Triton Explorer - Neptune Orbiter Mission Study

Triton is larger than the planet Pluto, and its highly inclined, retrograde orbit suggests that it may be a captured object that initially formed somewhere else in the solar system. Its composition (and its inventory of organic materials) is thus of considerable interest. Triton possesses an appreciable atmosphere, and its circulation, like that of Mars, is one of seasonal condensation flow between the southern and northern hemispheres. Although the surface pressure is only ~ 16 microbar, winds of 5-15 m / sec flow toward the equator from the sunlit hemisphere. Voyager 2 detected a number of plumes extending from the surface to approximately 8 km elevation within the atmosphere. Triton exhibits a variety of puzzling surface features; among these are structural features that suggest extensive faulting in the past, together with ice volcanism, and dark streaks that may be associated with the plumes. The Solar System Exploration Decadal Survey (NRC, 2003) lists a Neptune Orbiter / Triton Explorer as a "Deferred High-Priority Flight Mission" that may be considered for the second decade of this century. Likely science objectives for a Triton Lander mission would include a more complete characterization of the composition and circulation of the atmosphere; investigation of the physical processes responsible for plume formation; surface composition measurements; and geophysical monitoring, including seismological measurements that could potentially constrain the physics of plume eruptions. We describe here a conceptual dual-Lander mission to explore Triton's surface. Each of the two Landers would be powered by a standard multi-mission radioisotope thermoelectric generator (MMRTG). These Landers could operate on the surface of Triton for several years. A companion Neptune Orbiter would provide telecommunication links between the Landers and the Earth, and would be instrumented to observe both Triton and Neptune. Although a Jupiter Icy Moons Orbiter (JIMO) follow-on mission architecture was initially specified, we subsequently determined that the Landers could be also delivered by conventional propulsion. Our study addressed all key aspects of the mission architecture, including the science instruments, the main subsystems, various trade options for the power system, and a conceptual design for the Landers. We found that RPS power systems are necessary for long-duration missions to the Neptune system.