Reduced Gravity Testing
The problem
Section titled “The problem”A rover, lander leg, sampler or manipulator that will work at 1/6 g, 0.38 g or 1e-3 g has to be tested on Earth, where the weight, the soil and the fluid dynamics are all at 1 g. Four ground methods reduce the apparent weight of a test article: free fall in a drop tower or aircraft [1], mechanical offload of a fraction of the weight through a cable or counterweight [3], flotation on an air bearing over a level floor [7], and submersion in water at neutral buoyancy [5]. Each removes weight along a different axis, for a different duration, and each leaves a different residual that shows up in the data. Orbit does not supply a clean reference either: measurements on Spacelab, SPACEHAB, the shuttle middeck and Mir show that crew exercise dominates the 1 to 4 Hz band and pump activations produce transients of about 1 mg [19].
What has flown
Section titled “What has flown”Pogo, JSC. A pneumatic lifting cylinder hangs the subject from a gimbal support assembly through a chest harness and seat, and a two-stage vertical servo, a flapper-nozzle pressure amplifier driving intake and exhaust servovalves, holds the commanded lifting force as the subject moves [3]. Against walking and running subjects at lunar and Martian levels, Pogo held the simulated body force to within 2.2 to 10 percent, depending on the type of locomotion input. Mean stride frequencies at running speeds were 12 percent below the Earth values, and foot to ground reaction forces were 62 percent lower at lunar gravity and 51 percent lower at Martian gravity than on Earth [3].
ARGOS, JSC. The Active Response Gravity Offload System is a robot built like an overhead bridge crane: the active machine supplies the three translational degrees of freedom while a passive gimbal at the attachment point supplies the three rotational ones, and every translational axis is actively driven so that the machine’s own inertia is kept off the subject [4]. It has been in development since 2007, covers a test volume of more than 340 cubic meters, moves at up to 3.3 m/s and detects faults in under 20 milliseconds [22]. Human testing has covered the MKIII prototype planetary suit and the xEMU with a mass-representative PLSS mockup [4][5].
ATHLETE low-gravity testbed, JPL. The vehicle is suspended from six computer-controlled winches so that measured contact forces drive a simulated microgravity trajectory. The reported result is a simulated 5 microgee hop reaching almost 2 meters, stated without error bars or independent verification, plus a helical auger anchor driven in soft simulant until the test bin ran out of depth [23].
Chinese three dimensional follow-up system. It qualified Chang’e 3, Chang’e 5 and Tianwen 1 for hover, hazard avoidance, slow descent and landing, and cascades a fast cable-parallel platform for coarse tracking with a fine horizontal stage and a tension trim device; the measured maximum cable inclination was 0.17 degrees, about 12 mm of tracking error at a 4 m mean cable length [15]. The moving mass of the fast platform exceeds 20 t, far above the test spacecraft, and accelerating it is the source of the coupled sway the design has to suppress [15].
LAMA, DLR. The Philae landing gear was touched down from a KUKA KR500 robot on a 12 m rail through a weight offloading suspension whose cardanic joint was trimmed to the test object’s center of gravity, giving about 30 degrees of rotation in x and 17 in y [33]. The authors state that comet soil cannot be reproduced there and that rig artifacts, such as a suspension end-block impact removed by hand from one trace, were acknowledged but not bounded [33].
Parabolic aircraft. The Falcon 20 campaigns of the NRC carried a single wheel testbed and a cone penetrometer into effective lunar gravity [13][20]. The Bremen facilities and the DC-9 class aircraft give the durations in the table below [2][28].
Neutral buoyancy. The Neutral Buoyancy Laboratory trained the EVA corps through ISS assembly; its measured costs are in the neutral buoyancy section.
What is being tried now
Section titled “What is being tried now”Active and passive offload at the soil bin. Cable rigs for rovers continue to be built at small scale. IRGO, a Michigan Tech overhead two-axis trolley with a passive counterweight, tracks an infrared beacon on the rover to stay above it; the control loop is described as undeveloped and mostly untested, and its accuracy numbers come from a stationary beacon [34]. At the other end of the trade a counterweighted air bearing stack with a cylindrical bearing that doubles as an air duct reached under 0.1 N of vertical friction, but only 0.2 m of vertical stroke, plus or minus 45 degrees of rotation and about 1 minute of free motion on one 8 liter tank [9]. That prototype was shown on the bench; its manipulator testing, docking and EVA training uses appear only as concept figures [9]. All of these are ground tests.
Air bearing floors and mobile chasers. The ESTEC Orbital Robotics and GNC Laboratory combines a flat epoxy floor carrying three degree of freedom platforms with a robot arm on a 33 m track that replays rendezvous and descent trajectories in six degrees of freedom [24]. In the ESTEC ROOTLESS prototype the air bearings face upward from an omnidirectional mobile robot, the 4.7 kg payload floats on a glass and honeycomb plate, and the robot chases the payload with a contactless line-scan sensor so that no hose or cable touches it [16]. Contact experiments reproduced post-impact free-floating motion at 20 to 200 mm/s with position error under 10 percent [16]. A later three point automatically leveled plate turns the parasitic tilt into a control input, giving a settable simulated gravity from 4.756e-5 m/s2, the inclinometer resolution floor, to 0.1712 m/s2; Phobos gravity of 0.0057 m/s2 needs a 0.0333 degree tilt, and the rig remains planar [31]. The MSFC Flight Robotics Laboratory was built around a 4000 square foot precision air bearing floor carrying a motion base with six pressurized air tanks feeding 32 thrusters, and an eight degree of freedom overhead X-Y crane [7]. The Flat Floor Robotics Laboratory at the same center now carries tethered spacecraft simulators on a 44 by 86 foot self-leveling epoxy floor [8].
Parabolic and drop tower tests of soil and sampling hardware. Parabolic flight and drop towers are the only methods here that change the body force on a granular bed. Rigid wheel drawbar tests on lunar simulant FJS-1 and Toyoura sand compared gravity levels from 1/6 to 2 g with equivalent wheel load variation at 1 g, using a 150 mm aluminum wheel [17]. Penetrometry for the MMX sampler was flown with eight actuator-driven penetrometers, four tip shapes and six materials; tip shape changed the reaction force substantially, cohesion dominated the response, and in 2 mm glass beads the bed behaved like a liquid with grain strikes that produced force spikes large enough to obscure the trend [18]. In the GraviTower Bremen Pro a JSC-1A hourglass flowed to completion at 1 g but jammed and stayed clogged at 0.19 g, which the authors attribute to the granular Bond number, the cohesive force over particle weight; one run per gravity level was shown [26]. A Shear and Compression Cell for drop tower falls is built to measure cohesion and friction angle, but only two curves at 1 g have been reported [27]. These are aircraft or tower tests of seconds to tens of seconds; none is a flight on another body.
Longer low gravity by other means. A centrifuge on the ISS held 0.063 to 2.0 g for seven hours while vacuum hourglasses of eight granular media flowed; mass flow deviated from Beverloo’s law at low gravity, which the authors attribute to falling bulk density, so simulations carrying a 1 g bulk density overestimate rover traction and underestimate footpad sinkage [25]. This is a flown experiment in Earth orbit, with the centrifuge field and Coriolis effect handled by DEM comparison. Magnetic levitation in the GMMT rig holds a steady one sixth g on a magnetic simulant, and static cone penetration resistance at 1/6 g was about 29.1 percent below 1 g at 40 percent relative density but only about 8.8 percent below at the denser state [29]. The simulant is 40 percent magnetite, and the field adds an apparent cohesion, so it is a ground test with an acknowledged artifact [29].
Simulation and scaling laws. Discrete element models of single wheels, including a parallel-particle variant cheaper than conventional DEM, are being validated against wheel tests [14]. Physics-based simulation and granular scaling laws are proposed as the route from 1 g tests of flight-mass vehicles to lunar predictions [6]. A thesis built on two Falcon 20 campaigns graded three Earth-based methods against the lunar gravity data: reduced-weight testing at matched wheel load overstates mobility, with drawbar pull to weight falling 20 percent or more and sinkage rising up to 40 percent at equal load [20]. NASA’s current mobility test practice document recommends full-vehicle and single-wheel methods and lists the missing south pole simulant and terrain data as an open gap [21].
