Norishige Kanai
Norishige Kanai of the Japan Aerospace Exploration Agency REUTERS/Kirill Kudryavtsev/Pool

A Japanese astronaut has tweeted of his confusion after growing a massive 9cm (3 inches) during while in space.

Nourishige Kanai has been on board the International Space Station for less than three weeks, but has already noticed a dramatic spurt in his height.He wrote:

"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!"

The Tokyo-born medic and deep sea diver says he is worried that he may not fit into his spacecraft for his return home to Earth later this year.

"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," he said.

Most astronauts go into space can expect to grow by two to five centimetres, but Kanai's growth is quite unusual. The lack of gravity on the astronauts's spine causes the verabrae to spread apart, thus increasing height. However, the spurt is short-lived; most astronauts will return to their normal height after only a couple of weeks upon their return to Earth.

The Russian-built Soyuz space shuttles have a 6' 4" height limit for its passengers, and above average hopefuls are often not selected to participate in flight training.

41-year-old Kanai is a qualified doctor, having earned his medical degree in 2002. He became a certified ISS astronaut in 2011 following a number of years as a diving medical officer for the Japan Maritime Self-Defence Force.