The proposed rendezvous of a spacecraft with Halley's Comet when it swings around the sun in 1986 has been abandoned by the space agency because of its high cost.

Long the darling of space scientists around the world, the scheme of flying a spacecraft alongside Halley's Comet for as long as six months has been studied by the National Aeronautics and Space Administration for the last six years. The space agency has now concluded that the $500 million would take to conduct such a rendezvous is not worth the expense.

"It was very, very difficult to initiate a Halley's mission," said one Space agency source who asked not to be identified. "It demanded that to spend too much money this year and next, which we simply can't afford."

A rendezvous with Halley's Comet would have cost $30 million this year and more than $100 million next year to prepare a spacecraft to leave the earth in 1982. It would have to be launched in 1982 if it is to meet up with the comet when it circles the earth at 20,000 miles an hour in February of 1986.

The big expense in a rendezvous mission is development of an engine that can speed up a spacecraft or [WORD ILLEGIBLE] it down for weeks or even months at a time to match the changing speed of the comet.

The space agency has under development an electric engine named the [WORD ILLEGIBLE] which has never been used to propel a spacecraft through space. An ion engine involves the electrification (or ionization) of microscopic gas particles that can be blown out an exhaust like combustion products of a chemical fuel but can be burned for months without significant fuel loss.

The estimated cost of an ion engine for a Halley's rendenzvous is $200 million to $250 million, almost half the cost of the entire mission. The estimated cost of the spacecraft that would be used in a rendezvous is $300 million.

The purpose of a rendezvouz is the prolonged measurements and observations that can be taken of a comet, which is known to undergo drastic changes as it approaches the sun, swings around it and then flies away from it. The sun's heat alone would burn off tons of snow and dust that scientists believe make up most comets and their tails.

One theory of the origin of comets is that they were formed outside the solar system when they are disturbed by a nearby star one of the big outer planets like Uranus or Neptune.

Many scientiest are convinced that comets are made of the pristine matter that formed after creation and would furnish at least as many answers to the origins of the cosmos as the sun, moon and planets.

One reason scientists have wanted to rendezvous with Halley's Comet is sheer sentiment, for it was 18th Century British Astronomer Edmund Halley who predicted that the comet now bearing his name would be visible near the sun every 75 years.

In giving up plans to send a spacecraft to welcome Halley's Comet, the space agency is not giving up on the idea of someday devising a rendezvous with another comet. The comet Encke and the comet Giacobini-Zinner fly in close to the sun every four years, making them prime candidates for rendezvous missions when the space agency feels it can afford them.