SCIENCE FICTION has become big business for a few well known writers. There are a number of reasons for this phenomenon, not the least of which is the predominance of "sci-fi" action-adventure films in the movie theaters and on television. Some venerable practitioners of the genre command advances for their books that would have been virtually unimaginable when they were at the top of their form in the '50s and '60s. Now, some of their more youthful colleagues are profiting from the seemingly endless sf boom of the '80s, too.

Among the latter is Robert Silverberg, a prolific science fiction author from about the time he began shaving in the mid-'50s. Indeed, nearly three decades ago, while little more than a boy, Silverberg won a Hugo Award as the most promising new sf writer of the year. Today, many other awards gather dust on his mantel, including four Nebulas and two Hugos. Science fiction fans were shocked in the mid-'70s when Silverberg announced his retirement from the field, but he soon returned as the author of a fat fantasy novel, Lord Valentine's Castle, and has steadily produced fiction through the early and mid-'80s. While some readers and reviewers expressed delight over the "new" Silverberg, others were disappointed that Valentine and its sequels contained few of the qualities his work had been noted for in the '60s and early '70s: painfully honest explorations of a near future Earth, attempts to analogize the religious experience to the science-fictional "sense of wonder," and an ambivalence that is extremely rare in genre fiction -- particularly sf.

The old Silverberg seems to be back in Tom O'Bedlam (Donald I. Fine, $16.95), a post-apocalyptic odyssey through a California that has been bizarrely transformed by the Dust War, and yet remains oddly familiar in many ways. The eponymous character is a visionary who wanders the desolate landscape alone, pretending to be mad for the sake of survival. While the remnants of civilization erode, Tom is the key to a possible rebirth of the human race on Proxima Centauri, though he himself doesn't know this at the beginning of the novel.

Intricately plotted, Tom O'Bedlam works towards its inevitable climax with precision, though the pacing is at times a bit too deliberate. There is also a problem with tone here and there, as in the shooting of two young boys by the "scratchers," a group of murderous scavengers with whom Tom travels for much of the novel's length. The murders are described so matter-of-factly that this reader almost had to go back to see how the boys died. There is little emotion on display here, at least until the book's magnificent climax, but that makes the ending all the more powerful and evocative.

There are some finely drawn characters in Tom O'Bedlam. Jaspin, a former professor of cultural anthropology who joins the bizarre tumbond,e cult in an effort to understand the metaphysical truth behind what is happening to the Earth, is a detached, intriguing fellow, similar to characters in two of Silverberg's finest novels, Thorns and Dying Inside. Silverberg is a student of anthropology himself, author of the classic study The Mound Builders, and his sf frequently deals with tribal cultures, particularly on the mystical level.

It is finally the mysticism at the core of Tom O'Bedlam that is its most significant element, the point of its author's considerable art and artifice. Silverberg offers the patient reader a transcendental experience, its nature culled not from drugs, but from a detailed study of such exotica that make the story really work. It is love that prevails in this universe finally, but not in a saccharine, artificial way. The aliens who save humankind from itself are wondrous and godlike, informed by Silverberg's knowledge of the metaphysics of myriad cultures. Elephants From Space

SADLY, the same cannot be said of the new Larry Niven-Jerry Pournelle collaboration, Footfall (Del Rey, $17.95). Where Silverberg's aliens remain offstage most of the time, Niven and Pournelle's are on all too often. Since they resemble baby elephants with bifurcated trunks, they are not frightening, though they indiscriminately kill millions of people; the Fithp are merely ludicrous. Not so ludicrous as some of the humans who scurry for cover when the alien starship starts walloping the Earth with meteors, however.

For example, Captain Jeanette Crichton of the U.S. Army, when ordered to Washington on the double, barks: "If I'm going to the White House, I am damned if I'll go bare- legged!" Just like a woman, huh guys?

