1. Why Gravity Keeps Your Spine Squeezed on Earth
Your spine is a stack of bones with soft pads of cartilage-like tissue between them, called intervertebral discs. On Earth, whenever you are standing or sitting, the weight of your head, arms and body presses down on that stack. Lying down reduces the load on your spine, which is one reason the idea of "unloading" the spine comes up so often in space medicine.
In orbit, an astronaut and the station fall around the Earth together, so nothing presses the body down against the floor. Researchers call this unloading of the spine. A NASA-funded paper cites about 5 cm as a reported lengthening of body height during long missions; its own six-person study looked at spinal MRI and muscle changes [1]. So the basic answer to the title is simple. The question that keeps scientists busy is how big the stretch is and exactly which parts of the body make it up.
2. How Much Taller Do Astronauts Actually Get?
The numbers depend on who is measured and how. A report on NASA research said astronauts can grow up to about 3 percent taller in microgravity, which would add about 2 inches (5 cm) to a person who is 6 feet (1.8 m) tall [2]. A news report on a Japanese astronaut's mission gave a more typical range of 2 to 5 cm [4]. And a NASA-funded paper on six crew members cites about 5 cm for long missions as background [1].
These numbers are not meant to match exactly. They come from different people, different mission lengths and different ways of measuring, so it is safer to think of "a few centimeters, up to about 5" than of one official number. The size of the stretch also varies from person to person, and no source I read explains why.
3. Why Did One Astronaut's "9 cm" Turn Into 2 cm?
In January 2018, the Japanese astronaut Norishige Kanai had been on the ISS for about three weeks. He posted that his height had grown by about 9 cm (around 3.5 inches). He also worried about getting home: the Russian Soyuz capsule that carries astronauts back to Earth has limited room, and reports gave a maximum astronaut height of roughly six feet three or four inches (about 1.9 m), depending on the source [3][4].
Then he measured himself again, found he had grown only about 2 cm, and apologized for the mistake [3][4]. The 2 cm result sits right inside the usual range, so the first number was most likely a measuring error. The sources I read do not say exactly what went wrong. His first measurement was surprising enough to check again, and the second was very different. Measuring a person floating in a station is harder than it sounds.
4. Do the Spinal Discs Fully Explain the Stretch?
One proposed explanation is that reduced loading lets the discs expand, so the spine lengthens a little at every level. That fits the idea of unloading, but the mechanism has not been confirmed. In a study published in 2016, six NASA crew members had MRI scans of their lower spine before a space station mission, right after landing, and one to two months later. The researchers measured the height of the discs and found no clear change in disc height [1][5].
That result is easy to misread, so here are its limits. Only six people were scanned. The scans were taken lying down on Earth, not in orbit, and the astronauts had already been back in gravity for some time. So the study does not show that discs play no part. It shows that disc height, measured this way and at those times, is not enough to explain the whole gain. Another report says scientists still are not sure about the exact mechanics [4]. Some of that stretch may come from other changes in the spine, but I did not find a source that says which.
5. Do Astronauts Stay Taller After They Come Home?
No. When Scott Kelly returned in March 2016 after 340 days on the ISS, he was reported to have gained about an inch and a half, and two days after landing a news report said he had already lost it again. His twin brother Mark joked that he had been "squished back to normal height," and Kelly said, "gravity pushes you back down to size" [6]. Another report says that back on Earth, astronauts' spines compress again and they return to their original height [4].
So the growth is not like the growing you do as a child. It is a temporary change tied to being in orbit, One rough comparison is a sponge that expands when pressure is released and flattens again when you press on it, though a real spine is much stiffer and more complex.
6. Does Getting Taller Hurt?
Sometimes. Back pain is common on long missions, and the 2016 study's authors say more than half of crew members report spinal pain [5]. The same research also found that muscles supporting the lower spine can get weaker during a mission and had not fully recovered a month or two after return [5]. This is one reason space agencies study the spine closely and plan exercise for crews, since a longer trip, such as a mission to Mars, would put the spine through more.
It does not mean everyone feels the same. The stretch, the aches and the recovery appear to differ from person to person, and the sources I read do not give a full explanation for that.
7. Try Measuring Your Own Height Twice
You can practice the careful measuring that Kanai's story calls for. Ask an adult to help. Stand against a wall while they place a flat book on your head, then mark the height on a strip of paper taped to the wall. Measure the same person three times in a row, standing the same way each time, and compare the marks. Then try once in the morning and once in the evening. Do the marks line up exactly, or do they move a little?
If they move, think about why. Did you stand the same way? Did the book sit flat? Even on Earth, small changes in posture can change a measurement. Writing down what you changed is how scientists decide whether a number can be trusted.