When US astronaut Mark Kelly first visited China in 2003 as part of a National Committee mission on US-China Relations, he probably didn't expect that he would marry another delegate, US congresswoman Gabrielle Giffords, four years later.

Kelly was thrust into the spotlight in January 2011 after his wife was critically injured in an assassination attempt near Tucson, Arizona, that left six people -including a federal judge and a 9-year-old girl - dead.

"I found my wife in China, but the shooting completely changed our lives", he told China Daily on Sunday.

Kelly was in Beijing - his third trip to China - to deliver a speech to employees at Shaklee Corporation.

Kelly, 48, enlisted in the US navy in 1987 and served as a pilot before joining NASA in 1996. He was a pilot on space shuttle flights in 2001 and 2006, and then commanded missions in 2008 and 2011.

During the 2006 flight on space shuttle Discovery - the second mission after space shuttle Columbia disintegrated during re-entry in 2003 - Kelly said he realized the risk but still called it "a big reward" and "worthwhile".

"When you're going to take that amount of risk ... you have to make a decision based on what's worthwhile It's not what's worthwhile personally. It's what's worthwhile for society," he said.

A vibrant, attractive, forward-looking space program that tries new technologies benefits society, Kelly said. "China is experiencing this right now."

"Astronauts like Yang Liwei are under tremendous pressure, but it has got to benefit the country," he said.

Yang was China's first astronaut to go into space in 2003.

The same sense of responsibility also motivated Kelly to accept his last mission in May, when Giffords was still recovering from the gunshot wound she suffered to her head.

"It was a hard decision, but I eventually realized that it was what my wife wanted me to do, even though she could not speak at that time," Kelly said. "It would put my crew at risk without a commander."

Kelly retired from NASA and the US navy in June. He wrote on his Facebook page that he wanted to devote more time to helping Giffords recover from her injuries and spend more time with his daughters.

"This was not an easy decision. Public service has been more than a job for me and for my family," he wrote.

US Vice-President Joe Biden attended Kelly's retirement ceremony in October, calling the couple "examples of sheer, sheer courage and selflessness and dedication" that people cannot encounter every day.

As for his future, Kelly said a children's book inspired by mice he took into space during his missions will be released in October. Kelly's first book, Gabby: A Story of Courage and Hope, was co-written with Giffords in collaboration with Jeffrey Zaslow, and was a New York Times bestselling memoir after released in November.

"She's a very courageous person, and faced her disability with a great attitude. She often tells me in the morning: fight, fight, fight!" Kelly said.