WHEN SIR Ranulph Twistleton-Wyckenham Fiennes, the Third Earl of Banbury, chugged into Greenwich Sunday at the end of history's first recorded bipolar jaunt around the planet, who among us was not moved? Who among the people who still think of mother when they think of England could fail to be cheered by the return of this hardy Briton, so reminiscent in profile of the late Commander Whitehead and so obviously well-endowed with Schweppervescence? Who could quarrel with Prince Charles' assessment of the venture as "gloriously, refreshingly mad . . . mad but marvelous"?

By an inspiring fluke of timing, Sir Ranulph came into port amidst a sudden flowering of other heroic journeys--Bill Dunlop traversing the Atlantic in a 9-foot sailboat, Lon Haldeman bicycling across the U.S. in less than 10 days, and 65-year-old Ashby Harper and 54-year-old Ionthe Rhodes swimming the English Channel and Chesapeake Bay, respectively.

Such coincidences always seem pregnant with meaning, so it was hard not to interpret this one as a resounding proclamation that the spirit of adventure was alive and well. But the spirit of adventure is scarcely alive and not at all well. It has been on its deathbed for some time now, and a close scrutiny of the most recent achievements only confirms the diagnosis.

Take Sir Ranulph's Transglobe Expedition. To the casual scanner of headlines, it sounded splendid and simple. Sir Ranulph (whose surname is pronounced "fines," as in traffic) set himself an unprecedented task and, dash it all, went out and did it. For three years, through the Iranian hostage crisis and the 1980 elections, while wars raged in the Atlantic and in the Middle East, while the rest of humanity went about its workaday business, Fiennes, his companion Charles Burton (clearly a selfless man, for what other kind would enter such a wisp of a name into partnership with 10-syllabled nobility?), the baronet's wife, Virginia, and their comrades pursued a glorious contest against nature, circling the globe vertically, by land, by sea or wherever the Greenwich Meridian, that divinely inscribed line of bifurcation, might take them.

But in the fine print of the various accounts, certain curious details emerged. To wit:

* While the expedition began and ended in Greenwich, it frequently detoured thousands of miles from the meridian of that name to camp in such rugged outposts as Paris and Hollywood,where the travelers rested and set up shop as salespeople for the latest in British-made exploring gadgets. (At the end of the $17.5-million expedition, Prince Charles proudly announced that $5 million worth of orders had been received -- a ratio of income to outflow that may, perhaps, explain something about the recent state of the British economy.)

* When Sir Ranulph and Co. were not piloting their Land Rovers across the Sahara, or their snowmobiles across the Antarctic and Arctic icecaps, or their motorized rafts up the Yukon amd MacKenzie Rivers, they could be found aboard their 12,050-ton support ship, the Benjamin Bowring, accompanied by a crew of 23.

* During Fiennes and Burton's lonely journey across the Arctic, the expedition's Twin Otter supply plane made food and fuel drops every five or six days and provided no fewer than half a dozen replacement snowmobiles.

So, if you set out to define the ground rules of this mission, you would end up with more caveats than there are in all the collected speeches of Alexander Haig. But it would be wrong to hold this against Sir Ranulph and his teammates, who clearly represent the species at its bit-chomping best. The brutal truth is that Sir Ranulph, like the rest of us, is up against one of the immutable, irreversible and decidedly gloomy facts of 20th-century existence. The frontier, to all intents and purposes, is closed. Civilization is consuming the last isolated tribes and uncharted pieces of terra firma as surely as a Rolaids tablet consumes 47 times its weight in stomach acid.

There is no longer any mountain so high, any ocean so stormy, any jungle so thick, that modern mechanized mankind cannot stomp over it at will. The North Pole, which obsessed a whole generation of adventurers a few decades back, has become a major hub of the commercial air network. Tours to Antarctica are advertised in the New Yorker. Nature, in short, has said "Uncle," and it is only by elaborate contrivance and calculated sacrifice -- taking A, B and C items of modern technology along while leaving X, Y and Z items behind -- that humanity keeps the contest going.

