Contingency plans to set an American space shuttle down among the ancient stone statues that brood over this rocky island offer yet another instance of western civilization crashing through the barriers of time and distance.

Blithely remote from the customs and inventions of the modern age, Chile's celebrated speck of land in the South Pacific recently has been chosen as an emergency landing site for future U.S. space vehicles. Some of the 2,060 inhabitants are not sure they care to enter the space age.

Already thrust into modern times, Easter Island is also involved these days in an intensified search for a mysterious and disputed past. Clues are thought to be hidden under layers of ruins, earth and sand now being excavated. The statues, of disputed age and provenance, look on.

U.S. blueprints to extend the island's single runway and install advanced navigational equipment have won the approval of Chilean authorities who govern this 45-square-mile chunk of volcanic real estate.

The island, 2,300 miles west of Chile, is the easternmost spot in the Polynesians. The nearest inhabited neighbor is Pitcairn Island, 1,200 miles away, settled in 1790 by mutineers from the HMS Bounty.

Local residents, many of them descendants of settlers dating back at least 1,000 years, support the airport improvements, hoping to attract more jumbo jetloads of tourists. Meanwhile, Chile's military government sees a link to the space shuttle as a ticket to closer scientific cooperation with the United States.

But the project has also aroused suspicion among some islanders and Chilean mainlanders of possible U.S. military designs on the island. There are fears that it somehow will get drawn into the Reagan administration's Strategic Defense Initiative ("Star Wars") program and into a U.S.-Soviet confrontation. There are concerns as well about the damage the airport construction might do to archeological treasures here.

While American and Chilean authorities push ahead with the U.S.-financed runway extension -- a Chilean contractor is to start construction this month or next -- the island's historical importance has been highlighted anew by the recent return of Thor Heyerdahl, the Norwegian explorer who led an expedition here 31 years ago.

Heyerdahl's conclusion -- that Easter Island initially was occupied by South American Indians, not by Polynesian mariners as conventional wisdom held -- was scorned by many scientists. Despite Heyerdahl's famed 1947 Kon Tiki voyage, senior archeologists still doubted that ancient South Americans crossed the Pacific on balsa rafts before the Polynesians.

Heyerdahl came back to the island this month to inaugurate a joint Chilean-Norwegian project to excavate at one of the island's few white-sand beaches in hopes of solving the puzzle of the first settlers.

As a special attraction, the 71-year-old anthropologist, who has a flair for publicity, is also planning to demonstrate how the island's colossal stone statues could have been transported by people who had no knowledge of the wheel. Legend tells of the elongated, beetle-browed figures -- some several dozen feet tall and weighing as much as 80 tons -- having "walked" to stone terraces, or altars, called ahus, suggesting that they were moved upright over the island's hilly surface.

Heyerdahl left the island in 1956 still perplexed about how the gigantic carved structures could have been maneuvered. A 28-year-old Czechoslovak mechanical engineer, Pavel Pavel, inspired by the Norwegian adventurer's account of Easter Island in the book "Aku-Aku," wrote Heyerdahl several years ago after hitting upon a possible answer to the riddle. Heyerdahl invited him along to test the method, which involves several sets of ropes lashed around a statue's top and base.

Much has changed since Heyerdahl's first visit. Some here still remember the days when clothes were not worn and money was not used, when people ate only what they could grow or fish, when babies were born at home and horses were the fastest means of island travel, and when the only contact with the outside world was an occasional passing ship.

Today, jeeps, vans and motorbikes bounce around the island. Tourists arrive in wide-bodied jets. Imports of canned and cured food are served. A hospital exists, as does television, and 100 telephones connect to the outside world.

Still, Easter Island has not lost its rusticity. No road is paved -- not even the main street in Hanga Roa, a hilly collection of small, shaded houses, government offices and general stores overlooking the ocean and constituting the island's sole village.

Only one scheduled flight per week travels here from the mainland. Rapa Nui, a Polynesian language native to the island, is taught in school along with Spanish.

Foreign development is discouraged by a law restricting land ownership to natives -- or to the Chilean government, which holds title to 90 percent of the island.

Apart from working for the government or in the tourist trade, there is little work for islanders. The government employs just over half the eligible work force of 700. Of the 2,060 who live here, about 1,200 are considered natives.

With one church and two discotheques, life can get boring. Alcohol has become a problem among the youth. But islanders who move away have tended to return.

It was against the backdrop of New World forces vying with old island ways that news of the airport extension arrived here in the middle of last year. Some islanders got the impression that U.S. and Chilean officials had already agreed on the plan, and this was resented. The Council of Rapa Nui chiefs, said to represent 36 of the 42 native family clans, grumbled about potential environmental damage and a loss of sovereignty to the United States.

Sergio Rapu, 36, the governor, has strongly supported the project. He is the first native in recent times to be appointed governor. Educated in Nevada and Hawaii, he favors gradual development here and views local opposition to the U.S. plan in generational terms.

Politics on the island, he said in an interview, is a constant debate between the old and a more educated, younger group. Rapu said his aim is to prevent a rupture. "If we split and the older group declines to participate in the development of the island, we're bound to disappear as an ethnic group," he said.

Plans call for the Mataveri airport runway here to be lengthened 1,400 feet to a total of 11,000 feet. Sophisticated landing aids are to be installed and a small storage building constructed. The National Aeronautics and Space Administration says the improvements are necessary in the event a space shuttle, launched southward into polar orbit from Vandenberg Air Force Base in California, has a faulty liftoff. Before the explosion of the Challenger craft last month, the first launch from Vandenberg was set for this spring. The California-based flights are to have largely military purposes.

"This is not simply an emergency landing strip for the shuttle," said Radomiro Tomic, a former ambassador to the United States and 1970 Christian Democratic presidential candidate who led opposition to the project in Chile. "The real purpose is to enlarge U.S. strategic possibilities in the event of war with the Soviet Union."

U.S. and Chilean officials deny there is anything more to the agreement than publicly stated. As part of the accord, the United States has promised to look at ways Chile can become more involved in the American space program.

Before the Challenger disaster, NASA spokesman Joel Cassman had assured Chilean officials that the chances of a shuttle ever needing to land on Easter Island were practically zero. On the day Challenger exploded after liftoff, killing all seven astronauts on board, the stunned attache said: "Now . . . there'll be some doubt about that."