The most critical task planned for Discovery's mission is arguably the one that must be accomplished to permit continued assembly of the international outpost: repair of the stalled mobile transporter.

The transporter is an ingenious robotic cart designed to creep along rails on the front face of the station's unfinished solar array truss, carrying the station's Canadian-built robot arm from one work station to another. At each work station, the transporter and arm can be locked down to provide stability. The long truss eventually will sport four huge sun-tracking solar array panels that will generate the electrical energy needed to support a six-member crew and a full suite of scientific experiments.

But the outer segments of the truss cannot be built without the station's robot arm and the arm cannot be moved from point to point unless the transporter is operational and has redundant power, video and data cables.

As the transporter moves along the truss it plays out or rolls up ribbon-like power and data cables. To provide redundancy, two trailing umbilical system - TUS - reel assemblies play out and rewind separate cables. The transporter was launched with powerful cable cutters in devices on the transporter itself, called interface umbilical assemblies, in case of a jam in either TUS reel that might otherwise strand the work platform.

On Dec. 16, TUS cable No. 1 on the Earth-facing, or nadir, side of the transporter was severed when the cable cutter in the nadir interface umbilical assembly suddenly fired for no apparent reason, slamming down with 960 pounds of force. Engineers still do not know why the spring-driven cutter fired.

But the incident left the mobile transporter with just one set of power and data cables and NASA flight rules forbid its movement along the truss unless full redundancy is available. The concern is that a second failure could leave the transporter stranded between work sites and unable to be safely latched down. That, in turn, would pose a risk during shuttle dockings or other events when unwanted movement could prove dangerous.

"We have rules in place that say we do not base the (station's robot) arm on the mobile transporter during that time and we do not maneuver the mobile transporter," Shireman said. "It's very important for us to restore full functionality before we continue assembly. That's because to continue assembly, we have to move the mobile transporter out on the truss, base the arm out there to actually install the next piece of truss. That's a very important piece of this mission for us, to return to full functionality."

Said Sellers: "Basically it killed itself just before Christmas. It's a complicated task. If we don't get this thing fixed, we can't move this truck that moves up and down the front face of the station and we can't continue with assembly. So we absolutely have to get it fixed before the next mission.

"And it's a complicated business because it's a large, difficult-to-handle object, it sits inside a bay and it's got multiple, multiple electrical connections. So there are a number of significant obstacles to getting this done. We've thought about it a lot. I think we'll manage to do it OK, the engineering team has put an enormous amount of effort into this and I think it's going to work out. But it's one of the more daunting tasks."

Earlier this year, worried that the cable cutter on the upward facing, or zenith, side of the transporter might fire, Expedition 12 commander William McArthur and flight engineer Valery Tokarev attempted to disable the zenith cable cutter during an already planned spacewalk. But they were unable to drive in a "safing" bolt and rather than leave the cable in place, they removed it from the cutting mechanism and tied it off. That had the effect of stranding the mobile transporter at work site 4.

Sellers and Fossum will attempt to fix the zenith cable system during the first of two planned spacewalks when they will insert a wedge-like blade blocker designed to prevent any damage to the cable even if the cutter fires later. If the blade blocker fits properly, they will re-insert the zenith cable in the interface umbilical assembly and thus restore the transporter to single-cable operation.

That's important because the crew needs to move the transporter from work site 4 to work site 5 to get the access needed to repair the nadir cable system during a second spacewalk.

Assuming the zenith blade blocker works, Sellers and Fossum will venture back out to the truss during their second spacewalk and replace the interface umbilical assembly and its deployed cable cutter with a new unit carried up aboard Discovery.

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

07/07/06
Fri 03:49 AM...05...12...00...STS crew wakeup
Fri 04:19 AM...05...12...30...ISS crew wakeup
Fri 05:04 AM...05...13...15...EVA-2: Preparations with ISS oxygen
Fri 07:54 AM...05...16...05...EVA-2: Spacesuit purge
Fri 07:54 AM...05...16...05...MPLM transfers
Fri 08:09 AM...05...16...20...EVA-2: Spacesuit prebreathe
Fri 09:09 AM...05...17...20...EVA-2: Crew airlock depressurization
Fri 09:54 AM...05...18...05...EVA-2: Airlock egress
Fri 10:09 AM...05...18...20...EVA-2: EV1: FGB retrieval
Fri 10:09 AM...05...18...20...EVA-2: EV2: APFR setup
Fri 10:49 AM...05...19...00...EVA-2: Pump module retrieval
Fri 11:09 AM...05...19...20...EVA-2: EV1: Nadir IUA replacement
Fri 11:09 AM...05...19...20...EVA-2: EV2: APFR reconfig and TUS setup
Fri 11:59 AM...05...20...10...EVA-2: Pump module installation
Fri 12:39 PM...05...20...50...EVA-2: Nadir TUS replacement
Fri 03:24 PM...05...23...35...EVA-2: TUS cable routing
Fri 04:09 PM...06...00...20...EVA-2: Airlock ingress
Fri 04:24 PM...06...00...35...EVA-2: Airlock repressurization
Fri 07:49 PM...06...04...00...STS/ISS crew sleep begins

