Japanese astronaut Norishige Kanai says he has grown by 9cm in height during the past three weeks aboard the International Space Station. He expressed concern as to whether he would fit into the spacecraft to return to Earth.

Kanai, a flight engineer on the Expedition 54/55, shared his observation on Twitter. He said he had not experienced such a rapid growth spurt since he was in school.

みなさま、おはようございます。今日は重大なご報告があります。実は、宇宙に着いてからの身体計測があったのですが、な、な、なんと、身長が9センチも伸びていたんです！たった3週間でニョキニョキと。こんなの中高生のとき以来です。帰りのソユーズの座席に体が収まるか、ちょっと心配です。 — 金井 宣茂 (@Astro_Kanai) January 8, 2018

An increase of height is a well-studied phenomenon on space missions. A human body changes in the absence of normal gravity, with the spine becoming straighter and more elongated. Growth is usually recorded between 3cm and 5cm – a far cry from the 9cm reported by Kanai, who is also a medical doctor.

He is scheduled to return from the ISS in a Soyuz space capsule, the seats of which were modified for the Soyuz-TMA version in the early 2000s. Called Kazbek-UM, they were upgraded to accommodate crew members of up to 190cm in height and up to 95kg in weight. Previously, the limits had been 182cm and 85kg.

This is the first mission to the ISS for the 41-year-old Kanai. He arrived at the station in December 2017, along with NASA’s Scott Tingle and Russia’s Anton Shkaplerov.