The failure to install a replacement electrical distribution box during a marathon spacewalk Thursday has temporarily left the International Space Station unable to take in power from two of its eight solar array wings. But mission managers said jumpers installed before the excursion will allow near-normal operations while engineers study possible spacewalk repair options.

Credit: NASA

"We are down to six power channels," said Flight Director Ed Van Cise. "We have flight rules in place that give us a maximum loading of kilowatts per channel, and that's what we work to balance across the channels we have available.

"The space station, now that it's fully assembled and operational, has a lot of flexibility to route power around through different means so we can be creative with how we share power across the power channels to be able to provide as much balance as we can. But that said, there are some things that do require more power and are only connected to certain power feeds."

Until the installation issue is resolved and main bus switching unit No. 1 is re-connected to the station's electrical grid, equipment that is not considered crucial will be powered down or used when scheduling permits to avoid conflicts. A robotics work station in the U.S. lab module, for example, will be powered down until it is needed. Likewise automatic heaters may be operated manually to ease the load.

Major, on-going experiments like the Alpha Magnetic Spectrometer will continue to draw power, but others that operate on a more transient basis may require tighter scheduling.

"Really, on a bigger scale, we're just balancing the loads across the six channels we have without minimizing or reducing our operational redundancy we need to actually go fly the vehicle", Van Cise said. "Obviously, we want to keep doing that world-class science that we do, we just have to be smart on which facilities we have up at a given time so we don't have two facilities that use a lot of power on the same feed."

Power generated by the station's eight solar array wings, four on each end of the lab's main truss, is routed to four main bus switching units, all located in the central S0 truss segment. Two power channels feed into each 220-pound MBSU, which in turn deliver 160-volt array power to a pair of transformers known as DC-to-DC converter units. The DDCUs step the primary power down to the 124 volts used inside the station.

The MBSUs are critical to station operation, providing grounding and allowing flight controllers or station astronauts to crosstie power channels or to isolate them as needed when problems develop.

Several months ago, MBSU No. 1 stopped responding to commands, although it still delivered electricity from power channels 1A and 1B, generated by a right-side inboard solar array and one starboard outboard panel respectively.

Replacing MBSU No. 1 was the top priority of a spacewalk Thursday by astronauts Sunita Williams and Akihiko Hoshide. They successfully removed the malfunctioning unit but were unable to install a replacement when a bolt used to pull the 220-pound device down onto a gang of electrical connectors and cooling fins would not drive home.

The replacement MBSU was left partially torqued down, but not connected to power channels 1A and 1B.

"The team installed some jumpers prior to powering down the MBSU in preparation for the EVA and those jumpers allowed us to get power to all the ISS systems and payloads," said Program Manager Mike Suffredini. "So really, what the team has to do, they can get power to all the systems but we only have three quarters of the power available that we normally have.

"Depending on the task at hand over the next several days, the team may have to manage power loads a little bit, but this is familiar territory and we'll be able to deal with that while we decide what our next plan is."

The problem comes at a busy time for the station crew, with the departure of two cargo ships next month and three of the station's six crew members.

"While we think about our plan and when we might do another EVA if we decide that's necessary, we'll have to take all of that into account and move things around in order to accommodate that," Suffredini said. "But if we do decide to do an EVA ... you'll hear us talking about doing that sooner rather than later in the program."

If engineers can figure out what went wrong with the bolt on the replacement MBSU and how to fix it or work around the problem, Williams and Hoshide possibly could stage another spacewalk as early as next week.

But as of this writing, that's far from certain. For the MBSU's electrical connections and cooling fins to engage their counterparts in the truss housing, the box must be firmly seated.

Engineers will use ground spares to study possible techniques to free and then drive a possibly galled bolt without breaking it off or stripping its threads. Another option may be to partially disassemble the attachment fittings to work around the bolt.

In the meantime, Van Cise thanked Williams and Hoshide for "an awesome job today."

"Everybody down here passes on their congratulations for going way above and beyond and they expressed their thanks, all the way up to the program manager. He wanted me to specifically thank you for all that you put into making this as successful as it could be given the circumstances", Van Cise radioed.

"So thank you very much, job well done. Get some rest tonight, and I'll make sure (Flight Director) Tony (Ceccacci) gives you plenty of time to sleep in. Good job."

"Thank you very much for those words," astronaut Joseph Acaba replied from the Quest airlock, where he was helping Williams and Hoshide out of their spacesuits. "As you can see, these guys are happy, we're glad they went out and got back safe, that's what's important.

"You guys worked just as hard as we did, so thank you very much. We'll get these guys out and they can relax a little bit. Have a great night and we'll touch bases with you guys tomorrow."

Williams and Hoshide began repressurizing the space station's Quest airlock at 4:33 p.m. EDT to officially conclude the eight-hour 17-minute spacewalk, NASA's first since the last shuttle flight. Thanks to the problems, the spacewalk was the third longest in NASA history. The record holder, a station assembly flight in 2001, lasted eight hours and 56 minutes.

This was the 164th spacewalk devoted to station construction and maintenance since assembly began in 1998, the third so far this year, the fifth for Williams and the first for Hoshide. With this EVA, 109 astronauts and cosmonauts have logged 1,035 hours and 55 minutes -- 43.2 days -- of station EVA time.

Williams has now logged 37 hours and 34 minutes of EVA time, moving her up to 27th on the list of most experienced spacewalkers. She is second only to Peggy Whitson in total EVA time for a female astronaut. Hoshide is the third Japanese astronaut to walk in space.