A little more than three months from now, on the fourth Thursday in July, a mile stone will be reached in the age of space flight that began when Yuri Gagarin flew his tiny Vostok into orbit 16 years ago.

Sitting in the pilot's seat of the massive spaceship Enterprise, astronaut Fred W. Haise will reach down and press a button with his left hand while moving the stick in front of him back gently with his right. Alongside each other in the cockpit, Haise and copilot Gordon FUllerton will feel a sudden lurch, then a lift as the 75-ton Enterprise soars free of the Boeing 747 that has carried it 24,000 feet above California's Mojava Desert.

Five seconds after the spaceship's delta wings lift the Enterprise up and away from the 747, Haise will move the stick toward his copilot and roll the big craft in a 20-degree turn to the right.

At precisely the same time, the pilots of the 747 just beneath them will bank the jumbo jet in a 40-degree turn to the left to put as much room between them and the astronauts as they can.

Haise and Fullerton will now be on their way down toward earth at the controls of the black-and-white Enterprise for the first time.

They've both trained hard for this mission: Haise has flown more than 1,000 practice approaches in aircraft and no fewer than 2,000 in the $20 million simulator at Houston; Fullerton's practice runs are only a handful behind Haise's.

But that's not surprising, because "at the controls" of the Enterprise means dead-stick flight and landing all the way from orbit hundreds of miles above earth, since the spaceship will offer the two crewmen no throttle help while in the atmosphere. It will be a little like asking two sailors to take the power boat they're handling for the first time back to its mooring by sail.

The Enterprise has five permanent aft-mounted engines, just beneath the tail. But these five engines are used only at liftoff from earth and for maneuvering in space. The tanks that hold the fuel for these engines will not even be aboard the Enterprise when Maise and Fullerton take it aloft for its approach-and-landing test in July.

The National Aeronautics and Space Administration has long known what happens to men and their vehicles in space. What it doesn't know is how a huge spacecraft with a tail and a pair of wings acts when it leaves space to return to.

"It's going to be real good to have a pair of rudders to kick and a stick to move around again," Maise said in an interview last week at the Johnson Space Center outside Houston, where he has spent most of his 11 astronaut years training to fly spacecraft that have no tail or wings. "But this airplane isn't like any other I've ever flown. I tell you, we're going to fly a pretty impressive landing pattern when we go down in July."

Once clear of the 747 carrier plane, Maise will push down on the stick, nosing the big bird toward the hard desert floors that surround the Enterprise test center at Edwards Air Force Base. Clear of the carrier, Maise and Fullerton will be moving at 290 miles an hour. By the time Maise noses it down, the Enterprise will be speeding without engine throttle control at more than 310 miles an hour.

At that speed, Haise and Fullerton have less than six minutes to bring the $500 million Enterprise safely back to earth. But on what they do that six minutes lies much of the future of the $1 billion space shuttle program, in which the Enterprise is pust the first of five orbiting spaceships to be built and used as backbones of the U.S. space program for the next 20 years.

When they're down to about 18,000 feet, Haise and Fullerton will practice their landing maneuver for the first time. This involves "flaring" the Enterprise upward from its descent, so the nose suddenly points skyward and the flat-iron-shaped underbelly of the craft can deflect the wind coming by to give the huge spaceship more lift on its way down.

This maneuver serves to slow the Enterprise down to 200 miles an hour, giving the crew time to lower the nose and raise the airspeed again to bank lft toward the seven-mile-long runway which lies fewer than 10 miles away.

In the last leg of the banking turn toward Edwards, Haise and Fullerton line the Enterprise up with the runway and for the first time begin to use the unique Enterprise controls to maneuver the spaceship to earth.

Haise will deploy the shuttle speedbrake, which really means spreading the aft panels of the 30-foot-high tail rudder to the sides to slow the ship down. He will also flare the "elevons" at the aft parts of the wings, structures so big that when they're moved out to slow is speed they completely shift the center of gravity, lowering the nose and speeding the craft up for split second.

Finally, haise will employ the spaceship's body flap, a flat-boarded metal tab that was put on the Enterprise originally to protect the aftmounted engine bells from the hot gaes pouring off the wings during the speed of re-entry from space. Enginers figured the flap could also be used to slow the craft down at landing, which Haise plans to test on the first flight in July.

Through all this, Haise and Fullerton will be guided to the runway by some of the most remarkable computers built for space flight. The Enterprise will carry four of these computers, which are told the positions of every movable surface on the wings and tail of the spaceship 500 times a second. If they're not right, the computer tells them to move to the positions that are right.

All four computers inform each other 500 times a second too, so that if one fails or a sensor feeding one of the computers fails the other three computers can eliminate the failed computer from the flight.

But all four computers are built to last for 100 space flights, which suggests they will all be working when Haise and Fullertom flare the Enterprise upward at the end of its final runway approach in July. First, the main landing gear will touch down and then the nose gear, all at 200 miles an hour. The first rocketship ever to fly like an airplane will have made its maiden landing on earth.