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.''