Solar sail propulsion: An enabling technology for fundamental physics missions

Solar sails enable a wide range of high-energy missions, many of which are difficult or even impossible to accomplish with any other type of conventional propulsion system. They are also an enabling propulsion technology for two types of deep-space missions that are very favorable for testing current gravitational theories and the large-scale gravitational field of the solar system: the first type comprises missions that go very close to the Sun (<8 solar radii) and the second one comprises missions that go fast very far away from the Sun (similar to 200 AU). Being propelled solely by the freely available solar radiation pressure, solar sails do not consume any propellant. Therefore, their capability to gain (or reduce) orbital energy is theoretically unlimited and practically only limited by their lifetime in the space environment and their distance from the Sun (because the solar radiation pressure decreases with the square of solar distance). Nevertheless, solar sails make also missions that go far away from the Sun feasible because they can gain a large amount of orbital energy by first making one or more close solar approaches that turn the trajectory hyperbolic. For both mission types, the temperature limit of the sail film is a critical issue. In this chapter, we briefly review the physics and the current technological status of solar sails, and then present mission outlines and trade-offs for both mission types. Thereby, we will show that even near- or medium-term solar sails with a relatively moderate performance enable these kinds of missions.