Soft Robotic Perspective and Concept for Planetary Small Body Exploration

Abstract

Tens of thousands of planetary small bodies (asteroids, comets, and small moons) are flying beside our Earth with little understanding. Explorers on the surfaces of these bodies, unlike the Lunar or Mars rovers, have only few attempts and no sophisticated solution. Current concerns mainly focus on landing uncertainties and mobility limitations, which soft robots may suitably aid utilizing their compliance and adaptivity. In this study, we present a perspective of designating soft robots for the surface exploration. Based on the lessons from recent space missions and an astronomy survey, we summarize the surface features of typical small bodies and the associated challenges for possible soft robotic design. Different kinds of soft mobile robots are reviewed, whose morphology and locomotion are analyzed for the microgravity, rugged environment. We also propose an alternative to current asteroid hoppers, as a case of applying progress in soft material. Specifically, the structure is a deployable cube whose outer shell is made of shape memory polymer, so that it can achieve morphing and variable stiffness between liftoff and landing phases. Dynamic simulations of the free-fall landing are carried out with a rigid counterpart for comparison. The results show that the soft deployed shell can effectively contribute to dissipating the kinetic energy and attenuating the excessive rebounds.

Introduction

Planetary small bodies have received broad attention for their scientific value, resource utilization possibility, and threats to Earth.¹ Some of these bodies preserve the traces of the evolution of our solar system,² themselves or their fragments may have brought the early water, organics,³ and catastrophic collisions.⁴ An effective solution for surface exploration may refer to soft robots,^5,6 which have made a niche in outer space^7,8 and deep sea.^9,10

Surface exploration can provide plenty of ground truths at low costs.^11,12 The geological contexts of an asteroid, like the size and distribution of boulders on its surface, give access to study its original formation and geological evolution.^13,14 An analysis of spatial material heterogeneity based on in situ sampling may help validate the evolution hypotheses.¹⁴ The onboard camera¹⁵ carries human sight to the surface, while the radar¹⁶ can show the internal structure once it locates at an ideal zone. The mobility enables rovers to enter these science-rich sites or escape from possible fatal areas.¹⁷ Besides, the surface mechanical properties can also be inferred based on the contact data.¹⁸

However, small body surface exploration is difficult with only one recent success.¹² Compared to other aerospace missions, its unique challenges are mainly landing steadiness and surface mobility, caused by the weak gravity and the rugged surface. In 2012, the landing system for small bodies was ranked as NASA's medium priority for technology development, while extreme terrain mobility and small body mobility were both high priorities.¹⁹

Since the initial landings are conducted as free falls in microgravity, the impact speeds (<1 m/s) are generally tolerable for the landers,²⁰ and the concerns focus more on reducing kinetic energy and rebound uncertainties. For example, the lander MASCOT2 is estimated to have an impact velocity of two to three times larger than its target's escape velocity,¹⁷ which means it may bounce off the surface and lose contact forever, unless the kinetic energy is effectively attenuated within the first touch. In 2014, the lander Philae¹⁸ suffered accidental bounces on comet 67P due to failures of both hold-down thrust and anchoring harpoons, and the bounces lasted about 2 h, covering 1.2 km. The energy consumption of the first impact was estimated¹⁸ that although most of the lander's kinetic energy had been dissipated in the soil and the mechanisms, the remaining 10% still led to the undesirable rebound flight.

Releasing the lander at a lower altitude can help reduce the descent speed; however, the close distance between mothership and surface would bring control difficulties and threats. Several landing equipments have been proposed, such as the landing gears with damper,²¹ the granular bag,²² and the mass release mechanisms,^23–25 which generally require bulky design or accurate timing control. Grimm et al.²⁶ designed a deformable protective shell, which must be discarded after the touchdown so that the lander can have further free mobility.

