Scott Kelly
NASA astronaut Scott Kelly takes a selfie from the International Space Station during his year in orbit. (Credit: Scott Kelly / NASA)

Godspeed, Scott Kelly.

Ten days after finishing up nearly a year in space, the veteran of four spaceflights announced today that he would retire from NASA on April 1. But he also promised to stay involved in the space effort, even after adding 143.8 million miles to his orbital odometer.

“Our universe is a big place, and we have many millions of miles yet to explore,” he said in a Facebook posting. “My departure from NASA is my next step on that journey. I remain ever committed and dedicated to the service of human exploration and advancement whether in space or on Earth.”

With 540 days in space under his belt, Kelly is almost certain to have surpassed NASA’s lifetime limits for radiation exposure. Scientists acknowledged even before the 52-year-old’s return to Earth on March 1 that his chances of taking on another spaceflight for NASA were virtually nil. But Kelly hasn’t ruled out the idea of flying into space for a commercial venture like SpaceX or Boeing.

He doesn’t have to look far for an example. His twin brother, Mark Kelly, retired from the astronaut corps in 2011 and is now director of flight crew operations for World View Enterprises, which aims to send passengers into the stratosphere in a balloon-borne capsule.

For more than a year, Scott and Mark Kelly have been giving samples and undergoing medical tests for a NASA study into the effects of long-term spaceflight. Today Scott said he would continue to participate in that study “for as long as is necessary.”

Before his selection as a NASA astronaut in 1996, Scott Kelly served as an aviator and test pilot for the U.S. Navy. He referred to the past as well as the future in his Facebook post:

“My career with the Navy and NASA gave me an incredible chance to showcase public service to which I am dedicated and what we can accomplish on the big challenges of our day.

“I am humbled and excited by new opportunities for me to support and share the amazing work NASA is doing to help us travel farther into the solar system and work with the next generation of science and technology leaders. I look forward to continuing my 30 years of public service in a new role.

“To continue toward any journey, we must always challenge ourselves to take the next step.”

NASA was quick to pay tribute to Kelly in return. The space agency’s administrator, Charles Bolden, said in a statement that he was “one of the finest astronauts in the history of the space program” and deserved to get some “rest and time on the same planet as his family and friends.”

“All of us in the NASA family — and indeed in the broader scientific community — are grateful that he was willing to sacrifice time with his loved ones, meals that don’t come in a bag, a cold beer, hot showers, cool autumn breezes, the sounds of birds chirping, the ability to lay his head on an actual pillow, and so much more of the pleasures of life during his year of research and experimentation on the International Space Station,” Bolden said.

Here are some of the Twitter tributes from the past week:

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