What Happens to Your Body in Space?
The short answer
You get taller, by up to about 5 cm (2 inches). About two litres of fluid moves up out of your legs, so your face puffs up and your legs thin out. The hard skin on your soles peels off. And your weight bearing bones can lose 1 to 1.5% of their density a month.
Back on Earth, your height returns within days. Your bones can take years. People have lived with these changes every day since November 2, 2000, when the first crew moved into the International Space Station.
Why you grow
On Earth, your weight presses down on your spine all day. In orbit nothing presses, so its curves straighten out and you get taller. NASA measured astronauts growing by up to 3% of their height. For a person 1.8 m (6 ft) tall, that's about 5 cm. Older reports go as high as 7 cm.
The old explanation was that the soft discs between the bones swell. But a 2016 MRI study of six station astronauts found no consistent change in disc height. A 2018 review points mainly to the lower back losing its curve.
In January 2018 the Japanese astronaut Norishige Kanai posted that he had grown 9 cm (3.5 inches) in three weeks. He said he was a bit worried about fitting in his Soyuz seat for the trip home. A colleague suggested he measure again. The answer was 2 cm (under an inch), and he apologized for his "terrible fake news."
Why bone is the bad part
Bone that carries no weight gets thinner. NASA says weight bearing bones like the spine and hips lose on average 1 to 1.5% of their density each month on missions of four to six months. Older people lose about 0.5 to 1% a year at comparable sites. So one month in orbit can equal one to three years of an older person's loss.
Exercise slows it. Crews work out about two hours a day, and on the treadmill bungee cords hold them down. With today's machines, NASA's 2017 report puts the loss at about 0.3 to 0.5% a month.
Getting it back is slow. The same report gives a recovery half life of 3 to 9 months: in each of those stretches, half of what's still missing comes back. Substantial recovery takes more than four of them, which is one to three years. In a 2022 study, 9 of 17 astronauts still hadn't recovered their shin bone density a year after landing.
Key numbers
| What | Number | Source |
|---|---|---|
| Height gained in orbit | up to 3% of height | Rajulu, Young and Kim, NASA Body Measures, 2019 |
| That gain for a person 1.8 m (6 ft) tall | about 5 cm (2 inches) | Our arithmetic from Rajulu, Young and Kim, NASA Body Measures, 2019 |
| Kanai's first measurement, after three weeks in orbit | 9 cm (3.5 inches) taller | TIME, January 10, 2018 |
| Kanai's second measurement | 2 cm (under an inch) taller | TIME, January 10, 2018 |
| Fluid lost from the calf and thigh by mission day 3 (Skylab) | more than 2 litres | Thornton, Hoffler and Rummel 1974, Skylab |
| Density lost by weight bearing bones per month in flight | 1 to 1.5% | NASA, Risk of Spaceflight Induced Bone Changes, 2025 |
| Bone lost per year by older people at comparable sites | 0.5 to 1% | NASA Human Research Program, Risk of Early Onset Osteoporosis, 2017 |
| One month in orbit, in years of an older person's bone loss | 1 to 3 years | Our arithmetic from NASA, Risk of Spaceflight Induced Bone Changes, 2025 and NASA Human Research Program, Risk of Early Onset Osteoporosis, 2017 |
| Monthly bone loss with today's exercise machines | about 0.3 to 0.5% | NASA Human Research Program, Risk of Early Onset Osteoporosis, 2017 |
| Recovery half life of bone after flight | 3 to 9 months | NASA Human Research Program, Risk of Early Onset Osteoporosis, 2017 |
| Astronauts whose shin bone density hadn't recovered 12 months after landing | 9 of 17 | Gabel et al. 2022, Scientific Reports |
| First crew arrives on the space station | November 2, 2000 | NASA History, 25 Years of Continuous Human Presence |
What the film simplifies
- The film's "up to five centimetres" is NASA's 3% for a person about 1.8 m tall. It isn't a hard limit: older reports go up to 7 cm.
- "In one month, your bones can lose what a grandma's lose in a year" uses the loss rate from before today's exercise machines. With current machines the loss is down to about 0.3 to 0.5% a month. That is still about four months to a year of an older person's loss.
- The two hours of exercise aren't all spent on the treadmill. A treadmill session averages about 27 minutes. The rest is a weights machine and an exercise bike.
- "Twenty six years ago today" is true only on November 2, 2026, the day the film was scheduled to post. The first crew launched two days earlier and moved in on November 2, 2000.
