The International Space Station (ISS) has had a rough go of late, with no fewer than three launch pad accidents that prevented cargo vehicles from getting to orbit in the past year alone. Now another problem aboard the ISS itself may prevent the vehicles that do get to space from actually docking with the station—and one of those vehicles is on its way soon. That could necessitate an emergency spacewalk by commander Scott Kelly and astronaut Tim Kopra on Monday or Tuesday.

The problem involves the space station’s robotic arm, which moves from place to place along the station’s central truss aboard a sort of miniature rail car. When an uncrewed cargo vehicle arrives, it’s the job of the arm to reach out and grab it, then ease it in for a docking. But two days ago, as the arm was moving along the truss, it got stuck just four inches (1o cm) from where it needs to be to grapple an incoming ship.

“Late Wednesday, the Mobile Transporter rail car on the truss was being moved by robotic flight controllers… to worksite 4 just starboard of the centermost position on the truss for payload operations when it stopped moving,” NASA spokesman Rob Navias told TIME in an email. “Cause is still being evaluated, but might be a stuck brake handle.”

That comes at a very bad time. On Monday, the Progress 62 cargo vehicle is scheduled to launch from Baikonur, Kazakhstan in preparation for a docking with the station two days later. NASA and the Russians could face the twin dramas of the Progress countdown proceeding at the same time Kelly and Kopra are preparing to fix the transporter car.

“If the issue is not resolved we would have a discussion with our Russian colleagues,” Navias said. “The launch on Monday is scheduled at 3:44 AM Eastern time. The EVA would occur about 4 hours later.”

The ISS crew is in no danger of running out of supplies no matter what. A cargo vehicle did successfully get off the pad and into space on Dec. 6, and the station is always heavily provisioned precisely against the possibility of launch delays. Still, the current problem and the high-wire act that may be necessary to sort it out is one more reminder that even for an orbital outpost that has been continually occupied for 15 years now, space never has—and never will be—easy.

TIME is producing a series of documentary films about Scott Kelly’s yearlong mission in space. Watch them here.

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Write to Jeffrey Kluger at jeffrey.kluger@time.com.