With the pump module safely stowed, Sellers and Fossum will install the new interface umbilical assembly and then remove the 334-pound trailing umbilical system reel assembly. Fossum, riding on the end of the station's robot arm, will hand-carry it down to the shuttle cargo bay. Sellers will hand him the new reel assembly and both astronauts will move back up to the truss for its installation. After routing the cable back though the IUA, Sellers and Fossum will be done and the mobile transporter will be restored to normal, fully redundant operation.

If the zenith blade blocker cannot be installed for some reason during the first spacewalk, the spacewalkers will remove the zenith IUA and replace it with the new unit, restoring the transporter to single-cable operation.

In that scenario, they would take the old IUA back inside the station, remove its blade and install it on the nadir side of the transporter during the second spacewalk.

"I'm excited by the TUS task," Fossum said in an interview. "It failed in December and within weeks, when we came back after ... Christmas, they had one in the (training) pool all wrapped up with a big bow on it. And we started working on this task.

"It's very important to get that redundancy in the mobile base system, the mobile transporter that moves up and down the truss. If anything was to go wrong with that remaining one we would not be able to move it to a different location. Finishing assembly, adding on pieces of the truss, the solar arrays out on the ends, requires this mobile transporter to be mobile. And so it's been a big effort."

The next day, the astronauts will once again focus on transferring equipment and supplies from the logistics module to the space station.

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

JULY 8
Sat 03:49 AM...06...12...00...STS crew wakeup
Sat 04:19 AM...06...12...30...ISS crew wakeup
Sat 06:19 AM...06...14...30...MPLM transfers
Sat 08:39 AM...06...16...50...Middeck transfers
Sat 10:09 AM...06...18...20...Nitrogen transfer terminated
Sat 10:29 AM...06...18...40...PAO event
Sat 10:49 AM...06...19...00...Joint crew meal
Sat 11:49 AM...06...20...00...MPLM, middeck transfers resume
Sat 07:19 PM...07...03...30...STS/ISS crew sleep begins

If the mission extension is not granted, the astronauts will take most of the day off following the second spacewalk. The day after that, Monday, July 10, the Leonardo cargo module, now loaded with about two tons of no-longer-needed equipment and trash, will be unbolted from the station and restowed in the cargo bay for return to Earth. Nowak and Wilson then would begin the late inspection work, using the OBSS to examine Discovery's port wing.

Undocking would be targeted for around 7:43 a.m. on July 11, followed by OBSS inspections of the shuttle's nose cap and starboard wing leading edge panels. Lindsey and company would spend their last full day in orbit packing up and reviewing landing procedures before firing their braking rockets around 9:45 a.m. on Thursday, July 13, to set up a landing back at the Kennedy Space Center about 10:46 a.m.

Here is the flight plan for the remainder of a non-extended mission:

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

07/09/06
Sun 03:19 AM...07...11...30...STS crew wakeup
Sun 03:49 AM...07...12...00...ISS crew wakeup
Sun 06:04 AM...07...14...15...ISS CEVIS removal
Sun 06:19 AM...07...14...30...ISS message window R&R
Sun 06:24 AM...07...14...35...STS crew off duty time begins
Sun 10:19 AM...07...18...30...ISS PAO event 1
Sun 10:39 AM...07...18...50...ISS PAO event 2
Sun 10:59 AM...07...19...10...ISS meals
Sun 11:09 AM...07...19...20...STS meals
Sun 12:09 PM...07...20...20...STS off duty time resumes
Sun 12:29 PM...07...20...40...ISS CPA installation
Sun 03:04 PM...07...23...15...MPLM cleanup
Sun 03:49 PM...08...00...00...MPLM racks configured for entry
Sun 07:19 PM...08...03...30...STS/ISS crew sleep begins

