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Space Robotics

Guidance and control

The problem

Section titled “The problem”

Closed-loop control means turning a commanded state into actuator effort on a vehicle whose dynamics, sensing and processor are all worse than the plan assumed. Four problems recur: the vehicle moves through a medium it cannot steer directly (entry, descent, rotor flight), contact with terrain is poorly known (wheel slip, soil), the base the arm sits on moves when the arm moves, and the processor is slow and shared with everything else on board. The subtopics are attitude and trajectory control, traction and slip control, manipulator and force control, and free-flyer guidance.

What has flown

Section titled “What has flown”

Entry and descent. Entry is guided by modulating bank angle, since the capsule has no direct steering. The Mars 2020 entry controller is documented by its designers with flight results [1] and by the project’s as-flown reconstruction [2]. Powered descent on Mars 2020 used a heritage controller: the pre-flight design record states what terrain relative navigation was expected to buy and cost [24], and the as-flown reconstruction reports the divert, the vertical phase and an engine bias inferred by fitting a simulation to telemetry [23].

Rotorcraft. Ingenuity extended the control problem to an aerodynamically unstable aircraft in the thin Martian atmosphere. Its first dedicated in-flight system identification flights reached 15 m/s airspeed without instability or sustained control saturation [6, p. 22], and the program overview traces the move from technology demonstration to operational scout [5]. The pre-flight flight control paper gives a four-part architecture of mode commander, guidance, navigation and control, and an avionics split between a lockstep flight computer and a separate navigation computer; its figures are design values, not flight data [21].

Surface traction. Curiosity’s traction control commands wheel rates from proprioceptive estimates of terrain contact angle and is the one flown wheel-level controller in the corpus [7]. The wheel damage it addresses arises from the drive control mode combined with rocker-bogie kinematics rather than from static load [10].

Manipulators. Cartesian arm control starts with resolved motion rate control [11], and Khatib moved collision avoidance into the servo loop as a potential field [12]. Planetary arms flown to date use position control with contact handled open loop, partly because force sensing is costly to trust; a comparison of five sensing techniques on an arm equivalent to the MER deployment device found the wrist force-torque sensor best [13]. For free-floating bases, the ETS-VII experiments are the one orbital test of the reaction-dynamics theory [16].

Intravehicular free flyers. These navigate without GPS inside a module. SPHERES used ultrasonic beacons, and its autonomous docking to a tumbling target is documented in the guidance, navigation and control architecture thesis behind it [17]. Astrobee localizes against a prebuilt feature map [18], with a later graph-based localizer addressing drift when the map no longer matches the module [19]. A joint review compares the Astrobee, CIMON and Int-Ball operating experience [20]. Int-Ball2 flew visual SLAM navigation and magnetic docking on the ISS; its report compares on-orbit control error with the ground simulator, and the authors note the comparison is not like for like [22].

What is being tried now

Section titled “What is being tried now”

Convex powered descent guidance. The formulation reduces landing guidance to a second-order cone problem solved to global optimality in polynomial time [3]. The published timing is a workstation prototype at two to three seconds, not flight hardware [3, p. 12]. Convex formulations have been analyzed in simulation only [3][25], and successive convexification applied to Titan parafoil descent found no control authority along one axis and reported no landing dispersion [25].

Surface trajectory optimization. A planner for the CADRE lunar rovers poses driving as nonlinear trajectory optimization with signed distance fields for obstacles; it is designed, with no planetary flight result [4].

Slip prediction and terramechanics. Slip prediction from imagery learns a per-terrain model so that slip can be estimated ahead of the rover; it has been tested on Earth only [9]. Terramechanics models of steering on loose soil supply the physics such predictors approximate [8]. Reduced-gravity parabolic flight tests show that a 1 g test can mislead: at equal wheel load, drawbar pull to weight fell 20% or more and sinkage rose up to 40% in lunar gravity, and granular scaling laws predicted lunar results with smaller, conservative error than reduced-weight or equal-cone-index soil preparation [31, p. 81].

Free-floating base control. A review of control categories and ground test beds [14] and the structural comparison with fixed-base equations [15] set out the theory. A velocity-level analysis of a redundant arm on a free-flying carrier separates manipulator singularities from algorithmic ones that have no fixed-base counterpart; the paper is analytical and contains no experiment [27]. Allocating thrusters to carrier translation and wheels and joints to attitude and end-effector pose saved fuel against a thrusters-only baseline in a hardware-in-the-loop test where only the arm was physical [29].