Unlike this discarded shell, soft robots can combine landing energy absorption and mobility into one system, thus may save complexity and cost for the overall mission.^8,27,28 One example of such dual use is the tensegrity robots^27,28 for relatively large gravity, while other compliant concepts have been assessed for microgravity asteroids.^29–32 Compared to rigid landers, soft structures undergo more considerable deformation during the contact, dissipating the kinetic energy through internal damping.³³ In addition, the rubbery materials that build up soft robots intrinsically have higher friction and lower coefficient of restitution. The soft hopper developed by Bartlett et al.³⁴ that uses gradient stiffness to balance jumping performance and landing protection also proves a robust interface between its soft body and rigid payloads.

Various motion strategies have been studied for mobile robots; among them, the small body missions require traction-free and terrain-adaptable solutions. Most locomotion modes based on friction or even anisotropic friction are not feasible in microgravity, such as snake-like serpentine,³⁵ inchworm-like crawling,³⁶ or earthworm-like peristaltic locomotion,³⁷ unless they are combined with adhesion mechanisms.^38–40 The most practical strategy using state-of-art techniques might be jumping. In 2018, a decimeter-sized hopper MINERVA-II¹² successfully traveled about 800 m on the surface of asteroid Ryugu, whose liftoff is triggered by a flywheel's torque (similar to Cubli⁴¹). Since MINERVA- II has only a limited budget,¹² its upgraded generations^42,43 have already been planned by several space agencies.

In this work, we discuss the possibility of applying soft robots to small body exploration. First, typical small bodies' surface environments are summarized from precursive researches, which guide an analysis of robotic morphology and locomotion. In the following sections, several soft mobile robots are reviewed, and a novel hopper is further presented based on space-verified techniques. The final discussion identifies the application gaps of possible soft robotic options.

Materials and Methods

Targets' surface environments

In this section, we carry out a survey on the surface environments of some small bodies that mainly belong to Near-Earth Objects, which have acceptable flight distance, temperature condition, and photovoltaic source.

Small bodies generally have weak and nonuniform surface gravities (a typical order¹⁷ of 10⁻⁴ m/s²) because of their small irregular shapes.^44,45 The escape velocity^46,47 (the speed required to leave from the gravitational attraction when vertically ejected) limits the upper speed of robots to several centimeters per second. Besides, some asteroids are observed to have fast rotation; the centrifugal force and the Coriolis force make the dynamic environment more complex. The centrifugal forces may dominate over self-gravity on a large area of the surface, resulting in an outward net acceleration⁴⁸; also, the acceleration can have tangential components to the local ground, which may push the standing objects sliding away.⁴⁹

The rotation without atmosphere also leads to diurnal temperature fluctuations,^50–52 causing equipment degeneration and operation shutdown.⁵³ However, the periodical environmental fluctuations (temperature and illumination) encourage discussions of self-propelling robots^54,55 or self-anchoring structures.^56,57 For some freezing targets far from Earth,⁵⁸ the ice robot is imagined⁵⁹ while substantial technological gaps exist.

Multiple observations have shown that the surfaces are covered with numerous ridges, craters, ices, boulders, and granular regolith^60–64 (Fig. 1a, b). Take Ryugu as an example, the craters can have diameters over hundreds of meters and depths over a few tens of meters,² and nearly 4400 rocks larger than 5 m lie on its small surface.⁶⁴ Rather than coherent bodies or monoliths, some asteroids are thought to be loosely collected in the form of rubble piles^14,65 through the Van der Waals forces of fine grains, with cohesive strength on a level of tens of Pa.^48,66 The granular regolith with such tenuous cohesion would require specialized designs of locomotion,^67,68 anchoring,^69,70 and sampling.⁷¹

FIG. 1.

The surface environment and its challenges. (a) Image taken from the orbit above the asteroid Ryugu.⁴⁴ Image credit Seiji Sugita.² (b) Image taken on the surface of Ryugu.¹¹ Reprinted by permission from AAAS. (c) The asteroid hopper MINERVA-II.²⁰ Photo credit Yusuke Oki. (d) Simulation of the landing shows bifurcated trajectories. The marks of the landers are amplified 100 times for clearance. Color images are available online.

