As space missions become longer and longer, a point will be reached at which a system that will produce food at least partly from metabolic wastes will be necessary. A number of predictions have been made as to how long such a mission would have to be. In 1966, a report by the General Dynamics Company concluded that, for a two-year (or Mars-type) mission, regeneration of carbohydrates by physical-chemical means would allow the weight and volume of the food supply to be smaller than with stored food alone. In a parallel study, the Lockheed Missiles and Space Company came to a very similar conclusion. More recently Spurlock and his colleagues presented arguments to demonstrate that a crossover point occurs at mission lengths of about eight years, on the average (1, 2, 3).