Flight programs and proposals. The DLR and CNES MMX rover is planned as the first wheeled vehicle to drive in milli-gravity, where motions scale with the square root of the gravity ratio, about 50, so 5 cm/s on Earth corresponds to about 1 mm/s on Phobos; the paper reports expectations, not results [30]. A proposal to fly a vacuum chamber on Mars-gravity parabolas to test the Mars Helicopter contains one simulation, a 26-second maneuver inside a 1.3 g airframe load limit, and nothing built [32].
What an offload rig distorts
Section titled “What an offload rig distorts”The offload is applied at a single gimbal pivot point, so only the mass acting through that point is offloaded. Limbs and appendages that move outside the line of action are still accelerating against full terrestrial gravity, which is most severe in prone postures where the lower torso lies entirely outside the region served by the cable [5].
The system center of gravity is computed for a static neutral posture and moves when the subject bends, climbs or goes prone, and the gimbal cannot follow it dynamically or be readjusted in real time. Mapping the reachable pivot point envelope of the spreader bar gimbal showed large dead zones in which the pivot cannot be placed at all, so a pivot coincident with the system center of gravity was not achievable for every subject, and where it was achievable the resulting neutral equilibrium was hard to control and allowed the subject to rotate upside down about the pivot during initial suspension [5]. Incorrect center of gravity alignment, a high inertia lifting mechanism, or too few degrees of freedom each produce false data: the earlier Pogo rig gave only two translational degrees of freedom and carried substantial overhead inertia, which is why ARGOS drives all three translational axes and its gimbals carry an adjustable center of gravity [4].
The offload fallacy in terramechanics
Section titled “The offload fallacy in terramechanics”Offloading a vehicle, or reducing its mass, changes the load the soil carries but does not change the gravitational body force acting on the soil grains. Terrain under Earth gravity has a higher shear yield point than the same terrain under lunar or Martian gravity, so a lightened vehicle on Earth soil is supported by material stronger than it will meet in flight, and the trafficability result is optimistic [6]. The overhead cable holds the vehicle at reduced weight while the soil stays at 1 g, so nothing about soil strength, sinkage or traction at lunar gravity is tested by the rig itself [22]. The Curiosity case is explicit: the rover was stripped to 340 kg from a nominal 907 kg so that its Earth weight matched its Mars weight, leaving the Mojave terrain under full Earth gravity.
Quantified against VIPER using continuum simulation correlated with Glenn SLOPE lab physical tests, on GRC-3b at a friction angle of 47.8 degrees and bulk density 1839 kg/m3 the 73 kg MGRU3 test rover climbs a 30 degree slope at about 42 percent slip on Earth, while the 440 kg VIPER on the same terrain in lunar gravity would run at about 85 percent slip, above the 80 percent threshold associated with dig-in [6]. On the softer case, friction angle 37.8 degrees and density 1627 kg/m3, MGRU3 climbs 10 degrees at 18 percent slip and VIPER on the Moon would see roughly 75 percent. The nominal, unlightened vehicle produced the same slip against slope curve on the Moon as on Earth, so the correct procedure is to test the flight-mass vehicle and transfer the result through granular scaling laws rather than to offload it.
The effect has been measured directly, in the one facility class that changes the body force on the soil as well as on the vehicle. A single ExoMars flexible wheel was run in ES-2 Martian simulant aboard the NRC Falcon 20 parabolic flight aircraft at 10 to 70 percent slip, with wheel load held equal between gravity levels [13]. Maximum sinkage rose 38 percent in lunar gravity against 1 g at 164 N wheel load, and 27 percent in Martian gravity at 164 N and 47 percent at 225 N, all statistically significant at p of 0.01, 0.001 and 0.02 respectively [13]. The authors record that the sinkage effect was far more conclusive than the drawbar pull effect at every gravity level.
The load sensitivity moves too, which is the part that does not transfer. Raising wheel load 37 percent, 164 to 225 N, raised maximum sinkage 14 percent in 1 g but 31 percent in Martian gravity [13]: sinkage grows almost proportionally with load in Martian gravity and much less than proportionally in 1 g. A pressure-sinkage exponent fitted at 1 g is therefore not the exponent that applies in flight, and neither is a load sweep run at 1 g. Sinkage was inferred from hub displacement on a flexible wheel assuming constant average deflection, not measured at the soil surface [13].
Air bearing residuals
Section titled “Air bearing residuals”What the floor reproduces is planar free-floating motion in three degrees of freedom, two translations and yaw [10]. Out-of-plane rotation, out-of-plane translation and orbital dynamics are absent, and any residual slope of the floor appears as a constant in-plane acceleration. Three air pads carrying a 6.3 kg satellite model on an 1800 by 1800 mm glass table leave a horizontal acceleration of order 1e-3 g, set by air film viscosity and described as unavoidable [12]. A control law validated there is validated at 1e-3 g and against zero initial system momentum, which is what an air bearing emulates; full weight is still carried by the pads [12].
Closed-loop results on the fixed ESTEC floor show how much a controller loses to the facility: the same plan-and-track controller moved from a simulated flat floor to the 222 kg platform raised mean position error from 3.3 cm to 32 to 44 cm and heading error from 4.5 degrees to 24 to 51 degrees [10]. Reaction wheel saturation, not thruster resolution, destroyed attitude tracking, and only one run per trajectory was made, so the numbers describe that floor and that implementation [10].
Neutral buoyancy
Section titled “Neutral buoyancy”Neutral buoyancy cancels weight in every orientation and gives untethered freedom of movement over a large work volume with several suited operators at once, which no other method offers [5]. It substitutes water drag and added mass for weight, so dynamic motions are damped in a way orbital motion is not, and slow motion is favored over fast.
The residual 1 g inside the suit is a measured consequence: the EVA community holds training to a target of no more than ten hours of pool time per hour of EVA flown, across International Space Station assembly the ratio averaged 11.6 to 1, and the maximum training rate is three six hour runs in a five day week [11]. In the tiger team review of the EVA astronaut corps, suspected causes were collected for seventeen cases of training-related shoulder pain, and of the thirty-one causes named the inverted working position accounted for 25.8 percent and the planar hard upper torso for 22.6 percent; three astronauts in the study group had shoulder surgery, two of which were judged training related [11]. The planar torso replaced the pivoted design between 1997 and 2000 after a pivot failure during a manned test, and the fixed scye that replaced the gimbal cut overhead and downward reach.
The numbers
Section titled “The numbers”| Method | Duration per run | Gravity level | Axes offloaded |
|---|---|---|---|
| Drop tower, NASA GRC 2.2 Second Drop Tower | 2.2 s, 27 m free fall inside a drag shield in open air, 8 to 12 tests per day [1] | Below 1e-5 g [1] | All three |
| Evacuated drop tower, ZARM Bremen | 4.7 s, 9.3 s with catapult, three catapult runs per day [28] | 1e-6 g [28] | All three |
| ZARM GraviTower Bremen Pro | Up to 2.5 s, twenty runs per hour; partial gravity mode 2.5 s lunar, 3.0 s Martian [28] | 1e-4 g; 1e-2 g in partial gravity mode, preliminary [28] | All three |
| Reduced-gravity aircraft parabola | 20 +/- 5 s per parabola, about 45 parabolas per 2 to 3 hour flight [2] | Selectable 0, 1/10, 1/6, 1/3 or 1/2 g, each parabola bracketed by a 1.8 to 2.0 g pull-up [2] | All three |
| Active offload gantry, NASA JSC ARGOS | Unlimited [4] | Lunar, Martian or microgravity, set in software [4] | Vertical only, at one attachment point [5] |
| Air bearing floor | Unlimited | 1 g normal to the floor is carried by the bearing; motion is free in the horizontal plane [7] | Two translations and yaw; three degrees of freedom [10] |
| Tilted air bearing plate (ROOTLESS) | Unlimited | 4.756e-5 to 0.1712 m/s2 by commanded tilt [31] | Planar only [31] |
| ISS centrifuge | Seven hours | 0.063 to 2.0 g [25] | Granular bed and hardware |
| Neutral buoyancy | Limited by consumables and crew, hours | Weight canceled by buoyancy in all orientations [5] | All three, at the cost of drag and added mass [5] |
Package size follows from the facility: the 2.2 Second Drop Tower takes hardware up to 125 kg, and deceleration at the end of a drop tower run can exceed 70 g [1]. Parabolic task duration cannot exceed roughly 30 s, and cabin volume constrains what can be flown [5].
