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Station crew to venture outside for spacewalk
Posted: December 22, 2008

Space station commander Mike Fincke and flight engineer Yury Lonchakov plan to carry out a six-hour 10-minute spacewalk Monday night to install an instrument to measure the electrical environment around the station; to install and remove space exposure experiments; and to conduct minor maintenance on the Russian segment of the lab complex. Flight engineer Sandy Magnus will monitor the excursion from inside the space station.

Credit: NASA
Floating in the Russian Pirs airlock module, Fincke and Lonchakov plan to begin the spacewalk around 7:10 p.m. EST. This will be the 119th spacewalk devoted to station assembly and maintenance since construction began in 1998, the 19th this year, the fifth for Fincke and the first for Lonchakov.

For identification, Lonchakov will be wearing a Russian suit with blue stripes and go by the radio call sign EV-1. Fincke's suit will have red markings and he will use the call sign EV-2.

After exiting the airlock, the spacewalkers will install a Langmuir probe on the compartment's hull, just to one side of the airlock hatch. The probe will measure electron density, temperature and the plasma potential around the station, part of an ongoing investigation to pin down what might have caused explosive bolts to malfunction during two recent Soyuz re-entries.

Fincke and Lonchakov then will mount two experiments on the Zvezda command module's hull. One, called EXPOSE-R, is a joint Russian-European Space Agency experiment to expose a variety of biological samples to the space environment for a full year. Among the samples are seeds, bacteria spores and fungi. The other experiment, called IMPULSE, will investigate ionospheric disturbances in the electrically charged plasma environment around the station.

The spacewalkers then will re-orient a space exposure "witness plate" that apparently was bumped out of its normal orientation earlier. They also plan to remove a Biorisk experiment canister, cut away some loose straps near the Pirs docking port and secure an insulation flap. If time is available, the spacewalkers will carry out photo documentation of the station's Russian segment, paying special attention to a navigation antenna on a docked Progress supply ship that failed to operate normally.

The Langmuir probe is part of an ongoing Russian investigation into what caused an explosive bolt on two recent Soyuz spacecraft to malfunction during re-entry, delaying separation of the modules making up the spacecraft and resulting in steep, off-course descents.

"Our Russian colleagues have spent an enormous amount of time, a very structured effort, to create a fault tree and then work off the fault tree to come to a most probable cause," Mike Suffredini, NASA space station program manager, told reporters in late September. "They have concluded the most probable cause to be related to the (electrically charged) plasma environment and the (difference in) potential between the plasma environment, the environment around space station, and the space station itself.

"The way the space station is wired together, there actually is a voltage, a delta voltage potential, between the environment around the ISS and the ISS itself and what happens over time is occasionally that potential tries to equalize. And when it tries to equalize, you get what essentially is a spark to the ISS. This is an environment we've been living in since we starting flying ISS, it's why (NASA has) what we refer to as the plasma contactor units that we operate during (spacewalks) to prevent that from occurring during EVAs."

The plasma contactor units use xenon gas to equalize electrical potential around the station. The xenon supply is limited and the PCUs are only operated during spacewalks to prevent any chance of arcing while astronauts are working outside. Testing showed no obvious threat to the station's systems when the PCUs were turned off.

"What our Russian colleagues have determined is because of the very specific configuration of the pyros ... on the Soyuz, where it's failed to separate each time, they've determined that there's been some arcing, if you will, or equalizing of voltage in this area," Suffredini said. "And this over time has caused the pyros to be ineffective."

The culprit apparently involves insulation on the Soyuz near the pyro in question that includes an ungrounded aluminized layer.

The huge U.S. solar arrays that provide most of the station's power affect the electrically charge plasma around the lab. The recent Soyuz problems occurred after a major change in the configuration of the arrays. Another major change is expected in February, when a fourth and final set of arrays is added to the starboard side of the station's main power truss.

While some NASA engineers are not convinced the evidence supporting the arcing theory is conclusive, "it's a plausible scenario," Suffredini said.

"The space station's this big, old, huge chunk of metal flying through a magnetic field," deputy program manager Kirk Shireman said last week. "So measuring the potential at one point doesn't necessary tell you what the potential is at another point. We can do that by calculations, but it's also good to get a measurement closer to where the environment is where these bolts were living. That's why the Russian community wanted to have this probe in place."

The Langmuir probe will be mounted on the Pirs module, just above a Soyuz docking port. "There's an electron cloud, if you will, flowing around the station at all times," Shireman said. "And then the station itself generates electricity. The way the U.S. solar arrays are grounded, we actually put a charge on board the space station structure. In fact, we go to great pains to make sure that every piece of structure we've added to the ISS is grounded with very low resistance so the current can flow freely back and forth through not only the truss, but also through the pressurized modules.

"It's a phenomenon we've known about for decades," Shireman said. "On sunrise, there's a big rise in potential. And the potential I'm talking about is between the metal structure of the ISS and the surrounding electron cloud, if you will. And that changes over the orbit period. ... In darkness, it tends to be near zero. It tends to be highest right at sunrise, it comes down and then levels off to a standard level during most of the (daylight pass) and then goes to zero in the darkness. It also turns out that when you have a big metal bar and you run it through a magnetic field you get a current along that bar. So it's really the sum of those two effects that we have."

The potential is different along the station's solar power truss, which extends at right angles to either side of the lab's pressurized modules. Engineers initially predicted a peak voltage of around 140 volts. With the current electron density and plasma temperature in low-Earth orbit, Shireman said the actual potential is less than 40 volts. When the plasma contactors are turned on for spacewalks, it drops to less than 5 volts.

"We've designed the station so every metal part on the ISS would have the same potential with respect to the surrounding plasma," Shireman said. "But because we're this huge metal structure and we go through the magnetic field, the potential relative to the surrounding plasma is different along the truss. Along the pressurized section, it tends to be about the same."

Russian engineers are evaluating an explosive bolt that was removed from a Soyuz during a spacewalk in July and returned to Earth. Data from the Langmuir probe will be assessed and folded into the investigation.

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