EPSC Abstracts, Vol. 4, EPSC2009-270, 2009 European Planetary Science Congress, © Author(s) 2009 Wind-Assisted Aerobot Navigation on Titan: Implications for Mission Planning and Science Exploration A. Elfes (1), K. Reh (1), P. Beauchamp (1), N. Fathpour (1), L. Blackmore (1), C. Newman (2), Y. Kuwata (1), M. Wolf (1), C. Assad (1) (1) Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA ([email protected]/Fax: +1-818-393-5007) (2) Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA Abstract an aerobot and thereby enhance the scientific return of the mission. The recent Titan Saturn System Mission (TSSM) proposal incorporates a montgolfière (hot-air Titan Exploration Using Aerobots balloon) as part of its architecture. The authors The main LTA options for a Titan exploration have conducted a study to determine the impact of aerobot [1, 2, 3] include passive balloons using the Titan wind field to extend the scientific (drifters), propelled airships, unpropelled reach of the balloon. The results show that a wind- montgolfières (hot air balloons), and hybrid assisted unpropelled montgolfière will be able to designs such as a propelled montgolfière. Passive reach a broad set of science targets, while a wind- balloons are simple in design, but have no assisted propelled montgolfière could reach any navigation controllability, being primarily area of interest on Titan, and do so in a fraction of dependent on where the winds take them; airships, the time needed by the unpropelled balloon. on the other hand, are highly maneuverable and In-Situ Exploration of Titan could navigate to pre-specified science targets on Titan, but for multi-month missions would require The results obtained from the Huygens probe and a replenishment system for the lifting gas lost over from continuing observations by the Cassini time. Standard montgolfière balloons generate lift spacecraft have revealed that Saturn’s moon Titan through heating of the atmospheric gases inside has methane lakes and seas, river channels and the envelope, and use a vent valve for altitude drainage basins, dunes of organic sands, sierras control. A hybrid montgolfière design could have and impact craters, and possibly cryovolcanoes propellers mounted on the gondola to generate and other fascinating features. Different classes of horizontal thrust; in spite of the unfavorable high-value science targets (such as dunes or lakes) aerodynamic drag caused by the shape of the are found at geographically distant locations on balloon, a limited amount of lateral controllability Titan (such as the equatorial region and the poles), can be achieved. A Titan aerobot would have to while Titan’s cloud cover inhibits high-resolution use radioisotope thermoelectric generators (RTGs) investigation of the lower atmosphere and surface. for electric power, and the excess heat generated Both issues can be addressed by a highly mobile in can be used to provide thermal lift for a situ platform, and the dense atmosphere, low montgolfière [4]. This concept was baselined in temperature and low gravity of Titan have led to the recent NASA/ESA Titan Saturn System various proposals to deploy a lighter-than-air Mission (TSSM) proposal [5]. (LTA) robotic vehicle, or aerobot. An aerobot would be able to traverse vast distances on Titan, Wind-Assisted Navigation operate below the upper atmosphere cloud cover, In this paper we explore what guidance, navigation conduct high-resolution surveys of the surface, and and control capabilities can be achieved by a possibly even perform surface sampling. In vehicle that uses the Titan wind field. Results from planning an aerial mission at Titan, it is extremely our studies are applicable to aerobots with vertical important to assess how the moon-wide wind field controllability (montgolfières), as well as aerobots can be used to extend the navigation capabilities of with horizontal and vertical controllability (propelled montgolfières, airships). The control EPSC Abstracts, Vol. 4, EPSC2009-270, 2009 European Planetary Science Congress, © Author(s) 2009 planning approach is based on passive wind field Significance riding. The aerobot would use vertical control to Many of our results, as exemplified by Fig. 2, select wind layers that would lead it towards a verify that a simple unpropelled montgolfière predefined science target, adding horizontal without horizontal actuation will be able to reach a propulsion if available. Results presented in this broad array of science targets within the paper are based on a) aerodynamic models that constraints of the wind field. The study also characterize balloon performance at Titan, and b) a indicates that even a small amount of horizontal Titan WRF (Weather Research and Forecasting) thrust allows the balloon to reach any area of model [6] that incorporates heat convection, interest on Titan, and to do so in a fraction of the circulation, radiation, Titan haze properties, time needed by the unpropelled balloon. The Saturn’s tidal forcing, and other planetary results show that using the Titan wind field allows phenomena. It builds on the PlanetWRF model [7]. an aerobot to significantly extend its scientific For navigation planning, the conditions of the reach, and that a montgolfière (unpropelled or Titan atmosphere are sampled using a 3D spatial propelled) is a highly desirable architecture that lattice, with the temporal variability of the wind can very significantly enhance the scientific return field providing an additional dimension. Both of a future Titan mission. deterministic and stochastic wind models have been used in the analysis [8, 9]. Optimal search methods based on dynamic programming and Markov decision process algorithms compute time-optimal trajectories between any given start and goal locations on Titan (Fig. 1). We assume that the vertical and horizontal dynamics of the aerobot are decoupled, and that acceleration times when changing wind layers are negligible compared to total navigation times. Figure 2: Percentage of areas reachable on Titan and the corresponding traversal times for 24 different starting locations. The results are based on Ls = 90 deg (Northern Summer solstice), 4 different horizontal actuation levels (0.0 m/s, 0.25 m/s, 0.50 m/s, and 1.0 m/s), start altitude = 5km, goal altitude = 250m, sink rate = 0.6 m/s, and rise rate = 0.3 m/s. Figure 1: Titan reachability map showing the time (in Earth days) required for an aerobot starting at References (15oN, 5oE) to reach any target on Titan (Ls = 90o, start altitude = 5km, goal altitude = 250m, sink [1] Cutts, J. et al. (2004) “Scientific Ballooning at rate = 0.6 m/s, rise rate = 0.3 m/s, no horizontal the Planets”. Proc. 2004 COSPAR Scientific actuation). The white cells are not reachable. Assembly. [2] Hall, J. et al. (2006) “An Aerobot for Global in Results situ Exploration of Titan”. Advances in Space We conducted a large number of simulations [8, 9], Research, 37(11). varying Titan’s seasons (planetocentric solar [3] Elfes, A. et al. (2008) “An Autonomy longitude Ls = 0, 90, 180, 270 degs), balloon Architecture for Aerobot Exploration of the parameters (rise rate, sink rate, horizontal Saturnian Moon Titan”. Proc. IEEE Aerospace actuation speed), lattice grid size, etc. Some Conference. representative results are shown in Figs. 1 and 2. [4] Elliott, J., Reh, K. and Spilker, T. (2007) “Concept for Titan Exploration Using a EPSC Abstracts, Vol. 4, EPSC2009-270, 2009 European Planetary Science Congress, © Author(s) 2009 Radioisotopically Heated Montgolfiere”, Proc. IEEE Aerospace Conference. [5] Reh, K. et al. (2009) “Titan Saturn System Mission Study”, NASA. [6] Newman, C. E. (2008) “Modeling Titan's Atmosphere with the TitanWRF GCM”, Geological and Planetary Sciences, Caltech, Pasadena. [7] Richardson, M.I, Toigo, A. D. and Newman, C. E. (2007) “PlanetWRF: A general purpose, local to global numerical model for planetary atmospheric and climate dynamics”, Geophysical Res. 112. [8] Blackmore, L. et al. (2009) “Path Planning and Global Reachability for Planetary Exploration with Montgolfiere Balloons”, submitted for publication. [9] Elfes, A. et al. (2009) “Navigation Planning and Global Reachability for Planetary Exploration Balloons”, JPL tech report D-61013. .