Space weapons -- a useful briefing; The New High Ground, by Thomas Karas. New York: Simon & Schuster. 224 pp. $14 .95.

President Reagan has dramatized the concept of space weapons. But the militarization of space - the exploitation of ''the new high ground'' - has actually been under way for 25 years.

It has given us spies-in-the-sky and killer satellites. Will it also give us laser ray guns, particle-beam weapons, and orbiting military command posts? Will the President's ''dream'' of a space-based missile defense one day come true?

Such questions are hard to answer. The technical barriers are formidable. No one can foretell whether or when they might be overcome. Happily, though, for those who would think sensibly about this subject, space analyst Thomas Karas has produced a highly useful briefing.

Here is both a short history of the subject and an informative outline of the technical challenges. Even a quick read through its lucid chapters will dispel any romantic notions that ''Star Wars'' weaponry and orbital warfare loom in the near future. The real military challenge, especially for the United States, is to decide how far down the road of space militarization it is wise to go.

This is the essential point of Karas's essay. Indeed, he sees his book as background for what he hopes will be a national debate on space policy. In a short epilogue, he offers a few ''theses'' with which he thinks any realistic debate must grapple. They are worth considering.

* ''Space laser weapons will not protect us from the threat of nuclear war.'' Effective countermeasures will develop.

* ''With or without new space systems, we can't win a nuclear war.''

* ''We can't hope to make the Earth safe from warfare by moving (it) into space.'' The purpose of space weaponry will be to gain advantage on the ground, and that is where war will continue, mainly, to be fought.

* ''Only in a limited sense is space the new 'high ground.' '' You can see more from space and communicate over longer distances. But being in space is not equivalent to occupying a hill up which the enemy must struggle.

* ''We should think twice . . . before moving weapons into space.'' The Soviets would match US installations, leaving the US with no new advantage, yet escalating the arms race significantly.

* ''We will pay a price for the increasing militarization of our (US) national space program.'' This will jeopardize the prestige, national pride, and technological spinoffs that have come from the largely civilian space program. It could also retard US ability to meet foreign competition for the rapidly growing civil space business.

Certainly the question of how far to go with militarization of space is a vital issue. And the Karas ''theses'' raise important questions. At the same time, they show the author's bias. He is skeptical of space warfare and convinced that space weapons have dubious value. Thus his book, while valuable, has a definite point of view. There is need for a second book, by a different author, that would make the case for space militarization.

Karas is right in saying it is time to debate this issue. But for such a debate to be conducted sensibly, other views of the subject should be brought forward with the same high degree of competence and lucidity which Karas's work represents.