The urge to be firstest with the mostest (or firstest with the weirdest, in Evel Knievel's case), has turned adventure into something murky, kinky and wildly complicated -- not to mention colossally expensive. Consider the world of the long-distance bicyclist. Four of them made the transcontinental crossing last month, and pace-setter Lon Haldeman finished in an amazing 9 days, 20 hours and 2 minutes. But if you are conjuring up idyllic visions of the lone athlete out on the open road, conjure again. Trailing close on this foursome's heels was a small panzer division of recreational vehicles bearing doctors, nutritionists, masseurs, mechanics, spare parts, food, stereo headphones, and even a supply of Rubik's Cubes and other portable gimcracks in case the competitors got bored.

Or consider the 27-year-old Frenchman who ran 2,050 miles north to south across the Sahara -- accompanied by three vehicles, a first-aid specialist, a mechanic, a masseur and a film crew. Or consider bruised and bitten Stella Taylor, who abandoned her 1978 swim from the Bahamas to Florida to widespread applause for her valor -- and general indifference to the fact that she had been hauled into a boat on three separate occasions before she threw in the towel.

Swimming may be the most polluted of all endurance sports. "The last really authentic swimmer was probably Florence Chadwick," says William Graves, adventure editor at National Geographic. "She just put a lot of bear grease on and swam the channel." Chadwick once explained that "life in the water is less complicated," but she could hardly have been anticipating Diana Nyad's 1978 attempt to swim from Cuba to Florida inside a $42,000 motorized shark cage attended by a crew of 17. (In subsequent swims, Nyad traded her cage for a hit squad of armed shark-killers on her support boat, and having adopted this simpler program, she complained that another open-ocean swimmer, retired cookie-baker Walter Poenisch, "makes most of his swims on a boat." (Poenisch promptly sued.)

Clearly, self-sufficiency is no longer the rule in the adventure business. And nothing else is quite what it appears, either. We are living in the age of the asterisk, when the grandest achievements logged in the Guinness Book of World Records are subject to the qualification that "five-minute rest intervals . . . are permitted after each completed hour in marathon events."

Like Sir Ranulph, Diana Nyad is a formidable athlete and competitor, whose successful Bahamas-to-Florida swim in 1979 was justly celebrated on front pages the world over. The fact that people of this high caliber have been driven to concoct such bizarre challenges only underlines the desperation of their predicament -- a predicament that snags all of us, one way or another. We sense that our well-being as individuals and our survival as a species require us to stretch ourselves, to press on into the unknown, and we simply don't know how to go about it. No matter how we address the dilemna -- whether on the grand scale of Sir Ranulph or the modest scale of the weekend camper who drives hundreds of miles by car in order to hike a few miles by foot--contradictions and absurdities abound.

Even mountain-climbing and transoceanic sailing, two of the last reasonably pure outlets of adventure, are in a bad way. Last winter an American team decided to ski-trek around Mt. Everest, because, as Ned Gillette, the team leader, explained, "In the mountains today, style is everything. You can't climb unclimbed peaks anymore, and you can't explore unexplored country." But westerners are forbidden to cross from Nepal into Tibet and back, so Gillette's group had to break its journey in halves, spending eight weeks in Nepal during December and January, and six weeks in Tibet during April and May. Adventure on the installment plan.

The organizers of the 1983 American-Tibetan Everest expedition hope to take a small TV camera with them and to offer live coverage of the final assault. If the plan comes off, it should provide some memorable home entertainment, but Mt. Everest -- and mountain-climbing in general -- will never quite be the same again.

