A walking balloon could one day explore Titan or the Earth's seabed

07/15/2024, Andy Tomasvik, universetoday.com

(a) Screenshot of an animation of BALLET performing a single-step motion, and (b) visualization of BALLET stepping over a large rock to reach a destination represented by a cylinder on the rock. Credit: Hari D. Nayar, Jet Propulsion Laboratory, California Institute of Technology — (a) Screenshot of the animation of BALLET performing a one-step move, and (b) visualization of BALLET stepping over a large rock to reach its destination, represented by a cylinder on the rock. Credit: Hari D. Nayar, Jet Propulsion Laboratory, California Institute of Technology

New ways to move around other celestial bodies always capture the attention of the space exploration community. We’ve reported on many developments at the University of Texas, from robots that hang from the walls of Martian caves to robots that jump using jets of locally mined gas. But we haven’t yet reported on the idea of a balloon that “walks.” But that’s the idea behind BALloon Locomotion for Extreme Terrain, or BALLET, a project by NASA’s Jet Propulsion Laboratory’s chief roboticist Hari Nayar and his colleagues.

How exactly does the balloon “walk”? By lifting and moving one of its six legs. BALLET's architecture includes a positively buoyant balloon supported by six “legs” attached to adjustable tethers. The “legs” are small scientific packages capable of taking small surface samples or analyzing the chemical composition of the portion of the surface it touches.

Each foot is attached to three cables, individually controlled by pulleys. When a foot has completed its scientific work in a given location, BALLET retracts the cables for the foot, lifting it off the surface. It then extends the cables, using different cable lengths, to place the foot in a new location.

A rendering of the BALLET concept, with an aerostat and six suspended payloads that serve as legs. Each payload is connected to the aerostat by three tethers, allowing the payload to be positioned relative to the aerostat. Credit: Hari D. Nayar, Jet Propulsion Laboratory, California Institute of Technology

A preliminary study of the concept was conducted under a NASA Institute for Advanced Concepts (NIAC) grant in 2018. This study showed that it is better to lift two opposite legs off the ground at the same time to ensure the aerostat’s stability. It also demonstrated where the concept would be most useful: on Titan.

BALLET on the surface of Titan. Credit: Jet Propulsion Laboratory/ Shutterstock/ Professional Engineering

Typically, the balloon would be considered for deployment somewhere on Venus, where it could float in the atmosphere under Earth-like conditions. However, at such an altitude, controlling a payload placed on the surface would be extremely difficult. In addition, the harsh conditions on the surface would make the material requirements for the system unacceptable.

Similarly, a balloon could work on Mars, but the high wind speeds in the thin atmosphere would make it difficult to control. Titan offers better conditions – a relatively stable, dense atmosphere where a floating balloon would be feasible, and stable environmental conditions where BALLET would not be blown away by atmospheric winds.

The current plan for Titan exploration is a helicopter called Dragonfly

Titan also has many interesting places to explore, including cryovolcanoes and methane lakes. BALLET will allow even some of the most challenging terrain to be traversed without having to deal with considerations that could significantly affect the capabilities of a rover or helicopter, such as the planned Dragonfly mission.

Cassini radar image of dunes at Shangri-La, Titan. Credit: NASA/JPL-Caltech/ASI

However, there are still many design considerations, such as the difficulty of simultaneously managing all the different variables such as the balloon's orientation, the length of each of the 18 cables, and pathfinding. With the Phase I project completed, the concept appears to be on hold pending further funding from NASA at this stage.

However, in terms of applications, BALLET also has some obvious uses on Earth. The one that immediately came to mind was collecting “nodules” as part of underwater mining operations. Given the increased demand for cobalt and other materials contained in these nodules, and the bad publicity that comes with destroying the seafloor using traditional mining methods, this may be one of those rare space exploration ideas that is more likely to find application on Earth than beyond.

Additional Information:
Research Article – Balloon Locomotion for Extreme TerrainNayar et al.
UT – A Robot With Expandable Appendages Could Explore Martian Caves And Cliffs
UT – A Hopping Robot Could Explore Europa Using Locally Harvested Water
UT – Drones Could Help Map the Lunar Surface with Extreme Precision

The payload assembly in each rack, showing three winches, computing and control electronics units, two battery packs, and a suite of scientific instruments. Credit: Hari D. Nayar, Jet Propulsion Laboratory, California Institute of Technology

Description of the concept of a robotic vehicle (from the scientific paper Balloon Locomotion for Extreme Terrain)

BALLET achieves its advantages through several innovations:
(1) using the balloon for buoyancy and as a structural platform for locomotion,
(2) limbs consisting of taut cables with significantly less mass than legs consisting of dense modules,
(3) dividing the payload into six modular elements and lifting only one or two at a time, which significantly reduces the required buoyancy and the size of the balloon,
(4) Placing the payload in the feet, which keeps the center of gravity very low and the platform very stable.

Each payload will be housed in a 2000 cc – 2U enclosure as shown, with an estimated mass of 2.5 kg. Three winches (arranged symmetrically at 120 degrees offset), computing, control, and power components will occupy about half of this mass and volume (10 cm x 10 cm x 10 cm). The cable will be 0.5 mm in diameter, made of braided stainless steel with a nominal strength of 10 kg. With a spool diameter of 2 cm and a width of 0.5 cm, a 1 m length of wire will take up no more than two layers of winding on the spool. Each foot will have three cable winch components. Each cable winch component will include a spool, a spring-loaded cable tension sensor, a spool rotation sensor, a brushless DC motor, and a motor controller. To control the cable length, an initial calibration process will use the spool rotation position to determine the cable length. Power will be supplied by two lithium-polymer batteries with a total weight of 0.5 kg and an energy capacity of 75 Wh with cooling fans, which fit into a volume of 10 cm x 10 cm x 3 cm.

Each leg payload will contain a self-contained computing system with the software and power necessary to operate its assigned instruments and winches. A single-board computer will run the control and sensor software. Power will be provided by two battery packs housed in each payload, as shown in the figure. The computer in each leg will also be used to interface with the instrument or instruments inside the leg. Ultimately, during the course of the mission, coordination of instruments and services for control and data processing, mobility, asset monitoring, troubleshooting, and communications will be distributed among the leg computers.

To ensure mobility, one of the six computers will act as a coordinator, synchronizing movement between payloads for different gaits. This unit will receive commands from the wireless controller and transmit them wirelessly to the other five computers. To implement this model, each payload module must have Wi-Fi or Bluetooth connectivity.