Flight controllers are revising plans for a second spacewalk Wednesday to replace a coolant pump aboard the International Space Station, adding work to isolate an ammonia leak that should clear the way for installation of a new pump during a third spacewalk Sunday.

Credit: NASA

A formal decision on whether to proceed with the Wednesday spacewalk is expected after NASA's space station Mission Management Team reviews preparations during a meeting Tuesday at the Johnson Space Center in Houston.

The pump module, one of two in independent coolant loops, circulates ammonia through huge radiators to dissipate the heat generated by the lab's electronics. The loop A pump shorted out July 31, forcing the six-member crew to implement an extensive powerdown to prevent equipment from overheating.

The station can operate safely with just one coolant loop, but both are needed for normal operations. Adding a bit of urgency is the lack of redundancy in a critical system. If the loop B system should suffer a shutdown before loop A is repaired, the space station would face a much more serious problem with little time to resolve it.

Astronauts Douglas Wheelock and Tracy Caldwell Dyson conducted the first of what was expected to be a two-spacewalk repair job Saturday. They had hoped to disconnect the old pump, demating five electrical lines and four ammonia quick-disconnects, before installing a spare pump at the end of the excursion. Ammonia lines were to be reconnected during a second spacewalk Wednesday.

But Wheelock and Caldwell Dyson ran into major problems with a quick-disconnect fitting on one of the ammonia lines attached to the failed pump. After struggling to simply disconnect the M3 fitting, they ran into a significant leak that forced them to leave the line in place while troubleshooters considered what to do next.

Because of the leak, the spacewalk ran eight hours and three minutes and required a decontamination procedure at the end to make sure the astronauts did not bring any toxic ammonia back into the station's pressurize modules.

Engineers believe the leak is due to problems with one of two valves in the quick-disconnect fitting. The valve in question is outboard of the pump module and cannot easily be vented to stop the leak.

As a result, engineers have come up with a revised plan for the crew's second spacewalk Wednesday, one that would require them to first close an ammonia jumper between the central S0 truss segment and the starboard one, or S1, segment. Another quick-disconnect fitting near the outboard end of the S1 segment also must be closed.

To lower pressure in that segment of the line, flight controllers plan to activate a pressure relief valve before the spacewalk begins to make it easier for the astronauts to close the S0/S1 segment quick disconnect. Wheelock and Caldwell Dyson then can use a vent tool to release any residual ammonia trapped in the line leading to the M3 quick-disconnect.

Assuming that works, the M3 line will be removed, clearing the way for Wheelock to disconnect the five electrical cables and loosen the four bolts holding the failed pump module in place. Caldwell Dyson, meanwhile, will prepare a spare pump module on external storage platform No. 2 for removal during the third spacewalk.

With that work done, Wheelock, assisted by Caldwell Dyson, will use an adjustable grapple bar to move the failed unit to a powered payload attachment fitting at the base of the robot arm's mobile transporter.

At that point, the spacewalkers will collect their tools and return to the Quest airlock for possible decontamination procedures.

In yet another complication for spacewalk planners, Wheelock and Caldwell Dyson will be using rechargeable carbon dioxide removal cartidges in their spacesuits, which cannot operate as long as the lithium hydroxide scrubbers used during the first spacewalk.

Because they must protect a block of time at the end of the spacewalk for decontamination, Wheelock and Caldwell Dyson will face a more compressed timeline with more limited objectives.

But if all goes well, the astronauts will be able to carry out a third spacewalk Sunday to install the spare pump module, disconnect a pressure-regulating jumper box and reconnect the electrical cables and ammonia lines. The failed pump module eventually will be moved to external storage platform No. 2, but it's not clear when that might happen.

Here is a preliminary timeline for the revised second spacewalk, assuming it is approved by the space station MMT (in EDT and elapsed time; best viewed with a fixed-width font):

EDT........HH...MM...EVENT

06:55 AM...00...00...Suits to battery power
07:00 AM...00...05...Post depress/airlock egress
07:10 AM...00...15...Setup
07:40 AM...00...45...EV-2: CETA relocate; close outboard S1 QD; close M2
07:55 AM...01...00...EV-1: Close segment jumper QD; vent PM; close M2
10:00 AM...03...05...EV-2: Spare PM preps: open MLI; break torque
10:15 AM...03...20...EV-1: Failed PM electrical demate; break torque
11:00 AM...04...05...EV-1: Retrieve adjustable grapple bar (AGB)
11:20 AM...04...25...EV-2: Attach AGB to failed PM
11:45 AM...04...50...EV-1: Remove failed PM from S1 truss
12:05 PM...05...10...EV-2: Cleanup
12:30 PM...05...35...EV-1: Move failed PM to temporary mounting location
12:40 PM...05...45...EV-2: Airlock ingress; pre-press
01:00 PM...06...05...EV-1: Cleanup; airlock ingress
01:25 PM...06...30...Airlock pre-press
01:30 PM...06...35...Airlock repressurization (if no decontamination)