- YOU, Rocket, Grandma and Gravity are cartoon characters. Gravity is drawn as a figure who presses his palm on her head, and the mug she tries to park in mid air back home is a gag. The "free face lift" is Rocket's joke. The puffiness is fluid that has moved up from the legs.
At a glance
Runtime 1 minute 4 seconds · Topics: Human spaceflight, The human body, Bones, Space station · Captions on YouTube · Full script below
Test yourself
In orbit nothing presses down on your spine. What happens to your height, and how fast does it come back on Earth?
Answer
The spine's curves straighten and you get taller, by up to about 3%, around 5 cm for a person 1.8 m tall. The height comes back within days of landing.
Astronauts can lose 1 to 1.5% of their bone density a month. Older people lose about 0.5 to 1% a year. How many years of an older person's loss can one month in orbit equal?
Answer
Between one and three years. 1% a month against 1% a year is one year, and 1.5% a month against 0.5% a year is three years.
Why do astronauts exercise about two hours a day in orbit, and does it fully solve the problem?
Answer
Bones that carry no weight get thinner, and exercise cuts the loss to about 0.3 to 0.5% a month. That is smaller but not zero, and recovery on Earth is slow: in a 2022 study, 9 of 17 astronauts still hadn't recovered their shin bone density a year after landing.
The script
Rocket: Space makes you taller!
Host: Ask Norishige Kanai. In 2018, he said he grew nine centimetres up there.
Host: He worried he wouldn't fit in his seat home.
Host: Then he measured again. Two.
Rocket: Still counts!
Host: Down here, gravity squashes your spine all day. Up there, it straightens out, and you grow up to five centimetres.
Rocket: And you get a free face lift!
Host: That's two litres of fluid moving up from your legs.
Host: Puffy face. Bird legs.
Rocket: Plus baby soft feet!
Host: Your old foot skin peels off. And floats.
Host: Now the bad part. In one month, your bones can lose what a grandma's lose in a year.
Host: So astronauts work out two hours a day. On the treadmill, bungee cords hold them down.
Host: Then you come home.
Gravity: Missed me?
Host: Your height drops back in days. Your bones can take years.
Host: Twenty six years ago today, people moved into the space station. If you're under twenty six, someone's lived up there every day of your life.
Rocket: And there's always room for one more!
Sources
- Weight bearing bones such as the spine and hips lose on average 1 to 1.5% of their density each month on missions of four to six months.NASA, Risk of Spaceflight Induced Bone Changes, 2025
- Body fluids shift upward to the head; weight bearing bones lose on average 1 to 1.5% of mineral density per month.NASA, The Human Body in Space
- Long missions cost about 1 to 1.5% of bone density a month; older people lose 0.5 to 1% a year at comparable sites; with today's exercise machines the loss is about 0.3 to 0.5% a month; recovery half life is 3 to 9 months.NASA Human Research Program, Risk of Early Onset Osteoporosis, 2017
- Astronauts' height increased by up to 3% in orbit.Rajulu, Young and Kim, NASA Body Measures, 2019
- Skylab crews got taller, and by mission day 3 every crewman had lost more than two litres of fluid from the calf and thigh, shifted toward the head.Thornton, Hoffler and Rummel 1974, Skylab
- An MRI study of six station astronauts found lumbar disc heights not appreciably different after long flights.Chang et al. 2016, Spine
- Height gains in spaceflight of up to 7 cm; disc swelling was the older explanation, and recent evidence points more to loss of lumbar curvature.Green and Scott 2018, Frontiers in Physiology
- Kanai posted that he had grown 9 cm (3.5 inches) in three weeks, worried about his Soyuz seat, remeasured 2 cm (less than one inch) after a colleague's suggestion, and apologized for "terrible fake news".TIME, January 10, 2018
- Kanai's 9 cm post and his worry about fitting in the Soyuz seat when he went back.WGBH, January 10, 2018
- Kanai claimed 9 cm in three weeks, worried about the Soyuz seats, then remeasured 2 cm.Gulf News (Reuters), January 2018
- The first crew arrived at the station on November 2, 2000, two days after launch from Baikonur, and people have lived aboard continuously since.NASA History, 25 Years of Continuous Human Presence
- Nine of 17 astronauts had not fully recovered shin bone (tibia) density 12 months after long missions.Gabel et al. 2022, Scientific Reports