Hubble gets a brain transplant with the installation of a new computer. Photo: NASA

The next day, Foale and Nicollier will take over, beginning the second excursion by replacing Hubble's Intel 386 computer with a faster 486 version, prompting jokes at NASA about the most expensive 486 upgrade in history.

"Intrinsically it's not a difficult task," Grunsfeld said. "Except all the connectors are on the side of the box where you can't see them. So Mike has to do that basically without the aid of stereoscopic vision. He'll have one eye as he's reaching around to do that.

"And on the left of him on the doors is the data management unit, or DMU, and huge bundles of delicate cables," Grunsfeld continued. "So the challenge there is how do you jam yourself as close as you can to that without touching it so you can see the connectors you have to disconnect without damaging the cables. So Claude is going to be free floating by the door, telling him his clearance."

Again, ground controllers will carry out an aliveness test while the spacewalk continues to ensure the DF-224 computer is functioning properly.

Engineers initially were hesitant to swap out the computer.

Installation of the baby piano-sized Fine Guidance Sensor. Photo: NASA

"There was some concern about going up on a so-called rescue mission ... to take care of a spacecraft problem and do we really want to do a brain transplant on the same mission and risk having maybe other problems?" Leckrone said.

In the end, Hubble project managers approved the changeout, arguing the benefits outweighed any risks.

With the computer in place, Foale and Nicollier will replace one of Hubble's three fine guidance sensors before calling it a day and returning to the safety of Discovery's crew cabin.