Proximity operations and learned control. Linear model predictive control that linearizes the keep-out sphere into a time-varying half-plane reduces constrained docking to one quadratic program per sample, shown in simulation with the chaser attitude assumed synchronized to the target [30]. Reinforcement learning with an MPC reward reference for docking with a partially filled tank reached 96% success against 78% for the learning method alone in simulation, with hardware evidence limited to planar stabilization on air-bearing platforms [26, p. 7]. The debris-removal mission ADRIOS ClearSpace-1 sets out a ladder of reversible approach steps; launch is planned for 2028 and no performance has been measured [28].

The numbers

Section titled “The numbers”
QuantityValueStatusSource
Mars 2020 entry mass3369 kgreconstructed flight[2, p. 128]
Mars 2020 touchdown miss from safe target5 mflight[2, p. 182]
Mars 2020 miss before terrain relative navigation1.7 kmflight[2, p. 182]
Ingenuity airspeed in identification flights15 m/sflight[6, p. 22]
Convex descent solver time2 to 3 sworkstation[3, p. 12]
RL with MPC reward docking success96% vs 78%simulation[26, p. 7]
Granular scaling law error, drawbar pull to weight8.5%parabolic flight[31, p. 81]
Equal cone index gradient error, drawbar pull to weight44%parabolic flight[31, p. 81]

What is not established

Section titled “What is not established”

No onboard solver for convexified powered descent has a published timing on a flight-class processor [3]. The entry controller results are one flight reported by its developers [1]. Ingenuity’s low-frequency stability derivatives are poorly determined and its airspeed is inferred, so its lateral model is only partly validated [6]. Traction control benefit cannot be separated from terrain differences between drives [7]. Continuous force control, slip prediction and the CRESCENT planner have no planetary flight result in the corpus [4][9][13]. The ETS-VII reactionless result is one flight [16]. Proximity-operation controllers [30][26] and the thruster and arm allocation scheme [29] have no flight result in the corpus. Landing precision for the Titan parafoil formulation is not reported [25]. The reduced-gravity traction results come from 20 to 30 s parabolas with soil prepared in 1 g, so each run is transient [31].

