It was a different Ronald Reagan who went to the Johnson Space Center two years ago than the one who was in Houston last week. But then, it was a different space agency.The National Aeronautics and Space Administration, Air Force, aerospace contractors, the rest of the American public, and even the president were in shock in 1986 from the national tragedy that played again and again on TV screens everywhere.

The unbelievable happened Jan. 28, 1986. The Challenger space shuttle burst into flames just over a minute after launch. Its occupants - Francis Scobee, Michael John Smith, Ellison S. Onizuka, Judith Arlene Resnik, Ronald Erwin McNair, Sharon Christa McAuliffe and Gregory Bruce Jarvis - were killed in the crew cabin's fiery journey from sky to the Atlantic.

As hokey as the idea was, NASA included among its highly trained specialists in engineering and physics, a teacher, McAuliffe, who was trained especially for that launch. School children, some herded to places with huge television screens and satellite dishes, were awaiting their most exciting lesson plan. McAuliffe was to give a shuttle tour while in space.

Like most of us, Reagan hadn't been watching the shuttle launch. His White House communications director, told him what had happened.

"Sir, the shuttle blew up."

The next day, in what was to be a traditional, televised State of the Union address, Reagan offered consolation to a nation in mourning.

"Nancy and I are pained to the core by the tragedy of the shuttle Challenger," Reagan said. "We know we share this pain with all the people of our country. This is truly a national loss.

Three days after the disaster, Reagan was at Johnson to console more than 200 relatives of the Challenger crew.

"Man will continue his conquest of space, to reach for new goals and even greater achievements," he promised. "That is the way we shall commemorate our seven Challenger heroes."

Last week marked the first time since the 1986 memorial service that Reagan has been to Johnson. It was as if a shroud had been lifted.

His visit symbolized the end of a period of national mourning, of lack of faith in NASA and of ridicule by aerospace specialists who had accused the agency of lacking the guts to go up where European Space Agency cargoes and Soviet cosmonauts already are.

After 32 months of painful testimony, congressional oversight, management shuffles and mechanical modifications, an American shuttle is scheduled to launch this week.

Discovery, equipped with an emergency escape system for the astronauts, is scheduled to leave Earth Thursday morning, carrying with it a $100 million communications satellite.

Discovery's five astronauts, all shuttle veterans and four with military backgrounds, met at Johnson with the president. Rick Hauck, Dick Covey, Mike Lounge, David Hilmers and George Nelson are the next Americans in space.

Happy days are here again. Happy talk, as well.

"Soon the world will be watching as five brave Americans lift off from Earth," Reagan told NASA employees at Johnson. "America is going to space again - and we're going to stay . . . Neither can we stand still, nor be content, and we are not afraid. Ill fortune can slow us down, but it cannot stop us. . . . When the Discovery takes off, seven precious souls will soar beside it - the seven heroes of the Challenger."