In an unparalleled event for modern space exploration, four crew members from the International Space Station (ISS) began an early return flight to Earth on Wednesday due to a confidential medical condition affecting one of the astronauts aboard.

NASA announced its decision to return the agency’s SpaceX Crew-11 mission to Earth earlier than originally planned. This decision comes as teams monitor a medical concern involving a crew member who is currently stable and living aboard the orbital laboratory.

The Crew Dragon capsule carrying two U.S. NASA astronauts including a Japanese crewmate and a Russian cosmonaut detached from the space station and began its descent from orbit at about 5:20 p.m. (2220 GMT). They were headed towards a splashdown in the Pacific Ocean off the California coast early on Thursday.

Live video from a NASA webcast of the departure showed the capsule separating from the ISS and drifting away from the orbiting laboratory as the two vehicles soared some 260 miles (418km) over the Earth, south of Australia.

Earlier NASA plans to bring all four members of Crew-11 home a week ahead of schedule were announced on January 8. According to NASA Administrator Jared Isaacman, astronauts have faced a “serious medical condition” that requires immediate emergency attention on the ground.

According to Reuters, NASA Chief Health and Medical Officer James Polk later shed light on the current situation, stating that it is not a medical emergency and did not involve any injury sustained in the pursuit of operations.

The Crew-11 early return will not impact the timeline for the Artemis II moon mission; meanwhile a replacement is expected to launch in mid-February with four more astronauts.

In this time, the space station remains occupied by NASA astronaut Christopher Williams and two cosmonauts who flew to the ISS aboard a Russian Soyuz spacecraft in November.

From the perspective of NASA officials, it was riskier to leave the astronaut in space without proper medical attention than to temporarily reduce the size of the space station crew.

We have to wait until SpaceX delivers another crew, and the station will have to stand down from any routine or even emergency spacewalks.

The medical evaluation was the major decision made by NASA’s new administrator Jared Isaacman. In announcing the decision last week, he said: “The health and the well-being of our astronauts is always and will be our highest priority.”