REPORTS SOFTWARE ASPECTS OF STRATEGIC DEFENSE SYSTEMS A former member of the SD10 Panel on Computing in Support of Battle Management explains why he believes the “star wars” effort will not achieve its stated goals. DAVID LORGE PARNAS On 28 lune 2985, David Large Parnas, a respected computer scientist who has consulted extensively on United States defense projects, resigned from the Panel on Computing in Support of Battle Management, convened by the Strategic Defense lnitiattve Organization (SDIO). With his letter of resignation, he submitted eight short essays explaining why he believed the software required by the Strategic Defense Initiative would not be trustworthy. Excerpts from Dr. Par- nas’s letter and the accompanying papers have appeared widely in the p.yess. The Editors of American Scientist be- lieved that it would be useful to the scientific community to publish these essa,ys in their entirety to stimulate scientific discussion of the feasibility of the project. As part of the activity of the Forum on Risks to the Public in the use of computer systems the Editors of Communications are pleased to reprint these essays.” This report comprises eight short papers that were com- pleted while I was a member of the Panel on Comput- ing in Support of Battle Management, convened by the Strategic Defense Initiative Organization (SDIO). SD10 is part of the Office of the U.S. Secretary of Defense. The panel was, asked to identify the computer science problems that would have to be solved before an effec- tive antiballistic missile (ABM) system could be de- ployed. It is clear to everyone that computers must play a critical role in the systems that SD10 is considering. The essays that constitute this report were written to organize my thoughts on these topics and were submit- ted to SD10 wj th my resignation from the panel. - j Reprinted by perrnissicm of Anwricar~ Sciewfisf. journal of Sigma Xi: Software Aspects c,f Strategic Defense Systems. David Large Parnas. Vol. 73. No. 5. pp. 432-440 Cc 1985 ACM OflOl-0782/85/120o-1326 750 My conclusions are not based on political or policy judgments. Unlike many other academic critics of the SD1 effort, I have not, in the past, objected to defense efforts or defense-sponsored research. I have been deeply involved in such research and have consulted extensively on defense projects. My conclusions are based on more than 20 years of research on software engineering, including more than 8 years of work on real-time software used in military aircraft. They are based on familiarity with both operational military soft- ware and computer science research. My conclusions are based on characteristics peculiar to this particular effort, not objections to weapons development in general. I am publishing the papers that accompanied my let- ter of resignation so that interested people can under- stand why many computer scientists believe that sys- tems of the sort being considered by the SD10 cannot be built. These essays address the software engineering aspects of SD10 and the organization of engineering research. They avoid political issues: those have been widely discussed elsewhere, and I have nothing to add. In these essays I have attempted to avoid technical jargon, and readers need not be computer programmers to understand them. They may be read in any order. The individual essays explain: 1. The fundamental technological differences be- tween software engineering and other areas of engi- neering and why software is unreliable: 2. The properties of the proposed SD1 software that make it unattainable: 3. Why the techniques commonly used to build mili- tary software are inadequate for this job; 4. The nature of research in software engineering, and why the improvements that it can effect will not be 1326 Communications of the ACM December 1985 Volume 28 Number 12