What promises to be the most ambitious and challenging 10 years in the short history of the space age are to begin in 1985.

The National Aeronautics and Space Administration plans 13 space shuttle flights next year, 15 in 1986, 20 in 1987, 23 in 1988, 24 in 1989 and an average of 24 each year after that until 1995.

Even better, from NASA's point of view, is that the flight "mix" for the next 10 years is evolving just the way the agency hoped. One-third of the missions are to be science flights financed by the United States, Japan and the European Space Agency; one-third are commercial flights paid for by worldwide business customers, and one-third secret flights paid for by the Defense Department and the North Atlantic Treaty Organization.

"By the end of this decade, we fully expect our revenues will pay for what it will cost to operate the space shuttle," Jesse W. Moore, NASA's associate administrator for space flight, said in an interview recently. "By 1990, we expect the money we pay for a shuttle launch will be the same as the money we collect from the cargo we carry on a launch."

If one year in the next 10 best symbolizes the promise of the next decade, it is 1986.

In that year, the space probe Voyager 2, launched in August 1977, is expected to reach faraway Uranus, so mysterious that scientists do not even know the length of its day. "It's somewhere between 16 and 24 hours," Voyager 2 chief scientist Edward C. Stone said.

In the first close encounter with the planet seventh in distance from the sun, Voyager 2 is to pass Uranus just beyond its outermost rings, a scant 29,000 kilometers from Miranda, the innermost of the planet's five known moons.

So dark are these moons -- Miranda, Ariel, Umbriel, Titania and Oberon -- that some scientists suspect that they are composed of the most exotic ices in the solar system.

Also in 1986, the German-American spacecraft Ulysses is to leave Earth on a three-year voyage by Jupiter that, in another three years, would make it the first man-made machine to move around the north and south poles of the sun, where magnetic activity complicates study of the sun.

In 1986, a Galileo spacecraft is to depart Earth to orbit Jupiter and its four largest moons, and the space shuttle is to deploy in Earth orbit the $1.2 billion Space Telescope, most expensive instrument carried into space and one that can peer 10 times farther into the heavens than any telescope on Earth.

"If Olympus Mons [the largest volcano in the solar system at 15 miles high] were to erupt on Mars, we'll see it with the Space Telescope," said Dr. James Westphal of the California Institute of Technology, referring to the largest volcano in the solar system.

The expected highlight of the busy year is the trip around the sun for the first time since 1910 of Halley's Comet, which appears only once every 76 years and can truly be called a once-in-a-lifetime experience.

Expected to be in space to greet the comet are two Soviet spacecraft named Vega, a European spacecraft named Giotto and two Japanese spacecraft trailing the comet to photograph its 100 million-mile tail. In addition, a space shuttle mission, called Astro One, is scheduled to observe the comet with three ultraviolet telescopes and two wide-angle telescopes built specially for the reappearance of the most famous comet in the cosmos.

"There's no question 1986 will be a banner year for space science," NASA's Moore said. "There will even come a time in 1986 when we have two birds [space shuttles] on two launch pads at the same time, as we get ready to launch the Ulysses and Galileo missions one week apart."

The 1985 shuttle manifest shows that at least one shuttle is to be launched each month as a second shuttle launch pad is opened at Kennedy Space Center, the new shuttle Atlantis joins Columbia, Challenger and Discovery and the spaceport at California's Vandenberg Air Force Base becomes operational.

The oft-postponed first shuttle flight involving a secret Defense Department payload is scheduled Jan. 21. Two such secret missions are planned in September and October, with the latter inaugurating shuttle service from Vandenberg.

In February, NASA plans to orbit its second $100 million Tracking Data and Relay Satellite and, in March, hopes to retrieve and return to Earth the experimental Long Duration Exposure Facility satellite orbited by a shuttle crew earlier this year.

Subsequent plans call for a second shuttle flight of the European-built Spacelab in April, a third in June or July and a fourth in October financed by West German science and industry. On this last flight, two German scientists and a Dutch scientist are to join a crew of five Americans in becoming the largest space crew orbited simultaneously.

Even as European-American cooperation in space ventures increases, Europeans are expected to gain a greater share of the worldwide commercial market.

The European Space Agency currently attracts slightly more than 10 percent of commercial satellite business, using the French-built Ariane launch vehicle. Subsidized by ESA, Ariane loses money, but officials expect its market share to increase.

"It's good, healthy competition," Moore said, "but I expect they'll never have more than 25 percent of the world market . . . ."

Deep-space exploration is to resume later this decade. In April 1988, a shuttle is to orbit the Venus Radar Mapper whose radar dish is three times the size of the spacecraft.

Three months later, the mapper is expected to begin peering through sulfuric-acid clouds that obscure Venus from Earth. The mapper is expected to produce a global map of almost all of Venus' surface.

On Aug. 24, 1988, Voyager 2 is to encounter Neptune, the eighth planet from the sun and last to be explored in this century. The spacecraft is to make the closest planetary flyby attempted in an effort to solve longstanding mysteries about the planet, including a puzzling internal heat source that radiates twice as much heat as it receives from the sun.

Sometime between 1987 and 1988, NASA plans to raise its charge to carry commercial cargo in the shuttle from the current $71 million for a full cargo bay to perhaps $100 million. "We want to start earning revenue from the shuttle by the end of the decade," Moore said.

In the late 1980s, NASA also hopes to start preparations for erecting the nation's first permanent space station, due to be occupied in 1992.

In January 1987, a shuttle crew is to attempt refueling an orbiting Landsat satellite that for three years has not been able to position itself to aim cameras at Earth. In subsequent missions, space-walking astronauts are to practice erecting large metal structures as forerunners of the permanent space station.

NASA officials said they believe that the space station will open space to commercialization. Johnson & Johnson and McDonnell-Douglas Corp. are continuing a program on the shuttle to produce hormones that cannot be produced in quantity on Earth.

On the last shuttle flight, 3M Co. tested a process to produce organic crystals, apparently so successfully that other companies are considering chemical manufacture in space.

NASA has signed an agreement with Deere & Co. to test-manufacture a wide range of metals in space and is understood to be talking with General Motors Corp. about a similar agreement.

"I think we're on the edge of a boom in space commercialization," Moore said. "I think it will happen as soon as private industry realizes we are serious about the space station."

Critics of the station program include Congress' Office of Technical Assessment, which says that, before NASA commits $8 billion to the station, the agency should get private industry and foreign countries involved to share the cost.

Some space scientists have said they believe that funding the station will adversely affect such major scientific projects as expeditions to Mars to acquire study samples.

"The space station is too modest an endeavor for a nation that aspires to greatness," said Dr. Gerald K. Wasserburg of the California Institute of Technology.

The Soviets have said they plan to orbit Mars in 1988 and perhaps land a small probe on Phobos, larger of its two moons. NASA officials have expressed little apprehension that the Soviet space program will catch that of the United States.

"The Russians are 10 years behind us in space technology and exploration and will stay 10 years behind us for some time to come," NASA Administrator James M. Beggs said.