A major power shortage at the International Space Station has delayed this week’s SpaceX supply run.

SpaceX was supposed to launch a shipment Wednesday. But an old power-switching unit malfunctioned at the space station Monday and knocked two power channels offline. The six remaining power channels are working normally, according to NASA.

NASA stressed Tuesday that the station and its six astronauts are safe. But because of the hobbled solar-power grid, the SpaceX launch is off until at least Friday. NASA wants to replace the failed unit to restore full power, before sending up the SpaceX Dragon cargo capsule.

The breakdown has left the station’s big robot arm outside with one functioning power channel instead of two. Two power sources are required — one as a backup — when the robot arm is used to capture visiting spacecraft like the Dragon.

Flight controllers will use the robot arm to replace the bad unit with a spare later this week, saving the astronauts from going out on a spacewalk.

There’s no rush for this delivery. Northrop Grumman launched supplies two weeks ago.

Solar wings collect and generate electricity for the entire space station. Any breakdown in this critical system can cut into power and affect operations.

SpaceX, meanwhile, is still investigating this month’s fiery loss of its new Dragon capsule designed for astronauts.

Six weeks after a successful test flight without a crew to the space station, the crew Dragon was engulfed in flames during a ground test. SpaceX was in the process of firing the capsule’s thrusters on a test stand. The April 20 accident — which occurred right before or during the firing of the launch-abort thrusters — sent thick smoke billowing into the sky.

SpaceX and NASA have offered few details. But the accident is sure to delay launching a crew Dragon with two NASA astronauts on board. SpaceX had been aiming for a summertime flight.

The company still needs to conduct a launch-abort test, before astronauts strap in. The Dragon that flew last month was supposed to be used for this test in June.

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