Free fall gives correct physics for a short interval and is the only ground method that offloads a granular bed as well as the hardware sitting on it, which is why the case against gravity offset rests on parabolic flight data and scaling laws rather than on gantry results [6]. Offload rigs give unlimited duration at the cost of a single point of application [5]. Air bearings give unlimited duration in a plane but only three degrees of freedom [10]. Neutral buoyancy gives unlimited orientation at the cost of water drag on dynamic motion and of a 1 g load path through the operator [11].
What is not established
Section titled “What is not established”The ARGOS suit evidence rests on two and three subjects with subjective simulation-quality ratings, and the authors could not separate gimbal artifacts from lunar-gravity effects or suit mass from offload effects [4]. Offload accuracy, residual force and control bandwidth of the ARGOS family are not reported in that work [4], and the ARGOS capability sheet gives specifications without a fidelity assessment [22]. The ATHLETE microgravity hop is qualitative [23]. The air bearing results are for one region of one floor, with no dispersion, and the sim-to-real ratio does not transfer to another floor [10]. The counterweighted three-dimensional air bearing has no tracking data under a moving payload [9]. The wheel sinkage results in Martian simulant come from one flexible wheel with sinkage inferred from hub displacement [13], and all Falcon 20 lunar gravity data are 20 to 30 second parabolas, transients approaching quasi-steady state [20]. The low-gravity strength of regolith simulants is not yet reported from the drop tower cell [27], and the MMX locomotion results are expectations [30]. No method listed here reproduces both the body force on the soil and an unlimited duration with the full six degrees of freedom of a free-flying article.
References
- Lekan, J. (1989). Microgravity Research in NASA Ground-Based Facilities
. AIAA Aerospace Sciences Meeting, NASA TM-101397. Source
BibTeX
@inproceedings{lekan1989microgravity, title = {Microgravity Research in NASA Ground-Based Facilities}, author = {Lekan, Jack}, booktitle = {AIAA Aerospace Sciences Meeting}, number = {NASA TM-101397}, institution = {NASA Lewis Research Center}, year = {1989}, doi = {10.2514/6.1989-236}, abstract = {An overview of reduced gravity research performed in NASA ground-based facilities sponsored by the Microgravity Science and Applications Program of the NASA Office of Space Science and Applications is presented. A brief description and summary of the operations and capabilities of each of these facilities along with an overview of the historical usage of them is included. The goals and program elements of the Microgravity Science and Applications programs are described and the specific programs that utilize the low gravity facilities are identified. Results from two particular investigations in combustion (flame spread over solid fuels) and fluid physics (gas-liquid flows at microgravity conditions) are presented.} } - Neumann, E. S., Withrow, J. P. and Yaniec, J. S. (1996). Users Guide for NASA Lewis Research Center DC-9 Reduced-Gravity Aircraft Program
. NASA, NASA-TM-106755. Source
BibTeX
@techreport{neumann1996users, title = {Users Guide for NASA Lewis Research Center DC-9 Reduced-Gravity Aircraft Program}, author = {Neumann, Eric S. and Withrow, James P. and Yaniec, John S.}, number = {NASA-TM-106755}, institution = {NASA}, year = {1996}, url = {https://ntrs.nasa.gov/citations/19970010375}, abstract = {The document provides guidelines and information for users of the DC-9 Reduced-Gravity Aircraft Program. It describes the facilities, requirements for test personnel, equipment design and installation, mission preparation, and in-flight procedures. Those who have used the KC-135 reduced-gravity aircraft will recognize that many of the procedures and guidelines are the same.} } - Ray, D. M. (1994). Partial Gravity Simulation Using a Pneumatic Actuator with Closed Loop Mechanical Amplification
. NASA Lyndon B. Johnson Space Center, NASA TM-104798. Source
BibTeX
@techreport{ray1994partial, title = {Partial Gravity Simulation Using a Pneumatic Actuator with Closed Loop Mechanical Amplification}, author = {Ray, David M.}, number = {NASA TM-104798}, institution = {NASA Lyndon B. Johnson Space Center}, month = {June}, year = {1994}, url = {https://ntrs.nasa.gov/citations/19940029889}, abstract = {To support future manned missions to the surface of the Moon and Mars or missions requiring manipulation of payloads and locomotion in space, a training device is required to simulate the conditions of both partial and microgravity as compared to the gravity on Earth. The focus of this paper is to present the development, construction, and testing of a partial gravity simulator which uses a pneumatic actuator with closed loop mechanical amplification. Results of the testing show that this type of simulator maintains a constant partial gravity simulation with a variation of the simulated body force between 2.2 percent and 10 percent, depending on the type of locomotion inputs. The data collected using the simulator show that mean stride frequencies at running speeds at lunar and Martian gravity levels are 12 percent less than those at Earth gravity. The data also show that foot/ground reaction forces at lunar and Martian gravity are, respectively, 62 percent and 51 percent less than those on Earth.} } - Bekdash, O. S., Valle, P. S., Kim, K. J., Jarvis, S. L., Dunn, J. T., Norcross, J. R. and Abercromby, A. F. J. (2020). Development and Evaluation of the Active Response Gravity Offload System as a Lunar and Martian EVA Simulation Environment
. International Conference on Environmental Systems, 20205003610. Source
BibTeX
@inproceedings{bekdash2020development, title = {Development and Evaluation of the Active Response Gravity Offload System as a Lunar and Martian EVA Simulation Environment}, author = {Bekdash, Omar S. and Valle, Paul S. and Kim, Kyoung Jae and Jarvis, Sarah L. and Dunn, Jocelyn T. and Norcross, Jason R. and Abercromby, Andrew F. J.}, booktitle = {International Conference on Environmental Systems}, number = {20205003610}, institution = {NASA}, year = {2020}, url = {https://ntrs.nasa.gov/citations/20205003610}, abstract = {In preparation for future exploration missions, NASA seeks the ability to simulate partial-gravity operations for use in ground-based research, crew training, and engineering design evaluations. The Active Response Gravity Offload System (ARGOS) at the Johnson Space Center (JSC) is designed to simulate reduced gravity environments, such as lunar, Martian, or microgravity, using a robotic system similar to an overhead bridge crane. ARGOS continuously offloads a portion of a suited human’s weight during all dynamic motions within the test facility, which can include basic functional movements such as walking, running, and jumping, as well as a wide range of planetary surface activities. This system will be used as part of a metabolic-rate task characterization study to determine the workload associated with partial-gravity extravehicular activity (EVA). Pilot testing was conducted using the MKIII prototype planetary space suit and two gimbal designs to determine the ability of the ARGOS test environment to simulate planetary EVA operations. This paper will describe the lessons learned from the feasibility testing, simulation-environment mockup design, and the results from the pilot tests and their influence into the final study design. Being able to effectively simulate partial-gravity environments and characterize the performance of crewmembers will have an impact on multiple domains including suit design, task design, thermal models, and life-support-system capacity verification plans, among others.} } - Jarvis, S. L., Vu, L. Q., Gupta, G., Benson, E., Kim, K. H., Rhodes, R. and Rajulu, S. L. (2023). Development of ARGOS (Active Response Gravity Offload System) Offloading Assessments and Methodology for Lunar EVA Simulations
. International Conference on Environmental Systems, 20230002430. Source
BibTeX
