Fifteen years ago, shuttle astronaut Rich Clifford launched into space with a secret — a diagnosis of Parkinson’s disease.

The story, told by a Houston television reporter who also has Parkinson’s, comes from Clifford himself.

“I knew I couldn’t tell anyone. It would make them answer some difficult questions at a press conference sometime, and I didn’t want to do that to them,” Clifford said.

PHOTOS: For Space Shuttle Atlantis, The End Is Here

The reporter, KHOU’s Dan Lauck, writes that only Clifford and his commander knew of the diagnosis. The commander was Kevin Chilton, who left NASA in 1998 after three spaceflights to return to his military career. Now a four-star general, Chilton recently retired as commander of the U.S. Strategic Command.

Clifford left NASA after his third shuttle flight in March 1996, two years after he was diagnosed with Parkinson’s disease, a progressive nervous system disorder characterized by shaking, tremors and problems with walking and coordination. There is no known cure for the disease.

“I decided after (the March 1996 shuttle mission) that I probably shouldn’t fly again,” Clifford said. “Because I didn’t know how fast it was going to progress.”

Clifford went to work for Boeing and remains in Houston today. The reporter left KHOU in 2006 due to his medical condition. He underwent deep brain stimulation surgery last May to try to alleviate some of the symptoms. The story about Clifford is Lauck’s first since then.

Image: Shuttle astronaut Rich Clifford preparing for launch in March 1996 — two years after he was diagnosed with Parkinson’s disease. Credit: NASA