(CNN) What seemed like an out-of-this-world claim has turned out to be just that.

Japanese astronaut Norishige Kanai apologized Wednesday for suggesting he had "grown" 3½ inches (9 centimeters) while on the International Space Station.

Kanai, 41, had been on the ISS for only a few weeks as part of a six-month mission when he tweeted that he had some "important news" to share.

"I got my body measured after arriving in the space. Surprise, surprise, I was grown by 9 centimeter! only in 3 weeks. it's first time since my junior high/high school days. I am bit worried if I can fit into my seat in Soyuz on the way back to the earth," Kanai wrote in his first tweet on Tuesday.

He then said he was worried the unusual growth spurt could leave him stuck in orbit because he'd be unable to fit in to the seat on the Soyuz rocket used to return astronauts back to our big blue planet.