@inproceedings{jarvis2023development, title = {Development of ARGOS (Active Response Gravity Offload System) Offloading Assessments and Methodology for Lunar EVA Simulations}, author = {Jarvis, Sarah L. and Vu, Linh Q. and Gupta, Garima and Benson, Elizabeth and Kim, K. Han and Rhodes, Richard and Rajulu, Sudhakar L.}, booktitle = {International Conference on Environmental Systems}, number = {20230002430}, institution = {NASA}, address = {Calgary}, year = {2023}, url = {https://ntrs.nasa.gov/citations/20230002430}, abstract = {The Active Response Gravity Offload System (ARGOS) at NASA Johnson Space Center (JSC) is an analog environment that can offload pressurized suited subjects for various reduced gravity simulations. The suit is suspended from a robotic overhead crane by a cable connected to the suit via a gimbal with an adjustable pivot point (i.e. offload attachment). There has been increased interest in providing planetary pressurized suited training at ARGOS in preparation for lunar missions. Determination of the appropriate gimbal pivot point location for a given subject is vital for a high-fidelity functional lunar simulation. Interactions between the pivot point location and human-spacesuit center of gravity (CG) can result in righting moments that may lead to artificially stable or unrealistically challenging configurations. Changing the pivot point location is time consuming and repeated adjustment can result in loss of valuable pressurized suited time. This paper aims to share knowledge obtained from the offloading characterization efforts during pressurized suited testing at ARGOS and document the ongoing process to define an appropriate pivot point location through iterative quantitative and qualitative assessments. Human-spacesuit CG locations for the ARGOS lunar simulation were estimated using a 3D body scan and density model combined with spacesuit hardware CAD and specifications. Early pilot testing of the gimbal revealed that setting the pivot point coincident with the modeled CG location was not always possible due to the current gimbal design, and small pivot point shifts had noticeable effects on subject stability. Fourteen subjects performed a series of CG-related tasks in the Exploration Extravehicular Mobility Unit (xEMU) to assess simulation characteristics. Through iterative testing, this task list evolved to streamline the process needed to efficiently identify a suitable pivot point for a given subject. The developed methodology will be critical for pivot point selection during astronaut training in the ARGOS environment.} } - Hu, W., Li, P., Rogg, A., Schepelmann, A., Chandler, S., Kamrin, K. and Negrut, D. (2025). A Study Demonstrating that Using Gravitational Offset to Prepare Extraterrestrial Mobility Missions is Misleading
. NASA, 20250001809. Source
BibTeX
@techreport{hu2025study, title = {A Study Demonstrating that Using Gravitational Offset to Prepare Extraterrestrial Mobility Missions is Misleading}, author = {Hu, Wei and Li, Pei and Rogg, Arno and Schepelmann, Alexander and Chandler, Samuel and Kamrin, Ken and Negrut, Dan}, number = {20250001809}, institution = {NASA}, year = {2025}, doi = {10.1002/rob.22597}, abstract = {Recently, there has been a surge of international interest in extraterrestrial exploration targeting the Moon, Mars, the moons of Mars, and various asteroids. This contribution discusses how current state-of-the-art Earth-based testing for designing rovers and landers for these missions currently leads to overly optimistic conclusions about the behavior of these devices upon deployment on the targeted celestial bodies. The key misconception is that gravitational offset is necessary during the terramechanics testing of rover and lander prototypes on Earth. The body of evidence supporting our argument is tied to a small number of studies conducted during parabolic flights and insights derived from newly revised scaling laws. We argue that what has prevented the community from fully diagnosing the problem at hand is the absence of effective physics-based models capable of simulating terramechanics under low gravity conditions. We developed such a physics-based simulator and utilized it to gauge the mobility of early prototypes of the Volatiles Investigating Polar Exploration Rover (VIPER). This contribution discusses the results generated by this simulator, how they correlate with physical test results from the NASA-Glenn SLOPE lab, and the fallacy of the gravitational offset in rover and lander testing. The simulator, which is open-sourced and publicly available, supports trafficability analysis and facilitates principled studies into in-situ resource utilization activities like digging, bulldozing, and berming in low gravity environments.} } - Tobbe, P. A., Williamson, M. J. and Glaese, J. R. (1988). The flight robotics laboratory
. NASA, 19890003224. Source
BibTeX
@techreport{tobbe1988flight, title = {The flight robotics laboratory}, author = {Tobbe, Patrick A. and Williamson, Marlin J. and Glaese, John R.}, number = {19890003224}, institution = {NASA}, year = {1988}, url = {https://ntrs.nasa.gov/citations/19890003224}, abstract = {The Flight Robotics Laboratory of the Marshall Space Flight Center is described in detail. This facility, containing an eight degree of freedom manipulator, precision air bearing floor, teleoperated motion base, reconfigurable operator's console, and VAX 11/750 computer system, provides simulation capability to study human/system interactions of remote systems. The facility hardware, software and subsequent integration of these components into a real time man-in-the-loop simulation for the evaluation of spacecraft contact proximity and dynamics are described.} } - Jaynes, E., Inness, J., Garg, K., Lee, J. and Putman, A. (2024). Design and Applications of Tethered Spacecraft Simulators
. International Conference on Tethers in Space, 20240005792. Source
BibTeX
@inproceedings{jaynes2024design, title = {Design and Applications of Tethered Spacecraft Simulators}, author = {Jaynes, E. and Inness, J. and Garg, K. and Lee, J. and Putman, A.}, booktitle = {International Conference on Tethers in Space}, number = {20240005792}, institution = {NASA}, address = {Toronto}, year = {2024}, url = {https://ntrs.nasa.gov/citations/20240005792}, abstract = {Small, tethered spacecraft, or space tugs, are a promising space capability. They can be deployed on tethers and used for a wide variety of in-space activities; a large portion of which fall under in-space servicing, assembly, manufacturing (ISAM) functions. This includes functions such as structural mating and assembly, servicing, proximity operations, capture, docking, mating, and relocation. However, there is a great deal of work to be done to raise the Technology Readiness Level (TRL) of the capabilities needed to fully realize these capabilities. The Flat Floor Robotics Lab (FFRL) at NASA’s Marshall Spaceflight Center (MSFC) has been working on space tug development for several years. The eponymous floor is 44 by 86 feet, made from a self-leveling epoxy. This creates an extremely smooth surface that air bearings can float on with very little friction, creating a simulation of zero gravity in a two-dimensional plane. The lab has several platforms of various sizes that act as vehicle simulators for testing sensors, control algorithms, mechanisms, etc. One of these platforms is a small spacecraft simulator, which can be tethered to simulate a space tug. In the last several months, the FFRL team has been working with MSFC’s welding group to conduct a laser beam welding demonstration on air bearing platforms. This demo is utilizing a space tug simulator to fly up to a floating weld platform, where two parts are clamped together and laser welded. The space tug simulator has an air bearing to provide float, as well as several actuated air thrusters to provide propulsion and make the simulator maneuverable. Currently, flights of this simulator are completed by a human operator. An RF hand controller sends signals to both actuate and fire three sets of thrusters on the simulator. This makes any maneuvers of the space tug highly subject to human error, and operators must have dedicated practice time before being able to perform maneuvers with any sufficient reliability. For the current laser welding demonstration, human operated performance is sufficient, but future implementations of in-space welding and other ISAM efforts will require higher precision, reliability, and autonomy. MSFC has funded a project that will allow a small team in the FFRL to develop the next generation of space tug simulator, which will be automated. Closed loop control will allow a user to command a position, or set of positions, that the simulator will be able to “fly” to on its own. This Maneuverable Automated Tethered Spacecraft (MATS) simulator will have configurable design, to allow a variety of payloads and mission configurations to be demonstrated on the air bearing floor. This will be an invaluable test bed for low and mid TRL advancement for space tether mission subsystems, such as more advanced demonstrations of in-space welding. Once one MATSS is constructed and demonstrated effectively, several more can be built, enabling more advanced tether missions; Electric Sail test beds, sensor arrays, and other science-enabling missions. This paper will detail the design and test of the FFRL’s MATS simulator, including efforts to establish closed loop control and autonomous capabilities. It will also detail use cases for this technology, and how it will further develop both develop science and exploration missions} } - Okamoto, O., Nakaya, T. and Pokines, B. (1994). Concept verification of three dimensional free motion simulator for space robot
. Workshop for Computational Fluid Dynamic Applications in Rocket Propulsion, 19950017282. Source
BibTeX