References

  1. Brugarolas, P. and Way, D. W. (2021). Mars 2020 Perseverance Entry Controller Design and Flight Reconstruction . AAS/AIAA Astrodynamics Specialist Conference, AAS 21-601. Source
    BibTeX
    @inproceedings{brugarolas2021entry,
  title = {Mars 2020 Perseverance Entry Controller Design and Flight Reconstruction},
  author = {Brugarolas, Paul and Way, David W.},
  booktitle = {AAS/AIAA Astrodynamics Specialist Conference},
  number = {AAS 21-601},
  year = {2021},
  url = {https://dataverse.jpl.nasa.gov/dataset.xhtml?persistentId=hdl:2014/55195}
}
  2. Mars 2020 Entry, D. and Landing Team. (2023). Mars 2020 Entry, Descent & Landing As-Flown Reconstruction Report . Jet Propulsion Laboratory, California Institute of Technology, JPL Publication 22-1. Source
    BibTeX
    @techreport{mars2023mars,
  title = {Mars 2020 Entry, Descent & Landing As-Flown Reconstruction Report},
  author = {{Mars 2020 Entry, Descent} and {Landing Team}},
  number = {JPL Publication 22-1},
  institution = {Jet Propulsion Laboratory, California Institute of Technology},
  year = {2023},
  doi = {10.48577/jpl.16mnyf},
  abstract = {On February 18, 2021, amidst a global pandemic sweeping the nation and the world, the Perseverance rover safely completed its autonomous landing in Jezero Crater. The largest, heaviest, and most complex planetary rover ever built depended on its seven-minute entry, descent, and landing (EDL) sequence to touch down safely at the most dangerous landing site ever attempted on Mars. Building upon the success of the Mars Science Laboratory (MSL) mission, the Mars 2020 EDL system preserved as much of the MSL EDL architecture as possible while simultaneously fixing issues identified both pre- and post-MSL’s landing, adding mission enabling new EDL capabilities, and accommodating new mission-enhancing payloads. Reconstruction results to date have shown that the Mars 2020 EDL system performed largely as expected. Issues identified with MSL’s landing were successfully addressed during the Mars 2020 landing. The mission’s two new landing capabilities, range trigger and terrain relative navigation, proved critical in identifying and reaching a safe landing target within the hazardous Jezero Crater landing site, a site deemed too hazardous for previous landing systems. Finally, new EDL instrumentation and a new EDL camera suite gave the team greater insight into the landing performance and brought the world along for the ride. This As-Flown Reconstruction Report details the design, development challenges, pre-flight expectations, and flight performance of almost all aspects of the Mars 2020 EDL system. While this report captures the best understanding of the system as of the time of publication, no one knows better than this team that additional time and scrutiny will yield further insights; the team’s scrutiny of the MSL EDL system throughout the development of Mars 2020 led to several fixes and improvements to the EDL design. As a result, this report is a snapshot in time of the team’s best understanding of the performance of the system; expect that understanding to grow and evolve with future efforts. The Mars 2020 EDL team faced numerous challenges, some expected, some not, over the project’s lifecycle. Their engineering prowess, creativity, dedication, and can-do attitude allowed the tight-knit team to face each of these challenges and overcome them. This report documents their story. With Mars 2020’s successful landing, the edge of the Mars landing envelope has been pushed out once again, enabling the first controlled aircraft flights on another planet by the Ingenuity helicopter, the search for evidence of past life by the Perseverance rover, and the beginning of the long-envision Mars sample return campaign.}
}
  3. Açıkmeşe, B., Blackmore, L., Scharf, D. P. and Wolf, A. (2008). Enhancements on the Convex Programming Based Powered Descent Guidance Algorithm for Mars Landing . AIAA/AAS Astrodynamics Specialist Conference. Source
    BibTeX
    @inproceedings{acikmese2008enhancements,
  title = {Enhancements on the Convex Programming Based Powered Descent Guidance Algorithm for Mars Landing},
  author = {Açıkmeşe, Behçet and Blackmore, Lars and Scharf, Daniel P. and Wolf, Aron},
  booktitle = {AIAA/AAS Astrodynamics Specialist Conference},
  year = {2008},
  doi = {10.2514/6.2008-6426},
  abstract = {In this paper, we present enhancements on the powered descent guidance algorithm developed for Mars pinpoint landing. The guidance algorithm solves the powered descent minimum fuel trajectory optimization problem via a direct numerical method. Our main contribution is to formulate the trajectory optimization problem, which has nonconvex control constraints, as a finite dimensional convex optimization problem, specifically as a finite dimensional second order cone programming (SOCP) problem. SOCP is a subclass of convex programming, and there are efficient SOCP solvers with deterministic convergence properties. Hence, the resulting guidance algorithm can potentially be implemented onboard a spacecraft for real-time applications. Particularly, this paper discusses the algorithmic improvements obtained by: (i) Using an efficient approach to choose the optimal time-of-flight; (ii) Using a computationally inexpensive way to detect the feasibility/ infeasibility of the problem due to the thrust-to-weight constraint; (iii) Incorporating the rotation rate of the planet into the problem formulation; (iv) Developing additional constraints on the position and velocity to guarantee no-subsurface flight between the time samples of the temporal discretization; (v) Developing a fuel-limited targeting algorithm; (vi) Initial result on developing an onboard table lookup method to obtain almost fuel optimal solutions in real-time.}
