Astronaut McArthur during his first spacewalk on Sunday. Photo: NASA TV/Spaceflight Now

With two successful spacewalks under their belts, the Discovery astronauts spent Monday evening preparing for a third excursion Tuesday to ready the international space station for the attachment of a huge set of solar arrays in early December.

Astronauts Leroy Chiao and William McArthur plan to begin their second spacewalk - the third of the 100th shuttle mission - around 10:30 a.m.

"Somebody accused us on this flight of being very greedy," McArthur said in a NASA interview. "This is my first opportunity to do any spacewalks and, by golly, each team's going to get to do two of them! So we have no complaints. This is a great mission."

During the flight's first spacewalk Sunday, Chiao and McArthur electrically hooked up a massive structural truss that was mounted on the space station's Unity module the day before.

Crewmates Michael Lopez-Alegria and Peter "Jeff" Wisoff then staged a second spacewalk, or EVA, Monday to attach a new shuttle docking port.

The major goal of Tuesday's excursion is to mount two DC-to-DC converter units, or DDCUS, on the new Z1 truss. Chiao and McArthur also plan to connect additional umbilicals and to mount tools on the station's hull for future assembly teams.

The Z1, or zenith, truss houses the space station's main antenna systems as well as four massive gyroscopes needed to keep the outpost stable and properly oriented in space.

The 18,300-pound truss also will serve as the mounting point for NASA's first set of solar arrays. But the power generated by the arrays must be stepped down to a level the station's electronic systems can handle. That's where the DDCUs come in.

"They're sort of like DC transformers, if you will. Pretty sophisticated ones," McArthur said. "The solar arrays will produce electricity at a voltage that's (higher) than the voltage we need on board the station.

The DDCUs convert power from 115 to 173 volts DC to a steady 123 to 126 volts DC. They will not be activated until after the arrays are installed.

"You know, it's a little bit analogous to the fact that power from the electrical company comes on these large high-voltage power lines," McArthur said. "And so you have to reduce the voltage. And that's sort of what we're doing.

"We're also trying to keep it regulated to a very steady level so that we don't see power fluctuations in our equipment inside the space station," he said.

McArthur rides the shuttle's robot arm on Sunday. Photo: NASA TV/Spaceflight Now

During a spacewalk Sunday, McArthur was attached to Discovery's robot arm while Chiao assisted him as a free floater. Their roles will be reversed Tuesday.

Here is McArthur's description of exactly what they plan to do:

"Leroy's going to be attached to the end of the (robot) arm and I'll be sort of like (his) assistant mechanic," McArthur said. "I'll go get tools and set out tools that he needs. He's going to go underneath Z1 on the end of the arm and release these two DC-to-DC converter units, one at a time.

"He'll bring them up and he'll initially attach one bolt that holds each of them in place, and then he goes to get the second one. And I scamper up, scamper around to the side and drive the second bolt. And he brings in the second DC-to-DC Converter Unit, and in each case, I'll help him guide them into position.

"And then again, I'll come up on the side and attach the second bolt as he's going on to do the next task. Once they're in place, they're a couple of more cables that we have to reconfigure, now that the DDCUs are in place, from the zenith truss over to Unity.

"There's one more toolbox back on the pallet that previously had held the pressurized mating adapter No 3," McArthur continued. "And so, Leroy will go on the end of the arm and release that box, and he'll bring it over to the left side of the zenith truss and attach it. And I'll help him guide it into place if that's necessary.

"Then we'll look at some other task we might be able to perform to get ahead. We'll look at some of the (tasks planned for a spacewalk Wednesday) and if we have time, maybe try to pull those forward.

"For example, we're going to relocate the keel pin, a big pin that was on the bottom side of the zenith truss to hold it in the payload bay. Well it turns out, it's now still pointed down into the payload bay.

"And it can't be there for subsequent station assembly flights. It will interfere with some of the other equipment such as the P6 (solar array) truss. And so Leroy will release that and he'll move it over to a different side of the zenith truss.

"And then I'll go up on the (Unity module) where we have a bag with a lot of tools in it. And I'll carry a significant number of those tools over to the toolboxes ... and that pretty well will wrap up EVA three."