@inproceedings{okamoto1994concept, title = {Concept verification of three dimensional free motion simulator for space robot}, author = {Okamoto, Osamu and Nakaya, Teruomi and Pokines, Brett}, booktitle = {Workshop for Computational Fluid Dynamic Applications in Rocket Propulsion}, number = {19950017282}, institution = {NASA}, year = {1994}, url = {https://ntrs.nasa.gov/citations/19950017282}, abstract = {In the development of automatic assembling technologies for space structures, it is an indispensable matter to investigate and simulate the movements of robot satellites concerned with mission operation. The movement investigation and simulation on the ground will be effectively realized by a free motion simulator. Various types of ground systems for simulating free motion have been proposed and utilized. Some of these methods are a neutral buoyancy system, an air or magnetic suspension system, a passive suspension balance system, and a free flying aircraft or drop tower system. In addition, systems can be simulated by computers using an analytical model. Each free motion simulation method has limitations and well known problems, specifically, disturbance by water viscosity, limited number of degrees-of-freedom, complex dynamics induced by the attachment of the simulation system, short experiment time, and the lack of high speed super-computer simulation systems, respectively. The basic idea presented here is to realize 3-dimensional free motion. This is achieved by combining a spherical air bearing, a cylindrical air bearing, and a flat air bearing. A conventional air bearing system has difficulty realizing free vertical motion suspension. The idea of free vertical suspension is that a cylindrical air bearing and counter balance weight realize vertical free motion. This paper presents a design concept, configuration, and basic performance characteristics of an innovative free motion simulator. A prototype simulator verifies the feasibility of 3-dimensional free motion simulation.} } - Bredenbeck, A., Vyas, S., Suter, W., Zwick, M., Borrmann, D., Olivares-Mendez, M. and Nüchter, A. (2022). Finding and Following Optimal Trajectories for an Overactuated Floating Robotic Platform
. arXiv preprint. Source
BibTeX
@article{bredenbeck2022finding, title = {Finding and Following Optimal Trajectories for an Overactuated Floating Robotic Platform}, author = {Bredenbeck, Anton and Vyas, Shubham and Suter, Willem and Zwick, Martin and Borrmann, Dorit and Olivares-Mendez, Miguel and Nüchter, Andreas}, journal = {arXiv preprint}, booktitle = {Symposium on Advanced Space Technologies in Robotics and Automation}, year = {2022}, doi = {10.48550/arxiv.2206.03993}, abstract = {The recent increase in yearly spacecraft launches and the high number of planned launches have raised questions about maintaining accessibility to space for all interested parties. A key to sustaining the future of space-flight is the ability to service malfunctioning - and actively remove dysfunctional spacecraft from orbit. Robotic platforms that autonomously perform these tasks are a topic of ongoing research and thus must undergo thorough testing before launch. For representative system-level testing, the European Space Agency (ESA) uses, among other things, the Orbital Robotics and GNC Lab (ORGL), a flat-floor facility where air-bearing based platforms exhibit free-floating behavior in three Degrees of Freedom (DoF). This work introduces a representative simulation of a free-floating platform in the testing environment and a software framework for controller development. Finally, this work proposes a controller within that framework for finding and following optimal trajectories between arbitrary states, which is evaluated in simulation and reality.} } - Williams, D. R. and Johnson, B. J. (2003). EMU Shoulder Injury Tiger Team Report
. NASA Lyndon B. Johnson Space Center, NASA/TM-2003-212058. Source
BibTeX
@techreport{williams2003emu, title = {EMU Shoulder Injury Tiger Team Report}, author = {Williams, David R. and Johnson, Brian J.}, number = {NASA/TM-2003-212058}, institution = {NASA Lyndon B. Johnson Space Center}, month = {September}, year = {2003}, url = {https://ntrs.nasa.gov/citations/20040200968}, abstract = {The number and complexity of extravehicular activities required for the completion and maintenance of the International Space Station is unprecedented. It is not surprising that training to perform these space walks presents a risk of overuse musculoskeletal injuries. The goal of this tiger team, created in December 2002, was to identify the different factors contributing to the risk of EVA training-related shoulder injury in the Neutral Buoyancy Lab at the Sonny Carter Training Facility and to make recommendations that would either significantly reduce or eliminate those risks. Since 1999, concerns have been expressed about the risk of shoulder injury associated with EVA training at the NBL, particularly in inverted body positions (McMonigal, 1999). A survey was developed and administered to 42 astronauts and astronaut candidates; the results suggest a causal relationship between EVA training at the NBL and the observed injuries. Also, during the tiger team review, it became evident that training in the extravehicular mobility unit may also result in other types of injuries, including fingernail delamination, elbow pain, knee pain, foot pain, and nerve compression leading to transient loss of sensation in certain areas of the upper or lower extremity. A multi-directorate team to detect, evaluate and respond to the medical issues associated with EVA training should be implemented immediately and given the appropriate resources and authority to reduce the risk of injury to crew during training to a level as low as reasonably achievable.} } - Umetani, Y. and Yoshida, K. (1989). Experimental Study on Two-Dimensional Free-Flying Robot Satellite Model
. NASA Conference Publication: Space Telerobotics. Source
BibTeX
@inproceedings{umetani1989experimental, title = {Experimental Study on Two-Dimensional Free-Flying Robot Satellite Model}, author = {Umetani, Yoji and Yoshida, Kazuya}, booktitle = {NASA Conference Publication: Space Telerobotics}, year = {1989}, url = {https://ntrs.nasa.gov/citations/19900020580}, abstract = {The experimental study on a control method for a free flying space robotic arm was treated by means of a two-dimensional laboratory model. The main target is to develop a new control method for trajectory tracking or target capturing, considering dynamical interaction between the manipulator arm and the base vehicle in space micro-gravity environment. In order to simulate the micro-gravity environment mechanically, a laboratory model of a robot satellite supported on air bearings was developed. The model comprises a base equipped with power and air supplies and a two-link manipulator arm. This model has relatively low gravitational and frictional disturbance in planar motion. An on-line resolved motion rate control scheme with vision feedback is developed for experimenting capture operations. The scheme utilizes the Generalized Jacobian Matrix. In experiment, the acceleration environment of the model is evaluated firstly, then target capture operations are examined. The manipulator can properly chase and capture both a standing target and a moving target in spite of complex satellite/manipulator dynamical interactions. The experimental results confirm the validity of the Generalized Jacobian Matrix concept and the proposed control method.} } - Niksirat, P., Daca, A. and Skonieczny, K. (2020). The effects of reduced-gravity on planetary rover mobility
. IEEE Transactions on Nuclear Science, 7. Source
BibTeX
@article{niksirat2020effects, title = {The effects of reduced-gravity on planetary rover mobility}, author = {Niksirat, Parna and Daca, Adriana and Skonieczny, Krzysztof}, journal = {IEEE Transactions on Nuclear Science}, volume = {39}, number = {7}, pages = {797-811}, publisher = {SAGE Publications}, year = {2020}, doi = {10.1177/0278364920913945}, abstract = {One of the major challenges faced by planetary exploration rovers today is the negotiation of difficult terrain, such as fine granular regolith commonly found on the Moon and Mars. Current testing methods on Earth fail to account for the effect of reduced gravity on the soil itself. This work characterizes the effects of reduced gravity on wheel–soil interactions between an ExoMars rover wheel prototype and a martian soil simulant aboard parabolic flights producing effective martian and lunar gravitational accelerations. These experiments are the first to collect wheel–soil interaction imagery and force/torque sensor data alongside wheel sinkage data. Results from reduced-gravity flights are compared with on-ground experiments with all parameters equal, including wheel load, such that the only difference between the experiments is the effect of gravity on the soil itself. In lunar gravity, a statistically significant average reduction in traction of 20% is observed compared with 1 g, and in martian gravity an average traction reduction of 5–10% is observed. Subsurface soil imaging shows that soil mobilization increases as gravity decreases, suggesting a deterioration in soil strength, which could be the cause of the reduction in traction. Statistically significant increases in wheel sinkage in both martian and lunar gravity provide additional evidence for decreased soil strength. All of these observations (decreased traction, increased soil mobilization, and increased sinkage) hinder a rover’s ability to drive, and should be considered when interpreting results from reduced-load mobility tests conducted on Earth.} } - Karpman, E. (2019). Discrete Element Modelling for Wheel-Soil Interaction and the Analysis of the Effect of Gravity
. Journal of Terramechanics. Source
BibTeX
