Mercury sun-synchronous polar orbits using solar sail propulsion

As the oblateness of Mercury is nearly zero, an orbit about the planet will not precess without propulsive acceleration but instead will remain inertial. This causes the spacecraft to encounter a very severe thermal environment when passing near the subsolar point of Mercury's hot surface. An innovative concept applying solar sail propulsion in Mercury orbit is described where the low-thrust propulsion capability is used to realize a sun-synchronous orbit about the planet. The spacecraft is able to continuously move near the terminator, which substantially reduces the thermal hazards and also allows suitable conditions for remote sensing. Circular as well as eccentric polar orbits were investigated concerning their potential to realize the required rotation rate of the line of nodes. For the elliptical orbits, the periapsis is located above Mercury's north pole. Because of Mercury's 3:2 spin-orbit coupling, relative motion between the spacecraft's orbital plane and Mercury will result, allowing complete coverage of the planet's surface. The variation of the solar flux over a Mercury year caused by the high orbit eccentricity of the planet around the sun is also considered. Alternatively to solar sailing, chemical and electric propulsion are analyzed concerning the applicability for orbit precession at Mercury.