The first task will be to replace Hubble's failed gyroscopes with the RSU packages. Photo: NASA

The first item on the agenda for the first spacewalk, or EVA, is replacing all six of Hubble's gyroscopes, housed two each inside three compact devices called rate sensing units.

"On the RSUs, the task itself is not intrinsically difficult," Grunsfeld said. "You climb inside the telescope, you disconnect some connectors, run some bolts out and swap out the gyros. But ... everything around us is tremendously fragile. You can't move your arm in other than a very pre-planned way when you reach for a connector. So from the ballet aspect, it has to be a highly choreographed activity."

Smith will actually float completely inside the telescope's aft section, anchoring his feet in temporary restraints.

"I'll put his feet in while he tries to avoid touching anything except a handhold in the telescope," Grunsfeld said. "Then he will gently rotate so his mini work station, which is the part that holds all the tools on his stomach, is inches away from one of the fixed head star tracker light shields, which we're told we absolutely cannot touch. Then on his left side, as he's doing RSU 3, will be a couple of hundred-million-dollar scientific instruments and an optical bench, which we can't touch."

Smith will detach electrical connectors and physically hold each RSU while Grunsfeld, reaching in with a large power tool extension, drives out the bolts holding each device in place. Smith will then pass the old unit out and position its replacement so Grunsfeld can drive in the necessary bolts.

Concern about brittle internal insulation is a major issue. The focal plane structure "is covered with some MLI (multi-layer insulation) to protect it and what the Hubble community is concerned about is when a crew member climbs in there, if he rubs up against it, particulate is going to come loose and we're going to contaminate the telescope," said Kieth Johnson, the lead spacewalk trainer for SM-3A. "That's our biggest fear."

Installation of the Voltage/Temperature Improvement Kit. Photo: NASA

Once the three RSU boxes are connected, engineers on Earth will begin a series of "aliveness" tests to make sure they are working properly. While those tests are going on, Smith and Grunsfeld will move to the bottom of the telescope and open a valve to vent any traces of nitrogen in the NICMOS cooling system that might interfere with operation of the replacement cryocooler in 2001.

Grunsfeld and Smith will close out the first spacewalk by installing six voltage regulator kits, or VIKs, to improve battery conditioning and to protect against an unlikely failure scenario that could lead to a battery rupture. Finally, protective covers will be installed on a handrail near fine guidance sensor No. 3 to prevent possibly peeling paint from flaking off the second spacewalk.