'Grew like a plant': Astronaut's fears after growing 9cm in space

Tall story: ISS astronaut Norishige Kanai. Photo / Norishige Kanai / Twitter

Japanese astronaut Norishige Kanai has grown 9cm after going to the International Space Station, according to posts from his social media feed.

He said he had experienced the remarkable growth spurt since arriving in the orbiter just over three weeks ago.

This is far more than the usual 2-5cm most astronauts grow in space.

The growth is because a lack of gravity slowly allows the vertebrae in their spines to spread apart, the Telegraph reported.

Kanai said he was worried that he wouldn't fit in his spacecraft on his return home.

He tweeted: "Good morning, everybody. I have a major announcement today. We had our bodies measured after reaching space, and wow, wow, wow, I had actually grown by as much as 9cm!

"I grew like some plant in just three weeks. Nothing like this since high school. I'm a bit worried whether I'll fit in the Soyuz seat when I go back."

pic.twitter.com/Z0U1VN4FX5

— 金井 宣茂 (@Astro_Kanai) December 25, 2017

The Russian-built craft Soyuz, which transports astronauts to and from Earth, has a seating height limit.

Although most people who go to space experience the growth phenomenon, they do not stay taller when they go back to Earth, where gravity forces vertebrae to close together.

Kanai, on his first space mission, was previously a diving medical officer with the Japan Maritime Self-Defence Force.