When the Voyager II spacecraft flew by Saturn tonight it was the last time an American spacecraft will encounter another planet until 1986, when the same spacecraft is supposed to reach Uranus, and 1989, when it is to arrive at Neptune.

The United States is readying only one new mission to the planets, the Galileo spacecraft, which is scheduled to orbit Jupiter in 1987 but has which been so beset by delays and cost overruns that nobody in the space business believes it will get there on time.

The only spacecraft American scientists are counting on for the foreseeable future is Voyager II, which left the Earth four years ago last week.

Galileo is tied to a 1985 launch schedule from the space shuttle using a hydrogen-fueled engine called Centaur, which must undergo major modifications to fit in the shuttle's 65-foot-long cargo bay. If it cannot be modified in time, Galileo must be launched from Earth on a Delta-Vega rocket that may not get it to Jupiter until late 1989.

"If we don't get a new exploratory start in the next budget cycle, the outcome will be the end of a glorious era in U.S. history," said Dr. Bruce C. Murray, director of the Jet Propulsion Laboratory, where Voyager was designed, built and directed.

"There are no Darwinian imperatives that say we have to keep exploring, but if we stop doing it, that tells us something about ourselves," he said.

In the last 18 years the United States has flown unmanned spacecraft to other planets 23 times. Five flights have been to Venus, eight to Mars, three to Mercury, four to Jupiter and three to Saturn. Only American spacecraft have flown to Jupiter and Saturn, which are too far for spacecraft from the Soviet Union to reach.

"What's at stake here is a history that goes back 20 years to Explorer, the first American satellite," Murray said in an interview. "It's our involvement in exploration that goes back to all the Rangers, Surveyors, Mariners, Vikings and Voyagers that we've flown and that nobody flies better than we do."

In addition to the Galileo, the United States has two unexercised exploratory options before it.

One is a plan to orbit Venus with a huge radar dish in 1988 to peer through its 16-mile-deep clouds onto the planet's searing surface. First proposed over four years ago, this was turned down for three years by President Carter, who reversed himself one week before the 1980 election.

"I suppose that meant if he had won, it would have happened," Murray said, "but frankly, that's a sad way to run a railroad."

A second unexercised option is a mission to Halley's comet, which swings by the sun in February, 1986, for the first time in 75 years.

The Jet Propulsion Laboratory is studying a plan to design a spacecraft that will photograph the comet from up close and then fly through its tail to collect a sample of the material in it and return it to Earth.

It was only this summer, Murray said, that JPL scientists realized that they could fly a spacecraft through the comet's tail on a trajectory that would return the spacecraft to Earth in less than five years. Murray said it is also possible to use secret Air Force spacecraft to return the sample of the comet unscathed through the Earth's atmosphere to the United States.

"If you can't come back, there is not much point in going after a sample return," Murray said. "We've also discovered that the secret technology involved is mature enough to make it economically feasible to do."

Murray put the cost of a photographic and sample return mission to Halley's Comet at $280 million, just $30 million more than a simple photographic mission. NASA administrator James M. Beggs has said recently that he favored a mission to Halley that would return a sample, but only if its cost is not excessive.

"What we would do is drop some instruments off the spacecraft that will in any case be carried by the Europeans on their mission to Halley," Murray said. "The Europeans could make those measurements and we would save ourselves $50 million in the process."

Murray said that if a decision is not made this year the United States can forget the comet option.

"The spacecraft has to be launched in 1985, which in the time measured to design and build it is right around the corner," Murray said. "We've living on borrowed time."