Now in Tucson's University Hospital, Kelly is closely monitoring his wife's recovery, tackling every detail with the same focus he brings to his job as a space shuttle commander. In the first days after the attack, he slept in the hospital and only recently has begun to stay in a hotel across the street.
Although the couple owns a condo two miles away from the hospital, Kelly prefers to be as close as possible to his wife's bedside, where he tells her he loves her and reads out loud some of the thousands of supportive letters and e-mails that have flooded in since the attack.
So far, doctors say, Giffords' recovery has been nothing short of miraculous. She was removed from a ventilator Saturday and her condition was upgraded Monday to serious from critical.
Doctors have inserted a tracheotomy tube in her throat to help her breathe and put a feeding tube in her stomach.
While his wife has yet to speak or hint at how much she understands, Kelly has witnessed Giffords following instructions, like a command to hold up two fingers.
When Giffords does begin to speak, Kelly knows they have plenty to talk about. One of the "hardest things," he said, will be having to tell her about the six people who died in the attack, including one of her dedicated staffers, Gabe Zimmerman, and 9-year-old Christina-Taylor Green.
"I was at her funeral the other day," Kelly said. "For a 9-year-old girl to die, you know, just because she was interested in democracy? I mean, it's just not fair."
In the hospital's ICU, Kelly has also comforted Susan Heilman, the woman who brought Green to the supermarket and was also injured in the attack. He said Heilman spoke to Green seconds before the shoo0ting began.
"She leaned down next to her and said, 'You know, someday, you know, you could grow up and be like Gabrielle Giffords.' And those were the last words she heard," Kelly said. "I mean, you just can't explain it."
Capt. Kelly is also facing the question of his own future; whether he will be the commander of the final space shuttle mission April 19, known as STS-134.
His crew sent a supportive tweet this weekend, saying, "Gabby is improving. Mark is strong ... STS134 will succeed."
Whatever the final decision, Kelly's heart will be right there with his wife.
Inside his wife's wedding ring, he had a message inscribed -- "You're the closest to heaven that I've ever been."
Hear Sawyer's exclusive conversation with Kelly tonight on a special edition of "20/20," "The Congresswoman and the Astronaut: An American Story of Love and Strength," at 10 p.m. ET.