The long-distance sailors, meanwhile, are vying to see who can come up with the smallest contraption capable of carrying a human across the Atlantic. Among the west-to-east group, America's Bill Dunlop just undercut England's Tom McLean by a margin of 8 inches (9'1" against to 9'10"). In the other direction, the champion is America's Hugo Vihlen, who managed the trip, incredibly, in a 6-foot vessel (the April Fool). Short of a bathtub or an ice-bucket, it is hard to see where this contest can go next.

In any case, the game has been turned topsy-turvy. Where our forbears applied their wits to the challenge of subduing nature, we apply ours to the challenge of giving nature a sporting chance. Where they used state-of-the-art equipment, we go rummaging in technology's attic for obsolete playthings like sailboats, balloons, bicycles and, most obsolete of all, the unaided human foot.

It used to be an article of national faith that every American was born with the chance to be President, should his aspirations lean that way -- and the possibility was reputed to help build character and spur initiative. Be that as it may, it was surely true that every American was born with the chance to pull up his roots and throw himself into the wild. And other nationalities, to varying degrees, had their own frontier options. For the ordinary, frustrated, alienated, stifled citizen, this was a shot in the arm, whether he exercised the option or not. The fact that some people did, that their exploits could be read about in newspapers and novels, was part of what made us who we were.

Today, the frontier saga strains credibility in life or fiction. The desert-island adventure is no longer convincing because all the islands have been discovered, and, if worth inhabiting, are inhabited. (Only by setting these stories back in time can we make them believable. And similarly, the most appealing of the recent real-life adventures have been historical recreations such as Tim Severin's leather-boat journey from Ireland to Newfoundland, to test the theory that Irish monks discovered America in the 6th century A.D. But here, too, legitimate ideas are in short supply, and the shortage has produced a profusion of wacky proposals to recreate voyages that never occurred, or could have occurred, in the first place.)

Why have things come to this sorry pass? The obvious culprit is the planet itself, which began looking small and inadequate around the turn of the century. The poles took up the slack for a while, and then, abruptly and cruelly, the frontier gave out. Frederick Jackson Turner and others predicted that America would be a poorer and duller place as a result, and they could hardly have been more on target.

But before you blame inanimate fate, recall that a few short years ago, history offered up a new frontier more spectacular than all the rest. A generation was reared on the idea that it would get to witness, or even take part in, the exploration of outer space. Then something -- we still don't know just what -- happened. The economy turned, our attitude toward technology shifted, the big powers built missiles instead of rockets, and a magnificent dream gave way to the feeble reality of the space program in the 1980s: the occasional, tiny steplet for man, the long, slow crawl for mankind.

Whatever happened, it was a painful blow and we have repressed it -- repressed the memory of the excitement we felt then, repressed the disappointment and resentment we ought to feel now. Things haven't turned out the way we figured, so we blame our own youthful naivete for figuring wrong and we go about our lives. And how do we express the pool of unfulfilled yearning that lurks beneath the sober surface? In very small ways indeed -- we cart our sedentary carcasses out onto the basketball court, we hop a taxi to the health club, or we switch on the TV set for one of those oddball adventure-substitutes that crowd the Sunday schedule.

So if Sir Ranulph's way looks eccentric, at least it is dramatic. We are all stuck in the same muck, and most of us can manage only a small yelp. He has let loose with a great howl that speaks for everybody.

And the adventure goes on. A few hundred miles west of Los Angeles, Peter Bird was stroking fiercely yesterday in his effort to become the first solo rower across the Pacific. Bird radioed that one of the water tanks aboard his 35-foot vessel was leaking, but the other three tanks were okay, and so was his seawater distilling still. "I'm delighted to be making this speed," he said. "The weather is fine, and I'm right on track." CAPTION: Pictures 1 through 4, James W. Whittaker, the first American to conquer Mt. Everes; boat used in Fiennes' expedition; Lon Haldeman after record-setting bike trip; Bill Dunlop arrives in 9-foot craft. Copyright (c) National Geographic Society; Picture 5, Sir Ranulph Fiennes, and Charles Burton talk with Prince Charles after their arrival Sunday, by AP