There is apt to be much uncertainty in any program of planetary exploration. This uncertainty naturally leads to possible uncertainty in the time period in which planetary quarantine is desirable, to possible uncertainty in the total number of missions to be launched in the vicinity of any given planet, and indeed, to possible uncertainty in the meaning of the word ‘contamination’. A model is developed in this paper which makes possible the derivation of mission non-contamination requirements withouta priori knowledge of either the time period in which planetary quarantine is to be observed or the total number of missions to be used in exploring the planet being quarantined. On the basis of this model, some general observations are made about the need for carefully defining ‘contamination’.