In addition, the spacewalkers will make preparations for Servicing Mission 3B, currently scheduled for launch aboard the shuttle Columbia in June 2001.

The major goals of SM-3B are installation of two new solar arrays, the Advanced Camera for Surveys and a new cooling system to extend the life of the Near Infrared Camera and Multi-Object Spectrometer, or NICMOS. A so-called "thermal short" caused the instrument to use up its nitrogen ice coolant faster than expected and it currently is unusable. During SM-3B an innovative gaseous neon cooling system will be installed that should bring NICMOS back to life.

Astronauts Steve Smith (left) and Mark Lee, pictured during the second Hubble Servicing mission in February 1997. Photo: NASA/JSC

Breaking the third servicing mission into two parts "has the virtue of taking a mission now scheduled for June of 2000 that was shaping up to be quite a lengthy and complicated mission and breaking it up into simpler, core components," said Hubble program scientist David Leckrone. "This allows us to avoid pressing the envelope."

Planners initially believed SM-3A could be carried out with just three spacewalks. But lead flight director Linda Ham said it made sense to add a fourth "to pick up some optional tasks that if we don't do this time, we'll have to do next time. So we figured as long as we're going up there, we ought to do as much as we can."

It will not be easy. And even though the four spacewalkers have been training since August 1998, the margin of error will be small. Even so, Ham said in an interview, "if there aren't any major hiccups, we're going to get everything done and we'll probably have a little extra time on some of the EVAs if not all."

"In the last few flights where we've had EVAs, they ran long," she said. "So things are different in space than they are in the [training] pool on the ground. But Steve Smith and John Grunsfeld have been working this flight for a long time. They've had a lot of experience. All four of them are cross trained for all the events on these four days. They're really proficient."