}
  4. Cauligi, A., Albee, K., de la Croix, J.-P. and Brockers, R. (2025). CRESCENT: Collision-Free Highly-Constrained Trajectory Optimization for Driving on the Moon . IEEE Transactions on Field Robotics. Source
    BibTeX
    @article{cauligi2025crescent,
  title = {CRESCENT: Collision-Free Highly-Constrained Trajectory Optimization for Driving on the Moon},
  author = {Cauligi, Abhishek and Albee, Keenan and de la Croix, Jean-Pierre and Brockers, Roland},
  journal = {IEEE Transactions on Field Robotics},
  publisher = {JPL Open Repository},
  year = {2025},
  doi = {10.48577/jpl.m3hu8h},
  abstract = {Rovers have been a mainstay of planetary exploration missions, significantly expanding ourknowledge in planetary science. However, past rover missions have involved significant human supervisionto oversee rover operations, a state-of-practice that scales poorly for the next generation of missions. Inthis work, we present the development of CRESCENT, a motion planning algorithm developed for theupcoming multi-agent CADRE Lunar rover mission. CRESCENT was designed to safely drive a miniaturerover platform in a highly cluttered unmapped Lunar environment, executing complex motion directivesfrom CADRE’s team-level autonomy while meeting far stricter dynamical and temporal constraints thanexisting on-board planetary rover planning algorithms are capable of satisfying. Our hierarchical approachformulates an efficient numerical trajectory optimization-based motion planning algorithm that makesuse of nonlinear optimization to solve the planning problem in real-time. We demonstrate the efficiencyof our proposed approach through extensive simulations and hardware testing in a representative Lunarenvironment. Following CADRE’s upcoming deployment on the Lunar surface, CRESCENT will be thefirst nonlinear optimization-based trajectory optimization approach used on another celestial body.}
}
  5. Tzanetos, T., Aung, M., Balaram, J. (., Grip, H. F., Karras, J. T., Canham, T. K., Kubiak, G., Anderson, J., Merewether, G., Starch, M., Pauken, M., Cappucci, S., Chase, M., Golombek, M., Toupet, O., Smart, M. C., Dawson, S., Blandon Ramirez, E., Lam, J., Stern, R., Chahat, N., Ravich, J., Hogg, R., Pipenberg, B., Keennon, M. and Williford, K. H. (2022). Ingenuity Mars Helicopter: From Technology Demonstration to Extraterrestrial Scout . IEEE Aerospace Conference. Source
    BibTeX
    @inproceedings{tzanetos2022ingenuity,
  title = {Ingenuity Mars Helicopter: From Technology Demonstration to Extraterrestrial Scout},
  author = {Tzanetos, Theodore and Aung, MiMi and Balaram, J. (Bob) and Grip, Håvard Fjær and Karras, Jaakko T. and Canham, Timothy K. and Kubiak, Gerik and Anderson, Joshua and Merewether, Gene and Starch, Michael and Pauken, Mike and Cappucci, Stefano and Chase, Matthew and Golombek, Matthew and Toupet, Olivier and Smart, Marshall C. and Dawson, Stephen and Blandon Ramirez, Erick and Lam, Johnny and Stern, Ryan and Chahat, Nacer and Ravich, Joshua and Hogg, Robert and Pipenberg, Benjamin and Keennon, Matthew and Williford, Kenneth H.},
  booktitle = {IEEE Aerospace Conference},
  pages = {01-19},
  publisher = {IEEE},
  year = {2022},
  doi = {10.1109/aero53065.2022.9843428},
  abstract = {On April 19, 2021, NASA's Ingenuity Mars Helicopter successfully executed humanity's historic first flight on Mars. In the flights that followed, Ingenuity continued to explore the boundaries of what was aerodynamically, energetically, and operationally possible, and took on increasingly daring missions in the process. Over time, Ingenuity's mission has evolved from a “Technology Demonstration” of the first rotorcraft to fly on Mars to an “Operations Demonstration” of scientific aerial exploration scenarios on Mars. Ingenuity's activities to date have yielded a rich first-of-its-kind data set and extensive operational experience. This paper describes the Ingenuity Mars Helicopter's operation and technical performance. It details the approach and considerations involved in remotely operating a rotorcraft on Mars from Earth. It presents the performance of the vehicle in the extremely thin Martian atmosphere compared to predicted design values, based on analysis and testing on Earth. The agreement between predicted and observed performance has been excellent. This paper also discusses scientific impacts that Ingenuity Mars Helicopter has been able to contribute to the Mars 2020 Perseverance mission during Ingenuity's Operations Demonstration phase. The performance of the vehicle, the operational experiences and lessons learned, anomalies encountered and their resolutions presented in this paper critically inform the formulation, design, and development of the next generation of advanced aerial rotorcraft platforms for Mars and other extra-terrestrial bodies.11Copyright 2021 Jet Propulsion Laboratory, California Institute of Technology. Government sponsorship acknowledged.22The research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004).}
