The fifth day of the mission is highlighted by the first of at least two planned spacewalks, a six-and-a-half-hour excursion by Sellers and Fossum to test Discovery's robot arm/OBSS combination as a potential platform to make heat shield repairs down the road.

"The question we want to answer is, can you use the boom as a worksite, as a platform to repairing underneath the orbiter?" Lindsey said in a NASA interview. " So, the scenario is you have a problem, you want to go repair a tile or a leading edge (panel) or something like that. Can you put (one or two spacewalkers) out on the end of this boom, maneuver them underneath the vehicle, and is the platform stable enough to allow them to do repairs?"

The shuttle's robot arm is 50 feet long as is the OBSS extension.

"If an EVA crewmember puts a load or pushes at the end of the boom, how much flex do you get in that boom, how much flex do you get in the arm? Do the joints on the arm back drive? How much do they back drive? And, is it stable enough to be able to do work, precise work, where you have to be careful in how you do it?

"We're going to put one and then two crewmembers on that boom, go to a couple of positions, and they're going to do some typical EVA-type maneuvers. For example, they'll lean back aggressively while they're standing at the end of the arm to see what kind of drive you get on the joint. They might reach back for a tool and just kind of simulate those kinds of motions.

"And then, one of the test points will go actually to structure, where they're pushing against a truss structure in the payload bay, and see what kind of flex you're doing. So, we're going to get two things out of that. We're going to get loads information; in other words, how much force you're putting into the arm, how much do the joints move? And we're going to get an operational assessment, if you will. The EVA crewmember will simulate doing a repair and say, 'Can I do a repair with the arm in this configuration?' So, the goal is to be able to figure out at what points underneath the Orbiter do we need a further way to stabilize the crew, like some, putting something to stabilize them on the bottom of the vehicle itself? Or, can we get away without doing that at all?"

DAY.ET.........DD...HH...MM...EVENT

07/05/06
Wed 04:19 AM...03...12...30...STS crew wakeup
Wed 04:49 AM...03...13...00...ISS crew wakeup
Wed 05:34 AM...03...13...45...EVA-1: Pre-breathe with ISS oxygen
Wed 08:24 AM...03...16...35...EVA-1: Spacesuit purge
Wed 08:24 AM...03...16...35...MPLM transfers
Wed 08:39 AM...03...16...50...EVA-1: Spacesuit prebreathe
Wed 09:34 AM...03...17...45...SSRMS ungrapples lab module
Wed 09:39 AM...03...17...50...EVA-1: Crew lock depressurization
Wed 10:24 AM...03...18...35...EVA-1: Airlock egress; tool setup
Wed 10:44 AM...03...18...55...EVA-1: Zenith IUA replacement
Wed 11:14 AM...03...19...25...EVA-1: Tool setup
Wed 11:54 AM...03...20...05...EVA-1: OBSS setup (part 1)
Wed 11:59 AM...03...20...10...MELFI transfer
Wed 12:44 PM...03...20...55...EVA-1: 1 EV position 1 evaluation
Wed 01:24 PM...03...21...35...EVA-1: OBSS setup (part 2)
Wed 01:54 PM...03...22...05...EVA-1: 2 EV position 3 evaluation
Wed 02:29 PM...03...22...40...EVA-1: 2 EV push evaluation
Wed 02:59 PM...03...23...10...EVA-1: OBSS cleanup
Wed 03:49 PM...04...00...00...EVA-1: Tool cleanup
Wed 04:39 PM...04...00...50...EVA-1: Airlock ingress
Wed 04:54 PM...04...01...05...EVA-1: Airlock repressurization
Wed 08:19 PM...04...04...30...STS/ISS crew sleep begins

Making their way down to the shuttle's payload bay, Sellers and Fossum will rig safety tethers and attach a foot restraint to the end of the OBSS along with sensors to record how much force is imparted to the boom when they move about. Sellers then will climb onto the OBSS, connected to the shuttle's robot arm by a long tether and to the sensor boom by another, shorter tether. Fossum said they refer to the latter as "our sissy tether."

