FEW QUESTIONS are as enticing, as able to seduce our minds into timeless speculation and fantasy, as one of the oldest: Are we here on Earth really alone in the universe? And, if we are, why did it happen on this planet, and how did it happen? Is the miraculous phenomenon of life that is everything to us possibly just a one-time experiment in an otherwise empty cosmos?

But science is edging closer to partial answers. The latest in a series of reports by the Space Science Board of the National Academy of Sciences (Origin and Evolution of Life--Implications for the Planets: A Scientific Strategy for the 1980s) reveals some surprising progress.

The board reports, first of all, that on the basis of studies of the information sent back by the Viking missions to Mars, "we conclude that Mars is no longer a target for the direct search for life in the solar system . . . . There is no evidence for current life on Mars." And because there is "strong evidence" that none of the other planets or their satellites provides appropriate conditions, "We view the search for present life in the solar system as completed."

On the other hand, the chemical precursors of life, especially molecules containing the key element carbon, have been found elsewhere in the solar system, in comets, asteroids, interstellar space and in the atmospheres of the outer planets and their satellites. Studies of these still largely unexplored parts of space may tell us a lot about what is necessary for the beginning of life. And there is still the hope that "although we are probably alone in the solar system, we may not be alone in the universe."

It is to the study of our own planet, the Space Science Board believes, that science must turn to unravel further the mysteries of life. Satellite technology and remote sensing techniques for the first time make it possible to do this. Scientists now believe that life does not passively fit itself into an acceptable physical environment. Instead, recent studies lead them to think that life can alter and has profoundly altered the physical characteristics of this planet--its atmosphere, solid surface and water. Therefore, concludes the board, a major scientific goal of the coming decades "becomes the untangling of the dynamic processes that maintain Earth as a planet and sustain life."

That research will lead in two directions: to an understanding of our past and to the hope of maintaining a healthy planet in the future. Far more than other forms of life, human technology can alter the planet's ability to sustain life--and may already be doing so, perhaps in irreversible ways. The board mentions deforestation, the accumulation of wastes and the rapid extinction of species as examples. Planetary research may eventually make it possible to predict, and possibly to control, these and other trends.