Astronaut and retired Navy Captain Scott Kelly returns to Earth after a year in space

Even after 340 days in zero-gravity weakened his muscles, astronaut and retired Navy Captain Scott Kelly successfully returned to Earth strong enough to give a fist pump and thumbs up.

Kelly's 144 million-mile star trek ended when his Souyuz capsule landed in Kazakhstan.

"The air feels great out here," Kelly reportedly said as he was lifted out of the capsule.

His yearlong stay on on the International Space Station (ISS) gives him more days in space than any other U.S. astronaut. While on board, he worked alongside Russian, European, and Japanese personnel, circling the Earth 5,440 times.

Mars is a 2.5 year round-trip journey. Trouble starts with muscular atrophy.

Maintaining muscle is tough in zero gravity. Astronaut calf muscles compared after a six month mission on the ISS show even after aerobic exercise five hours a week and resistance exercise three to six days per week, muscle volume and power both still decrease significantly. In one of the more extreme cases, Canadian astronaut Chris Hadfield returned to Earth after two months and had to undergo strength training for a few weeks to re-acclimate to Earth's gravity.

Kelly is part of a NASA experiment on the effects of extended time in space on the human body. It just so happens his brother Mark is also an astronaut, but more importantly, he's a genetic twin. Mark Kelly, husband of former Arizona Congresswoman Gabrielle Giffords, was the control subject on the ground. Both gave blood, saliva and urine samples, ultrasounds and bone scans, received flu shots and more, to be compared when Scott returned.

While in space, Kelly sent more than a thousand tweets, including beautiful images of Earth.

And he watched as the newsworthy events of the year unfolded from his high perch.

He stood with France after the terrorist attacks in Paris, even though his feet couldn't reach the ground.

And he saw epic sunrises we on Earth could only dream.

Welcome home, Captain Kelly.