NASA's weird BALLET rover would float and walk across the surface of Titan

The surface of Titan, the largest moon of Saturn, is similar to that of Earth – just substitute extremely cold methane for the water in rivers and seas, add a dense atmosphere absorbing 90% of sunlight, and volcanic features spewing ‘lava’ of ammonia and water.

Titan’s surface gravity is weaker than that of the Earth’s Moon, and the origin of its craggy, mountainous features has inspired debate. 

Exploring such an inhospitable environment will require an ingenious solution. Hari Nayar from NASA’s Jet Propulsion Laboratory hopes it could be a rover called the Ballet – BALloon Locomotion for Extreme Terrain.  

“The concept is basically a balloon with six legs,” he says. “The system is designed so there is enough buoyancy in the balloon that you can lift two feet at a time, but not enough that it can completely take-off from the ground.”

It is a striking design – but one with a solid engineering basis, says Nayar. “In general, walking machines or limbed machines are more capable of negotiating rough terrain compared to wheeled vehicles,” he says. Although wheels are generally much more efficient, vehicles using them struggle with slopes of more than 20º, and some of the most promising areas of future exploration could be the most difficult to reach. 

Step by step

Nayar’s solution uses several recognisable features in novel ways. The balloon inflates using compressed helium after landing. Titan’s dense atmosphere and low gravitational pull mean that a relatively small balloon could lift a decent amount of mass. A 3m-diameter balloon might be able to carry a 45kg radioisotope thermoelectric generator, a nuclear device commonly used for space missions where batteries would run out – or, as on Titan, where there is too little sunshine for solar panels.

With the generator on board, the Ballet should be able to carry another 5kg per ‘foot’. Mission equipment goes in the feet, allowing scientists on Earth to examine the alien world. Each foot is controlled by three cables of changing lengths, while a balloon-mounted camera looks ahead as the rover gently perambulates at about two or three metres per minute. 

A question of balance

Will it work? “I think it has potential but we are still in the very early stages of the development,” says Nayar. “There is a long road ahead of us in terms of trying to prove this is a viable concept… a lot of work to get it to a stage where it will be considered for a future mission. But I do hope that some day it will.”

Fittingly, the Ballet concept comes down to questions of balance. Instruments must be small and light enough for the feet to move easily, yet sensitive enough for mission applications. The balloon must be buoyant enough to stop cables going slack, but not so inflated that it floats off. 

Nayar hopes the rover could also be used elsewhere, such as on Mars or Venus, but that would bring even more challenges – fluctuating temperatures and gusting winds, for example. 

Nayar and his team have run simulations and analyses, and he hopes to build a prototype to show the world his idea is ready for the solar system. 

“It’s kind of a fun thing to work on,” he says. “It’s pretty unusual – but at least the physics says it could work.”