NASA is currently working to select its next major deep space exploration mission as part of the New Frontiers program. This fall, the space agency will select a few proposals for further study, and the final selection will be made sometime in 2019.
There are a number of intriguing options, such as sending a spacecraft back to Saturn's moon Enceladus to study the water environment there, or to Jupiter's moon Io to study the highly active volcanoes and surface lava lakes. One of the most captivating mission proposals, however, from Johns Hopkins University's Applied Physics Laboratory (APL), is to send a dual-quadcopter to Saturn's hydrocarbon moon of liquid methane, Titan.
The proposed spacecraft, Dragonfly, would land on Titan using a conventional parachute, take samples and spectroscopic readings to determine the composition of the surface and the atmosphere, and then fire up its eight rotors, two at each corner, and fly to another location. The primary mission of the Titan drone would be to study the prebiotic chemistry of Titan and search for signs of liquid water, habitability, and even life.
While the only liquids on Titan's frigid surface—about minus 290 degrees Fahrenheit (minus 180 Celsius) on average—are flowing pools and channels of hydrocarbons such as methane, researchers believe that just below the surface Titan harbors a vast, global liquid water ocean. The liquid hydrocarbons on the surface and in the atmosphere go through a cycle of evaporation and rain similar to the hydrological cycle on Earth, and the large moon, bigger than the planet Mercury, might be similar in structure and chemical makeup to the ancient Earth, before life took hold and transformed our home planet.
Dragonfly would take four types of readings to learn more about this mysterious world—mass spectrometry to determine the composition of the atmosphere and surface, gamma-ray spectrometry to determine the composition of subsurface material and search for a water ocean, meteorological and geophysical sensing to measure the winds and weather conditions, and visual camera observations to map the physical structures of the moon and scout out future landing sites.
"We could take a lander, put it on Titan, take these four measurements at one place, and significantly increase our understanding of Titan and similar moons," said Dragonfly project manager Peter Bedini of APL. "However, we can multiply the value of the mission if we add aerial mobility, which would enable us to access a variety of geologic settings, maximizing the science return and lowering mission risk by going over or around obstacles."
NASA's Cassini spacecraft deposited the European Space Agency's Huygens probe on Titan in January 2005, which became the first craft to land on a planetary body in the outer solar system when it touched down in a dry lakebed on the moon. Along with radar and spectroscopic observations from Cassini, Huygens revealed Titan to be a world covered in mountains of water ice instead of rock, flowing lakes and channels of liquid methane, and rich organic material blanketing the mysterious world. Huygens' successful landing paved the way for another craft to return to the hazy orange moon of Saturn, and if the Dragonfly mission is accepted, it could become the first spacecraft to achieve aerial flight in an atmosphere beyond Earth.
The atmosphere on Titan, composed primarily of nitrogen with notable amounts of methane and hydrogen, is approximately 1.5 bars at the surface, or 1.5 times the pressure of Earth's atmosphere at the terrestrial surface. The dense atmosphere and low gravity of Saturn's largest moon mean a helicopter-style drone would achieve more lift on Titan than Earth, making it easier to fly with less power. While NASA's Jet Propulsion Laboratory has been working on a helicopter drone that could fly on Mars for years, the thin atmosphere on the Red Planet, only about one percent of that of Earth, makes designing a Martian aircraft extremely difficult. Flying a drone on Titan could be much easier and more productive.
The APL mission proposal describes a Titan drone that uses a Multi-Mission Radioisotope Thermoelectric Generator, or MMRTG, to power the spacecraft. A MMRTG uses heat from decaying radioactive material, specifically plutonium-238, to generate electricity and power the spacecraft for years. After flying to a new location, Dragonfly would need to recharge its flight systems and operate its science instruments, and under the thick haze of Titan, solar panels cannot be used to generate power.
Titan has become one of the most tantalizing celestial bodies in our entire solar system, rivaled only by the icy moons. The world of methane rains and ice mountains could be undergoing a process similar to the early Earth, it may be that life is on its way to taking root there, and it may even be that life exists on Titan now—either in the water ocean below the surface or, perhaps, an entirely alien form of life like nothing on Earth that does not rely on liquid water to survive. These mysteries, along with the possibility of exploring miles and miles of Titan with an aircraft, make Dragonfly one of the most thrilling mission proposals on the desks of NASA's New Frontiers program offices.