The extreme terrains bring uncertainties to both landers¹⁸ and jumping robots (Fig. 1c) in each takeoff or touchdown,^42,72 and the microgravity then amplifies the uncertain flights. We simulated the landing scenario of a rigid lander as presented by Van wal et al.,⁷³ and the settling positions show clear bifurcation (Fig. 1d). The landing site choosing and the path planning suffer from such indeterministic trajectories.⁷⁴ Since the communication with Earth has minutes of time delay,⁵³ rovers should have a high level of adaptivity, through autonomous environment-analyzing and decision-making algorithms⁷⁵ (computational intelligence) and through the virtue of their physical designs (mechanical intelligence⁷⁶).

Table 1 lists key features of some visited small bodies. Although future revisits can benefit from such knowledge, it should be noted that the map-like details are seldom known a priori. For example, recent observation of Bennu by spacecraft OSIRIS-Rex reveals unexpected texture against the prelaunch prediction, which is beyond the spacecraft's sampling design specifications.⁷⁷

Table 1.

Documented Knowledge of Four Small Bodies

Asteroid/comet	Ryugu	Bennu	Itokawa	67P
Spacecraft	Hayabusa2 in 2018; 3 successful hoppers¹¹	OSIRIS-Rex in 2018; touchdown sampling⁷⁷	Hayabusa in 2005; lander MINERVA failed⁶⁵	Rosetta in 2014; lander Philae severely bounced¹⁸
Shape	Spinning-top¹⁴	Spinning-top⁷⁷	Sea otter with small head, large body⁶⁵	Two lobes connected by a neck⁶³
Surface area	2.7 km^{2 64}	0.782 km^{2 77}	0.393 km^{2 61}	36.4 km^{2 62}
Surface gravity¹⁷ (m/s²)	$1.5 6_{- 0.21}^{+ 0.28} \times 1 0^{- 4}$	8.67 ± 0.48 × 10^–5	9.12 ± 0.55 × 10^–5	2.46 ± 0.16 × 10^–4
Escape velocity	0.24–0.46 m/s⁴⁶	0.13–0.23 m/s⁴⁷	∼0.2 m/s⁶¹	∼0.9 m/s¹⁸
Rotation period¹⁷	7.63 h	4.30 h	12.13 h	12.76 h
Daytime surface temperature	290–375 K with a global average of 339 ± 12 K⁵¹	Maximum of 303–390 K at equator⁵⁰	A global average of 340 ± 10 K⁵²	180–230 K⁵⁸
Possible obstacles (sizes in diameter)	4400 boulders⁶⁴ >5 m 30 craters² >20 m	208 ± 40 boulders⁷⁷>10 m 12 craters⁶⁰ >10 m	373 boulders⁶¹ >5 m 9 craters⁶¹>50 m	3546 boulders⁶² >7 m A 900-m-high cliff⁶³

Soft robotic solutions for roving the surface

Hopping is efficient to travel long distances and over obstacles in microgravity. Many untethered soft hopping robots (Fig. 2a), mainly designed for Earth's gravity, are powered by combustion^34,78,79 to obtain intense output force. Since a slight kick can achieve enough jumping performance, soft robots like JelloCube^80,81 (Fig. 2b) are competent, which uses the strike of an elastic strip, and Limpet^82,83 that uses flexible linear motors. Slowly storing and fast releasing the potential of deformable shells can also lead to jumping,^84–87 inside which the scientific payloads can be mounted. Rolling or tumbling is also applied to shell-like robots^87–89 by reaching their mass centers out of supporting areas (Fig. 2c, d). This strategy reduces sliding; however, it requires multiple coordinated actuators^90,91 or oscillators.⁹²

FIG. 2.

