When Scott Kelly returned to Earth in March 2016 after 340 days aboard the International Space Station, he stepped off the recovery vehicle almost two inches taller than his identical twin brother Mark.
The twins had, until Scott’s launch, been the same height for their entire adult lives — approximately 6 feet tall. When Scott came down, Mark had to look up to meet his brother’s eyes. Not by much. But measurably, in a way that was easy to see when they stood next to each other.
Neither of them was particularly surprised. Every astronaut who spends a significant time on the ISS comes back taller than they left. NASA has known this for decades, and the specific engineering headache it creates is one of the small quiet problems the agency has been quietly solving for the entire history of long-duration human spaceflight.
The mechanism
The human spine is designed to be compressed.
Between each of the 24 vertebrae in the spinal column sits a small cushioning disc — a spongy structure made of tough outer cartilage surrounding a gel-like core. On Earth, gravity constantly presses down on this stack of vertebrae and discs, squeezing the gel cores slightly flatter and the discs slightly thinner. Standing up, walking, sitting, lying down — all involve some version of this compression, applied continuously to the spine over the course of an ordinary day.
Even under normal Earth conditions, this creates a small daily height variation. Most people are about half an inch to a full inch taller when they wake up in the morning than they are before going to bed. During the night’s sleep, without weight bearing down on the spine, the discs absorb fluid and expand slightly. During the day, standing pressure squeezes that fluid back out.
In space, the daily compression cycle essentially stops. The discs never get squeezed. They fill with fluid to their natural, unloaded size and stay there. Combined with paraspinal muscle relaxation — the deep muscles that normally hold the spine under tension against gravity also let go in microgravity — the cumulative effect stretches the entire spinal column by several centimetres over the course of a mission.
A 6-foot astronaut can gain up to 3 percent of their body height. That works out to approximately 2 inches, or 5 centimetres. The gain is measurable within days of reaching orbit and continues, more slowly, throughout the mission.
The engineering problem
None of this matters to Earth-bound people, but it matters substantially to spacecraft designers.
Flight suits — both the pressure suits worn during launch and reentry, and the extravehicular activity suits used during spacewalks — are fitted to specific astronauts before launch. They have to be. A pressure suit that doesn’t fit correctly cannot maintain the seal needed to keep an astronaut alive during a launch abort or a spacewalk emergency. Every joint has to be in the right place. Every seam has to line up with a specific body position.
If an astronaut is going to grow two inches during their mission, the suits have to accommodate that growth from day one. NASA designs flight suits with built-in expansion allowance, adjustable at the waist, the legs, and the torso, so that the same suit fits both at launch and at return without recalibration during the flight.
Similar constraints apply to seat couches — the moulded seats astronauts strap into during launch and landing. These are custom-fitted before launch to distribute forces evenly across the specific body they are designed for. A body that has grown two inches will not fit the same couch identically on return. Post-mission couches are typically fitted with adjustment padding or replaced entirely for the reentry vehicle, so that the returning astronaut is properly supported through the specific g-forces of atmospheric reentry — which are, incidentally, being applied to a spine that has just spent months without any compression at all.
Cabin dimensions and habitat design also have to account for it. Handhold placements, sleep restraint dimensions, exercise equipment adjustments, even the length of umbilicals used for spacewalks — all have to be designed for the taller version of the astronaut who will be occupying the cabin by the time the mission is complete.
What happens when they come back
The extra height is not permanent.
Within hours of returning to Earth’s gravity, the discs begin to compress again. Water is squeezed out of the gel cores. The paraspinal muscles slowly re-engage their normal Earth-gravity tone. Over the following days and weeks, the astronaut’s height gradually returns to their pre-flight baseline.
The process is not instant, and it is not always comfortable. Many returning astronauts report significant back pain during the reconsolidation period — the same discs that have been quietly expanded for six months are now being asked to compress rapidly under gravity again. In some cases, the compression drives fluid out of the discs unevenly, contributing to the four-fold increased risk of spinal disc herniation that persists for months after long-duration flights.
Full height reversion typically takes between two and six months, depending on the individual and the mission length. Scott Kelly was back to his pre-flight height within a few months of returning. So was his brother Mark’s small temporary advantage as the taller twin.
What this reveals about the body
The specific fact that human beings get taller in space is a small piece of a much larger picture about what gravity actually does to the body.
Every load-bearing structure in the human skeleton — the spine, the hips, the legs — is designed against a specific gravitational force that has been constant for the entire evolutionary history of the human species. Bones develop density in response to that force. Muscles maintain tone against that force. Cartilage compresses under that force. The specific mechanical shape of an adult human body reflects tens of thousands of years of evolution under one specific set of load conditions.
Take those conditions away, and the body slowly begins to change. The spine stretches. The bones thin. The muscles atrophy. Cardiovascular capacity shifts. Some of these changes reverse quickly on return. Some reverse partially. Some, particularly the bone density loss, may never fully reverse for astronauts on very long missions.
The two-inch height gain is the easiest of these changes to notice and the least consequential to worry about. It is also the specific one that requires NASA’s flight suit engineers to build in room for it, so that the astronauts who go up as one size come home safely in the same equipment as a slightly larger version of themselves.
