It has become very clear that if I'm going to be a space traveler too, there will have to be some alterations. Such as three inches off the waist and a diminution of the derriere.

I have come to the quick conclusion after climbing assorted ladders and squeezing my frame inside one of the two space shuttles at NASA's headquarters, 25 miles outside this hell-for-leather city.

Although the real space shuttle is now scheduled to take off from Kennedy Space Center in March, the two in Building 9A at NASA's 1,600-acre park are full-scale models that have been used to train the astronauts chosen for the shuttle program. The pilots ride up front, in the manner of a regular aircraft. Behind them is an enormous cargo cab, 15 feet by 60 feet, capapble of accommodating a 47-passenger bus.

The shuttle can carry 65,000 pounds and stay in space for a month. It will carry space labs where scientists and specialists -- both men and women (and not necessarily astronauts) -- will be able to work in a shirt-sleeve environment. The researchers who fly the space labs will be briefed on zero gravity and taught how to keep house in space, in addition to learning how to conduct their specialized tasks: earth surveys, astronomical observations and biomedical studies.

The shuttle carries freight into space in the same way a truck pulls a trailer. It must handle the freight with a manipulator system called MSR, which has been developed with a private Canadian contractor. Visitors to Building 9A will see a huge helium-filled elongated balloon that is used as a dummy because it reacts like freight in a zero-gravity environment.

No less a scientific seer than Isaac Asimov has said that space travel, and ultimately interplanetary travel, will start with the space shuttle. "The real beginning of space travel," Asimov has said, "is right now."

All that comes home to the visitor with palpable excitement at the Lyndon B. Johnson Space Center, named for the Texas president who wanted this venture into tomorrow to be on his home territory. Today the Johnson Space Center, despite its location, unhandy to downtown Houston, clocks more than a million visitors a year. They come by private car or by Gray Line tour, which leaves downtown by bus at 1 p.m. daily, takes 4 1/2 hours, costs $11 per person and is led by a guide.

Those who come on their own can also take a self-guided tour following green signs through an assortment of buildings including a visitor's information center, and a mission stimulation and training center where astronauts practice. The oft-pictured Mission Control Center is a facility included only on guided tours.

A mural painted by Robert McCall on the wall of Building 2, the visitor Information Center, sets the tone for the dazzling exhibits that rival those at the Air and Space Museum in Washington. Titled "Opening the Space Frontier -- The Next Giant Step," the mural, 71 feet long and 26 feet high, shows what man has accomplished in space and what might come. The peek into the future includes spacemen floating about in the outer world, Flash-Gordon style, using their own individual power modules.

With that stunning picture to set the style, one can inspect the model of the Lunar Roving Vehicle with all its gadgets and cameras, and view Apollo 17's command module, the last moon ship, which flew in December 1972, bringing back Eugene Cernan, Ronald Evans and Harrison Schmitt.

Everything from the hard hats worn by LBJ and Mrs. Johnson to glistening fragments of moon rock are on hand. Films are shown throughout the day in an 800-seat auditorium. Side exhibits remind the visitor that as far back as 1232 -- more than two centuries before Columbus -- the Chinese were firing incendiary rockets; by 1621 they were putting them to military use. One can moon over a lunar chart that was drawn in 1647.

Perhaps most awesome of all is the collection of rockets that are set up on the fields of the NASA park, starting with Alan Sherpard's Freedom 7, which rode into the skies atop a Redstone booster in 1961, staying aloft for 15 minutes and 22 seconds. Alongside are the sections of the giant Saturn, which fired all the moon men onto that silvery planet. Like everything else at NASA, this bit of Texas is tomorrowland. For real.