For Cmdr. Michael J. Smith, a decorated Navy pilot with more than 4,300 hours of flying time, the 11th mission of the Challenger was to have been the culmination of 24 years of work and a five-year wait for his first flight in space.

Instead, his were the last words from the space shuttle as it thundered upward yesterday.

"Roger, go at throttle up," he said, acknowledging a routine command from Mission Control a split second before Challenger exploded in a ball of orange flame.

It was the end to a lifetime dream for the 40-year-old North Carolinian, who was one of the most experienced aviators in the astronaut corps. Sen. Jake Garn (R-Utah) referred to him as "my mother hen" during Garn's preparation for the shuttle flight on which he was a crew member last year.

Smith had flown 28 types of civilian and military aircraft, was a veteran test pilot and had logged more than 4,000 hours of flying time in jet aircraft in pursuit of a goal he had set for himself as a teen-ager growing up in the small coastal town of Beaufort, N.C.

"That's been his dream since the first flight in space," said Curtis Lancaster, who was Smith's science teacher at Beaufort High School. "He had his private pilot's license before he graduated from high school."

Smith graduated from the U.S. Naval Academy in 1967, and a year later received a master's degree in aeronautical engineering from the U.S. Naval Postgraduate School in Monterey, Calif.

In 1969, Smith received his aviator wings and was assigned to the advanced jet training command as an instructor.

For the next two years, he flew A6 Intruders and completed a Vietnam cruise aboard the USS Kitty Hawk, coming home with the Navy Distinguished Flying Cross, three Air Medals and the Vietnamese Cross of Gallantry with Silver Star.

After completing Navy test pilot school in 1974, Smith worked on cruise missile guidance systems at a Navy installation on the Patuxent River in Maryland, later returning to the test pilot school as an instructor.

Smith served two tours of duty in the Mediterranean before being accepted as an astronaut candidate in May 1980. "He was totally involved in NASA's goals, but he was excited about getting his turn," his younger brother Patrick said yesterday.

"We all have dreams and that was his," Patrick Smith said. "He achieved his and for that we're grateful."

Survivors include his wife, Jane, and three children, Scott, 17, Alison, 15, and Erin, 9.