The National Aeronautics and Space Administration has killed plans for a much-anticipated rendezvous between an unmanned spacecraft and a comet, citing budgetary constraints.

NASA administrator James M. Beggs confirmed this week that the mission to the comet, known as Wild 2, had been eliminated from the fiscal 1987 budget, which is being drafted by the Reagan administration. "However," Beggs said, "the comet rendezvous proposal is a very promising candidate for a new start" in fiscal 1988.

The mission was expected to cost $400 million from fiscal 1987 to 1992, within the budgetary limits NASA established in 1981 for new planetary missions.

Delaying a decision on a comet rendezvous by even one year means the spacecraft will not be able to fly to Wild 2, the favorite candidate of space scientists because it is a bright, fresh comet.

The comet was discovered in 1978, and will be making its third and fourth appearances in 1990 and 1996. In order to rendezvous with the comet when it passes near the Earth, NASA would have to begin work on the mission next year to have the spacecraft ready to launch from a space shuttle in 1991.

Space scientists reacted with dismay to the NASA decision, particularly in the wake of decisions the agency made in 1980 and 1981 not to attempt a mission to fly by Halley's Comet. This comet, the biggest and brightest of those that pass the Earth at regular intervals, will swing around the sun early next year.

"Once again, the United States is passing up an opportunity to initiate the exploration of the comets, which contain the most unblemished record of the origins of our solar system," said Dr. Laurel L. Wilkening, vice president and dean of the graduate school of the University of Arizona. Wilkening, who is vice chairman of the President's Commission on Space and chairman of NASA's Comet Rendezvous Science Working Group, added, "Unfortunately, this only continues a tradition begun in the 1970s in which NASA repeatedly passed by opportunities to send spacecraft to comets, including Comet Halley . . . . "

Space scientists were also upset because NASA's science advisory groups had recommended that a comet rendezvous be the next new mission in planetary exploration.

"The decision is a major setback for the U.S. planetary exploration program," said a joint statement by Wilkening and Dr. David Morrison, a former NASA astronomer who now teaches at the University of Arizona.

Scientists had pushed for a mission to Wild 2 because the spacecraft would have been able to travel with the comet for 850 days, observing it before its long dust tail formed and obscured the comet's nucleus or damaged the spacecraft. The mission would have been the most ambitious that NASA had devised to explore the solar system.

The next comet that NASA could decide to approach would be Tempel 2, which will pass by the Earth in 1998. NASA would have to decide by the middle of next year whether to pursue that mission, which would also cost $400 million.