A medical issue affecting a Crew-11 astronaut aboard the International Space Station (ISS) has forced the agency to postpone Thursday’s spacewalk indefinitely and consider bringing the crew home early.

In a statement issued Wednesday, NASA said the agency is monitoring a health concern affecting a member of Crew-11 that arose Wednesday afternoon aboard the orbital laboratory. An emailed update to Space.com received early Thursday morning stated that NASA is “actively evaluating all options, including the possibility of an earlier end to Crew-11’s mission.” The agency has not disclosed what the health issue is nor which crew member is affected out of respect for their medical privacy.

“These are the situations NASA and our partners train for and prepare to execute safely,” the statement to Space.com reportedly reads. “We will provide further updates within the next 24 hours.”

Crew-11, which consists of space station commander Mike Fincke (58), flight engineer Zena Cardman (38), Japanese astronaut Kimiya Yui (55), and Russian cosmonaut Oleg Platonov (39), launched to the ISS aboard a SpaceX Crew Dragon capsule on August 1. The four astronauts are in the final weeks of their mission, scheduled to return to Earth in mid-February. As of now, that’s still the official plan.

NASA’s emergency contingency plan

NASA has never returned an astronaut from the ISS early due to a medical issue, but the agency is always prepared for the unlikely event it needs to do so. In this case, bringing Crew-11 home early would be relatively straightforward, as their Crew Dragon spacecraft is still docked to the ISS.

All four of the crew members would board the Dragon capsule and likely head for reentry and splashdown in the Pacific Ocean, as they would have in February. A SpaceX recovery vessel based in Southern California would then retrieve the spacecraft and crew.

Crew-11’s early departure would not leave the ISS uncrewed. NASA astronaut Chris Williams and two Russian cosmonauts—Sergey Kud-Sverchkov and Sergey Mikayev—who launched to the space station in November aboard a Russian Soyuz spacecraft would remain on the space station.

Rare medical spacewalk delay

It is unusual for NASA to postpone a spacewalk due to a medical issue, but they have previously been pushed back due to minor concerns, such as a pinched nerve or spacesuit discomfort.

NASA’s Fincke and Cardman were supposed to emerge from the space station’s Quest airlock at approximately 8 a.m. ET on Thursday. It would have been Cadman’s first spacewalk and Fincke’s 10th, tying him with Peggy Whitson for the most spacewalks by a NASA astronaut.

Their primary task was to prepare the 2A power channel for future installation of a roll-out solar array. This array will provide additional power to support the last few years of operation and, ultimately, the lab’s deorbit. Cardman and Fincke would have also installed jumper cables, photographed station hardware, and swabbed the exterior of the ISS to collect potential microorganism samples, according to NASA.

Another spacewalk is scheduled for 7:20 a.m. ET on Thursday, January 15, sending another pair of NASA astronauts outside the space station to replace a high-definition camera, install a new docking aid for visiting spacecraft, and relocate several cooling hoses. The names of the participating astronauts were supposed to be announced after the January 8 spacewalk. It’s currently unclear whether this medical issue will force NASA to postpone the January 15 EVA.