This picture shows the start of a servicing mission with the launch of a space shuttle.

Hubble is designed for on-orbit servicing by a team of astronauts. Servicing missions allow for planned, periodic replacement and improvement of Hubble's science instruments and other equipment. During a servicing mission the crew maneuvers the shuttle to rendezvous with Hubble, uses a robot arm to place the telescope in the shuttle's cargo bay, and then installs the new equipment. After the work is complete, the crew sets Hubble free and returns it to duty.

The Hubble Space Telescope's purpose is to spend 20 years probing the cosmos from the nearby planets of our own solar system to the farthest and faintest galaxies. Crucial to fulfilling this objective is a series of on-orbit servicing missions. Hubble was placed in orbit on April 25, 1990, and subsequent servicing followed in December 1993 and February 1997. The third servicing mission has been separated into two flights. The first of these flights, Servicing Mission 3A, was in December 1999, and the second, Servicing Mission 3B, is scheduled for 2001.

During the December 1999 mission, astronauts installed six fresh gyros, a powerful new computer, an enhanced Fine Guidance Sensor, a new solid-state data recorder, a new radio transmitter, and improvement kits for batteries and electronics. On the next service call, astronauts will install a powerful new instrument called the Advanced Camera for Surveys. This third generation science instrument will greatly enhance Hubble's imaging capabilities.