Japanese astronaut Norishige Kanai told a tall tale.

He said Monday on Twitter that he grew 3½ inches (9 centimetres) since arriving at the International Space Station on December 19.

Weightlessness has that effect: Without gravitational force compressing the spine, fluid between the discs fluctuates as they temporarily expand, like a coiled spring unspooled from the top.

That kind of growth would have been a little unusual. NASA has said growing an inch or two in space is fairly normal for space walkers. Nearly double that? Kind of a stretch.

"Good morning, everyone. Today I share some serious news. Since coming to space, I have grown 9 centimeters. This is the most I've grown in three weeks since junior high school," Kanai wrote.

But skepticism from a Russian colleague on board led Kanai to remeasure himself, and he found the more accurate spurt: two centimeters, or less than an inch.

In his retraction later posted on Twitter, he called his inaccurate announcement "fake news," The Japan Times reported.

Still, Kanai's initial viral tweet ignited the public's interest about the surprising ways that space affects the human body, and important context about how extraordinarily ordinary it is for the bodies of astronauts to elongate in space.

It even happens on Earth every night. Humans regularly grow and shrink in a similar way, said J.D. Polk, NASA's chief health and medical officer.

As you lie down to sleep, your spine decompresses by as much as half a centimeter. It compresses again while you are in a standing or sitting position. So the phenomenon is more earthbound than it would first seem.

While growth is temporary and astronauts revert to their normal height when they slip the bonds of space and return home, the height difference must be accounted for when figuring the dimensions of spacesuits, stations and vehicles.

Space is at a premium in, well, space, with each inch scrutinised to pack in instruments, tools, plants and insects for experiments and other essentials such as food and water.

That means the living and working quarters are tight. On the Russian Soyuz TMA spacecraft, the vehicle used to get astronauts to and from the ISS, personnel are limited to 6 feet 3 inches so they can fit inside the seats.

That means anyone at that limit on Earth would be restricted from ISS operations.

"I am a little worried I won't fit in my seat on the return trip on Soyuz," Kanai said in his initial tweet, when he thought he grew much taller than he did.

Polk said the spacesuits and seat liners inside the spacecraft were designed with fluctuating bodies in mind, including expanded spines.

He and others are not concerned about Kanai squeezing into the seat, each one fitted with a liner customized for and molded to the body of each astronaut and taken aboard the Soyuz to ensure a tight fit during the violent reintroduction to gravity.

"To help absorb the shock of landing, explosive charges fired and instantly pushed our seats forward so that our faces were very close to the instrument panel," astronaut Ron Garan wrote in October 2011, describing reentry from the Soyuz vehicle.

Once the vehicle reenters Earth's atmosphere, astronauts are again compressed to their normal height, Polk said.

Taller space program hopefuls had their dreams dashed in earlier decades.

The country's first astronauts, the legendary Mercury 7 crew including John Glenn and Alan Shepard, were all under 6 feet - it would have been too much for anyone taller inside the claustrophobic Mercury capsule.

Later recruits could exceed that limit in the space shuttle program, though some flirted with the restriction among the celestial bodies.

"According to my quick calculations here, I seem to have grown about an inch or so. So I'm now too tall to fly in space," said 6-foot-3 Columbia payload commander Richard Hieb in July 1994, after measuring himself as part of a medical experiment.

"And that's without slipper-socks."

While height differences are fleeting, NASA scientists and researchers are still learning about the longer-term effects of zero gravity on the human body, a vital lesson if humans reach beyond the moon to colonise Mars and other planets.

The agency got a rare opportunity in 2015 when astronaut Scott Kelly spent a year on the ISS, a record, providing researchers a wealth of metrics.

His twin brother, Mark, a retired astronaut, was studied so scientists could compare notes on terrestrial and extraterrestrial effects on the mind and body.

There are a host of concerns, such as plaque buildup in arteries and how shifts in bodily fluids affect eyesight.

Vision problems are a common issue among astronauts - gravity on Earth tends to draw fluids downward, but that does not occur in space, and scientists believe those fluids fluctuate and build in the skull and inflame the optic nerve.

An exam of John Phillips, an astronaut on the ISS in 2005, determined that the backs of his eyes were flatter and pushed his retinas forward. In six months, his eyesight went from 20/20 to 20/100.

His vision later improved to 20/50 and remained there, even years later. It's a battle against biology: "Your body wasn't made to pump blood away from your brain," Polk said.

But research yielded from the Kellys and others aboard the ISS suggest a resiliency in the human body, Polk said. Loss of bone density and vision issues appear to plateau after some time as the body, as with all life, bends toward homeostasis.

And yet, some dangers will always be present and risks only reduced, chief among them radiation from the sun and galactic cosmic rays.

A radiation detector aboard the Mars rover Curiosity concluded that a human would be bombarded with a minimum of 0.66 sieverts during a round-trip excursion to Mars, or the equivalent of receiving a CT body scan every five to six days. That would bolster the risk of cancer and other ailments.

Earth's magnetic field helps protect humans from the sun's radiation. Earthlings on average are subjected to a perfectly tolerable 10 microsieverts (0.00001 sieverts) daily.

But that isn't true for space, or for Mars. Cosmic rays penetrate just about every object, including astronauts, and the best way to reduce exposure is to get to your destination sooner, with bigger and faster engines. So plan accordingly.

2017 © The Washington Post

This article was originally published by The Washington Post.