In space, the near-absence of gravity causes astronauts’ spines to stretch, sometimes as much as three percent. The vertebra in their backs expand and relax without gravity constantly pushing them together. Kelly’s height-boost will be short lived, as most astronauts return to their original height after a few months. In 2013, Space.com reported that astronauts started using a new ultrasound machine to carefully measure the changes to their spines after certain intervals, and Kelly’s measurements may be particularly valuable after his unprecedented time in space.
Micro-gravity environments like the International Space Station have a laundry-list of effects on the human body — some of which we still don’t fully understand — which will be major factors in future manned missions to Mars. Micro-gravity can effect bone density, immune systems, vision, muscle composition, and more during long periods of exposure.
Kelly’s brother, astronaut Mark Kelly, is also a vital part of the process. Mark, Kelly’s twin, has spent significantly less time in space than his brother, so he’s a perfect control group for studying the effects of long-term spaceflight.
Now that Scott Kelly is home, NASA’s scientists will run exhaustive tests to compare the two brothers’ bodies and minds after their journeys through the stars.