NASA astronauts Scott Kelly and Tim Kopra will likely perform an unexpected spacewalk next week, to fix a rail car that stalled on the outside of the International Space Station. The spacewalk could happen as early as Monday, December 21st.

The unscheduled spacewalk will address a problem that arose on Wednesday with the "Mobile Transporter," a one-ton rail car attached to the outside of the space station. The transporter stalled while it was being moved by flight controllers at Mission Control in Houston. While NASA hasn't determined an official cause of the failure, it is believed to be a problem with one of the rail car's brake handles.

The transporter helps move equipment and the station's robotic arm (Canadarm2) along a 278-foot stretch of track at a speed of 1 inch per second. The agency was attempting to move the rail car out of the way before next week's arrival of a Russian Progress resupply spacecraft.

NASA says the ISS management team will meet on Sunday to determine the final course of action. Whenever they decide to perform it, NASA will livestream the spacewalk on NASA TV. The resupply mission is scheduled to launch from the Baikonur Cosmodrome in Kazakhstan at 3:44AM ET on Monday, but won't make it to the space station until Wednesday. This would be Kopra's second and Kelly's third spacewalk, as well as the 191st spacewalk going back to the beginning of the space station's construction.