07/10/06
Mon 03:19 AM...08...11...30...STS crew wakeup
Mon 03:49 AM...08...12...00...ISS crew wakeup
Mon 05:59 AM...08...14...10...MPLM egress
Mon 06:14 AM...08...14...25...MPLM deactivation
Mon 06:19 AM...08...14...30...PAO event
Mon 07:24 AM...08...15...35...Middeck transfers
Mon 08:04 AM...08...16...15...MPLM vestibule depressurization
Mon 09:24 AM...08...17...35...Crew meals begin
Mon 10:24 AM...08...18...35...SSRMS grapples MPLM
Mon 10:34 AM...08...18...45...MPLM bolts backed out
Mon 11:19 AM...08...19...30...MPLM pulled away from Unity
Mon 12:54 PM...08...21...05...MPLM berthed in shuttle bay
Mon 01:34 PM...08...21...45...SSRMS ungrapples MPLM
Mon 01:49 PM...08...22...00...SSRMS grapples MBS
Mon 02:39 PM...08...22...50...SSRMS ungrapples lab
Mon 03:29 PM...08...23...40...OBSS port survey
Mon 05:14 PM...09...01...25...Rendezvous tools checkout
Mon 07:19 PM...09...03...30...STS/ISS crew sleep begins

07/11/06
Tue 03:19 AM...09...11...30...STS crew wakeup
Tue 03:49 AM...09...12...00...ISS crew wakeup
Tue 05:24 AM...09...13...35...Farewell ceremony
Tue 05:39 AM...09...13...50...Hatch closure
Tue 06:14 AM...09...14...25...Centerline camera installation
Tue 06:24 AM...09...14...35...Group B powerup
Tue 06:24 AM...09...14...35...ODS leak check
Tue 06:59 AM...09...15...10...Undocking timeline begins
Tue 07:43 AM...09...15...54...ISS undocking
Tue 08:12 AM...09...16...23...Separation rocket firing No. 1
Tue 08:40 AM...09...16...51...Separation rocket firing No. 2
Tue 09:29 AM...09...17...40...Group B computer powerdown
Tue 09:29 AM...09...17...40...Crew meals begin
Tue 10:44 AM...09...18...55...OBSS starboard survey
Tue 11:19 AM...09...19...30...ISS: PMA-2 depressurization
Tue 01:24 PM...09...21...35...OBSS nose survey
Tue 01:59 PM...09...22...10...OBSS berthing
Tue 02:54 PM...09...23...05...Laptop post-undock reconfig
Tue 03:04 PM...09...23...15...RMS powerdown
Tue 04:28 PM...10...00...39...NC-5 rocket firing
Tue 06:49 PM...10...03...00...STS crew sleep begins

07/12/06
Wed 02:49 AM...10...11...00...Crew wakeup
Wed 05:04 AM...10...13...15...FCS checkout
Wed 05:49 AM...10...14...00...Cabin stow begins
Wed 06:14 AM...10...14...25...RCS hotfire
Wed 06:58 AM...10...15...09...NC-6 rocket firing
Wed 08:49 AM...10...17...00...PILOT landing training
Wed 10:09 AM...10...18...20...Crew meal
Wed 11:29 AM...10...19...40...Deorbit briefing
Wed 11:59 AM...10...20...10...L-1 communications check 1
Wed 12:19 PM...10...20...30...PAO event
Wed 12:39 PM...10...20...50...Wing leading edge gear stow
Wed 12:39 PM...10...20...50...Entry video setup
Wed 01:24 PM...10...21...35...L-1 communications check 2
Wed 03:09 PM...10...23...20...PGSC stow (part 1)
Wed 03:24 PM...10...23...35...Orbit adjust rocket firing
Wed 03:29 PM...10...23...40...KU antenna stow
Wed 06:49 PM...11...03...00...Crew sleep begins

07/13/06
Thu 02:49 AM...11...11...00...Crew wakeup
Thu 05:04 AM...11...13...15...Group B computer powerup
Thu 05:19 AM...11...13...30...IMU alignment
Thu 05:24 AM...11...13...35...GIRA stow; PGSC stow (part 2)
Thu 05:59 AM...11...14...10...Deorbit timeline begins
Thu 09:44 AM...11...17...55...Deorbit ignition (rev 186)
Thu 10:46 AM...11...18...57...Landing

PREVIEW REPORT PART 7 --->