Four astronauts aboard a SpaceX Crew Dragon capsule are slated to splash down early Thursday in the Pacific Ocean, ending a weeklong effort to get an unnamed crew member home to evaluate a medical issue.

The crew left the International Space Station at 5:20 p.m. ET Wednesday aboard the spacecraft and are on a 10-hour journey, gradually lowering in altitude and preparing to reenter Earth’s atmosphere.

The astronauts, part of a mission called Crew-11, are scheduled to splash down around 3:40 a.m ET Thursday off the coast of California.

NASA made the decision to bring the crew home last week after it announced it was canceling a planned spacewalk due to a medical issue.

“This is not an injury that occurred in the pursuit of operations,” said Dr. James Polk, chief health and medical officer at NASA headquarters, during a January 8 news conference.

Rather, the problem relates to having a “medical issue in the difficult areas of microgravity,” Polk added, noting that NASA wanted to bring the astronaut home to make use of diagnostic tools. While the International Space Station does have a suite of medical equipment, it does not have all the tools a typical emergency room would have.

NASA’s Mike Fincke and Zena Cardman would have carried out the spacewalk that was canceled — both are members of the Crew-11 team. The duo is returning home alongside crewmates Kimiya Yui of the Japan Aerospace Exploration Agency, or JAXA, and Russian cosmonaut Oleg Platonov of Roscosmos.

NASA has not disclosed which crew member has the medical issue, nor has the agency revealed details about the nature of that crewmate’s condition other than to say the person is in stable condition and would not require special arrangements for the return trip. Typically, medical information is kept confidential to ensure an astronaut’s privacy.

Returning to Earth

The journey home can take a toll on astronauts’ bodies; the g-forces experienced as the Crew Dragon capsule dives back toward Earth can reach more than five times the force of Earth’s gravity.

The mission’s final stretch is also among the most dangerous as the Crew Dragon capsule reenters the atmosphere at more than 22 times the speed of sound. The process can heat the exterior of the returning spacecraft to more than 3,500 degrees Fahrenheit (1,926 degrees Celsius), creating a buildup of plasma and triggering a communication blackout.

After the capsule splashes down in the Pacific, recovery ships will be waiting nearby to bring it out of the water. Once on board, the crew members will likely exit their capsule aboard medical stretchers — a common practice for transporting astronauts when they return to Earth and begin readjusting to gravity. Using a stretcher is not an indicator of whether an astronaut is experiencing a serious medical condition.

But NASA said the affected astronaut will be undergoing a medical workup upon the return to Earth.

Who is left on board the space station?

Crew-11 originally had been slated to depart the space station in mid-February, only after a replacement team — the Crew-12 astronauts — would have arrived to take over operations.

The Crew-11 astronauts’ premature departure leaves the football field-size space station with three staff members: two Russian cosmonauts, Sergey Kud-Sverchkov and Sergei Mikaev, as well as NASA astronaut Chris Williams, who traveled to the orbiting laboratory as part of a rideshare agreement with Russia.

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