@article{karpman2019discrete, title = {Discrete Element Modelling for Wheel-Soil Interaction and the Analysis of the Effect of Gravity}, author = {Karpman, Eric}, journal = {Journal of Terramechanics}, volume = {91}, pages = {139-153}, publisher = {Elsevier BV}, school = {McGill University}, address = {Montreal}, year = {2019}, doi = {10.1016/j.jterra.2020.06.002} } - Dong, Q., Chen, Q., Huang, K., Xing, W. and Shen, B. (2023). A Three-Dimensional Follow-Up System for a Spacecraft Low-Gravity Simulation Test Platform
. Journal of Tsinghua University, 3. Source
BibTeX
@article{dong2023three, title = {A Three-Dimensional Follow-Up System for a Spacecraft Low-Gravity Simulation Test Platform}, author = {Dong, Qiang and Chen, Qiang and Huang, Ke and Xing, Wei and Shen, Bing}, journal = {Journal of Tsinghua University}, volume = {63}, number = {3}, pages = {449--460}, year = {2023}, doi = {10.16511/j.cnki.qhdxxb.2022.26.056} } - Tomasek, J., Kolvenbach, H., Pagnamenta, M. and Wormnes, K. (2016). A Robotic Testbed for Low-Gravity Simulation
. International Symposium on Artificial Intelligence, Robotics and Automation in Space. Source
BibTeX
@inproceedings{tomasek2016robotic, title = {A Robotic Testbed for Low-Gravity Simulation}, author = {Tomasek, Jakub and Kolvenbach, Hendrik and Pagnamenta, Marco and Wormnes, Kjetil}, booktitle = {International Symposium on Artificial Intelligence, Robotics and Automation in Space}, organization = {European Space Agency}, address = {Beijing, China}, year = {2016}, url = {https://roboshare.esa.int/i-SAIRAS/isairas2016/Session3b/S-3b-4-HendrikKolvenbach.pdf} } - Kobayashi, T., Fujiwara, Y., Yamakawa, J., Yasufuku, N. and Omine, K. (2010). Mobility performance of a rigid wheel in low gravity environments
. Journal of Terramechanics, 4. Source
BibTeX
@article{kobayashi2010mobility, title = {Mobility performance of a rigid wheel in low gravity environments}, author = {Kobayashi, Taizo and Fujiwara, Yoichiro and Yamakawa, Junya and Yasufuku, Noriyuki and Omine, Kiyoshi}, journal = {Journal of Terramechanics}, volume = {47}, number = {4}, pages = {261-274}, publisher = {Elsevier BV}, year = {2010}, doi = {10.1016/j.jterra.2009.12.001} } - Smyth-Moore, A., Borg, J., Soria-Salinas, Á., Murdoch, N., Kato, H., Miyamoto, H., Usui, T., Kaufmann, E., Granvik, M. and Hagermann, A. (2025). Microgravity penetrometry flight campaign in support of MMX sampler science exploitation
. Progress in Earth and Planetary Science, 38. Source
BibTeX
@article{smythmoore2025microgravity, title = {Microgravity penetrometry flight campaign in support of MMX sampler science exploitation}, author = {Smyth-Moore, Alexander and Borg, Johan and Soria-Salinas, Álvaro and Murdoch, Naomi and Kato, Hiroki and Miyamoto, Hideaki and Usui, Tomohiro and Kaufmann, Erika and Granvik, Mikael and Hagermann, Axel}, journal = {Progress in Earth and Planetary Science}, volume = {12}, number = {38}, year = {2025}, doi = {10.1186/s40645-025-00704-8}, abstract = {Abstract Characterising the mechanical properties of minor bodies is essential for understanding their origin and evolution. Past missions such as Hayabusa2 have landed on asteroids to sample and discover what these bodies are made of. However, there has been conflicting evidence and reports into the physical properties of the granular surface material of these bodies. With future missions such as Japan Aerospace eXploration Agency’s Martian Moons eXploration mission landing on Phobos, the understanding and identification of these physical properties is crucial to maximising the scientific output from these missions. Penetrometry, the determination of the reaction force that an object experiences as it penetrates a surface, can help to understand the essential properties of regolith, such as grain size, porosity and cohesion. Results of penetrometry experiments are largely analysed based on empirical models, which presents us with a challenge if we want to apply them to understand granular materials on asteroid surfaces because gravity cannot be eliminated in the laboratory. Hence, it is essential to verify penetrometry as a method and validate penetrometry instrument designs in microgravity. For this purpose, we conducted a microgravity experiment onboard a parabolic flight campaign. Our experiment tested the use of penetrometry in asteroid-analogue environments by investigating samples with varying properties, such as grain size distribution and shape, and then compared to 1 g experiments to understand the role microgravity plays. The experiment provided a substantial database for future analysis. This paper will focus on the design of the experiment and the parabolic flight campaign in which the experiments were conducted. The design decisions and the variables adjusted during the experiment will be discussed, evaluating how these influenced the campaign and its outcomes. We will also provide a snapshot of preliminary results of the data captured during this experiment. For example, we show the effect of cohesion on penetrometer reaction force, with more cohesive materials providing larger reaction forces nearly of the same magnitude of their 1 g counterparts. We also show that penetrometer tip shapes provide different reaction forces and that flat tips provide the largest reaction force compared to the others. The influence of penetration velocity will be investigated further with the aid of theoretical models. Early indications from the results seen so far are promising for future analyses and will provide key information for the analysis of penetrometry data on future missions.} } - DeLombard, R., McPherson, K., Hrovat, K., Moskowitz, M., Rogers, M. J. B. and Reckart, T. (1997). Microgravity Environment Description Handbook
. NASA, NASA-TM-107486. Source
BibTeX
@techreport{delombard1997microgravity, title = {Microgravity Environment Description Handbook}, author = {DeLombard, Richard and McPherson, Kevin and Hrovat, Kenneth and Moskowitz, Milton and Rogers, Melissa J. B. and Reckart, Timothy}, number = {NASA-TM-107486}, institution = {NASA}, year = {1997}, url = {https://ntrs.nasa.gov/citations/19970022250}, abstract = {The Microgravity Measurement and Analysis Project (MMAP) at the NASA Lewis Research Center (LeRC) manages the Space Acceleration Measurement System (SAMS) and the Orbital Acceleration Research Experiment (OARE) instruments to measure the microgravity environment on orbiting space laboratories. These laboratories include the Spacelab payloads on the shuttle, the SPACEHAB module on the shuttle, the middeck area of the shuttle, and Russia's Mir space station. Experiments are performed in these laboratories to investigate scientific principles in the near-absence of gravity. The microgravity environment desired for most experiments would have zero acceleration across all frequency bands or a true weightless condition. This is not possible due to the nature of spaceflight where there are numerous factors which introduce accelerations to the environment. This handbook presents an overview of the major microgravity environment disturbances of these laboratories. These disturbances are characterized by their source (where known), their magnitude, frequency and duration, and their effect on the microgravity environment. Each disturbance is characterized on a single page for ease in understanding the effect of a particular disturbance. The handbook also contains a brief description of each laboratory.} } - Daca, A. (2023). Predicting Planetary Rover Mobility in Reduced Gravity Using 1-g Experiments
. Unpublished. Source
BibTeX
@phdthesis{daca2023predicting, title = {Predicting Planetary Rover Mobility in Reduced Gravity Using 1-g Experiments}, author = {Daca, Adriana}, journal = {Unpublished}, school = {Concordia University}, address = {Montreal, Quebec, Canada}, type = {PhD thesis}, month = {February}, year = {2023}, doi = {10.13140/rg.2.2.12718.28485}, abstract = {Traversing granular regolith, especially in reduced gravity environments, remains a potential challenge for wheeled rovers. Mitigating hazards for planetary rovers requires testing in representative environments, but direct Earth-based testing fails to account for the effect of reduced gravity on the soil itself. Here, experimental apparatus and techniques for reduced-gravity flight testing are used to systematically evaluate three existing Earth-based testing methods and develop guidelines for their use and interpretation: (i) reduced-weight testing, (ii) matching soil testing instrument response through soil simulant design, and (iii) granular scaling laws (GSL). \nExperimentation campaigns flying reduced-gravity parabolas, with soil and wheel both in lunar-g, have shown reductions in net traction of 20% or more and increases in sinkage of up to 40% compared to Earth-based testing methods (i) and (ii). Scaled-wheel testing, according to GSL (method iii) has shown better agreement with reduced-g tests (less than 10% error) and also tends to err on the side of conservative predictions. \nLimitations of GSL are investigated including a recently proposed cohesion constraint (that the wheel radius ratio must be the inverse of the gravity ratio) and the effects of wheel size and aspect ratio on GSL’s accuracy. It was found that the cohesion constraint can most likely be ignored for mildly cohesive soils such as lunar regolith. Limits on wheel sizes and aspect ratio variation are also proposed. \nThe application of GSL to planetary rover testing is demonstrated through two studies undertaken in collaboration with NASA’s Jet Propulsion Laboratory. One study compares wheel designs for a skid-steer lunar rover in single-wheel tests scaled by GSL, demonstrating that diagonal grousers improve turning performance without requiring larger wheels. The second study involves application of GSL to the design of two reconfigurable test platforms for evaluating steep-terrain mobility performance. \nAnother aspect of rover mobility testing—normal force control in single-wheel testbeds—is also investigated. An improved method for single-wheel testing, using a 4-bar mechanism, essentially eliminates normal force oscillations from frictional vertical sliders. \nFinally, guidelines for conducting and interpreting 1-g mobility tests for lunar rovers are presented, and potential avenues for future research are outlined.} } - Creager, C., Breckenridge, J., Johnson, K., Oravec, H., Moreland, S., Sobey, A. and McBryan, E. (2025). Best Practices for the Testing of Planetary Roving Vehicle Mobility Systems and Tires
