Installing a new S-band radio transmitter is one of the missions most trickiest tasks. Photo: NASA

Smith and Grunsfeld will carry out the third spacewalk, attaching an electronic control unit on the new fine guidance sensor, installing a new S-band radio transmitter, a new data recorder and beginning work to repair Hubble's exterior insulation. Of those tasks, the radio transmitter is perhaps the trickiest. It was not designed to be replaced by spacewalking astronauts and it includes small screws that could easily float free.

"It's kind of like if you had to repair a watch wearing winter gloves," Grunsfeld said. "I'm trying to deal with little screws that are non captive and if you drop one, obviously it'll float away. When we started this, we regarded that as a very difficult task, one of the big challenges. Now, when I go in and do it, it's just a very straightforward task. I'm pretty confident that'll go just fine."

The spacewalkers will end the third excursion by "putting up some new wallpaper on the telescope," Grunsfeld joked. "It's actually more like a huge cookie sheet on the lower part of the telescope over the electronics bays to improve the thermal condition of the telescope."

The fourth and final spacewalk, carried out by Nicollier and Foale, is devoted to installing new thermal insulation on the top half of the telescope and repairing balky latches on an equipment bay door that will need to be opened and closed during SM-3B.

The final task for Discovery's crew is to patch up peeling thermal insulation. Photo: NASA

"Compared to the other days, it's a lighter workload," Grunsfeld said of the fourth spacewalk. "It has a little over an hour for optional tasks. That's assuming everything goes perfectly on all the other days."

If all the work does, in fact, go well, Clervoy will release Hubble into open space the day after the fourth EVA. Unlike the last servicing mission, Hubble will not be boosted to a higher altitude by Discovery. The SM-3B flight in 2001 will be carried out by the veteran shuttle Columbia, which is too heavy to reach a higher orbit.

"For me, this is absolutely a dream flight," Grunsfeld reflected. "At the California Institute of Technology, I was a senior research fellow on the faculty of the physics and astronomy department. My whole life was writing proposals, getting observing time, going out and observing. I spent a lot of time up on Mount Palomar on the 60-inch telescope. So Hubble will actually be the largest optical telescope I've ever worked on."