US planetary exploration is on the back burner. President Reagan has disappointed the space community by not plumping for a manned space station. But unobtrusively in the background, the ''Mars Underground'' is preparing for the second giant leap for mankind - a manned landing on Mars.
As James Oberg makes clear in this fascinating book, this is not such a fantastic vision at all. Much of the technology is already in place or is soon to be put there. The cost would not be prohibitive - substantially less than for the US Apollo moon landing program, a fraction more than for the space shuttle. And the world does not have to wait upon the fate of Reaganomics. The Soviet Union is quite capable of mounting a Mars expedition in this century and seems to be preparing to do so. Moreover, the project is fully within the capacity of the European Space Agency or of a combination of the ESA and Japan.
Thus Mr. Oberg's book should be read as an overture to action that is quite likely to occur within the next few decades and not as a wishful dream.
Oberg is a veteran space expert who is especially noted as an analyst of the Soviet space program. For this book, he draws heavily, but not exclusively, on material presented at the Case for Mars Conference held in Boulder, Colo., last year. It was here that the ''Mars Underground'' (Oberg's term) first assembled as a group in which the members discovered that many US space experts and laymen were interested in serious analysis of a Mars landing.
Oberg himself is a Mars landing enthusiast; he makes no pretense here of being unbiased. Within that context, he does present a wealth of factual data and feasibility analysis that is quite sound. He does not gloss over the difficulties a Mars mission would face. On the other hand, he argues cogently that both the difficulties and the costs have been grossly overestimated in the past by skeptics who dismissed a Mars landing out of hand. On balance, his book is an excellent layman's briefing on this fascinating prospect.
Whether or not other readers will find Oberg's analysis as persuasive as I do is best left for them to judge. After all, I'm a Mars landing enthusiast, too. But it would take a coldly unimaginative reader indeed not to appreciate Oberg's vision, as when he ends his book by noting that, over the coming centuries, the Red Planet could become ''a soft-colored, gleaming, green-tinted jewel, reflecting the spread of life across its surface . . . an event of galactic significance.''