. NASA, 20250003470. Source
BibTeX
@techreport{creager2025best, title = {Best Practices for the Testing of Planetary Roving Vehicle Mobility Systems and Tires}, author = {Creager, Colin and Breckenridge, John and Johnson, Kyle and Oravec, Heather and Moreland, Scott and Sobey, Alexander and McBryan, Emily}, number = {20250003470}, institution = {NASA}, year = {2025}, url = {https://ntrs.nasa.gov/citations/20250003470}, abstract = {NASA has an extensive history of conducting mobility testing on planetary tires, rovers, and mobility subsystems. This paper contains a summary of the best practices and lessons learned from past test approaches, as well as from relevant test methodologies external to NASA, with a focus on tires and mobility systems. It is intended to be a resource for future planetary rover developments. This document, which will be released in two phases, serves two objectives: 1) compile the existing knowledge of mobility related testing within NASA; and 2) identify major gaps in mobility related test needs and suggest potential paths forward. This first release of the document (early 2025) captures information based on the authors’ existing knowledge and an abbreviated literature review. A more thorough version will be released in late 2025 and will include more information on test methods external to NASA, as well as additional lessons learned through further investigations. The Lunar Terrain Vehicle (LTV) is used as case study to help identify major test needs; however, this document is intended to be applicable to a wide range of potential missions. Though no current mobility standards exist for the testing of planetary tires and mobility subsystems, future motive is to utilize the information from this whitepaper to guide the development of NASA mobility test standards.} } - Valle, P. S. (2017). Reduced Gravity Testing of Robots (and Humans) Using the Active Response Gravity Offload System. ntrs.nasa.gov/citations/20170008856
BibTeX
@misc{valle2017reduced, title = {Reduced Gravity Testing of Robots (and Humans) Using the Active Response Gravity Offload System}, author = {Valle, Paul Steinar}, howpublished = {Presentation at the Institute of Electrical and Electronics Engineers}, year = {2017}, url = {https://ntrs.nasa.gov/citations/20170008856}, abstract = {No abstract available} } - Wilcox, B. H. (2011). ATHLETE: A Limbed Vehicle for Solar System Exploration
. IEEE Aerospace Conference. Source
BibTeX
@inproceedings{wilcox2011limbed, title = {ATHLETE: A Limbed Vehicle for Solar System Exploration}, author = {Wilcox, Brian H.}, booktitle = {IEEE Aerospace Conference}, pages = {1-9}, year = {2011}, doi = {10.1109/aero.2012.6187269}, abstract = {As part of the Human-Robot Systems project funded by NASA, the Jet Propulsion Laboratory has developed a vehicle called ATHLETE: the All-Terrain Hex-Limbed Extra-Terrestrial Explorer. Each vehicle is based on six wheels at the ends of six multi-degree-of-freedom limbs. Because each limb has enough degrees of freedom for use as a general-purpose leg, the wheels can be locked and used as feet to walk out of excessively soft or other extreme terrain. Since the vehicle has this alternative mode of traversing through or at least out of extreme terrain, the wheels and wheel actuators can be sized for nominal terrain. There are substantial mass savings in the wheel and wheel actuators associated with designing for nominal instead of extreme terrain. These mass savings are comparable-to or larger-than the extra mass associated with the articulated limbs. As a result, the entire mobility system, including wheels and limbs, can be about 25% lighter than a conventional mobility chassis. A side benefit of this approach is that each limb has sufficient degrees-of-freedom to use as a general-purpose manipulator (hence the name "limb" instead of "leg"). Our prototype ATHLETE vehicles have quick-disconnect tool adapters on the limbs that allow tools to be drawn out of a "tool belt" and maneuvered by the limb. A power-take-off from the wheel actuates the tools, so that they can take advantage of the 1+ horsepower motor in each wheel to enable drilling, gripping or other power-tool functions. This paper describes the applicability of the ATHLETE concept to exploration of the moon, Mars and Near-Earth Asteroids (NEAs). Recently, the focus of human exploration beyond LEO has been on NEAs. One scenario for exploration of a NEA has been likened to a submarine exploring a wrecked ship - humans would sit in a "bubble" and approach the asteroid for up-close examination and robotic manipulation. What is important is to ensure that the bubble doesn't collide with the asteroid surface, nor float away. Multiple limbs, such as available on ATHLETE, allow for precise positioning and anchoring so as to enable the human bubble to maximize its exploration potential. A microgravity testbed has been constructed in the ATHLETE lab, with six computer-controlled winches able to lift ATHLETE and payloads so as to simulate the motion of the system in the vicinity of a NEA. Accurate 6-axis force-torque sensors will measure the applied forces and moments wherever the vehicle touches a simulated asteroid surface. These measured forces can be used to compute the resultant motion of the vehicle in the microgravity environment, and the winches then move the vehicle along the computed trajectory. Preliminary test results from this system are described.} } - Zwick, M., Huertas, I., Gerdes, L. and Ortega, G. (2018). ORGL: ESA's Test Facility for Approach and Contact Operations in Orbital and Planetary Environments
. International Symposium on Artificial Intelligence, Robotics and Automation in Space. Source
BibTeX
@inproceedings{zwick2018orgl, title = {{ORGL}: {ESA}'s Test Facility for Approach and Contact Operations in Orbital and Planetary Environments}, author = {Zwick, Martin and Huertas, Irene and Gerdes, Levin and Ortega, Guillermo}, booktitle = {International Symposium on Artificial Intelligence, Robotics and Automation in Space}, organization = {European Space Agency}, address = {Madrid, Spain}, year = {2018}, url = {https://roboshare.esa.int/i-SAIRAS/isairas2018/} } - Ozaki, S., Ishigami, G., Otsuki, M., Miyamoto, H., Wada, K., Watanabe, Y., Nishino, T., Kojima, H., Soda, K., Nakao, Y., Sutoh, M. and Maeda, T. (2023). Granular Flow Experiment Using an Artificial Gravity Generator at the International Space Station
. npj Microgravity. Source
BibTeX
@article{kuroda2023granular, title = {Granular Flow Experiment Using an Artificial Gravity Generator at the {International Space Station}}, author = {Ozaki, Shingo and Ishigami, Genya and Otsuki, Masatsugu and Miyamoto, Hideaki and Wada, Koji and Watanabe, Y. and Nishino, T. and Kojima, Hirohisa and Soda, K. and Nakao, Y. and Sutoh, Masataku and Maeda, Takao}, journal = {npj Microgravity}, volume = {9}, pages = {61}, year = {2023}, doi = {10.1038/s41526-023-00308-w}, abstract = {Abstract Studying the gravity-dependent characteristics of regolith, fine-grained granular media covering extra-terrestrial bodies is essential for the reliable design and analysis of landers and rovers for space exploration. In this study, we propose an experimental approach to examine a granular flow under stable artificial gravity conditions for a long duration generated by a centrifuge at the International Space Station. We also perform a discrete element simulation of the granular flow in both artificial and natural gravity environments. The simulation results verify that the granular flows in artificial and natural gravity are consistent. Further, regression analysis of the experimental results reveals that the mass flow rate of granular flow quantitatively follows a well-known physics-based law with some deviations under low-gravity conditions, implying that the bulk density of the granular media decreases with gravity. This insight also indicates that the bulk density considered in simulation studies of space probes under low-gravity conditions needs to be tuned for their reliable design and analysis.} } - Madden, I. P., Muruganandam, S., Missaoui, A., Gries, O., Kollmer, J., D'Angelo, O. and Sinha-Ray, S. (2025). Behaviors of Lunar Regolith Simulants Under Varying Gravitational Conditions
