Kelly, who has been photographed by his wife's hospital bed holding her hand, said there is a habit of hers that has convinced him that she recognizes him.
"If I hold her hand, she'll play with my wedding ring," he said. "She'll move it up and down my finger. She'll take it off. ... She'll put it on her own finger. She'll move it to her thumb. And then she can put it back on my finger.
"The reason why I know that that means she recognizes me is because she's done that before. She'll do that if we're sitting in a restaurant. She'll do the same exact movements," he said.
Watch the full interview on a special edition of "20/20," "The Congresswoman and the Astronaut: An American Story of Love and Strength," at 10 p.m. on Channel 7.
Read more on abcnews.com.