Or how about the dour Soviets, who never know what's going on until a couple of hours after the Americans, but who do manage to kill a few aliens just to show that they are formidable, if not terribly bright. Through sheer tortoiselike obstinacy, they have managed to build a space station while the U.S. languishes in the hands of wimpy liberals who never saw an invasion coming! The authors even take time out to editorialize about how much things have improved under the indefatigable Ronald Reagan, but in this near-future world, a Carter clone continually wavers on the edge of surrender while sturdier souls save the planet.

But let us return, however briefly, to the Soviets, who all speak their native language so badly that the only Russian word any of them can muster in the entire 495 pages is Da. This neatly parallels the dialogue in the American sequences, which seems always to be prefaced by the word "Hell," as in "Hell, these antigravity toilets are the best . . . after all, they are made in the good old US of A.' Actually, I made that one up, but you get the idea.

One of the Americans is Roger Brooks, special assignments reporter for The Washington Post. Brooks is a reprehensible fellow who not only conks his buddy on the head to save his own worthless hide, but actually does this in the process of violating National Security. After escaping from the top secret site of Project Archangel Michael, the nasty Brooks stops in to return a pickup truck to a group of survivalists. He's thinking about publishing what he's seen and ruining Earth's chances of getting even with the aliens, so naturally he reveals this to a survivalist. The irate fanatic drowns the not only sneaky but obviously very stupid reporter in the toilet. How's that for pungent symbolism, gang?

Well, the Fithp turn out to be pretty stupid themselves. In much the same way that some conservatives view the third world as childish nations who should never be allowed to play with technology lest they hurt themselves and threaten us, te aliens have inherited all their rather familiar sf hardware from an advanced race. The British? The Germans? Hoosiers? No, they are cleverly labelled The Predecessors.

Footfall is at least twice as long as it ought to be, and it is extremely heavy-handed. The dust jacket heralds it as "Probably the finest novel of alien invasion ever written." This comment is not attributed to anyone, and H.G. Wells' admirers need not fear that the old master is outstripped by the authors of this dull-witted farrago. Rock Around the Clock

AS CLEVER and streamlined as Footfall is clumsy and bulky, The Nick of Time by George Alec Effinger (Doubleday, $12.95) is an amusing pastiche of virtually every time travel cliche to ever find its way between the covers of an sf novel. Frank Mihalik, certified hero, travels from 1996 to 1939. He visits the New York World's Fair for a pleasurable afternoon and evening, but awakens the following morning to discover that the same day is repeating itself. There has been a problem up ahead in the future, and Frank is marooned with the gawking depression-era crowds, seemingly for an eternity.

This time-loop-crisis section of the novel was first published as a 1983 novelette in The Magazine of Fantasy and Science Fiction under the title "The World of Pez Pavilion: Preliminary to the Ground-Breaking Ceremony." This 27-page story serves as a springboard for Frank and his girlfriend Cheryl to vault into the distant future, alternate realities, and even outside of time itself.

Among Effinger's engagingly silly characters are King Proximo and Queen Hesternia, who command the forces of the future and past, respectively; the Historian, who bears a resemblance to Cardinal Richelieu, and rules everything, past, future, present, in all universes. Richelieu himself puts in a token appearance: "He was dressed in his red robe and red skullcap; he looked like Charlton Heston with pasted-on beard." Richelieu, who believes Mihalik to be Queen Anne of Austria, is confronted with the truth in this comic gem of a scene. Mihalik announces that he is not a woman and tears open his filthy dress. "Mihalik," Effinger writes drily, "presented his most persuasive argument."

There are many other off-the-wall treats in The Nick of Time. Among these are an alien equation that makes travel between stars identical to travel between Morristown and Summit, New Jersey. Effinger even offers a contest, which he calls "son of Have You Been Paying Attention?" or "The Nick of Time Mystery Fallacy." The first ten readers to figure out the mystery get free copies of Effinger's next novel. Enter now, by all means.