HOUSTON It s been 15 years since astronaut Rich Clifford boarded the shuttle Atlantis with a secret a secret he s now willing to share.

Clifford had a problem with his shoulder, but the doctors had missed it. More than once, in fact.

So finally, in 1994, he got an answer that would change his life.

I asked him to look at my shoulder because my right arm wasn t swinging naturally, Clifford recalled.

First thing the next morning, Clifford and his flight surgeon arrived at the Texas Medical Center to see Dr. Joseph Jankovic.

It was the standard neurological stuff. You tap your toes, and you touch your nose. And he said it all looked pretty normal. Then he said, I want you to walk down the hall for me, Clifford said of his first meeting with Jankovic.

A walk down the hall was all it took to diagnose a progressive degenerative disease for which there is no known cure. Clifford had Parkinson s.

But with no diagnostic test for the disease, it s no wonder that some patients doubt their diagnoses. Clifford was one of those.

In fact, his wife said it took him 15 years to believe it.

But even in 1994, Clifford realized he didn t want to argue with Jankovic, who had personally diagnosed more than 10,000 patients and was the authority on Parkinson s in Houston.

What surprised Clifford then was how the doctors including Jankovic regarded his diagnosis as good news.

He said, I know you ve got Parkinson s, but I ve got to prove to NASA that it s not something else. Something worse, like ALS or MS, Clifford recalled.

By the time an astronaut is ready to fly, NASA has invested anywhere from $10 million to $20 million in each crew member. They ve already been pilots in the military, been to test pilot school, gotten a master s in engineering and have been at NASA for 10 years.

That s a lot of investment. So unless there s a really strong reason not to fly, we d prefer than they fly, Richard Jennings, the former chief flight surgeon for NASA, said.

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.

So, two years after he was diagnosed, in the spring of 1996, the shuttle Atlantis heaved its 4.5 million pounds from the launch pad, shaking Mother Earth awake in the middle of the night, with Clifford onboard.

Only he and his commander knew his secret.

Astronauts missions all have a level of danger to them. Rich Clifford s job on STS-76 involved spacewalks.

It s a hazardous environment out there, Clifford said.

When you re watching a spacewalk, it looks as if one lapse of concentration, one misstep during could send an astronaut floating out into space.

Of course, everyone who steps out the door during a spacewalk is tethered to safety by a steel cable.

That applies to every box and every tool, too.

When Clifford did his spacewalk, even after climbing all over the Mir for six hours, nothing floated away. He d performed perfectly.

But it would be his last performance.

I decided after that, that I probably shouldn t fly again, Clifford said. Because I didn t know how fast it was going to progress.

And with Parkinson s, that s the $64,000 question no one knows how fast it will progress.

That fact has been hardest on Clifford s two sons, Richard and Brandon.

From day one, they ve studied Parkinson s even more than their father.

In the aftermath of his six-hour spacewalk, Clifford said he was surprised to learn that his sons had watched the entire thing.

Sheepishly, they told him they d recorded it and watched in on fast-forward.

The boys tried to tell me, This is not going to be good. I said, Boys, I m doing fine. Don t worry about it, Clifford said.

Clifford s wife, Nancy, tears up sometimes when Clifford talks about the future.

We don t really talk about what will be, she said through tears. We just talk about how things are now. About what is now.

The first time a doctor tells you that you have Parkinson s disease, it sounds like a sentence.

For Rich Clifford, that day was 17 years ago. For this reporter, it was 10.

I d been reporting at KHOU 11 News for six years when a persistent tremor in my left hand first sent me to the doctor.

It had finally reached the point where there were days when I spent most of my time just trying not to have my left hand shake, which you can t do. But you try.

After my diagnosis, I continued reporting at KHOU for another seven years.

But in June of 2006, the physical symptoms of Parkinson s made it to where I needed to retire.

In May of 2010, I underwent deep brain stimulation surgery in a bid to ease some of those symptoms.

This is the first story I ve written since.

Though Clifford and I both have Parkinson s, we each have our own unique set of side-effects.

That s how it is with this disease. Every story is different.

These days, Clifford works for Boeing on the shuttle program.

After spending a good portion of his life braving the darkness of space, he s now confronting the mysteries of the human brain his brain ... the final frontier.

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Editor's note: The Houston Area Parkinson Society's event this Saturday is at full capacity and tickets are no longer available.For more informationabout the Houston Area Parkinson's Society,visit hapsonline.org.