text: Story highlights NASA says Scott Kelly is 2 inches taller after spending nearly a year in space 
   
   He's about to reunite with his twin brother in Houston 
   
   Since his return to Earth, the astronaut has been sharing photos of life on the ground 
   
   (CNN) Astronaut Scott Kelly and his twin brother used to be the same height. 
   
   But it's likely that won't be true when the two stand side by side for the first time in nearly a year. 
   
   NASA's Jeff Williams told CNN that Scott Kelly grew 2 inches during his time aboard the International Space Station 
   
   It was expected, and it's temporary, Williams said. 
   
   "Astronauts get taller in space as the spine elongates," Williams said, "but they return to preflight height after a short time back on Earth." ||||| 1 of 9 Full Screen Autoplay Close Skip Ad × Photos: Astronaut Scott Kelly lands after a year in space View Photos The Soyuz TMA-18M space capsule returns from space and lands near the town of Dzhezkazgan, Kazakhstan. Caption The Soyuz TMA-18M space capsule returns from space and lands near the town of Dzhezkazgan, Kazakhstan. March 2, 2016 Ground personnel help International Space Station crew member Scott Kelly of the United States to get off the Soyuz TMA-18M space capsule after landing near the town of Dzhezkazgan, Kazakhstan. Kirill Kudryavtsev/AP Buy Photo Wait 1 second to continue. 
   
   This post has been updated. 
   
   When astronaut Scott Kelly arrived in Houston on Thursday morning, he was about two inches taller than when he left for the International Space Station a year before, according to NASA representatives. That’s pretty normal for an astronaut: Without the full strength of gravity pressing down on gel-filled discs between the vertebrae, they expand and lengthen the spine. It’s a weird but temporary side effect of spaceflight. 
   
   But even if Kelly hadn’t had his vitals checked immediately upon landing, he might have noticed the slight height change: One of the first Earthlings he saw was his identical twin, retired astronaut Mark Kelly — a man now notably, if only temporarily, shorter. 
   
   NASA scientists already knew that Kelly would walk a little taller when he emerged from the Soyuz capsule. But he’ll have changed in other, less obvious ways, too — and that’s the whole point of his recordbreaking mission. Kelly and Russian cosmonaut Mikhail Kornienko spent 342 days on the ISS to help scientists measure the effects of long-term spaceflight on the human body. 
   
   [Watch live: Scott Kelly and NASA scientists discuss his record-breaking year in space] 
   
   The impressive jaunt is a record for the United States, and for the International Space Station. But astronauts actually have stayed in space for months longer than that: Back in the pre-ISS days of Russia’s Mir station, several cosmonauts broke this record. 
   
   Astronaut Scott Kelly spent 340 days aboard the International Space Station, longer than any American astronaut. Here's a look at the year he spent in space, through his eyes above Earth. (Victoria Walker/The Washington Post) 
   
   But this is the first attempt to really study the effects of such a long stay in space. The hope is that data collected will highlight some of the most worrying physiological and psychological hurdles to spending months or years in space — allowing scientists to tackle those problems before NASA attempts a long-haul flight to Mars or the planets beyond. NASA hopes to put astronauts on the Red Planet by the 2030s. 
   
   Kelly’s twin brother made him a perfect candidate for such a mission. The earthbound twin was used as a sort of “control” for experiments being run on his orbiting brother. Even identical twins aren’t alike enough that NASA can pin every difference between them on the effects of spaceflight, but finding changes that occurred in one man and not the other over the course of a year might give them some important clues. 
   
   [Why American astronauts drink Russian urine] 
   
   “Obviously, this is a tiny sample size, so we’re not really looking at how Scott and Mark are different during the year, exactly,” Johns Hopkins Medical School's Andrew Feinberg told The Washington Post just before Kelly's launch last year. “It’s not statistically valid to say that differences between them must be due to the spaceflight.” 
   
   But, added Feinberg — whose project for the mission focused on epigenetics, or the way different environments affect the expression of our genes — “if something happens after Scott departs, increases during his trip, and then goes back to normal after he comes back to Earth — if we don’t see that kind of sequential change in his twin, well, it’s not proof of anything, but it certainly suggests something interesting is going on.” 
   
   
   
   Kelly, wearing a body suit designed to fight the effects of low gravity on body fluids, takes ultrasound measurements. (NASA) 
   
   The scientific experiments involved in the twin study are ongoing. All year, Kelly has been taking blood samples each time a shuttle is about to head back to Earth, allowing scientists to study fresh, unfrozen cells just hours after they’re drawn. Meanwhile, Mark has donated countless hours to providing samples of his own, as well as undergoing the same psychological and cognitive tests his brother completed in space. Now scientists can take all of that data and make something of it. 
   