}
  6. Aagren, T. S., Ruan, A. W., Malpica, C., Withrow-Maser, S. and Meyn, L. (2025). In-flight System Identification of the Ingenuity Mars Helicopter . AIAA SCITECH Forum, 20240014856. Source
    BibTeX
    @inproceedings{aagren2025flight,
  title = {In-flight System Identification of the Ingenuity Mars Helicopter},
  author = {Aagren, Tove S. and Ruan, Allen W. and Malpica, Carlos and Withrow-Maser, Shannah and Meyn, Larry},
  booktitle = {AIAA SCITECH Forum},
  number = {20240014856},
  institution = {NASA},
  year = {2025},
  doi = {10.2514/6.2025-0007},
  abstract = {The 68th and 69th flights of NASA’s Ingenuity Mars Helicopter marked the first dedicated system identification flights of an aerial vehicle on another planet. Chirp signals were injected into the swashplate cyclic controls for both legs of the two out-and-back flights. Frequency responses were computed from the flight data, using both the Direct Method (DM) and the Joint-Input-Output (JIO) approach, for the identification of stability and control derivatives in forward flight conditions. The resulting identified state-space models were compared against existing flight dynamics simulation models, showing excellent correlation in the higher frequency range. External disturbances were seen to introduce a bias in the identified lower frequency responses, which was partially mitigated using the JIO method. These findings will inform future modeling and flight testing efforts of Mars rotorcraft.}
}
  7. Toupet, O., Biesiadecki, J., Rankin, A., Steffy, A., Meirion-Griffith, G., Levine, D., Schadegg, M. and Maimone, M. (2020). Traction Control on the Curiosity Mars Rover: Algorithm and Flight Results . Journal of Field Robotics. Source
    BibTeX
    @article{toupet2020traction,
  title = {Traction Control on the Curiosity Mars Rover: Algorithm and Flight Results},
  author = {Toupet, Olivier and Biesiadecki, Jeffrey and Rankin, Arturo and Steffy, Amanda and Meirion-Griffith, Gareth and Levine, Dan and Schadegg, Maximilian and Maimone, Mark},
  journal = {Journal of Field Robotics},
  year = {2020},
  doi = {10.48577/jpl.hkzuqs},
  abstract = {No abstract available.}
}
  8. Ishigami, G., Miwa, A., Nagatani, K. and Yoshida, K. (2007). Terramechanics-based model for steering maneuver of planetary exploration rovers on loose soil . Journal of Field Robotics, 3. Source
    BibTeX
    @article{ishigami2007terramechanics,
  title = {Terramechanics-based model for steering maneuver of planetary exploration rovers on loose soil},
  author = {Ishigami, Genya and Miwa, Akiko and Nagatani, Keiji and Yoshida, Kazuya},
  journal = {Journal of Field Robotics},
  volume = {24},
  number = {3},
  pages = {233-250},
  publisher = {Wiley},
  year = {2007},
  doi = {10.1002/rob.20187},
  abstract = {Abstract This paper presents analytical models to investigate the steering maneuvers of planetary exploration rovers on loose soil. The models are based on wheel‐soil interaction mechanics, or terramechanics, with which the traction and disturbance forces of a wheel are evaluated for various slip conditions. These traction forces are decomposed into the longitudinal and lateral directions of the wheel. The latter component, termed the side force has a major influence in characterizing the steering maneuvers of the rover. In this paper, the wheel‐soil mechanics models are developed with particular attention to the side force and the validity of the model is confirmed by using a single‐wheel test bed. The motion profile of the entire rover is numerically evaluated by incorporating the wheel‐soil models into an articulated multibody model that describes the motion dynamics of the vehicle's body and chassis. Steering maneuvers are investigated under different steering angles by using a four‐wheel rover test bed on simulated lunar soil regolith simulant. The experimental results are compared with the simulation results using the corresponding model parameters. The proposed wheel‐and‐vehicle model demonstrates better accuracy in predicting steering maneuvers as compared to the conventional kinematics‐based model. © 2006 Wiley Periodicals, Inc.}
}
  9. Angelova, A., Matthies, L., Helmick, D., Sibley, G. and Perona, P. (2006). Learning to Predict Slip for Ground Robots . IEEE International Conference on Robotics and Automation. Source
    BibTeX
    @inproceedings{angelova2006learning,
  title = {Learning to Predict Slip for Ground Robots},
  author = {Angelova, Anelia and Matthies, Larry and Helmick, Daniel and Sibley, Gabe and Perona, Pietro},
  booktitle = {IEEE International Conference on Robotics and Automation},
  pages = {3324-3331},
  year = {2006},
  doi = {10.1109/robot.2006.1642209},
