>> his capsule opens in kazakhstan. it was the first fresh air in 340 days. >> it's not like cold air coming into the capsule. >> help was there on the ground if he needed it to climb out. kelly did not appear. to despite almost one year in conditions of microgravity. leaving the space station, said, was bittersweet and he could have stayed even longer if necessary. >> i look forward to coming home and there's things that i miss. i felt like if it was for the right reason, i clear cloi have stayed. you know, however long it took. >> part of the goal of the endurance trip is to measure how the body toll rates long periods of space and increased exposure to radiation in preparation to possible travel to mars. that round trip alone would take 2 1/2 years. this mission generated tremendous interest around the world. fuelled in part by kelly's use of social media. he has one million followers on twitter and instagram and posted