Safely tethered, Sellers will be moved to a point just above the starboard sill of the cargo bay, about 14 feet from any other structure. This is considered a "strong" configuration for the boom. After moving about to impart various forces, he will be moved to a point high above the port wing for another round of "bouncing the boom" tests. Extended some 60 feet from the shuttle, the boom is expected to exhibit a slow back-and-forth sway as Sellers moves about.

"The idea is to see if you could do a repair while standing on the end of this," Sellers said in a NASA interview. "The problem is, of course, that it's a very long, bendy fishing pole with somebody standing on the end, so it's not a completely stable platform. It sways around a bit; we've tested it in virtual reality, we've tested it on an air-bearing floor. We know that it has some motion, some sway, associated with it."

In an interview with CBS News, he likened it to painting the side of a boat while standing in a raft bobbing up and down.

"It will have a long, slow sway, about four to eight seconds period," he said. "So I want you to imagine that you're standing in a little row boat that's going up and down in the swell, slowly, and you're trying to paint the side of a ship that's not moving, or the side of a dock next to you. You have to put quite a lot of compensation in, you have to be careful, but it's not impossible. You just have to think about what you're doing."

After the first two tests, the arm will be maneuvered back into the cargo bay and Fossum will climb aboard. The boom then will take both spacewalkers back out over the port wing and finally, to a cargo truss in the payload bay that Fossum will push against to mimic actual repair work.

"After we test it out with one person, we'll bring the boom back down to the payload bay and I'll get into the foot restraint," Fossum said. "Piers will be hanging on to the boom. This is to simulate the kind of situation where we'll actually (be) doing a repair, where I would be the one in the foot restraint, with both hands free so I can do the repair, and Piers will be moving around the outside.

"We'll have kind of a tool stanchion and some other support equipment set up, so he'll act as if he's getting tools out and handing them to me. We'll be able to evaluate the stability of the system with both of us moving around and putting force into the whole boom and arm just to make sure that it's stable enough. We'll be doing some basic maneuvers in free space.

"And then, we'll go down to some structure and I'll actually put force into the structure as if I'm tamping down, the repairs, like for a tile repair that requires a continuous tamping motion to push all of the repair material down into the patch site. We'll just do that to see if it holds us stable enough to accomplish those kinds of basic tasks."

The work could help clear the way for an eventual mission to service the Hubble Space Telescope. The observatory circles the globe in a different orbit from the space station and the crew of any proposed servicing mission would not be able to take advantage of the safe haven option.

As a result, NASA needs credible ways to make repairs if any impact damage is seen and with the OBSS, the astronauts would have a potential way to work on the belly of the shuttle.

Ground testing indicates the robot arm/OBSS combination will be relatively sturdy and the astronauts said they had no concerns about the possibility the OBSS - and the astronauts - could somehow break free of the shuttle's robot arm.

"We've thought about that and all the different ways to get ourselves out of trouble," Sellers said. "We'll still be tethered, by the way, to the main shuttle arm so if we came off, that's our first recourse. So the first thing you've got to do is get off the boom, then you've got to stabilize yourself (and) use the tether to get back to the arm. So a multi-step process, but I think pretty safe."

The astronauts will focus on equipment transfers from the logistics module the following day - flight day six - review plans for their second spacewalk and hold a traditional joint crew news conference.

DAY.ET.........DD...HH...MM...EVENT

07/06/06
Thu 04:09 AM...04...12...20...STS crew wakeup
Thu 04:49 AM...04...13...00...ISS crew wakeup
Thu 07:04 AM...04...15...15...MPLM transfers
Thu 08:54 AM...04...17...05...EVA tools configured
Thu 10:24 AM...04...18...35...Spacesuit checkout
Thu 11:19 AM...04...19...30...SAFER checkout
Thu 11:49 AM...04...20...00...Joint crew meal
Thu 12:49 PM...04...21...00...Joint crew news conference
Thu 01:29 PM...04...21...40...Joint crew photo
Thu 01:49 PM...04...22...00...MPLM transfers resume
Thu 03:49 PM...05...00...00...EVA-2: Procedures review
Thu 07:49 PM...05...04...00...STS/ISS crew sleep begins

PREVIEW REPORT PART 6 --->