Mercury Imaging from a Sun-Synchronous Solar Sail Orbiter

An innovative concept applying solar sailpropulsion in Mercury orbit is described where the low-thrust propulsion capability is used to realize a sun-synchrounous orbit around Mercury near the terminator. Due to Mercury's location deep within the solar gravitational well, an orbiter mission is difficult to realize with conventional propulsion. Moreover, because of the severe thermal conditions caused by a 10.6 Sun environment and reflection as wll as reradiation from Mercury's surface, orbit design about Mercury is important to ensure spacecraft survivability.If the orbit orientation is fixed in space, extreme thermal loading occurs when the spacecraft passes the subsolar point. On the other side, due to the vicinity of the Sun solar sails can operate very effectively at Mercury, allowing to perform imaging from a sun-synchronous orbit with a relatively moderate thermal environment. Solar sails posses a unique property for the realization of a sun-synchronous orbit around Mercury, because the rotation rate of the nodal line has the same inverse square variation concerning solar distance as the solar radiation pressure. Science objectives and potential payloads for such a mission considering the special orientation of the orbit are discussed. In order to show that acceptable illumination conditions exist for imaging, shadow lengths for several orbital plane alignments were calculated. Moreover, the relatively low altitudes ina sun-synchronous orbit would allow high ground resolution for imaging. Based on Mercury's 3:2 spin-orbit coupling complete coverageof Mercury's surface would be possible within two Mercury years