Composed of two studies conducted by the Union of Concerned Scientists--one on anti-satellite weapons and one on space-based ballistic missile defense--together with an introductory chapter outlining the history of space weapons, this volume should put well-founded doubt into the minds of even the most optimistic fan of President Reagan's Star Wars scenario. The UCS finds that while anti-satellite weapons--missiles that can find and destroy intelligence-gathering or forward-observation satellites--are now in a primitive state, making only low-orbit satellites immediately vulnerable, the technology to go after high-orbit satellites is feasible. This is a particular immediate threat to the USSR, whose satellites are mostly slow and low orbiting, but it can be a long-term threat to US satellites. The UCS foresees a new arms race in anti-satellite weapons if no treaty is forthcoming to prohibit their testing, and they bet that the Soviets will jump to further testing soon without such a treaty. Antiballistic missiles based in space are another matter. The UCS report, working on the assumption that an effective system would have to destroy enemy ballistic missiles in their early booster phase (before multiple warheads are launched and while the booster rocket and flame provide good targets), concludes that such a system would be unlikely to be successful, couldn't be shown to be effective in advance of an attack, and would cost hundreds of billions of dollars to attempt. They point to effective countermeasures--the launching of decoys, the deployment of vastly greater numbers of real missiles, the use of various screens to blind sensors, or simply faster booster rockets--as one enormous problem. But there is also the problem of developing the necessary particle beam or laser technologies required to intercept the ICBMs, or the inhumanly complicated softwear needed for the system--computer programming that would have to work under conditions of nuclear attack. No one could be sure, no matter how much testing was done, that all the components would work when the time came, and that doubt would render the system almost useless. Embarking on the development of this element of the Star Wars plan would result in more, faster ICBMs, and in more submarine-based cruise missiles, since these would be almost impossible for even a working system to track and counteract. Emphasizing that the Star Wars initiative is a radical break with the past, the UCS says Stop It Now, and their case, made on a high technical level but with summary chapters for the uninitiated, is a solid one.