Three years ago, President Reagan began one of his most ennobling projects. He established the Young Astronaut Program to prepare America's children for the greatest of all adventures: the conquest of space.

At no cost to the taxpayers, the program has introduced space-age education to 450,000 school children across America. In the affluent suburbs and the inner cities alike, the children have responded to the lure of space exploration. Science classes have been transformed into space adventures, with children eagerly pursuing the science and math they used to shun.

Beginning today, the Young Astronaut Council will also offer the program to the nation's preschool children. A special, yearlong program has been tested in Florida; it will permit parents and teachers to start early, preparing their children for the 21st century. The text presents fun-to-do activities that will introduce "astrotots," as they'll be called, to the basics of space-age education before they start school.

The federal Head Start program, which helped develop this course, will use it in its 25,000 classrooms. Some other nurseries and day schools also plan to adopt it. There is no similar curriculum for 3- to 5-year-olds.

The Young Astronaut Council will also announce plans to construct a multimillion-dollar Young Astronaut headquarters building near Washington's Dulles airport. It will be the centerpiece for a proposed National Space Park.

The power behind these plans is John Herrity, the Fairfax County board chairman, who has already brought several high-tech firms into his county and seeks to make it a national technological center.

The building will also become world headquarters for Young Astronauts/Young Cosmonauts International, which was formed in Tokyo this year. Nearly 100 nations have inquired about the program, which has already been adopted by several countries. It was introduced, this year for example, to the Chinese mainland.

The Young Astronaut Council is also crusading to improve the educational standing of American school children. Americans put up more school buildings and hand out more diplomas than any other people. But the quality of education has declined.

Graduates coming out of American schools, for example, are less qualified than their Japanese counterparts. This has reduced the productivity of American workers, restricted their technological skills and lowered their wage-earning ability. To put it bluntly, they have lost their competitive advantage.

But the most urgent need is for teachers. Four million Americans certified to teach aren't in the classroom, because of the low salaries.

Twenty years ago, 22 percent of all college freshmen planned on teaching. This has now fallen, according to the latest available figures, to an alarming 6 percent. As a consequence, a massive teacher shortage is looming in America. The reason is simply that the classroom offers no financial incentive.