Different soft robots that have appealing features for small body exploration. (a) A hopper with gradient stiffness.³⁴ (b) JelloCube.⁸¹ (c) An isoperimetric tumbling robot.⁹¹ (d) A rolling tensegrity robot.¹²³ Image credit Kyunam Kim. (e) Modular Limpet.⁸² Image credit Mohammed Sayed. (f) A multilegged robot with artificial intelligence package.¹⁰³ (g) A starfish robot and its gait.¹⁰⁰ Image credit William Scott. (h) Sea urchin-inspired robot.¹⁰² (i) A soft and spiny paw for climbing.¹²² (j) A wall climbing robot.³⁸ (k) A growing robot.¹³² Images reprinted with permissions. Color images are available online.

Possible bioinspired solutions may refer to the zoobenthos (animals inhabiting the seafloor), such as echinoderms, pelecypods, and crustaceans. The buoyant and turbulent environment has molded their morphology and locomotion styles, and many of them maintain conforming to the seabed at slow speeds rather than swimming.⁹³ For soft robots (Fig. 2e–h), the dynamic behavior of underwater running and crawling for octopus robots^94–96 and the coordinated gait design for starfish robots^97–100 are well studied. An actuator inspired by the tube feet of echinoderms combines magnetic adhesion and locomotion^101,102 (Fig. 2h). Similarly, a number of multilegged soft robots^103–108 have been developed, and in some cases, modular designs^104–106 are adopted. Whether using adhesion or not, the principle that can inspire microgravity rover design is to distribute the actuation system into multiple units (distributed actuation^109,110), constraining the main body's acceleration undulation to avoid the risks of detachment from the surface.

Equipped with adhesive grippers, the limped robots (Fig. 2i, j) are capable of crawling, drilling, and sampling.⁷⁰ The surface-grasping technologies that suit microgravity vacuum space include dry adhesion,^111–114 electrostatic adhesion,^{38,39,115–117} and micro-spine hooks.^118–122 The granular rough surface would require integrated load distribution, substrate conforming, and cleaning mechanisms. As mentioned, the rocks on some asteroids themselves are loosely aggregated, and on these fragile interfaces, the anchoring behaviors still need to be verified. In addition, limited initial preload can be exerted in the nearly floating state, as well as the axial force for auger anchors.⁶⁹

Tensegrity robots^123–126 (Fig. 2d) have been developed to survive from heavy drops and intellectually conduct multiple gaits. Since the landing impact on small bodies may be too weak to deform most tensegrity structures significantly, the overall stiffness can be tailored as Rieffel and Mouret's¹²⁶ for better damping and conforming performances. Besides, tensegrities are appealing for their easiness of carriage, for which reason origami or kirigami structures^127–131 are also preferred. Another promising strategy to cruise the surface is deploying soft growing robots,^132–134 so that they can relieve the base lander from mobility systems. The growing robot developed by Hawkes et al.¹³² (Fig. 2k) can lengthen itself from 28 cm to 72 m, which is close to one-third of the asteroid Itokawa's minimum diameter.⁶⁵

A hopper with stiffness and configuration modulation

In this study, we introduce a novel concept of hopping rover shown in Figure 3a, which is based on the experiences of MINERVA-II¹² and inspired by the aforementioned soft robots. The proposed structure is a deployable polyhedron like cubes (Fig. 3b), with ordinary internal instruments and an outer shell made of shape memory polymer (SMP). A tendon-driven system is used to fold the shell, and the SMP is thermally regulated by embedded flexible resistance heaters and thermocouples.

FIG. 3.

The proposed hopper concept. (a) The hopper undergoes morphing and stiffness variation between liftoff and landing phases. (b) The structure of the robot. A MINERVA-II like Cubli⁴¹ is wrapped by a deployable shell that is made of SMP. (c) The modulus variation of SMP in response to temperature changes. An optional strategy to utilize the environmental temperature is shown, which requires the spatial design of T_g between hinge and panel regions. SMP, shape memory polymer. Color images are available online.

