It's been reported that a planned mission to Titan will use a robotic boat to explore the liquid-methane seas. But the project will also have eyes in the sky: a hot-air balloon will circumnavigate Titan and observe its multiform topography.
The Titan Saturn System Mission, a joint endeavor between NASA and the European Space Agency, hopes to launch probes to Titan by 2020 with the aim of better understanding the seas, atmosphere, surface composition and subterranean dynamics of Saturn's largest moon. One of the probes would be a seagoing vessel, designed to sail on Titan's hydrocarbon lakes. Since the surface of Titan is so wildly varied, though, TSSM is also planning to set a hot-air balloon drifting for six months at an altitude of 10 kilometers, recording as it goes.
Proposals for sending a balloon to Titan have existed for a few years now. NASA reports use the term "montgolfiere" to describe the craft, a reference to the hot-air balloon design pioneered by the Montgolfier brothers in 1873. Like these nineteenth-century airships, Titan's balloon would achieve loft by capturing heated gas in a bulbous overhead bag, called an envelope. While the first hot-air balloons kept live fires burning in their gondolas, the heat for Titan's balloon would be generated by an onboard plutonium isotope.
"Hot-air" is a relative term in this case. Titan's mean surface temperature is about -290 Fahrenheit, so a balloon flying over Titan would require only about 1 percent of the heat it would need on Earth. This is one reason a ballooning mission makes sense, though there are others: Titan's slow rotation produces generally calm weather, and a system of trade winds would carry a drifting aircraft all the way around the moon. Just in case the wind doesn't do the trick, the team at TSSM is considering a mechanical propeller to help guide the balloon.
Titan features some of the most diverse terrain to be found anywhere in the solar system. Aside from the massive lakes (the only stable bodies of surface liquid in our system besides Earth's own), Titan also sports deserts, craters, cryovolcanoes and mountains of water ice. A terrestrial rover would be impractical on a world with such eclectic geology. It's hoped that not only could a balloon cover more ground than a wheeled vehicle, but it might even be able to land and scoop a bit of surface material for study.
Most current projections don't put the balloon or the boat on Titan sooner than 2029. An overview of the Titan Saturn System Mission's plans and goals is available online.