The third servicing mission to the Hubble Space Telescope includes four planned space walks by four astronauts completely cross-trained to perform any of the scheduled tasks on any day.

Hubble was designed to be serviced while on orbit through Extravehicular Activity (EVA). The telescope has two grapple fixtures for the shuttle's robot arm, handholds and handrails for use by the astronauts, and electrical connections and bolts designed for easier manipulation by the crew while wearing pressurized space suits.

As was the case on the first two servicing missions in December 1993 and February 1997, the tasks scheduled for the space walks have been prioritized to make maximum use of the time that the astronauts are in the payload bay.

Steve Smith holds one of the tools used during a spacewalk to service the Hubble Space Telescope during the STS-82 mission.

The four astronauts will work in teams of two to perform the work on alternating days to allow a rest day between each space walk. Steve Smith and John Grunsfeld will team together on the first and third excursions outside the crew compartment, and Mike Foale and Claude Nicollier will join forces on space walks two and four.

Recognizing the Spacewalkers:

Steve Smith	Solid Red Stripe
John Grunsfeld	Solid White (no markings)
Mike Foale	Broken Red Stripe
Claude Nicollier	Diagonally Broken Red Stripe

As was the case in the previous servicing missions, while the individual tasks have been prioritized, the various tasks have been designed to take into account the possibility that crew members may encounter unforeseen difficulties that could change the preplanned schedule of installation. The four space walkers have been trained so that any one is capable of performing any given task.

Mission Specialists Steve Smith and John Grunsfeld practice EVA Servicing Tasks

As currently planned, the first space walk will feature the replacement of all three Rate Sensor Units (RSU's) (two were replaced on the first servicing mission - STS 61). Each RSU houses two gyroscopes used to point the telescope for observations. In addition, Smith and Grunsfeld will then install voltage/temperature improvement kits (VIK) to protect Hubble's batteries from overheating as they get older. The VIK lowers each battery's charge termination voltage. They also will open coolant valves and remove caps on the Near Infrared Camera and Multi-Object Spectrometer, which was installed on the second visit to Hubble in 1997. NICMOS was found to be leaking its super-cold coolant shortly after activation. Opening the valves will allow any residual coolant frozen in the line to dissipate in the vacuum of space in preparation for work scheduled on the next visit in 2001.

The second space walk will include the removal of the old computer and installation of a new, improved computer with more power. Foale and Nicollier also will replace one of the Fine Guidance Sensors. An FGS was replaced on the second servicing mission.

The third space walk includes replacement of an S-band transmitter, installation of another Solid State Recorder, which is more efficient than the old-style tape recorder, and attachment of new insulation on equipment bays midway up the side of the telescope.

The fourth and final planned space walk by Foale and Nicollier includes activities termed 'Optional Tasks,' and includes attachment of more insulation around the top of the telescope and covers on handrails with peeling paint. Most of the insulation damage is on the side of the telescope that always faces the Sun.

Throughout the space walks, the flight support system will be rotated so that the area being worked on faces forward to provide better visibility and access by the robot arm.