For a hopper, it has been numerically tested that a firm kick on the ground is preferred for broader exploration coverage.^34,68 Accordingly, we adopt a variable stiffness strategy conducted by SMP,¹³⁵ whose glass transition temperature (T_g) marks a distinct transition of a glassy state (modulus ∼GPa) and a rubbery state (modulus ∼MPa) (Fig. 3c). The reinforced SMPs have demonstrated their qualification of direct and chronic exposure to space.^136,137 Besides, their low density, low cost, and high strength-weight ratio¹³⁸ make them ideal material for the robot.

During the initial landing process, the SMP shell is heated to be compliant and deployed. Based on the conservation of angular momentum, the flywheels are used for directional hopping and in-flight attitude control,^42,53 so that the robot touches the ground with a proper attitude. The unfold state on the ground ensures unobstructed visibility of internal instruments; also provides a larger illumination area on the solar cells than the cubic state.⁴³

Before liftoff, the heaters soften the SMP shell's hinge area, which is then folded by the tendons. Once completed folding, the heaters turn off while the tendons are kept tensioned until the shell cools down, so that the cubic configuration can be reserved. The folded shape has smaller momentum of inertia than the unfolded shape, and high stiffness is also obtained. These two features both benefit further jumping. Due to the weak gravity, the jumping flights can last a few minutes,¹² during which the shell is heated to recover its flat soft state, preparing for the next landing.

Results and Discussion

Simulation and operation of the robot

We use the planar discrete elastic rod model¹³⁹ with width adjustment¹⁴⁰ (discrete ribbon, see Supplementary Data and Supplementary Fig. S1) to simulate the bounces of a deployed, soft-rigid hybrid structure. A rigid counterpart is also simulated to illustrate possible advantages of soft robots, and the gravity is 10⁻⁴ m/s². As shown in Figure 4a and Supplementary Video S1, the robot with the soft shell tends to conform and slide on the ground, and the velocity of the carried rigid component is diminished before it touches down. Meanwhile, the rigid lander experiences transitory collision and sliding, which is similarly verified by experiment⁴² and simulation,⁷³ indicating a limited energy dissipation with little sliding. As a result, the bouncing distances of the soft-rigid hybrids are lower than the rigid counterparts, as shown in Figure 4b.

FIG. 4.

Simulation, deployment, and folding of the SMP shells. (a) Dynamic simulation of the microgravity bounces at initial attitude and velocity of 45°. A lander with a soft deployed shell is compared with its rigid counterpart. (b) Simulation of the trajectories of the first bounce with different initial attitudes. (c) Deployment of the shell through shape memory effect by heating. (d) Folding of the shell is driven by tendons. The process is conducted on a heating platform at 45°C, and the spatial T_g design enables the folding without embedded heaters. Color images are available online.

Figure 4c and Supplementary Video S2 show the unfolding of the SMP shell by shape memory effect. Heaters are glued at the hinge area (where the thickness is 2 mm), and each consumes a power of 4 W to realize unfolding at room temperature. The power consumption can be optimized by reducing the thickness or introducing conductive reinforcers.¹⁴¹ If controllable stiffness is required during the whole operation, T_g of the SMP shell should be higher than the target's maximum surface temperature. T_g is easily tailorable through various raw material and ratio choices; for example, a series of epoxy-based SMPs developed by Leng et al.^142,143 for space applications have the T_g range of 37–163°C.

An optional energy-saving strategy is to utilize the diurnal temperature fluctuation. To do so, the glass transition temperature of the hinge areas, T_g1 (40°C in the experiment), can be designed to be lower than the daytime temperature T_surf (assumed as 45°C). At the same time, the panels have higher glass transition temperature (T_g2 ≈ 75°C; Fig. 3c), so that they remain rigid and can be folded by tendons (Fig. 4d and Supplementary Video S3). Such spatial design can be manufactured in one piece through multimaterial 3D printing¹⁴⁴ or dual curing.¹⁴⁵

Future development of this prototype will consider a fully wrapped configuration, the honeycomb structure,²⁶ and gradient stiffness.³⁴ Besides, some SMPs that have self-healing property^141,146 enable active maintenance of the robot and increase its survival chance in outer space.