   There are 10 official experiments running as part of the twin study, selected from a pool of research proposals submitted by institutions across the country: Six projects (including Feinberg’s) look at the way cellular behavior may change due to low gravity, high levels of radiation exposure, and other space-centric environmental factors. These studies will look at things like how aging, gene expression, and immune system function may have been affected. 
   
   [How Scott Kelly made flowers bloom in space] 
   
   Two studies will focus on physiological changes. One will try to determine whether plaque buildup in the arteries is accelerated during spaceflight, and another will look at how shifts in body fluid affect eyesight. Vision problems are a common complaint among astronauts. 
   
   Another study will use psychological tests to determine whether Scott Kelly has had changes in perception, reasoning, decision making and alertness that aren’t typical for someone spending a year on Earth. 
   
   And one last experiment will study how the microbes inside the twins’ guts have changed over the course of the year. Diet and environment can have a huge (and surprisingly swift) influence on the microbes that live in and around us, and there’s more and more evidence that these bug colonies have a profound effect on our health and wellness. Figuring out how bacterial colonies change in space could help keep the first Mars crew happy and healthy. 
   
   [How filthy is the International Space Station? An astrobiologist weighs in.] 
   
   The hope is that scientists can come up with a plan for protecting the men and women who may eventually journey to Mars. It’s likely that the flight to our closest neighboring world will take about nine months each way. Once a crew touches down on the sandy surface, NASA will want them to stay awhile – after all, it would be a shame to spend 18 months in a tin can for a day or two on the surface of a new planet. So it seems likely that the first Mars mission will be a multiyear commitment. 
   
   “NASA is working on this science project that’s the greatest in the history of civilization,” Feinberg said last March. “They’re turning humankind from an Earth-dwelling species into a space-exploring species. One day, humankind will be a species that can settle on other planets. It might be a hundred years before we have humans living on Mars, but this is a whole new kind of science. It’s a multi-generational effort.” 
   
   
   
   Jill Biden, the wife of Vice President Biden, greets Expedition 46 Commander Scott Kelly of NASA. (Joel Kowsky/NASA) 
   
   Upon landing in Houston on Thursday morning, Kelly was greeted by his girlfriend (who works in NASA’s communications department) and his two daughters, as well as his twin brother and sister-in-law, former congresswoman Gabrielle Giffords (D-Ariz.). The last time Scott Kelly came home from the ISS, it was two months after Giffords had been the victim of a near-fatal assassination attempt. Kelly has cited this period — when he was unable to return home to be with his family — as the time when the isolation of spaceflight felt most profound. 
   
   In addition to the gathered family, Kelly was welcomed by second lady Jill Biden, who brought apple pie and beer for the astronaut. 
   
   “That’s what he said he wanted, so that’s what I brought him,” Biden said during a brief ceremony streamed by NASA TV. 
   
   “The beer is from the president, actually, from the garden, and the apple pie is just from all Americans,” she said. “So, we hold you in our hearts. Welcome home.” 
   
   [Why NASA is so excited to send a twin to space] 
   
   “It’s great to be back in Texas, on U.S. soil,” Kelly told the NASA press pool after landing. “It’s just an unbelievable feeling to be back here on planet Earth, back in our great country, and back with all my family and friends.” 
   
   And although Kelly has become something of a celebrity for his record-breaking flight, he remains humble. “I also want to say this was not my flight. They’ve talked about my year in space, it’s not my achievement, it’s NASA’s achievement and it’s our country’s achievement,” he said. 
   
   Kelly had said previously that a swim in his pool was a top priority upon arriving home. And it looks like he took the plunge as soon as he was able: 
   
   Kelly will address the public at a 2:00 p.m. briefing (EST) on Friday. Scientists will be on NASA TV starting at 1:00 p.m. to discuss the mission. 
   
   Read More: 
   
   Why Scott Kelly grew flowers in space 
   
   Why American astronauts drink Russian urine 
   
   NASA needs 14 new astronauts. A record-breaking 18,300 folks applied. 
   
   How much longer will the space station last? 
   
   Someone once smuggled a sandwich into space, which is a horrible idea 
   
   How filthy is the International Space Station? An astrobiologist weighs in.
target: – NASA will be studying how Scott Kelly's body reacted to almost a year in space, using his twin brother Mark Kelly, who stayed on Earth, as a control. One big difference to note right away: The brothers will no longer be the same height, as they were before Scott went into space, because he grew 2 inches while aboard the International Space Station, CNN reports. "Astronauts get taller in space as the spine elongates," NASA's Jeff Williams explains. That's because "without the full strength of gravity pressing down on gel-filled discs between the vertebrae, they expand and lengthen the spine," the Washington Post explains. But Scott won't be able to lord it over Mark for too long: Astronauts "return to preflight height after a short time back on Earth," Williams says.