Mike Fincke, the NASA astronaut who recently experienced a medical scare in space and was evacuated, is now revealing some details about the terrifying situation he faced. In an interview with the Associated Press, Fincke said doctors still don’t know the cause of his illness, but his medical emergency happened “completely out of the blue.”

“It was just amazingly quick,” Fincke told The Associated Press.

What happened when NASA astronaut Mike Fincke became ill in space?

Fincke said he ate dinner on Jan. 7 and prepared to embark on a spacewalk the next day. However, he suddenly started feeling severe pain before his walk, and he was unable to talk. Fincke’s crew then urgently called for help from surgeons on the ground.

The 59-year-old former Air Force colonel said he was in pain for about 20 minutes, but he then started feeling better.

“My crewmates definitely saw that I was in distress,” Fincke said. “It was all hands on deck within just a matter of seconds.”

Doctors told Fincke that he was not having a heart attack, but there are a number of other things that could have caused his illness. Fincke, who had stayed in space for nearly six months, may have suddenly become ill after experiencing more than 500 days without gravity, doctors said.

Fincke decided to hold back any more details about his experience because he didn’t want to compromise the medical privacy of other astronauts if something similar happens to them.

How is Mike Fincke feeling now?

Although he is now feeling healthy, Fincke said he’s sad that his spacewalk never happened. According to the Associated Press, that would have been Fincke’s 10th spacewalk, and it would have been the first for his crewmate Zena Cardman.

Due to the medical emergency, the two astronauts and their two other crew members returned from their mission on Jan. 15. They were expecting to stay in space for at least another month. While Fincke continued to apologize to his crew for the early return, they told him to stop feeling bad.

“This wasn’t you. This was space, right?” Fincke’s colleagues told him, per the Associated Press. “You didn’t let anybody down.”

Fincke hopes that he can still return to space in the future.