SCIENCE FICTION writers tend to have long careers. Jack Williamson, the dean of sf, has been active in the field since 1928; at least five other science fiction writers started their careers before World War II and are still flourishing. While it is a relatively rare feat for non-sf writers to sell their work for 50 years, nearly a dozen sf writers have done this.

Science fiction writers have long careers for two reasons. First, many writers begin to sell at an early age; Samuel Delany, John Brunner and Isaac Asimov all were published before their 20th birthdays.

Second, a science fiction writer's collected works substantially increase in value if the writer stays alive and keeps writing. Indeed, the demand for new works by authors with proven selling power is so great that sf publishers have created a variety of tricks to ensure that the bestselling author's byline continues to appear before the public. These tricks include series written by other hands based on an outline produced by a "big name" writer (a practice known as "sharecropping"), collaborations between a well-known writer and an unknown partner, and having other writers produce authorized sequels to classic works. Most of these forms of literary partnership are decades or even centuries old; the elder Alexandre Dumas is said to have farmed out so many works to so many lesser hands that he routinely forgot which of "his" novels were about to be published. But it is only in the late 1980s that sf editors, increasingly worried about the profitability of their lines, have allowed collaborations and pastiches to dominate the field.

A typical example of collaborative sf is Marion Zimmer Bradley, Julian May and Andre Norton's Black Trillium (Doubleday, $19.95). The authors are among the most experienced in the field; Bradley has been a novelist since 1961, May's first fiction appeared in 1952, and Norton has been active since 1934. Unlike much Doubleday sf, Black Trillium is handsomely produced, with a superb book design by Guenet Abraham, reinforced by Jamie S. Warren Youll's jacket design and Mark Harrison's jacket illustration. But despite the seasoned experience of the authors, Black Trillium is neither inventive nor imaginative.

The British sf critic Nick Lowe contends that most modern fantasy novels are what he describes as "plot coupon" fiction; in these novels, characters roam across an imaginary world collecting plot coupons -- talismans, spells, fragments of swords, what have you -- which are then cashed in at the end of the book for the "prize" -- ultimate power, the triumph of good over evil and so on.

Black Trillium only differs from the typical plot-coupon romance in that three characters are collecting coupons instead of one. After the sunny country of Ruwenda is taken over by the evil sorcerer Orogastus, the three surviving princesses of the Ruwendan royal house are sent on quests to gather pieces of the legendary Black Trillium, a four-leaf clover-like plant that, when united, will generate enough magical power to blast the forces of evil into oblivion. Each author created one of the three princesses, whose characters follow the habits of the heroines of the author's solo fiction. Thus the princess created by Marion Zimmer Bradley nearly gets raped; Julian May's princess spends most of the book being wistful and wearing flowing white robes; and Andre Norton's character blunders into a swamp and spends most of the book wandering around aimlessly until her sisters rescue her.

Black Trillium is not only predictable, it abounds with solecisms, misplaced metaphors and other forms of bad writing. The triune author creates knights who "smote their armored bosoms in a gesture of fealty," a princess who "let out a smothered squeak of fear" and another princess who "wanted to hurl both her ire and her hurt at this untouched stranger, to demand from her own lips why her magic had failed." Was Black Trillium edited? Poul Anderson A FAR better effort is Poul Anderson's The Shield of Time (Tor, $18.95). Anderson has been active since 1947. In his long career, he has written many good books and a few brilliant ones (The Broken Sword, Tau Zero), and even his rare failures are honest, honorable works.

In the 1980s, Anderson's career was in decline; much of that decade was spent producing The King of Ys (1987-88), a fantasy that might have been a brilliant 500 page novel but unfortunately appeared as a bloated 1,100 page tetralogy. But both The Shield of Time and its predecessor, the Hugo-nominated The Boat of a Million Years (1989), mark a return to form.

The Shield of Time revives the "Time Patrol" series, begun by Anderson in the late 1950s but abandoned since the publication of The Guardians of Time (1960; revised, 1981). The Time Patrol is a far future police force that travels through time to ensure that history remains untampered with.

Although billed as a novel, The Shield of Time is in fact three connected novellas. In the first novella, Time Patrol agent Manse Everard travels back to Afghanistan in 200 B.C. to prevent the pieces of the empire created by Alexander the Great from reuniting; the second pits agent Wanda Timberley against the chaos of the Pleistoscene era; and in the third adventure, Everard and Timberley travel to 12th-century Naples to ensure the creation of the Renaissance.

