Several recent studies have been undertaken to optimize mission strategies and to select appropriate instrumentation for in situ studies of short period comets (Farquhar et al, 1974; Bender, 1974; Newburn, 1973; Meissinger, 1972; Roberts, 1971). Although some studies have contrasted the physical characteristics of several proposed target comets, few have comprehensively studied the orbital history and ephemeris uncertainties of target comets. In general, the navigational accuracy of cometary flyby probes is almost entirely dependent upon the target comet’s position uncertainty at the time of intercept. Although cometary error analyses are necessary for realistic mission planning, such analyses cannot be conducted in the standard fashion. Comets are affected by nongravitational forces (Marsden et al, 1973), they occasionally exhibit slight discontinuities in their orbital motions, and at least one comet (Biela) has completely disintegrated (Marsden and Sekanina, 1971). Each comet is an individual. Comets have steadfastly resisted recent attempts at classification. Hence, it seems clear that, for each comet of interest in mission planning, a separate in-depth error analysis study must be undertaken to realisticly determine the target comet’s ephemeris uncertainty at the time of intercept. Such studies should consider a number of criteria in order to assure accurate ephemerides for prospective cometary targets. Using the 1980 apparition of comet Encke as an example, these criteria are outlined below.