CONTACT. By Carl Sagan. Simon and Schuster. 432 pp. $18.95.

IT'S BEEN a long wait, but at last Carl Sagan's aliens are here, and have made contact. Well, actually they're not here, they're at the center of the galaxy, but let's not quibble.

Rumors of these aliens first became widespread at the very beginning of 1981, when it was announced that Simon and Schuster had paid a world record $2 million advance for a novel that had not yet been written! On the basis of a screenplay co-authored by Sagan and his colleague Ann Druyan, they felt confident, it seems, that they were onto a winner. It seemed a fair bet. Sagan's science-fact book The Dragons of Eden was a best seller; he wrote and presented the popular television series Cosmos (which in its book form was another best seller); in short he seemed to be popular science's most successful guru.

Contact was originally announced for fall, 1982. It is now exactly three years later.

Those three years have been full of rumor about the strangely delayed novel. Simon and Schuster must have been cursing that it was not published in time for the explosion in the alien business set off by Spielberg's film E.T. Could it be that the conceptual jump from science journalism to fiction writing had been too much even for Sagan's Pulitzer prizewinning mind? At last we can judge for ourselves, and with considerable evidence, for at around 140,000 words this is not a short book.

The first thing to say is that Sagan's fans and Sagan's detractors (two large subcultures) may both have cause to feel disappointed. The book is neither brilliant nor disgraceful. One thing it isn't is a sell-out to the lowest common denominator; the scientific content requires some thought to digest, and the impeccably liberal sociology with which the book is crammed is pitched at a level higher than your average best seller. It is, in fact, a rather preachy book, or perhaps it would be kinder to say, educational. Discussions about exobiology, about race relations, about world politics and -- perhaps less predictable -- a good many discussions about religion in which the word "numinous" plays a prominent role; all this can be a little tiring, as when one reads too many encyclopedia entries in a row. The tone is earnest, very much that of a youngish university lecturer talking about the state of the world after a few, but not too many, drinks.

ALL THIS TALK is the flesh on a rather flimsy skeleton of plot. The plot itself is one that will be recognized by science fiction fans who like books by professors of astronomy, for it is very similar (in the first half at least) to a book called A for Andromeda, published in 1962. This was co-authored by a prominent scientist of the time, Fred Hoyle, and based on his successful British TV serial of the same title. It starred Julie Christie in her screen debut. The story of A for Andromeda concerns signals from the stars picked up by a radio telescope which, when eventually decoded, turn out to be instructions for building an enigmatic machine. This is precisely the plot of Contact too. The difference is that Hoyle's machine was found to be a computer which gave instructions for building a beautiful female android who turned out to be a very clever Julie Christie. Sagan's machine is a spacecraft which navigates interstellar space by way of a rapid transport system of wormholes, which are a kind of more manageable and lovable black hole.

With admirable doggedness but -- it has to be said -- a certain ponderous slowness as well, Contact rumbles towards its seemingly ultimate revelation which, as is traditional in first contact stories (of which there are probably more than a thousand), is dismayingly trite. The superbeings from the stars whose messages we on Earth have received and interpreted present themselves in person (at the heart of the Milky Way) in the form of our loved ones, and their sentiments are couched in impeccable middle American. At this point I felt cheated and ready to give up. But wait, this is not the end, there are another 60 pages to go. And to be fair, they are really pretty good and contain some genuine surprises; there is a certain bleak maturity about them which contrasts interestingly with the rather chirpy optimism (messages from the stars unifying the nations) of what has gone before, without their ending on too black a note. At the very end Sagan produces a scientific rabbit out of his hat which in me at least really did produce a sense of wonder, and which incidentally makes retrospective sense of the slightly tedious religious arguments that are so prominent earlier on. The rabbit in question concerns secret messages that may be found in th ultimate physical constants of the universe. Messages from Whom? Sagan seems to be asking.

The protagonist of the novel is a woman, an astronomer, who, like Sagan himself, specializes in attempts to make contact with life forms Out There. A cold woman, frightened by human (let alone alien) contact, she is characterized rather perfunctorily (as is everybody else in the book) and yet her very unattractiveness (one cannot tell whether this trait was deliberately put in by Sagan or just happened) gives the book a certain somber sadness. At the very beginning of the book, she is found as a child staring at the stars, hanging on to the earth for fear of tumbling into the void which though above seems below. One can see Sagan having done that as a kid (as many of us did), and one likes him for it.

Sagan is certainly a better scientist than a novelist. As a novel the book is slow; as a portrait of the way scientists think, it is quite interesting, in what it gives away as well as in what it consciously tells us.