Comparison of the solar sail with electric propulsion systems

The propulsive efficiencies of the solar sail and of electric propulsion systems are compared on the basis of specific impulse. It is shown that the solar sail is more efficient at one a.u. for mission durations greater than about two months, that the advantage is increased for missions toward the inner planets, and that the same conclusions are reached when the comparison is based on maximizing the momentum transferred to the payload. Other factors that will influence the choice of a propulsion system for a specific mission are mentioned.