An electrical distribution unit aboard the International Space Station malfunctioned Monday, taking down two of the lab's eight power channels. The crew is in no danger while engineers troubleshoot the problem, officials said, but if it can't be resolved, the electrical box in question might have to be replaced with a spare, possibly prompting NASA to delay launch of a SpaceX Dragon supply ship Wednesday.

Assuming an on-time launch atop a Falcon 9 rocket, the Dragon cargo capsule, loaded with nearly 5,500 pounds of equipment and supplies, is expected to catch up with the station early Friday and then stand by while the lab's robot arm locks on and pulls it in for berthing.

The Canadian-built robot arm can be operated with a single power source, but NASA flight rules require two for redundancy. As it turns out, one of the two power circuits supporting arm operations was knocked out by the main bus switching unit problem that cropped up Monday.

During a 2012 spacewalk, Sunita Williams, seen here, and Aki Hoshide, out of view, completed installation of a replacement main bus switching unit, or MBSU, one of which is visible at lower right. Four MBSUs distribute electrical power from the station's solar arrays through eight power channels. An MBSU malfunctioned Monday, taking down two of those channels. Engineers are studying how to re-route that power in the near term. Spares are available if the MBSU has to be replaced. NASA

A NASA official said power from the two channels fed by the main bus switching unit, or MBSU, could be re-routed using jumpers, but it was not clear whether that included power to the robot arm. NASA flight rules require redundant power for robot arm operations. If secondary power to the arm cannot somehow be re-routed or otherwise restored, and if the flight rule remains in force, the Dragon launch presumably would be delayed until after the MBSU could be replaced, either robotically or during a spacewalk.

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The space station's primary solar power system consists of four rotating solar wings, two on the right side of the outpost's pressurized modules and two on the left. Four MBSU's, mounted in the center of the station's solar array truss, distribute power from the arrays downstream to various electrical circuits. The MBSU that malfunctioned took down power channels 3A and 3B. The system was designed so channels can be "cross tied" as needed to work around failures.

In 2017, an MBSU powering channels 2A and 2B suffered a malfunction, prompting flight controllers to replace it using the station's robot arm. Another MBSU was replaced during two spacewalks in 2012.