When the space shuttle Discovery touches down today, the tallest astronaut, Scott Parazynski, a strapping, 6-foot-1-inch blond who looks like an actor and is a doctor, will have completed his third mission in space. But for all his achievements, Parazynski will land with his greatest dream still unfulfilled: to be a U.S. Olympic luger.

Those who have followed John Glenn's latest space odyssey have heard of Parazynski, the mission specialist and the doctor responsible for drawing most of Glenn's blood samples during Discovery's nine-day orbit of Earth.But few people know that before earning his Stanford medical degree, before his first launch, before his spacewalk outside the Russian space station Mir, he discovered luge.

"One of the greatest adventures of my life," Parazynski has called it.

He said his introduction to the sport was "a total fluke," but it was more like the natural course of events: In the mid-'80s if you lived anywhere near Stanford, Bonny Warner, a member of the 1984 Olympic luge team, was there recruiting ambitiously for her sport. Warner, then a Stanford student, conducted one-day summer clinics hoping to woo college-age athletes to the sport's Olympic effort.

Parazynski spent his 24th birthday in 1985 with 75 strangers dodging cones on a black road under the summer sun and falling in love with the supine slalom. At the time, Parazynski was an M.D./Ph.D. candidate in cancer biology at Stanford; on that day he became - and remains - a self-described "luge junkie."

Warner did not know in 1985 that Parazynski intended to be an astronaut, a doctor and an Olympian. When she found out, "I laughed to myself," she said recently. "He honestly thought he could do it. I remember thinking, `How dare you think you could do all three?' "

Warner selected him to represent the Western States Luge Club and train in Lake Placid, N.Y., that winter.

By 1987 - within a year of the Winter Games in Calgary - Para-zyn-ski was so determined to earn an Olympic berth that he was not only entering singles races on the treacherous Lake Placid course, but he was also attempting doubles.

Parazynski never made the Olympic cut, however. He finished ninth in the men's singles at the 1988 U.S. trials; only the top three in each event competed in Calgary. At 26, his dream of participating in the Olympics seemed over.

However, Parazynski did get to Calgary. He was the coach of another Camp Warner veteran, Raymond Ocampo. At 37, Parazynski has gone on to become the astronaut and doctor he said he would be. He did not become an Olympian, but he will never forget his adventures on ice.