. npj Microgravity. Source
BibTeX
@article{madden2025behaviors, title = {Behaviors of Lunar Regolith Simulants Under Varying Gravitational Conditions}, author = {Madden, Ian P. and Muruganandam, Sathyashri and Missaoui, Amine and Gries, Oliver and Kollmer, Jonathan and D'Angelo, Olfa and Sinha-Ray, Suman}, journal = {npj Microgravity}, volume = {11}, pages = {88}, year = {2025}, doi = {10.1038/s41526-025-00501-z}, abstract = {Abstract Understanding the behavior of regolith in varying gravity conditions, is critical for space exploration and future missions. In this work, the gravity-driven hopper flow of lunar regolith simulant in different gravitational accelerations (terrestrial, lunar) is first observed experimentally. Numerical simulations (DEM) are then developed to understand the role which cohesive interparticle forces play in such gravity-driven flow, using the theoretical framework of granular Bond number. Qualitative comparison between a terrestrial experiment and numerical simulation validated this framework. Following that, we numerically studied the dynamic behavior under varying gravitational conditions (from terrestrial to lunar to asteroid gravitational accelerations). We find that this behavior is extremely sensitive to the interplay of the gravity conditions and the attractive/cohesive forces among particles. The numerical and experimental results show that the complex interaction of these forces can drastically change the dynamics of the material producing effects relevant for variable gravity applications.} } - Duffey, C., Lea, M. and Brisset, J. (2025). Measuring regolith strength in reduced gravity environments in the laboratory
. Review of Scientific Instruments. Source
BibTeX
@article{duffey2025measuring, title = {Measuring regolith strength in reduced gravity environments in the laboratory}, author = {Duffey, C. and Lea, M. and Brisset, Julie}, journal = {Review of Scientific Instruments}, volume = {96}, year = {2025}, doi = {10.1063/5.0249495}, abstract = {This paper presents the design and development of a Shear and Compression Cell (SCC) for measuring the mechanical properties of granular materials in low-gravity environments. This research is motivated by the increasing interest in planetary exploration missions that involve surface interactions, such as those to asteroids and moons. The SCC is designed to measure key mechanical properties, including Young’s modulus, angle of internal friction, bulk cohesion, and tensile strength, under both reduced gravity and microgravity conditions. Using a drop tower with interchangeable configurations, we can simulate the gravitational environments of celestial bodies such as the Moon and Titan. The SCC, coupled with the drop tower, provides a valuable tool for understanding the behavior of regolith materials and their implications for future space exploration missions.} } - Könemann, T. and Cornelius, M. (2024). Bremen Drop Tower: Payload User's Guide, Version 1.6
. ZARM Fallturm-Betriebsgesellschaft mbH, University of Bremen. Source
BibTeX
@techreport{konemann2024bremen, title = {Bremen Drop Tower: Payload User's Guide, Version 1.6}, author = {Könemann, Thorben and Cornelius, Merle}, institution = {ZARM Fallturm-Betriebsgesellschaft mbH, University of Bremen}, year = {2024}, url = {https://www.zarm.uni-bremen.de/fileadmin/user_upload/Research/Testing_and_Facilities/ZARM_BDT_PUG.pdf} } - Chen, J., Li, R. and Fu, S. (2026). Influence of Low Gravity on the Penetration Resistance of Lunar Regolith
. npj Microgravity. Source
BibTeX
@article{chen2026influence, title = {Influence of Low Gravity on the Penetration Resistance of Lunar Regolith}, author = {Chen, Jun and Li, Ruilin and Fu, Shigen}, journal = {npj Microgravity}, volume = {12}, pages = {7}, year = {2026}, doi = {10.1038/s41526-026-00562-8}, abstract = {Abstract Lunar surface operations conducted by the United States and the Soviet Union confirmed that penetration resistance is a key indicator for evaluating the engineering properties of lunar regolith. To quantitatively assess the influence of reduced gravity on penetration resistance, this study employed a newly developed Geotechnical Magnetic-gravity Modeling Test (GMMT) system to perform cone penetration tests under controlled gravitational acceleration levels of 1/6 g, 1 g, and 2 g. The results indicated that the normalized penetration resistance increased as gravity decreased, and this effect was amplified at higher relative densities. To investigate the underlying mechanisms, discrete element method (DEM) simulations were conducted. The findings revealed that, in addition to gravity, in situ factors such as high relative density and irregular particle morphology also significantly enhanced penetration resistance by strengthening interparticle contact and friction. These non-gravitational effects partially offset the expected reduction in resistance under lower gravity, leading to a smaller-than-anticipated decline. This study provides new insights into the gravity-dependent penetration behavior of lunar regolith.} } - Buse, F., Barthelmes, S., Chalon, M., Langofer, V., Bertleff, W., Lichtenheldt, R., Skibbe, J., Bihler, M., Holderried, R., Reill, J., Vodermayer, B., Stubbig, L., Bertrand, J., Tardivel, S., Vernazza, P., Murdoch, N., Ulamec, S. and Michel, P. (2021). Wheeled Locomotion in Milli-Gravity: A Technology Experiment for the MMX Rover
. International Astronautical Congress, IAC-21,A3,4A,8,x64275. Source
BibTeX
@inproceedings{buse2021wheeled, title = {Wheeled Locomotion in Milli-Gravity: A Technology Experiment for the {MMX} Rover}, author = {Buse, Fabian and Barthelmes, Stefan and Chalon, Maxime and Langofer, Viktor and Bertleff, Wieland and Lichtenheldt, Roy and Skibbe, Juliane and Bihler, Markus and Holderried, Roman and Reill, Josef and Vodermayer, Bernhard and Stubbig, Leon and Bertrand, Jean and Tardivel, Simon and Vernazza, Pierre and Murdoch, Naomi and Ulamec, Stephan and Michel, Patrick}, booktitle = {International Astronautical Congress}, number = {IAC-21,A3,4A,8,x64275}, address = {Dubai, United Arab Emirates}, year = {2021}, url = {https://elib.dlr.de/144876/} } - Velosa, C. M. N., Gerdes, L. and Zwick, M. (2018). Automatic Levelling of a Platform to Achieve Artificial Gravity
. International Symposium on Artificial Intelligence, Robotics and Automation in Space. Source
BibTeX
@inproceedings{velosa2018automatic, title = {Automatic Levelling of a Platform to Achieve Artificial Gravity}, author = {Velosa, Carlos M. N. and Gerdes, Levin and Zwick, Martin}, booktitle = {International Symposium on Artificial Intelligence, Robotics and Automation in Space}, organization = {European Space Agency}, address = {Madrid, Spain}, year = {2018}, url = {https://roboshare.esa.int/i-SAIRAS/isairas2018/} } - Afman, J. P., Feron, E. and Walker, M. (2019). A full scale atmospheric flight experimental research environment for the Mars helicopter
. arXiv preprint. Source
BibTeX
@article{afman2019full, title = {A full scale atmospheric flight experimental research environment for the Mars helicopter}, author = {Afman, J. Pablo and Feron, Eric and Walker, Mitchell}, journal = {arXiv preprint}, year = {2019}, url = {https://arxiv.org/abs/1910.09290} } - Schröder, S., Biele, J., Block, J., Roll, R., Ulamec, S. and Witte, L. (2013). Philae Landing Test at the Landing and Mobility Test Facility (LAMA)
. International Astronautical Congress, IAC-13, A3, 4, 3. Source
BibTeX
@inproceedings{schroder2013philae, title = {Philae Landing Test at the Landing and Mobility Test Facility (LAMA)}, author = {Schröder, Silvio and Biele, Jens and Block, Joachim and Roll, Reinhard and Ulamec, Stephan and Witte, Lars}, booktitle = {International Astronautical Congress, IAC-13, A3, 4, 3}, address = {Beijing}, year = {2013}, url = {https://elib.dlr.de/103154/} } - Wavrunek, T. A. (2021). Active Gravity Offloading System with Infrared Tracking for Rover Testing
. Michigan Technological University. Source
BibTeX
@mastersthesis{wavrunek2021active, title = {Active Gravity Offloading System with Infrared Tracking for Rover Testing}, author = {Wavrunek, Travis A.}, publisher = {Michigan Technological University}, school = {Michigan Technological University}, type = {Master of Science report, Mechanical Engineering}, year = {2021}, doi = {10.37099/mtu.dc.etdr/1213}, abstract = {Gravity offloading is a tool used to test how different gravitational forces will impact the mobility of rovers bound for Lunar or Martian expeditions. Previous approaches have been successful in simulating partial gravity environments, and this report details how the Infrared- Gravity offload (IRGO) system, developed for the Planetary Surface Technology Development Laboratory (PSTDL) and lunar simulant sandbox, has a similar aim. Through a series of iterations, IRGO has been developed to actively track an infrared beacon and follow a rover within the test chamber to eliminate inertial and friction forces along two horizonal axes. A portion of a rover’s weight is offloaded using a passive counterweight system to provide a third translational degree of freedom. Future plans to incorporate a lightweight gimbal as well as an active vertical axis are also discussed as solutions to improving the IRGO system.} }