  abstract = {In this paper we predict the amount of slip an exploration rover would experience using stereo imagery by learning from previous examples of traversing similar terrain. To do that, the information of terrain appearance and geometry regarding some location is correlated to the slip measured by the rover while this location is being traversed. This relationship is learned from previous experience, so slip can be predicted later at a distance from visual information only. The advantages of the approach are: 1) learning from examples allows the system to adapt to unknown terrains rather than using fixed heuristics or predefined rules; 2) the feedback about the observed slip is received from the vehicle's own sensors which can fully automate the process; 3) learning slip from previous experience can replace complex mechanical modeling of vehicle or terrain, which is time consuming and not necessarily feasible. Predicting slip is motivated by the need to assess the risk of getting trapped before entering a particular terrain. For example, a planning algorithm can utilize slip information by taking into consideration that a slippery terrain is costly or hazardous to traverse. A generic nonlinear regression framework is proposed in which the terrain type is determined from appearance and then a nonlinear model of slip is learned for a particular terrain type. In this paper we focus only on the latter problem and provide slip learning and prediction results for terrain types, such as soil, sand, gravel, and asphalt. The slip prediction error achieved is about 15% which is comparable to the measurement errors for slip itself}
}
  10. Arvidson, R. E., DeGrosse, P., Grotzinger, J. P., Heverly, M. C., Shechet, J., Moreland, S. J., Newby, M. A., Stein, N., Steffy, A. C., Zhou, F., Zastrow, A. M., Vasavada, A. R., Fraeman, A. A. and Stilly, E. K. (2017). Relating geologic units and mobility system kinematics contributing to Curiosity wheel damage at Gale Crater, Mars . Journal of Terramechanics. Source
    BibTeX
    @article{arvidson2017relating,
  title = {Relating geologic units and mobility system kinematics contributing to Curiosity wheel damage at Gale Crater, Mars},
  author = {Arvidson, Raymond E. and DeGrosse, Patrick and Grotzinger, John P. and Heverly, M. C. and Shechet, J. and Moreland, Scott J. and Newby, M. A. and Stein, Nathan and Steffy, A. C. and Zhou, Feng and Zastrow, A. M. and Vasavada, Ashwin R. and Fraeman, Abigail A. and Stilly, E. K.},
  journal = {Journal of Terramechanics},
  volume = {73},
  pages = {73--93},
  year = {2017},
  doi = {10.1016/j.jterra.2017.03.001},
  abstract = {Curiosity landed on plains to the north of Mount Sharp in August 2012. By June 2016 the rover had traversed 12.9 km to the southwest, encountering extensive strata that were deposited in a fluvial-deltaic-lacustrine system. Initial drives across sharp sandstone outcrops initiated an unacceptably high rate of punctures and cracks in the thin aluminum wheel skin structures. Initial damage was found to be related to the drive control mode of the six wheel drive actuators and the kinematics of the rocker-bogie suspension. Wheels leading a suspension pivot were forced onto sharp, immobile surfaces by the other wheels as they maintained their commanded angular velocities. Wheel damage mechanisms such as geometry-induced stress concentration cracking and low-cycle fatigue were then exacerbated. A geomorphic map was generated to assist in planning traverses that would minimize further wheel damage. A steady increase in punctures and cracks between landing and June 2016 was due in part because of drives across the sharp sandstone outcrops that could not be avoided. Wheel lifetime estimates show that with careful path planning the wheels will be operational for an additional ten kilometers or more, allowing the rover to reach key strata exposed on the slopes of Mount Sharp.}
}
  11. Whitney, D. E. (1969). Resolved Motion Rate Control of Manipulators and Human Prostheses . IEEE Transactions on Man-Machine Systems. Source
    BibTeX
    @article{whitney1969resolved,
  title = {Resolved Motion Rate Control of Manipulators and Human Prostheses},
  author = {Whitney, D. E.},
  journal = {IEEE Transactions on Man-Machine Systems},
  volume = {10},
  pages = {47-53},
  year = {1969},
  doi = {10.1109/tmms.1969.299896},
  abstract = {The kinematics of remote manipulators and human prostheses is analyzed for the purpose of deriving resolved motion rate control. That is, the operator is enabled to call for the desired hand motion directly along axes relevant to the task environment. The approach suggests solutions to problems of coordination, motion under task constraints, and appreciation of forces encountered by the controlled hand.}
}
  12. Khatib, O. (1986). Real-Time Obstacle Avoidance for Manipulators and Mobile Robots . Earth and Space Science, 1. Source
    BibTeX
    @article{khatib1986real,
  title = {Real-Time Obstacle Avoidance for Manipulators and Mobile Robots},
  author = {Khatib, Oussama},
  journal = {Earth and Space Science},
  volume = {5},
  number = {1},
  pages = {90-98},
  publisher = {SAGE Publications},
