Astronauts on the space shuttle Endeavour finally put Hubble right. They installed two new instruments that entirely cleared up the telescope’s vision problem. From an imaging standpoint, the Wide Field and Planetary Camera 2 (WFPC2) was the most significant upgrade. It replaced WFPC1, which was unable to focus light sharply because Hubble’s primary mirror had been ground to the wrong shape. The astronauts also took out the High Speed Photometer and in its place installed the Corrective Optics Space Telescope Axial Replacement, which properly focused light from the primary mirror onto the other science instruments.
Nearly as important, the astronauts replaced the twin solar arrays, which had been troublemakers ever since the telescope was deployed. Every time Hubble passed from day to night or back again — twice during each 90-minute orbit of Earth — the change in temperature caused the arrays to contract or expand. The resulting jitter tended to blur images as well as cause the telescope to lose its lock on guide stars. In addition, the astronauts installed some new gyroscopes and other electronic units.
Astronauts on the space shuttle Discovery installed two science instruments during the second servicing mission. They first removed the Goddard High Resolution Spectrograph and replaced it with the Space Telescope Imaging Spectrograph (STIS). The new technology and greater capabilities incorporated into STIS also made the Faint Object Spectrograph nearly redundant, so the astronauts replaced it with the Near-Infrared Camera and Multi-Object Spectrometer (NICMOS). This extended Hubble’s vision to wavelengths longer than visible light. Astronauts also replaced one of the observatory’s Fine Guidance Sensors (used to point the telescope) and replaced an old reel-to-reel tape recorder with a Solid State Recorder having ten times the capacity.
Astronauts on the space shuttle Discovery headed back to Hubble sooner than originally planned because of major problems with the telescope’s gyroscopes. Hubble carries six gyros and needs at least three to keep itself properly oriented. When a fourth gyro failed in November 1999, the telescope went into “safe mode.” (Discovery had been having its own troubles — including damaged wiring, a contaminated engine, a dented fuel pipe, and bad weather — so its launch had been delayed nine times.) In addition to replacing all six gyros, the Discovery astronauts installed a more advanced central computer and a digital data recorder.
Astronauts on the space shuttle Columbia completed the tasks initially scheduled as part of an all-encompassing third servicing mission. The key upgrade on this flight was the installation of the Advanced Camera for Surveys, which replaced the Faint Object Camera (the last of Hubble’s five original science instruments). In addition, the astronauts installed an experimental cryogenic system to return NICMOS to operation. A small heat leak had caused the infrared camera’s coolant to evaporate faster than planned, and the new system returned it to full operation. The astronauts also installed a new set of solar arrays that delivers far more power to the telescope.
Although no one knows yet when the next mission will fly, the two main tasks are already clear. First, astronauts finally will replace WFPC2 with a new and better camera — the Wide Field Camera 3. Hard as it may be to believe, Hubble scientists think this camera should outperform WFPC2. The other key upgrade will be to install the Cosmic Origins Spectrograph in place of COSTAR