A problem with a rail car at the International Space Station might just call for an emergency spacewalk by NASA astronauts Scott Kelly and Tim Kopra.

A robotic arm attached to a rail car that moves along the central truss of the ISS got stuck. It is important that the arm is immediately fixed, as it is used to grab incoming cargo vehicles and ease it onto the station. Getting stuck four inches from where it should be positioned is enough for the arm to miss grabbing a cargo full of supplies for the astronauts.

"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 said, according to TIME. "Cause is still being evaluated, but might be a stuck brake handle."

A meeting by the ISS management team on Sunday will clarify what course of action the astronauts would have to take. A cargo from Baikonur Cosmodrome in Kazakhstan is set to launch Monday and arrive at the station on Wednesday, so astronauts might perform the emergency spacewalk as early as Monday, Dec. 21., according to The Verge.

This unexpected mission could mean Kelly's third and Kopra's second spacewalk.