NASA gives green light to space walk

International Space Station Commander Gennady Padalka and NASA ISS Science Officer Mike Fincke will leave the Pirs docking compartment’s airlock again Wednesday afternoon for a second try to repair a Station circuit breaker. Mission managers gave formal approval to the Wednesday spacewalk at a Tuesday morning meeting. Its purpose is to restore power to one of the 600-pound gyroscopes that orient the Station in space.

From NASA:
It’s a Go for Wednesday Spacewalk

International Space Station Commander Gennady Padalka and NASA ISS Science Officer Mike Fincke will leave the Pirs docking compartment’s airlock again Wednesday afternoon for a second try to repair a Station circuit breaker.

Mission managers gave formal approval to the Wednesday spacewalk at a Tuesday morning meeting. Its purpose is to restore power to one of the 600-pound gyroscopes that orient the Station in space.

Padalka and Fincke are scheduled to begin the spacewalk at 5:40 p.m. EDT. They will wear the same Russian Orlan spacesuits they used during their June 24 spacewalk, cut short after about 14 minutes because of a balky handle that activates a switch in Fincke’s suit. That caused an unexpected pressure drop in his main oxygen tank.

The goal of this spacewalk is to restore electricity to one of three functional Control Moment Gyros (CMGs). One of them, CMG 2, went off-line April 21 when it lost power. The two gyroscopes that are working can control the Station’s attitude, but a third operating CMG will provide greater backup capabilities. The fourth CMG failed two years ago, and will be replaced when Space Shuttle flights resume next year.

The spacewalk will take place during the arrival of the Cassini-Huygens spacecraft at Saturn. Spacewalk coverage will be broadcast beginning at 4:30 p.m. EDT. Both the spacewalk and the Cassini arrival at Saturn can be seen on live streaming video at http://www.nasa.gov/multimedia/nasatv/index.html.

The spacewalk could last up to six hours. The repair is planned to restore power to one of four gyroscopes that help orient the complex.

The crew?s first spacewalk was cut short June 24 when flight controllers in Moscow noticed the unexpectedly high rate of pressure loss in the primary oxygen bottle on Astronaut Mike Fincke’s Russian spacesuit.

Managers found the balky handle was not fully seated into the closed position before the spacewalk started. Russian technicians concluded that it was an isolated event and gave the crew an OK to use the same suits for Wednesday’s spacewalk.

Spacewalk preparation procedures have been updated to provide additional verification to ensure the handle is in the proper position.

The Wednesday spacewalk will follow the same plan crewmembers had set out to follow last week. That was to have been the first time a spacewalk was done in Russian spacesuits to replace a U.S. component on the U.S. segment of the Station.

When outside the Russian segment of the Station, at the beginning and end of the Wednesday spacewalk, flight controllers at Mission Control Moscow speak to Fincke and Padalka in Russian. When they are outside the U.S. segment of the Station, the team in Houston speaks to them in English.

Because the crewmembers are wearing the Russian spacesuits, using the Pirs airlock, and using a Russian crane to aid them in climbing to the repair site, the first and last parts of the spacewalk will be coordinated by engineers in Russia’s Mission Control Center outside of Moscow.

At the junction of the Russian and American-built segments of the Station, flight controllers at Mission Control in Houston take over to guide the spacewalkers the rest of the way to the repair site as they move, using handholds and tethers.

At the main truss, Padalka and Fincke replace a Remote Power Controller Module (RPCM) that houses the faulty circuit breaker. When that is done, and Fincke and Padalka return to the Russian crane, Russian ground controllers will again take over primary support.

Throughout the spacewalk (the second for Fincke and the fourth for Padalka, who did two previous spacewalks aboard the Mir Space Station in 1998), flight controllers in Houston and Moscow remain in constant contact with each other, virtually working side by side despite the physical distance between them. During previous spacewalks, a single control center, either in Russia or the United States, has been the lead for the spacewalk.

The spacewalk plan initially called for use of American spacesuits and the U.S. Quest Airlock. But the crew could not get the cooling system of one of the U.S. spacesuits to work.

Using the Russian spacesuits poses some additional challenges. The spacewalkers are using VHF radio antennas on the Russian end of the Station, and the truss structure may interfere at times with those radio signals.

They also have to go about twice as far to get to the worksite as they would have if they started from the U.S. airlock. That means they have to be outside the Station longer and use the Russian Strela cargo crane, attached to the Pirs, to reduce their travel time. Also, the Russian spacesuit gloves are not as supple as those of the American suits.

If the line-of-sight signal to the antennas is blocked, Padalka and Fincke may not be able to talk to the ground or to each other. If communications are interrupted, the control teams have developed the system of four simple hand signals and have identified a place near the worksite from which their radios’ signals can reach the antennas.

Mission Control also can use a ”pager signal” to alert the spacewalkers. Controllers can simply turn off the light at the end of Canadarm2, positioned to provide camera views of the repair work.

This will be the 54th spacewalk for station assembly and maintenance.

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