perform all service in orbit, with "servicing missions" scheduled to take place every three years.

There have been three servicing missions since the Hubble was placed in orbit in 1990. These updates and repairs have left the telescope an even better scientific instrument than it was at launch, the way putting new parts in a computer can make it work better than it did to begin with.

The first servicing mission, known as "SM1," was launched in December 1993. Its main goal was to correct a serious flaw in the telescope. Soon after it began operation, scientists realized that something was wrong with the Hubble's primary mirror: it had been made a tiny bit too flat. Though the shape of the mirror was off by no more than a fraction of the thickness of a human hair, the flaw was enough to make it impossible to precisely focus all the light it gathered, so the images sent back from the telescope were blurry. The Corrective Optics Space Telescope Axial Replacement (COSTAR) part of SM1 involved placing corrective mirrors—some no bigger than a coin—at various places on the telescope. These mirrors did the trick, enabling the HST to capture the amazing images we've seen ever since.

There were other tasks for SM1 as well. In addition to correcting the defective mirror, astronauts also replaced parts of the solar panel wings (still the largest structures ever replaced in orbit), added processors to the flight computer, added new sensors, and made a number of other enhancements.

Hubble Space Telescope while docked to the Space Shuttle Discovery's Payload Bay during the second servicing mission.

The second mission—SM2—went up a little more than three years later, in February 1997. This time, the task was to add new instruments and perform general maintenance. For instance, the astronauts added the Space Telescope Imaging Spectrograph (STIS), which included ultraviolet detectors. A spectrograph separates the gathered light into its component colors, sort of the way a prism makes a rainbow. By examining which colors are brightest, scientists can learn a lot about the composition, temperature, and other qualities of the source of the light. Today, information gathered by the HST's spectrograph is helping researchers study black holes, quasars, and star formation.

SM2 also brought up the Near Infrared Camera and Multi-Object Spectrometer (NICMOS), which can record electromagnetic waves in the near-infrared wavelength. The NICMOS sensors have let scientists see objects that were too faint for the HST's optical and ultraviolet sensors. The crew of SM2 also pioneered the use of space's first cordless power tool, the Pistol Grip Tool, which would be used again two years later.

Hubble's next regular servicing mission was originally planned for June 2000. However, since one of the telescope's six gyroscopes (which control its aiming) failed in early 1999, the telescope had only the minimum number needed to function properly. NASA immediately began planning a special "call-up" mission, SM3a, to replace the gyroscopes. As it turned out, the sense of urgency was not misplaced, because a fourth gyroscope failed in November 1999, sending the HST into a sort of "sleep" mode—safe, but unable to make any observations.

When the SM3a team went up in 1999, they replaced all six gyroscopes, upgraded a guidance sensor and the spacecraft's computer, and installed a voltage/temperature kit for the spacecraft's batteries. A new transmitter, solid state recorder, and thermal insulation blankets were also installed. As with the other missions, the maintenance left the HST a better instrument than it had been before.

The second part of SM3—SM3b—is scheduled for the second half of 2001. Its primary goal, as usual, is to upgrade Hubble systems that might be weak or failing. One task, for instance, will be to add a cooling unit for the NICMOS, which has fallen out of service. Cooling the NICMOS sensors will reduce electronic noise in its readings and enable scientists to start using it again.

The astronauts will install four new, more robust solar arrays to generate power for the telescope. In addition, they will make repairs to the deteriorating thermal insulation blanket that surrounds the telescope and install a new camera (the Advanced Camera for Surveys), replacing the Faint Object Camera that had been used for general sky surveys. If necessary, astronauts will also give the telescope a boost to keep it in orbit, as was done in SM1 and SM2. Even at a height of 360 miles (600 km), the HST is subject to atmospheric drag that would eventually cause it to lose altitude and make an unscheduled visit back home.

Read more about Hubble servicing missions .

Get the lowdown on the upcoming servicing mission , scheduled for November 2001.