NASA’s Crew-11 is returning to Earth today from the International Space Station (ISS), cutting short their six-month stint in space after an unknown crew member suffered an undisclosed medical condition.

“For NASA itself, this has been unprecedented,” said Dr. Farhan Asrar, associate dean at Toronto Metropolitan University’s school of medicine.

“When we look at the ISS 25-year history, this is the first time that any time a mission has been cut short specifically for a medical reason or undergoing a medical evacuation,” he said. “There have been … [ISS] medical issues over time, though, as well, but not to the extent that led to a medical evacuation.”

However, Asrar noted that in 1976, a Soviet mission was cut short due to a noxious smell in the spacecraft.

The Crew-11 return comes after NASA cancelled a scheduled spacewalk by astronauts Zena Cardman and Mike Fincke last week, later revealing a “medical issue” with an unnamed astronaut.

Although the crew is returning a month earlier than expected, three people will remain on the ISS: Russian cosmonauts Sergei Mikaev and Sergey Kud-Sverchkov and NASA astronaut Chris Williams.

Crew-12, consisting of NASA astronauts Jessica Meir and Jack Hathaway, along with Russian cosmonaut Andrey Fedyaev and European Space Agency astronaut Sophie Adenot, is scheduled to head to the ISS on Feb. 15.

Meanwhile, NASA is preparing for its Artemis II mission, which will see Canadian Space Agency astronaut Jeremy Hansen, along with NASA astronauts Reid Wiseman, Christina Koch and Victor Glover, go around the moon. The launch is scheduled for no earlier than Feb. 6.

Asrar said while this serves as a reminder about the challenges of space, it also shows how important crew safety is to NASA.

“Agencies and the decision-makers are willing to even cut [missions] short because [they view] astronaut health and safety as paramount,” he said.