Gregory B. Jarvis had been eagerly looking forward to the Challenger mission after being bumped from two previous flights: last year by Sen. Jake Garn (R-Utah) and earlier this month by Rep. Bill Nelson (D-Fla.).

Just before yesterday's flight, Jarvis, 41, said he was "excited, but not nervous."

"You get very comfortable that for any contingency they know what to do. So I feel very, very comfortable."

As a payload specialist, Jarvis said that he wouldn't have much to do during the liftoff and would "be holding onto my seat."

A civilian engineer for Hughes Aircraft Co., Jarvis was to have conducted six days of experiments on the effects of weightlessness on fluid carried in tanks. These had been planned to provide information on the design of liquid-fueled rockets.

Jarvis had been scheduled to make his first shuttle flight -- aboard the spacecraft Discovery -- last April, but his place was taken by Garn, who became the first member of Congress in space. He also was replaced, this time by Nelson, on a flight of the shuttle Columbia, which landed Jan. 18.

Jarvis, who had described himself as a "workaholic," spent his spare time practicing classical guitar, river rafting, riding his bicycle and playing squash and racquetball.

Born Aug. 24, 1944, in Detroit, Jarvis graduated from high school in Mohawk, N.Y. He earned a bachelor's degree at State University of New York in Buffalo in 1967 and a master's at Northeastern Univeristy in Boston in 1969. Both degrees were in electrical engineering.

In July 1969, he entered the Air Force and was assigned to the Space Division in El Segundo, Calif., where he specialized in advanced tactical communications satellites.

He joined Hughes in 1973, and ultimately served as the spacecraft manager responsible for the final assembly and testing of communications satellites.

Garn told reporters, "It's very hard for me to talk about it because these are my friends. "I don't know of any time I have been more shocked or more moved."

Nelson was watching the launch on television in his office with staff members. David Dickerson, Nelson's press secretary, said, "It was really just dead silence in the room. Frankly, no one said a word. The TV spoke for itself . . . . "

Jarvis is survived by his wife Marcia and by his parents, Bruce and Lucille.