Dragonfly: A rotorcraft lander concept for scientific exploration at titan

The major post-Cassini knowledge gap concerning Saturn’s icy moon Titan is in the composition of its diverse surface, and in particular how far its rich organics may have ascended up the ”ladder of life.” The NASA New Frontiers 4 solicitation sought mission concepts addressing Titan’s habitability and methane cycle. A team led by the Johns Hopkins University Applied Physics Laboratory (APL) proposed a revolutionary lander that uses rotors to land in Titan’s thick atmosphere and low gravity and can repeatedly transit to new sites, multiplying the mission’s science value from its capable instrument payload. Titan is an “ocean world” that is rich in both carbon and nitrogen.4,5 See Table 1 for data on Titan’s environment. FORMULATION OF THE DRAGONFLY CONCEPT The NASA community announcement in January 2016 identifying Titan as a possible target for the fourth New Frontiers mission opened new possibilities in Titan exploration (Box 1). Although the exploration of Titan’s seas had previously been considered, notably by the APL-led Titan Mare Explorer (TiME) Discovery concept,6,7 the timing mandated by the announcement of opportunity precluded such a mission. Specifically, with launch specified prior to the end of 2025, Titan arrival would be in the mid-2030s, during northern winter. This means the seas, near Titan’s north pole, are in darkness and direct-to-Earth (DTE) communication is impossible.8 Even with the higher budget threshold of New Frontiers 4 ($850 million plus launch and operations costs) compared with Discovery (~$450 million INTRODUCTION Saturn’s moon Titan is in many ways the most Earthlike body in the solar system.1–3 This strange world is larger than the planet Mercury and has a thick nitrogen atmosphere laden with organic smog, which partly hides its surface from view. Since cold Titan is far from the Sun, on Titan methane plays the active role that water plays on Earth, serving as a condensable greenhouse gas, forming clouds and rain, and pooling on the surface as lakes and seas. Titan’s carbon-rich surface is shaped not only by impact craters and by winds that sculpt drifts of aromatic organics into long linear dunes but also by methane rivers and possible eruptions of liquid water (“cryovolcanism”). While living things are ~70% water, and finding water has been a convenient initial focus for astrobiological investigations in the solar system, the chemical processes that conspire to lead to life rely on functions exerted by compounds of carbon, nitrogen, oxygen and hydrogen, with traces of sulfur and phosphorus (CHNOPS). In contrast to Europa (abundant in water, and perhaps sulfur), Dragonfly: A Rotorcraft Lander Concept for Scientific Exploration at Titan Johns Hopkins APL Technical Digest, Volume 34, Number 3 (2018), www.jhuapl.edu/techdigest 375 plus radioisotope power source and launch costs), it would be challenging indeed to provide a relay spacecraft and a sea probe. A lander with DTE communication would be possible at lower latitudes, however. The only detailed study of such a mission (see Box 2) was the 2007 Titan Explorer NASA Flagship Mission Study,9,10 led by the Johns Hopkins University Applied Physics Laboratory (APL). This study advocated the science that could be obtained from three platforms, an orbiter, a hot-air (Montgolfière) balloon, and a lander. The lander (designed before Titan’s seas had been discovered) was intended to be delivered to Titan’s Belet sand sea, a large—and thus easily targeted— dune field expected to be free of rock and gully hazards. After the lander’s parachute descent and landing on Pathfinder-like airbags (wherein if it landed on top of a dune, it would just roll down to the bottom), petals would unfold and science would begin, with cameras, a chemical analysis suite, a seismometer, and a meteorology package. Much of the science definition in the Titan Explorer Study was useful in formulating the Dragonfly proposal. A scientific limitation of a single lander, however, is that it explores only a single location. This limitation can be mitigated slightly at “grab-bag” landing sites where geological processes have gathered samples from a range of areas (in Mars Pathfinder’s case, a flood deposit of rocks; dune sands may similarly have material from a range of source locations). However, a lander with some kind of mobility, or augmented by some mobile element (e.g., a “fetch” rover), would help address the challenge of acquiring samples from sites more interesting than the landing point, a site that would be most likely selected for safety rather than for scientific interest. The concept of a rotorcraft lander on Titan tricklecharging a battery for brief atmospheric flights by using the power from a radioisotope power source had been proposed some 17 years ago.11,12 At that time, the vehicle was imagined to be a helicopter, a vehicle that is used on Earth for near-guaranteed access to a wide range of terrain, for personnel delivery, and for search and rescue. However, helicopters are mechanically complex (one