  year = {1986},
  doi = {10.1177/027836498600500106},
  abstract = {This paper presents a unique real-time obstacle avoidance approach for manipulators and mobile robots based on the artificial potential field concept. Collision avoidance, tradi tionally considered a high level planning problem, can be effectively distributed between different levels of control, al lowing real-time robot operations in a complex environment. This method has been extended to moving obstacles by using a time-varying artificial patential field. We have applied this obstacle avoidance scheme to robot arm mechanisms and have used a new approach to the general problem of real-time manipulator control. We reformulated the manipulator con trol problem as direct control of manipulator motion in oper ational space—the space in which the task is originally described—rather than as control of the task's corresponding joint space motion obtained only after geometric and kine matic transformation. Outside the obstacles' regions of influ ence, we caused the end effector to move in a straight line with an upper speed limit. The artificial potential field ap proach has been extended to collision avoidance for all ma nipulator links. In addition, a joint space artificial potential field is used to satisfy the manipulator internal joint con straints. This method has been implemented in the COSMOS system for a PUMA 560 robot. Real-time collision avoidance demonstrations on moving obstacles have been performed by using visual sensing.}
}
  13. Helmick, D., Okon, A. and DiCicco, M. (2006). A Comparison of Force Sensing Techniques for Planetary Manipulation . IEEE Aerospace Conference. Source
    BibTeX
    @inproceedings{helmick2006comparison,
  title = {A Comparison of Force Sensing Techniques for Planetary Manipulation},
  author = {Helmick, Daniel and Okon, Avi and DiCicco, Matt},
  booktitle = {IEEE Aerospace Conference},
  pages = {1-14},
  address = {Big Sky, Montana},
  year = {2006},
  doi = {10.1109/aero.2006.1655724},
  abstract = {Five techniques for sensing forces with a manipulator are compared analytically and experimentally. The techniques compared are: a six-axis wrist force/torque sensor, joint torque sensors, link strain gauges, motor current sensors, and flexibility modeling. The accuracy and repeatability of each technique is quantified and compared. The relative complexity and the impact on flight design of each technique are also compared. The results presented can be used in a trade study for missions requiring manipulator force sensing capabilities}
}
  14. Dubowsky, S. and Papadopoulos, E. (1993). The Kinematics, Dynamics, and Control of Free-Flying and Free-Floating Space Robotic Systems . IEEE Transactions on Robotics and Automation, 5. Source
    BibTeX
    @article{dubowsky1993kinematics,
  title = {The Kinematics, Dynamics, and Control of Free-Flying and Free-Floating Space Robotic Systems},
  author = {Dubowsky, Steven and Papadopoulos, Evangelos},
  journal = {IEEE Transactions on Robotics and Automation},
  volume = {9},
  number = {5},
  pages = {531--543},
  year = {1993},
  doi = {10.1109/70.258046}
}
  15. Papadopoulos, E. and Dubowsky, S. (1991). On the Nature of Control Algorithms for Free-Floating Space Manipulators . IEEE Transactions on Robotics and Automation, 6. Source
    BibTeX
    @article{papadopoulos1991nature,
  title = {On the Nature of Control Algorithms for Free-Floating Space Manipulators},
  author = {Papadopoulos, Evangelos and Dubowsky, Steven},
  journal = {IEEE Transactions on Robotics and Automation},
  volume = {7},
  number = {6},
  pages = {750--758},
  year = {1991},
  doi = {10.1109/70.105384},
  abstract = {It is suggested that nearly any control algorithm that can be used for fixed-based manipulators also can be employed in the control of free-floating space manipulator systems, with the additional conditions of estimating or measuring a spacecraft's orientation and of avoiding dynamic singularities. This result is based on the structural similarities between the kinematic and dynamic equations for the same manipulator but with a fixed base. Barycenters are used to formulate the kinematic and dynamic equations of free-floating space manipulators. A control algorithm for a space manipulator system is designed to demonstrate the value of the analysis.>}
}
  16. Yoshida, K. (2003). Engineering Test Satellite VII Flight Experiments for Space Robot Dynamics and Control: Theories on Laboratory Test Beds Ten Years Ago, Now in Orbit . The International Journal of Robotics Research, 5. Source
    BibTeX
    @article{yoshida2003engineering,
  title = {Engineering Test Satellite VII Flight Experiments for Space Robot Dynamics and Control: Theories on Laboratory Test Beds Ten Years Ago, Now in Orbit},
  author = {Yoshida, Kazuya},
  journal = {The International Journal of Robotics Research},
  volume = {22},
  number = {5},
  pages = {321--335},
  year = {2003},
  doi = {10.1177/0278364903022005003},
