The largest international exhibition of avant-garde contemporary art, Documenta 6, opened here June 24 with laser beams piercing the night sky and transatlantic television "performances" being bounced from New York to Kassel and back via satellite.

The opening's most unusual feature - a staged video event wherein an airplane crashed into a New York City garbage dump - was received here on a TV screen atop a 17th-century hillside park outside Kassel. Hundreds of art lovers and curiosity seekers from all over Europe plowed through a colossal traffic jam to watch the minute-long crashing of the plane under a giant statue of Hercules. "It is a monument to our time," said the artist, H. A. Schuldt of Germany.

Hercules remained cool, and even seemed pleased. He was, after all, rather avant-garde himself in his day. Hotel clerks in Kassel, however, were less well-tempered as the city groaned under the burden of hundreds of artists, dealers, curators, collectors and journalists who had come to see what the authoritative Documenta 6 had declared the most interesting new art of the 1970s.

On five previous occasions, since 1955, this unlikely provincial town, former home of Jerome Bonaparte and the Brothers Grimm, has been similarly transformed into the avant-garde headquarters of the world - a sort of Soho by the Fulda - when these ever-larger, quasi-quadrennial Documenta exhibitions have been staged. Originally conceived in 1955 by art professor Arnold Bode of Kassel to acquaint Europeans with 20th-century art, it was suppressed during the Hitler era. The Documenta cmmittee, made up of German scholars and curators, subsequently saw the need the desire for periodic surveys of the best art then proliferating all over the world, particularly in the trendy '60s and Documenta exhibitions have been held every four or five years ever since.

Though it was not Documenta's intention to be a market guide, it has become just that because of its scholarly and theoretically impartial base, though some artists insist it is as political and subjective as any other exhibition, depending on the taste of the curator in charge, this year the brilliant but beleaguered Manfred Schneckenburger, curator of the Cologne Kunsthalle. Be that as it may, Documenta is now the single most influential contemporary art exhibition in the world, and the closest thing the world has to a "museum of the now." For an artist, inclusion in a Documenta can mean instant fame, a fact underscored this year when close to 1,000 journalists turned up at the press office to cover the 750 artists in the show. A computer-operated moving light sign with words on the front of the Documenta's main building takes a sly look at this situation by processing nouns and adjectives being used in press reports on Documenta and flashing them to viewers here.

This vast exhibition, which occupies three museum buildings and miles of city parkland along the river Fulda, was not quite ready when the crowd of 10,000 paying visitors descended during the opening weekend. In front of Documenta headquarters at the 18th-century Fridericianum Museum, American conceptualist Walter de Maria is still hard at work drilling a kilometer-deep hole in the ground, in which he will ultimately sink a kilometer-long metal shaft. Also on the lawn in front of the Fridericianum stands Richard Serra's colossal sculpture made of four raw steel slabs which has become both billboard and magnet for an array of protestors, from disgruntled artists to a group of Iranian students staging a 14-hour hunger strike to protect the shah's regime and the savak, the Iranian secret police.

Art would seem to be considerably more political here than in America, though much of the hostility, resulting in widespread defacing of the works, may stem from the fact that this exhibition cost the federal state and city governments combined a total of 3 million marks, or $1.3 million. Private and corporate assistance will help pick up the total 6-million marks tab ($2.6 million), much of which went to commission 50 artists from as far away as Japan, Israel and America to come to Kassel and build interior "environments" and giant outdoor "site" sculptures designed for specific spaces in the park. The results are the show's most exciting and mind-expanding works and their impact will no doubt be felt for some time.

Though the site sculpture has ended up stealing the show, it was meant to be subsidiary to the basic theme of Documenta 6, which is "the media in art and art in the media," an examination of how artists are using mass media - photography, film, video and books - as a form of expression, in lieu of the traditional paint on canvas. To back up the theme, there is a seemingly endless and unedited history of photography show, evidently one of the first to be mounted in a contemporary art context on the continent. Though it may be a revelation for European audiences, not to mention a boost for the undeveloped photography market here it holds few surprises for Americans. There is also an extensive film program and video artists can be viewed easily and comfortably. If one chooses to be bored, one can do so in comfort.

In the maze of the Fridericianum, Nam June Paik, leading video artist, points up the stalled state of that art by using a dozen or so color TV sets as part of a tropical, lagoon-like environment in which they have no meaning beyond being decorative, nervously undulating light sources. Other video installations are similarly amusing, if meaningless as art, including Chiris Burden's replica of the first TV transmission. Though basically a gadget, of which there are several here (including an automatic drawing machine), Burden's piece provides a considerably more agreeable experience than Paul Sharit's simultaneous projection of two epileptic seizures.

If they pulled the plug on Documenta, the best works could still be seen, most of them outdoors. The wood and steel intersecting bridges of George Trakas, for example, comprise a rich and complex niece which requires the use of all the senses of the viewer-participant. The same is true of Robert Morris' mood-making stone environment, and the elegant concrete sculpture of Israeli Dani Karavan, who draws with light and water to evoke a cathedral-like exhilaration. Richard Fleischer's sod drawing, "floating square," is an esthetic wonder. Is it possible that any Washington institution would have the courage to commission Fleischner to transform the Mall?

There is a very large show of drawings filling the orangery, a museum show really, covering the years since 1964, and no doubt assuaging the feelings of dozens of worthy artists excluded by the theme of this show. There is also a paintings section, but it is Documenta's most complete disaster, filled with derivative paintings by unknown Europeans and second-rate examples by big-name Americans. Two incredibly bad De Koonings should never have left the studio. Even Frank Stella and Roy Lichtenstein manage to be boring. The only paintings that were fresh and new (which is, after all, what Documenta is about), are those of London-born Malcolm Morley, a sort of abstract-expressionist pop artist with a message. Four painters from East Berlin caused West Germany artists to walk out in protest. Viewer may leave for less idealistic reason, though one of the painters, surrealist Wolfgang Mattheuer, is worth a look.

There are, however, several interesting "environments" inside the Fridericianum, particularly a blue tile trompe I'oeil "room object" by German artist Peter Reuter, and other inventive works by Jochen Gerz, Nikolaus Lang and Anne and patrick Poirer. Klaus Rinke's circle of plumbobs inevitably recalls Washington's Yuri Schwebler, who does it better. In terms of what it might have been, Documenta 6's biggest disappointment is the book show. Many Americans are using book-related formats in highly expressive ways, but few of them are here. Fendrick's book shows have been far more interesting. The Neue Gallerie housing th book show is well worth a visit, however, if only to see the mini-retrospectives of Joseph Beuys, Germany's premier avant-gardist, and the late Belgian artist poet Marcel Broodthaers, who is far too little known in the U.S.

Conclusions about '70s art as revealed in Documenta 6 are as varied as the art itself, though it is clear that the emphasis is on a need to communicate the world's large problems, rather than upon mere stylistic novelty. The elitism of the '60s seems to have given way to a desire to bring art and life closer together.

One American critic has called the exhibition "a dinosaur." American dealer Virginia Zabriskie more correctly called it "mind-boggling, with enough new ideas to keep artists thinking for the next 10 years." The controversy will no doubt rage until the exhibition closes Oct. 2.