Hedgehog Robots Could Hop and Tumble on Comets and Asteroids

Researchers at NASA's Jet Propulsion Laboratory (JPL), the Stanford University, and the Massachusetts Institute of Technology, have developed 'Hedgehog' robots that are capable of hopping, tumbling and flipping in microgravity.

While a Mars rover can't operate upside down, the Hedgehog robot can function regardless of which side lands up. Credit: NASA/JPL-Caltech/Stanford

Traditional spacecraft are designed to roll and move around on wheels. They are not designed to move around in an upside-down manner. Asteroids, comets and other such small celestial bodies may have very rough surfaces, and the gravity conditions on them could be so low that they would be dangerous for traditional robots that roll around on wheels.

"Hedgehog is a different kind of robot that would hop and tumble on the surface instead of rolling on wheels. It is shaped like a cube and can operate no matter which side it lands on," said Issa Nesnas, leader of the JPL team.

The 'Hedgehog' robots have been designed to address the problems faced by traditional rovers. These robots are cubical in shape and have spikes on them. They have internal flywheels that they spin and brake in order to move in a desired manner. During the tumbling and hopping processes, the spikes on the robotic cube function as legs and also protect them.

"The spikes could also house instruments such as thermal probes to take the temperature of the surface as the robot tumbles," Nesnas said.

In June 2015, JPL and Stanford had each built a Hedgehog prototype, which were tested under microgravity conditions aboard NASA's C-9 aircraft. These Hedgehog robots demonstrated their capabilities for getting around small bodies on different types of surfaces including soft, crumbly surfaces; icy, slippery surfaces; rocky and rough surfaces; and sandy surfaces.

"We demonstrated for the first time our Hedgehog prototypes performing controlled hopping and tumbling in comet-like environments," said Robert Reid, lead engineer on the project at JPL.

A “turn in place” or “yaw” maneuver is the simplest type of motion that a Hedgehog robot can perform. The cube robot directs itself in a specific direction, and when the distance to the target is quiet long, it takes large hops using its spikes and moves towards its destination, and when the distance is quite short it tumbles by rotating its faces.

These robots were tested over 180 parabolas, and it was observed that the Hedgehog could spin itself forcefully for launching itself from the surface. This is considered to be a "tornado" maneuver, which could help the robot to escape if it got caught in sticky situations, such as a sandy sinkhole.

The Hedgehog prototype designed at JPL has three internal flywheels and eight spikes on its surface. The robot weighs about 5kg without any equipment. When cameras, spectrometers and other such equipment are added, the weight of the prototype could increase to about 9kg. The prototype designed at Stanford has comparatively shorter spikes, weighs less and is smaller in size.

The maneuvering mechanisms are the same in both the Hedgehog prototypes. The robots have three internal flywheels that spin and stop using motors and brakes for maneuvering. However, the Stanford's prototype uses as friction belts for braking, while JPL's prototype uses disc brakes.

"By controlling how you brake the flywheels, you can adjust Hedgehog's hopping angle. The idea was to test the two braking systems and understand their advantages and disadvantages," said Marco Pavone, leader of the Stanford team, who originally proposed Hedgehog with Nesnas in 2011.

"The geometry of the Hedgehog spikes has a great influence on its hopping trajectory. We have experimented with several spike configurations and found that a cube shape provides the best hopping performance. The cube structure is also easier to manufacture and package within a spacecraft," said Benjamin Hockman, lead engineer on the project at Stanford.

Currently, attempts are being made to increase the autonomy of the Hedgehog robot, to find out the functions that they would be able to perform by themselves, without being instructed from Earth to do specific functions. The researchers consider that in the same way as the Curiosity and Opportunity rovers on Mars communicate through the satellites orbiting Mars planet, a mothership that is in orbit around the celestial body could be used by the Hedgehog robots for communication.

The researchers state that when compared to the construction cost of a traditional rover, the cost of a Hedgehog robot is less. Additionally, many Hedgehog robots could be sent on a single flight. This would allow a mothership to send Hedgehog robots one by one or in a spread out manner to enable better discoveries.

Pavone is leading the Phase II development of the Hedgehog robot through the NASA Innovative Advanced Concepts (NIAC) Program. Nesnas led NASA's Center Innovation Fund (CIF) and NASA's Flight Opportunities Program (FOP), for the flight development and testing of the Hedgehog robots.

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