Spaceflight Insider

ISS astronauts set for spacewalk to replace failed power unit


Scott Kelly works outside the space station on a Nov. 6, 2015, spacewalk. (Click for full view.) Photo Credit: Kjell Lindgren / NASA

European Space Agency (ESA) astronaut Timothy Peake and NASA astronaut Timothy Kopra will take a walk outside the International Space Station (ISS) on Friday to replace a failed power unit and finish work from previous spacewalks.

Electrical Power Distribution

A diagram of electrical power distribution. The Sequential Shunt Unit (SSU) acts as a ‘dummy’ resistive load on each solar array to maintain a constant voltage. (Click to enlarge.) Image Credit: Boeing

The two Expedition 46 flight engineers were already involved in an extravehicular activity (EVA) last month, less than six days after arriving at the station on Dec. 15. That contingency spacewalk had Scott Kelly, the station commander, and Kopra free a stuck Mobile Transporter to allow for the docking of a Progress cargo ship a couple of days later. Peake assisted the two inside the ISS, including helping them don their spacesuits.

The Jan. 15 spacewalk will see both Tims venture outside the Quest airlock just before 7 a.m. CST (13:00 GMT).

“I am thrilled to be assigned a spacewalk in 10 days,” Peake tweeted last week about the assignment. “Lots of work to do before Tim and I can open the hatch.”

In an ESA press release, Peake said the primary task of the EVA will be to replace a failed Sequential Shunt Unit (SSU), which transfers electrical power generated by the solar panels.

That unit failed in November last year, compromising one of the station’s eight power channels. The outpost has been operating fine during the last two months on seven of eight channels and the crew was never in any danger. The SSU is relatively straightforward to replace, as it is a simple box that is removed by undoing one bolt.

After replacing the unit, the spacewalkers will finish laying cables across the Destiny lab module to the zenith port of the Harmony module where Pressurized Mating Adapter (PMA) 3 will eventually be moved in preparation for the arrival of International Docking Adapter (IDA) 3.

International Space Station Russian Soyuz spacecraft in orbit above Earth NASA photo posted on SpaceFlight Insider

Portions of the ISS have been on orbit since 1998. Various spacecraft, such as the Russian Soyuz seen here, have serviced the orbiting lab since it was placed on orbit. Photo Credit: NASA

IDA-3 should launch sometime in 2017, whereas IDA-2 is scheduled to be sent skywards this spring and will be attached to PMA-2, which is located at the forward port of the station’s Harmony module. IDA-3 is a replacement for IDA-1, which was lost in June 2015 due to a launch failure of a SpaceX Falcon 9 v1.1 rocket launched from Cape Canaveral’s SLC-40.

After routing cables, the astronauts will then reinstall a valve that was removed for the relocation of the Permanent Multipurpose Module (PMM) Leonardo last year. Both tasks are holdovers from previous spacewalks.

Before donning their spacesuits, the two will breath pure oxygen for about two hours, purging their bodies of nitrogen. Since spacesuit pressure is lower than the space station’s, this helps the spacewalking astronauts avoid decompression sickness, also known as the ‘bends’, where dissolved gasses in the body, such as nitrogen, form bubbles in body tissue—a condition that can cause pain in muscles and joints, cramps, nausea or worse. Scuba divers can experience similar pains if they rise too quickly to the water’s surface.

Kopra will be designated as EV1—the lead spacewalker—and will have red stripes on his spacesuit denoting his position. Peake, EV2, will have no stripes. Kelly and cosmonaut Sergey Volkov will assist them on the inside of the space station with NASA astronaut Reid Wiseman assisting from Mission Control in Houston.

NASA TV coverage of the spacewalk should begin at 5:30 a.m. CST (11:30 GMT).

This will be the 192nd spacewalk in support of space station’s assembly and maintenance and the 35th that will use the Quest airlock.

Sequential Shunt Unit

The station has eight Sequential Shunt Units (SSUs), each one is located on the Canaster Mast of their corresponding Solar Array Wing (SAW). The one that failed on Nov 13, 2015, was on the 1B channel on the S6 (starboard) Truss segment. Image Credit: NASA


Derek Richardson is a student studying mass media with an emphasis in contemporary journalism at Washburn University in Topeka, Kansas. He is currently the managing editor of the student run newspaper, the Washburn Review. He also writes a blog, called Orbital Velocity, about the space station. His passion for space ignited when he watched space shuttle Discovery leap to space on Oct. 29, 1998. He saw his first in-person launch on July 8, 2011 when the space shuttle launched for the final time. Today, this fervor has accelerated toward orbit and shows no signs of slowing down. After dabbling in math and engineering courses in college, he soon realized that his true calling was communicating to others about space exploration and spreading that passion.

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