When Scott Kelly came back to Earth in March 2016 after spending 340 days on the International Space Station, he exited the recovery vehicle nearly two inches taller than his identical twin brother Mark.<\/p>\n
The twins had, up until Scott\u2019s departure, maintained the same height for all of their adult lives \u2014 approximately 6 feet. Upon Scott\u2019s return, Mark found himself looking up to meet his brother\u2019s gaze. Not by a large margin. But it was noticeable, in a measurable way that was evident when they stood side by side.<\/p>\n
Neither was particularly taken aback. Every astronaut who spends an extended duration on the ISS returns taller than when they left. NASA has been aware of this phenomenon for decades, and the unique engineering challenge it brings is one of the many subtle issues the agency has been addressing throughout the history of long-duration human space exploration.<\/p>\n
The mechanism<\/p>\n
The human spine is structured to allow compression.<\/p>\n
Between each of the 24 vertebrae in the spinal column lies a small cushioning disc \u2014 a spongy entity composed of robust outer cartilage encasing a gel-like core. On Earth, gravity continuously exerts pressure on this stack of vertebrae and discs, slightly flattening the gel cores and thinning the discs. Activities such as standing, walking, sitting, or lying down all apply some form of this compression, exerted continuously on the spine throughout a typical day.<\/p>\n
Even under usual Earth conditions, this produces minor daily variations in height. Most individuals find they are about half an inch to a full inch taller in the morning than they are before bedtime. During sleep, without the weight pressing down on the spine, the discs take in fluid and slightly expand. Throughout the day, standing pressure expels that fluid back out.<\/p>\n
In space, this daily cycle of compression essentially ceases. The discs remain uncompressed. They hydrate to their natural, unloaded size and maintain that state. Coupled with the relaxation of paraspinal muscles \u2014 the deep muscles that typically keep the spine tense against gravity also release in microgravity \u2014 the overall effect elongates the entire spinal column by several centimeters over the course of a mission.<\/p>\n
A 6-foot astronaut can enhance their height by up to 3 percent. This translates to roughly 2 inches, or 5 centimeters. The increase can be detected within days of reaching orbit and continues, albeit at a slower pace, throughout the mission.<\/p>\n
The engineering challenge<\/p>\n
This phenomenon may not concern those on Earth, but it poses significant challenges for spacecraft engineers.<\/p>\n
Flight suits \u2014 including pressure suits worn during launch and reentry, and the extravehicular activity suits employed during spacewalks \u2014 are tailored to specific astronauts before launch. This is essential. A pressure suit that does not fit properly cannot maintain the necessary seal to protect an astronaut during a launch abort or a spacewalk crisis. Each joint must be precisely positioned. Every seam must align with a particular body posture.<\/p>\n
If an astronaut is expected to grow two inches during their mission, the suits must be designed to accommodate that change from day one. NASA engineers create flight suits with built-in expansion features, adjustable at the waist, legs, and torso, ensuring the same suit remains fitting from launch all the way to return without needing recalibration during the flight.<\/p>\n
Similar considerations apply to seat couches \u2014 the contoured seats astronauts secure themselves into during launch and landing. These are custom-fitted prior to launch to evenly distribute forces across the specific body they are designed for. A body that has increased in height by two inches will not align perfectly with the same couch upon return. Post-mission couches are generally equipped with adjustment padding or are entirely replaced for the reentry vehicle, so that the returning astronaut is adequately supported through the distinct g-forces experienced during atmospheric reentry \u2014 which are, notably, acting on a spine that has just endured months without any compression.<\/p>\n
Cabin measurements and habitat design also need to factor this in. Placements for handholds, dimensions for sleep restraints, adjustments for exercise equipment, even the length of umbilicals used for spacewalks \u2014 all must be crafted for the taller version of the astronaut who will occupy the cabin by the end of the mission.<\/p>\n
What happens upon their return<\/p>\n
The additional height is not permanent.<\/p>\n
Within hours of arriving back in Earth\u2019s gravity, the discs start compressing again. Water is expelled from the gel cores. The paraspinal muscles gradually re-engage their normal tone under Earth\u2019s gravity. Over the next days and weeks, the astronaut\u2019s height slowly reverts to their pre-flight baseline.<\/p>\n
This process is not immediate, nor is it always comfortable. Many returning astronauts experience considerable back pain during the reconsolidation phase \u2014 the same discs that have been gently expanded for months are now required to compress rapidly under gravity once more. In certain situations, this rapid compression can lead to uneven fluid expulsion from the discs, contributing to a four-fold increase in the risk of spinal disc herniation that lingers for several months following long-duration missions.<\/p>\n
Full reversion of height typically<\/p>\n","protected":false},"excerpt":{"rendered":"
When Scott Kelly came back to Earth in March 2016 after spending 340 days on the International Space Station, he exited the recovery vehicle nearly two inches taller than his identical twin brother Mark. The twins had, up until Scott\u2019s departure, maintained the same height for all of their adult lives \u2014 approximately 6 feet. […]<\/p>\n","protected":false},"author":2,"featured_media":374697,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"Default","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[179],"class_list":["post-374696","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-uncategorized","tag-source-scienceblog-com"],"_links":{"self":[{"href":"https:\/\/wolfscientific.com\/index.php?rest_route=\/wp\/v2\/posts\/374696","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/wolfscientific.com\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/wolfscientific.com\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/wolfscientific.com\/index.php?rest_route=\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/wolfscientific.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=374696"}],"version-history":[{"count":0,"href":"https:\/\/wolfscientific.com\/index.php?rest_route=\/wp\/v2\/posts\/374696\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/wolfscientific.com\/index.php?rest_route=\/wp\/v2\/media\/374697"}],"wp:attachment":[{"href":"https:\/\/wolfscientific.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=374696"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/wolfscientific.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=374696"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/wolfscientific.com\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=374696"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}