Two NASA astronauts conducted a seven-hour spacewalk outside the International Space Station (ISS) on Wednesday in order to complete a lengthy list of maintenance tasks 250 miles above the Earth.Scott Kelly, Commander of NASA's 45th expedition to the ISS, and flight engineer Kjell Lindgren were selected to perform the spacewalk. Kelly arrived at the ISS in May and plans to stay 342 days at the station-the longest time in space for any NASA astronaut.The American space agency broadcast live coverage of the 189 spacewalk in the history of the ISS via its website.Down at Mission Control in Houston, Texas, Tracy Caldwell Dyson, who has performed three spacewalks, interacted with Kelly and Lindgren.Lindgren performed maintenance on the Alpha Magnetic Spectrometer (AMS), which records and analyzes billions of cosmic ray events and helps astronomers understand dark matter in space. He covered the device with a thermal blanket, which will help it run cooler and extend its life.The AMS was delivered to the ISS in 2011 in a shuttle mission led by Mark Kelly, Scott's twin brother. Kelly removed "some insulation from a power switching unit on the station's truss to facilitate its future robotic replacement," according to NASA. He applied grease to components of the 17.6-meter (57.7-foot) Canadarm2 robotic arm, which is a crucial support for astronauts working in space alongside other tasks.Wednesday's walk was delayed slightly after Lindgren prematurely switched on the water flow in his spacesuit. The system is important in ensuring astronauts do not overheat, but can be a danger in itself. In 2013, an astronaut performing a spacewalk almost drowned when his helmet flooded with water. Another spacewalk is scheduled for Nov. 6, a few days after the 15-year anniversary of continuous human inhabitation of the ISS. Meanwhile, Thursday promises to be another banner day for Kelly.In the wee hours of Thursday morning, just after midnight Eastern time, Kelly will break the American record for NASA's longest single space shot. That 215-day record — more than seven months — was set in 2007.Kelly has been living at the orbiting lab since March. He is to remain there until March 2016. Russian Mikhail Kornienko is also part of the one-year experiment in preparation for eventual Mars expeditions, although it will fall shy of the 14-month world record held by a fellow cosmonaut, Valery Polyakov, who lived on Russia's old Mir space station in 1994 and 1995.Former space station resident Michael Lopez-Alegria has been counting down the days until Kelly surpasses his record of 215 days, eight hours and 22 minutes. "Proud to pass the baton," he said in a tweet.Earlier this month, Kelly broke the U.S. record for the most accumulative time in space: 383 days and counting over four missions. It will total 522 days by the time he returns to Earth, well short of the Russian record.