Two spacewalking astronauts began preparing the International Space Station (ISS) for new solar arrays on Sunday (Feb. 28), battling tough bolts to kick off a major power upgrade for the orbiting lab.

Expedition 64 flight engineers Kate Rubins and Victor Glover — both NASA astronauts — spent more than seven hours working outside the station during the spacewalk to install modification kits for the new solar arrays. They worked on the station's portside edge to install a bracket and support struts on two mast canisters at the base of the outpost's current solar wings there, but were only able to install one of the kits while assembling a second and storing it for later.

"They completed the construction of upper support hardware and secured it to the space station’s exterior structure until work can be completed on the next spacewalk on Friday, March 5," NASA officials said in an update.

The ISS, parts of which have been in orbit since 1998, is getting ready for new solar panels. NASA says the oldest set of panels have been operating continuously since December 2000, and are still doing well despite their advertised 15-year service life. (The other pairs were delivered in September 2006, June 2007 and March 2009.) But the arrays don't generate as much power as they used to, hence a series of spacewalks beginning now.

The new arrays will be smaller than the old ones due to advances in solar technology. They will be installed to roll out in front of the six current arrays, allowing the new installations to use the infrastructure already in place for the existing set, according to NASA. Boeing (the prime contractor for space station operations) will provide the arrays, with the help of its subsidiary Spectrolab and a major supplier, Deployable Space Systems.

The spacewalkers' goal was to install new array support structures to the station's 4B and 2B using a solar arrays modification kit and several tools, which came in a huge bag about 8 feet (2.5 meters) long, and 1 foot (0.3 m) wide and deep. Rubins and Glover hauled the kit and solar array struts for their work towards the right edge of the station, using a special "slingshot" device to use crew safety tethers far from the core of the ISS.

"Unfortunately, this mod kit is very large, and it doesn't fit out the door in its current state," said spacewalk officer Art Thomason in a news conference held Wednesday (Feb. 24). "So we have to bring it out in pieces, kind of like assembling furniture."

Thomason noted that the mass of the equipment is roughly 330 lbs. (150 kilograms), and the crew members will need to be cautious while bringing everything to the far edges of the space station, where the solar arrays are located.

"Even though we don't have gravity to deal with in space, we still have inertia and mass. The crew knows to be careful with this," he said. "As they're translating [moving] out there, they're going to take it easy and make sure when they're turning corners, and things like that, they help guide the bag because this is a larger thing than what they're used to."

The crew hoped to install two sets of struts at two worksites nearby the solar arrays, Thomason said. At each site, they were to use a portable foot restraint and tethers to anchor in place, before installing a left strut, a right strut and a midstrut. The astronauts planned to secure thermal blankets over each of the struts. Rubins also carried a new high-definition video camera on her spacesuit, a first for U.S. spacewalks, to provide clearer views.

In practice, however, the Glover and Rubins fell behind schedule after one of the bolts on the first strut did not fully engage at first.

"One of the bolts did not fully engage on the first attempt, so Rubins used a power drill to back it out and reseat it, then used a ratchet wrench to tighten the bolt, reaching a safe configuration," NASA officials wrote in the update. "The bolt likely will need to be secured further before installing one of the new solar arrays that will be delivered to the space station later this year aboard SpaceX’s 22nd commercial resupply services mission."

The astronauts then managed to assemble the upper support for the second set of arrays, then secured it to their worksite so it can be installed on an upcoming spacewalk on March 5.

Rubins and Japanese astronaut Soichi Noguchi will venture outside the station on that next excursion, which is expected to complete the work begun in today's spacewalk. It will also include a bundle of maintenance tasks: venting ammonia, removing and replacing a wireless video transceiver assembly, and installing a "stiffener" on the thermal cover of the Quest airlock to stop the cover from blowing out during spacewalks (when some residual air from the space station escapes into space.) For that excursion, Rubins will be EV 1 and Noguchi EV 2.

Solar arrays naturally degrade over time and the new array set will boost station current power levels by 20% to 30%, bringing the ISS back up to about what was available when the orbiting laboratory was first constructed decades ago, NASA said in a statement. (Batteries are also a factor in station power, especially for storage capabilities; spacewalking crews spent about four years upgrading the older batteries to newer and more efficient versions, finally completing that work in January.)

The eight solar arrays now in place provide about 160 kilowatts of power; half of that is stored for when the station is in orbital darkness, which occurs about 15 times a day. Once the new solar arrays are in place over the old ones, adding the new array power to what remains of the older arrays should provide up to 215 kilowatts of power, depending on factors such as whether the station is in sunlight or darkness, NASA said.

"The solar arrays will be delivered to the International Space Station in pairs in the unpressurized trunk of the SpaceX Dragon cargo spacecraft during three resupply missions starting in 2021, when the second pair of current arrays reaches the 15th year of its design life," NASA added in the statement. "The installation of each solar array will require two spacewalks: one to prepare the worksite with a modification kit and another to install the new solar array."

Solar array 4B has an interesting history. Back in 2007, astronauts on the space station and a space shuttle were deploying the newly delivered array when they noticed a tear developing. They stopped the deployment and consulted NASA's Mission Control in Houston for a fix.

The result was an epic spacewalk — the procedures for which were implemented in mere days — that saw astronaut Scott Parazynski spacewalking atop the Canadarm2 robotic arm and an extension piece for a repair. Parazynski used tools crafted in orbit to carefully sew together the tear in the fully powered array. The tricky work of Parazynski, his crewmates and ground controllers that November allowed the damaged array to finish deploying. The fix is still in place today.

This article is reproduced at www.space.com