William R. Alschuler. St. Martin's, $23.95 (256p) ISBN 0-312-26225-6

Have aliens visited Earth? Did the government conspire to cover up a UFO crash at Roswell, N.Mex.? Astronomer Alschuler (UFOs and Aliens)-while allowing for the possible existence of life elsewhere in our galaxy-remains skeptical that aliens have touched down on Earth. With a child's enthusiasm for the fantastic and a scientist's eye for detail, the author examines the science of alien technologies reported by eyewitnesses and featured in various media. Several reports, for example, suggest that aliens travel in silent, rapidly accelerating saucers. Applying the law of inertia, however, the author notes that extreme acceleration would exert a crushing pressure on alien passengers. In addition, since solid objects generate pressure waves as they move through a substance such as air, silent saucers are implausible. Other reports from abductees suggest that aliens can travel from the star Vega to Earth in a little over a day, an unlikely feat that Alschuler asserts would be possible only if the aliens had access to a tunnel through space (a highly unstable passage commonly known as a "worm hole") or the ability to bend space using a warp drive (which requires as much energy as the sun will radiate over its lifetime). Although Alschuler's analysis seems at times like an introductory physics text as it delves into the quantum mechanics and physics of prospective alien technologies, readers will appreciate his objective, fact-based analysis of a range of purported extraterrestrial phenomena. (Feb. 1)

Forecast:Many Americans think that UFOs are piloted by aliens, and many of them believe in alien abductions. This vast potential readership will be enticed by this book's title, only to slam the volume down. However, scientifically inclined readers who, like Alschuler, are drawn to science fiction-fans of books like ThePhysics of Star Trek -are the perfect target readership for his evaluation of what is or is not possible in the physical world as we know it; if booksellers can appeal to them, this book should do respectably well.