All three adventures show Anderson's love of ancient and medieval history. The areas in which Everard and Timberley have their adventures are unknown territory to most readers; even seasoned historians usually do not worry about what would have happened if Roger II failed to establish the Kingdom of the Two Sicilies. But Anderson not only shows why Roger II's reign was important, but projects the characters and the readers through two alternate histories that plausibly show the dire consequences of Roger II's defeat.

The Shield of Time could have been one of Anderson's best books if he had omitted Wanda Timberley's adventure in the Pleistoscene, which, in 100 pages, serves little purpose save to remind the reader that Anderson has a hard time creating convincing female protagonists. For when Anderson writes about Manse Everard, he produces confident, sure writing with a sweep and grandeur lacking in most modern sf. A 200-page Shield of Time would have been a memorable entertainment; the published book is uneven, but still worth reading. Philip Jose Farmer PHILIP JOSE FARMER has been writing science fiction for more than 35 years, and is still influential; future historians may regard him as a literary grandfather of the cyberpunk movement of the 1980s. In many ways, Farmer is the first postmodern sf author, since much of his writing consists of pastiches designed to ridicule or deconstruct such legendary pulp heroes as Tarzan and Doc Savage.

Farmer's latest novel, Dayworld Breakup (Tor, $18.95), concludes a trilogy begun with Dayworld and continued with Dayworld Rebel. In the world of 3000, overpopulation is so severe that people live one day of each week and are placed in suspended animation for the remaining six, allowing seven people to inhabit space formerly reserved for one. But one man, Jefferson Cervantes Caird, discovers the truth about the conspiracy that keeps humanity in bondage and he plots a revolution.

By the time of Dayworld Breakup, Caird and his colleague/lover, Panthea Pao Snick, are on the run from the authorities. The first half of Dayworld Breakup is a relentless chase, in the long-established sf tradition (begun by A.E. van Vogt more than half a century ago) of vigorous, fast-paced work which, on reflection, makes absolutely no sense. The second half is a mixture of psychological intrigue and global apocalypse, as Caird finds out the truth about himself and his society discovers that it is founded upon lies.

Since both Caird and his enemies have the ability to live every day of the week, the peculiarities that made Dayworld interesting no longer exist; all that remains is a generic oppressive city of the future, in which shallow characters act in shallow ways. There's a profound sense of weariness and exhaustion in Dayworld Breakup; this is a novel that reads as if it had been written for no other purpose than to fulfill a contract. One hopes that Farmer's next novel will mark a return to form. Terry Bisson WHILE Terry Bisson's style in no way resembles Farmer's, Bisson's work resembles Farmer's in one important way -- much of it is an attack on time-honored sf traditions. In his fourth novel, Voyage to the Red Planet (Morrow, $16.95), Bisson turns the first mission to Mars into a comedy.

It's early in the 21st century, and corporations have been busy buying up as much of the government as they can. Nabisco has taken over the Smithsonian, and people who drive to work buy Executive, Business or Freeway Class when they pay their tolls on the privatized highways. NASA has gone bankrupt, and the space ship Mary Poppins, which would have carried a joint Soviet-U.S. mission to Mars, lies abandoned in an outer space landfill until a frenzied Hollywood producer decides to retrieve and refurbish it: He aims to use the ship as the basis for a blockbuster movie. Can an aging American astronaut, his ferocious Soviet counterpart, two fatuous inbred film stars, one stowaway and a dimwitted director survive the trip?

There's a good deal of comic energy in Bisson's work; not since Harry Harrison's The Technicolor Time Machine has an sf writer so deftly skewered the Hollywood mind. What is surprising is that the science in Voyage to the Red Planet is thorough and accurate; Bisson's goal is to show both the heroic nature of spaceflight and the ways such heroism can be diluted or destroyed by marketing and advertising. He is both a traditionalist and a postmodernist.

Bisson's work is a fresh, imaginative attempt to confront some of the problems of our time. I hope he continues to pursue his independent path. For it is the Bissons of the field, not the sequelizers or the sharecroppers, upon whom the future of science fiction depends.

Martin Morse Wooster is the Washington editor of Reason magazine. He has contributed to "The New Encyclopedia of Science Fiction" and "Twentieth-Century Science Fiction Writers."