Though NASA hasn't flown as many space shuttle missions recently as it would have liked, business is booming enough that the Johnson Space Center wants to buy a fourth training aircraft to let pilots practice the techniques of landing the shuttle before their missions begin.

At least 24 pilots are now in training for 12 shuttle flights scheduled through June 1986.

Another 12 pilots will soon start training for six more flights scheduled through September 1986.

The shuttle training planes are manufactured by Grumman Corp. at a cost of $14 million each.

Grumman takes a used Gulfstream II executive jet, modifies its structural wiring and cockpit and installs engine controls designed to simulate the shuttle's behavior as closely as possible.

Shuttle pilots make at least 30 flights in the trainers, and 800 landing approaches.

But the astronauts say all that training is worth it. The worst mishap that has occurred in 15 shuttle landings has been a burned-out brake assembly.

A more serious accident -- for instance, the shuttle's front wheel collapsing on landing -- could end up costing $100 million to replace the electronic equipment that is housed in the shuttle's nose. FLIGHTS FOR ALL? . . .

The record number of 90 astronauts now training at the Johnson Space Center will soon grow when 12 new astronaut candidates from the bumper crop of 1984 applicants join those already trying to get a seat on a future space shuttle flight.

Is there room for all of them? Air Force Lt. Gen. James A. Abrahamson thinks so.

Abrahamson served as NASA's shuttle chief before he was appointed director of President Reagan's Strategic Defense Initiative. He predicts that 5,000 astronauts will have ventured into space by the year 2004.

"That includes Americans, Russians, Europeans and Japanese," he said, suggesting those other countries will be flying their own versions of the space shuttle by the turn of the century.

Abrahamson says the Pentagon's growing need for orbital surveillance and reconnaissance and the establishment of a civilian space station will probably provide most of the impetus. Says Abrahamson: "There will be a dramatic expansion in the commercialization of space," requiring a whole new corps of astronauts. HALLEY'S UPDATE . . .

Halley's comet has just been spotted about 400 million miles from Earth, inside Jupiter's orbit and appearing to linger against the edge of the constellation Orion.

"It's about to cross into the constellation Taurus and then will head back into Orion again before moving into Taurus again some time in October," said Donald K. Yeomans of the Jet Propulsion Laboratory, one of NASA's representatives to the worldwide International Halley Watch. "It's moving now in an S-shaped loop against the sky, mostly because of the Earth's own motion around the sun, not because it's doing a real loop out there in space."

When the comet swings around the sun in February 1986, it will be its 30th recorded visit to Earth since 240 B.C. Up to now, most scholars have counted only 28 recorded visits. But a group of British historians will soon report in Nature magazine that they discovered a set of Babylonian clay tablets in the British Museum, documenting a visit by the comet in 164 B.C. that historians previously had believed had not been recorded.