After a 20-year stint in NASA, American astronaut Scott Kelly officially started his retirement on April 1, and it's not an April Fool's joke. After setting a new record of 522 days in space, the 52-year-old astronaut announced his retirement when he returned to Earth in early March.

Kelly's 20-year career stint in space exploration has been marked with many firsts and several milestones. On his last space mission, he lifted off from Earth with Russian cosmonaut Mikhail Kornienko on March 27, 2015.

Upon their return on March 1, 2016, the two space men successfully completed one of mankind's most ambitious space missions, one that started a new era for space explorations. During their year in space, almost 400 experiments were completed in preparation for NASA's future missions.

Along with Kornienko, Kelly participated in several studies to test how the human body handles isolation, weightlessness, radiation and stress of long-term spaceflights.

"When we do things that are really hard, we can achieve great things - and that has worked as a great model for me," said Kelly in a March 25 interview.

While Kelly is now officially retired, his involvement in space missions continues at least here on Earth and to a certain degree. For instance, Kelly continues to be debriefed about the one-year mission. He is also undergoing several tests to analyze the long-term mission's effects on his body.

Scott's twin brother, Mark Kelly, also participated in parallel studies to determine genetic changes induced by the former during this recent space flight. They tested Scott's body in space and Mark's body on Earth. Mark is also a former NASA astronaut who stayed Earth-bound during Scott's one-year mission in the ISS.

On March 31, Kelly tweeted "#ThankYou, @NASA! It's been an incredible 20 years! #YearInSpace" and shared the YouTube video link for the video titled Speed of Sound #YearInSpace. The video showed a collection of clips from Kelly's recent one-year stay onboard the International Space Station.

It's been great, Scott! Thank you.

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