Chances and challenges for soft robots

One concern of developing small body robots is that, currently, it has limited relevance to the nonaerospace community¹⁹; however, challenges posed by future missions require interdisciplinary discussions. There is no one-fit-all solution for the unknown diversified targets, and a swarm of micro robots is of interest to space agencies.¹⁷ These are in accord with soft robotic subjects, from individual adaptivity^108,147 to swarm coordination.¹⁴⁸ In addition, soft robotic proposals can find intersections with other research interests such as benthic or adhesion technologies.

Considering the similarities to small satellites, robotic design can utilize their criteria,¹⁴⁹ and a convenient way to choose components is from those already having high technology readiness levels. Beyond that, a continuous study on basic techniques is demanded, such as miniature tenacious actuators^150,151 and corresponding manufacture equipment.^152,153 Under harsh environments, the actuation performance would degrade,¹⁰ and the systemic design should pass a series of ground tests such as radiation and vacuum thermal cycling.^136,142 The environmental impacts on polymers, which are used for seals and substrates in space, are summarized by Krishnamurthy.¹⁵⁴

Soft prototypes should be persuasive for arranging technology-driven missions¹² to complete tests, while pursuing a maximum scientific return. Integration of standard scientific payloads results in soft-rigid hybrids, and in some cases, elaborate management of rigid interfaces^34,121 and component distribution¹⁰ is necessary. Although microgravity releases the burden of carriage on untethered robots, the mobility system should still have meticulous weight and power occupation.²⁷

While the dynamic behaviors like contact and motion would be distinct in various gravity conditions, a key challenge on the path of soft robots is the difficulty in validating their performance in ground laboratories. Only few three-dimensional gravity-compensation test beds^42,155 developed for rigid rovers meet the requirements of the large workspace (several meters), long operation time (several minutes), high accuracy, and quick response. The deflection of large-scale soft robots under Earth's gravity cannot be compensated by these overhanging systems, making the validation more difficult, while emulating in-plane microgravity for soft robots with restricted morphology is possible using air-floating test beds.^31,156 Low-cost launches¹⁵⁷ and access to the International Space Station would significantly benefit key technical tests, such as the tested gripper.¹¹¹ Based on accurate modeling,^33,158,159 robotic optimization and evolution¹⁶⁰ can be carried out aiming at small body environments, which will be virtually constructed using the mentioned obstacle statistics.

Conclusions

Future planetary rovers should possess both computational intelligence and mechanical intelligence. From the mechanical aspect, this article invites a discussion of the soft robotic option for small body exploration, aiming to enhance survival chances and to open up science-rich but risky zones.

Past space missions and astronomy researches have revealed the major challenges of surface exploration: the near-floating microgravity with perturbations, the space exposure with temperature fluctuations, and the diversified geography with prearrival uncertainties. Soft robots can perform their fortes from the beginning landing process and provide versatile mobility choices. However, application gaps await further practical researches, such as the untested actuation or anchoring performances.

As a case for discussion, we present an alternative to the current asteroid hoppers. A shell that is made of SMP is used to transform the hopper's stiffness and configuration, pursuing a firm liftoff and a smooth touchdown. Dynamic simulation identifies the contribution of the soft deployed shell to reduce landing uncertainties. It is dependent on real mission objectives that soft robots should be designed, and we regard these as driving factors for future developments of our prototype.

Footnotes

Author Disclosure Statement

No competing financial interests exist.

Funding Information

This work was supported by the National Key R&D Program of China (Grant No. 2019YFA0706500).

Supplementary Material

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