Two U.S. astronauts stepped outside the International Space Station this morning, making the 100th spacewalk dedicated to the ongoing assembly of the orbiting outpost.

The six-hour, 56-minute extravehicular activity (EVA) performed by astronauts Peggy Whitson and Dan Tani came nine years and 11 days after the first station spacewalk in December 1998.

Today's excursion was devoted to inspecting two devices related to the station's power generating solar arrays, both of which have malfunctioned. Tani and Whitson were sent on the "fact-finding mission" to the Solar Alpha Rotary Joint (SARJ) and beta gimbal assembly (BGA) on the station's starboard-side in an attempt to diagnose what has caused the earlier to shed metal filings and the latter to trip circuit breakers.

The decision by mission managers to have Whitson and Tani conduct the EVA came after space shuttle Atlantis was delayed from launching earlier this month by a faulty low level fuel sensor system. NASA engineers began a tanking test at Kennedy Space Center while Whitson and Tani were outside in order to clear Atlantis for a January liftoff.

As the spacewalk was quickly scheduled, the Expedition 16 astronauts were surprised to learn it was the 100th in station history when collectSPACE.com spoke with them last week.

"That's actually news to us about the 100th EVA," said Tani in response to a question from collectSPACE.com. "It is kind of mind boggling. I remember pre-ISS talking about the hundreds, or more than one hundred, EVAs that are going to be required for assembly and thinking that was a huge mountain to climb."

"We're proud to be part of those hundred EVAs. It's very nice to know that we're going to make that 100 mark," he concluded.

In addition to making the 100th spacewalk, Whitson also set a record today for female total EVA time, surpassing Sunita Williams who had set the bar at 29 hours and 17 minutes last February. Whitson became the first woman to command the ISS when she arrived at the station three months ago.

Including today's pair of spacewalkers, 73 astronauts and cosmonauts have worked on the ISS's exterior, including three representatives from the European Space Agency (France, Germany and Sweden), three from the Canadian Space Agency and an astronaut from Japan's Aerospace Exploration Agency.

More than 624 hours -- over 26 days -- were logged by those 73 explorers while performing spacewalks. Almost a third of the EVAs began out the space shuttle's airlock; the remaining exited from the station, either from the US Quest or Russian Pirs airlocks. Two-thirds of the EVAs took place while a shuttle was docked at the station; the others, like today's, during the stages between missions.

The first ISS spacewalk was made on December 7, 1998, on the same flight that saw the docking of the first two station modules. STS-88 mission specialists Jerry Ross and Jim Newman completed the 7-hour, 21-minute EVA outside of Endeavour, connecting umbilicals on the Unity node.

The longest duration station EVA was performed by Jim Voss and Susan Helms during the 17th ISS spacewalk on March 11, 2001. For 8 hours and 56 minutes, the two astronauts worked to configure a docking port to receive a resupply module. The briefest EVA came three years later, after a loss of pressure in Mike Finke's oxygen tank cut short the 53rd spacewalk to just 13 minutes.

The 100th station spacewalk was also the 289th in space history. The 100th American and Russian EVAs also took place outside the ISS: the U.S. in February 2001 on the 15th ISS spacewalk; Russia, just 8 months later, on the 27th ISS EVA.

"That's an amazing record of resounding success," Tani radioed as he made his way back inside near the end of today's spacewalk. "A hundred space EVAs is fantastic and even though we lose a piece of tape here and there, I think we've done incredible work and I know we couldn't have built the space station without it."

"Of course we want to thank everyone involved in this EVA," Tani continued, "but also everybody who has worked on all hundred EVAs. Everybody who makes the suits, the gloves and the procedures and the tools... we thank them all. It's fantastic and fortunate to be the people out here doing the spacewalk but we know that all the hard work happens on the ground."

"Oh! Thank you to whoever makes Kapton tape, too!" he was quick to add.

Click to visit collectSPACE.com for a summary of the first 100 station spacewalks.

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