  abstract = {The Engineering Test Satellite VII (ETS-VII), an unmanned spacecraft equipped with a 2-m long, six-degree-of-freedom manipulator arm, was developed and launched by the National Space Development Agency of Japan (NASDA). ETS-VII has successfully carried out a variety of on-board experiments with its manipulator arm, and these key technologies are essential for an orbital free-flying robot. These results will provide a solid basis for future satellite servicing missions. This paper highlights manipulator control utilizing the concepts of the generalized Jacobian matrix and the reaction null-space. These concepts have been proposed and discussed for the past ten years using laboratory test beds, and their practical application has now been demonstrated in orbit.}
}
  17. Nolet, S. (2007). Development of a Guidance, Navigation and Control Architecture and Validation Process Enabling Autonomous Docking to a Tumbling Satellite. Source
    BibTeX
    @phdthesis{nolet2007development,
  title = {Development of a Guidance, Navigation and Control Architecture and Validation Process Enabling Autonomous Docking to a Tumbling Satellite},
  author = {Nolet, Simon},
  school = {Massachusetts Institute of Technology},
  type = {Ph.D. thesis},
  year = {2007},
  url = {https://dspace.mit.edu/handle/1721.1/38598}
}
  18. Alexandrov, O., Barlow, J., Benavides, J., Bualat, M., Carlino, R., Coltin, B., Cortez, J., Daley, E., Feller, J., Flückiger, L., Fong, T., Fusco, J., Garcia Ruiz, R., Hamilton, K., Kanis, S., Katterhagen, A., Kim, Y., Love, J. F., McIntyre, M., McLachlan, B., Mora Vargas, A., Moratto, Z., Moreira, M., Morse, T., Orosco, H., Park, I.-W., Provencher, C., Sanchez, H., Sharif, K., Smith, E., Smith, T., Soussan, R., Symington, A., Talavera, R. O., To, V., Wheeler, D. and Yoo, J. (2026). Astrobee: Free-Flying Robots for the International Space Station . IEEE Transactions on Field Robotics. Source
    BibTeX
    @article{alexandrov2026astrobee,
  title = {Astrobee: Free-Flying Robots for the International Space Station},
  author = {Alexandrov, Oleg and Barlow, Jonathan and Benavides, Jose and Bualat, Maria and Carlino, Roberto and Coltin, Brian and Cortez, Jose and Daley, Earl and Feller, Jeffrey and Flückiger, Lorenzo and Fong, Terrence and Fusco, Jesse and Garcia Ruiz, Ruben and Hamilton, Kathryn and Kanis, Simeon and Katterhagen, Aric and Kim, Yunkyung and Love, John F. and McIntyre, Michael and McLachlan, Blair and Mora Vargas, Andres and Moratto, Zack and Moreira, Marina and Morse, Theodore and Orosco, Henry and Park, In-Won and Provencher, Christopher and Sanchez, Hugo and Sharif, Khaled and Smith, Ernest and Smith, Trey and Soussan, Ryan and Symington, Andrew and Talavera, Rafael Omar and To, Vinh and Wheeler, DW and Yoo, Jongwoon},
  journal = {IEEE Transactions on Field Robotics},
  year = {2026},
  url = {https://ntrs.nasa.gov/citations/20260001396},
  abstract = {The Astrobees are free-flying robots that operate inside the International Space Station (ISS) and were launched to the ISS
in 2019. Since then they have successfully performed hundreds of activities in space supporting almost two dozen separate
research projects. The robots were designed to overcome multiple challenges unique to the ISS environment, including safety,
upgradeability and maintainability, limited mass and computation, and unique localization challenges from lack of gravity and a
constantly changing environment. This article provides an overview of Astrobee, from hardware and software design to deployment
results and activities.}
}
  19. Soussan, R., Kumar, V., Coltin, B. and Smith, T. (2022). AstroLoc: An Efficient and Robust Localizer for a Free-Flying Robot . International Conference on Robotics and Automation. Source
    BibTeX
    @inproceedings{soussan2022astroloc,
  title = {AstroLoc: An Efficient and Robust Localizer for a Free-Flying Robot},
  author = {Soussan, Ryan and Kumar, Varsha and Coltin, Brian and Smith, Trey},
  booktitle = {International Conference on Robotics and Automation},
  pages = {4106-4112},
  year = {2022},
  doi = {10.1109/icra46639.2022.9811919},
  abstract = {We present AstroLoc, an efficient and robust monocular visual-inertial graph-based localization system used by the Astrobee free-flying robots onboard the International Space Station (ISS). We provide a novel localization system that limits the traditionally higher computation times for graph-based localization systems and enables the resource constrained Astrobee robots to benefit from their increased accuracy. We also introduce methods for handling cheirality issues for visual odometry and localization factors that further increase localization robustness. We evaluate the performance of AstroLoc on a dataset of ISS activities and show that it greatly improves pose, velocity, and IMU bias estimation accuracy while efficiently running in a limited computation environment. AstroLoc has improved the localization accuracy for the Astrobee robots on the ISS and has led to more successful and longer duration activities. While the AstroLoc system is tuned for the Astrobee robots, it can be configured for any resource constrained platform. The source code for AstroLoc is released to the public.}
}
  20. Yamaguchi, S. P., Mora Vargas, A., Eisenberg, T., Rogon, C., Yamamoto, T., Inoue, S., Kössl, C., Coltin, B., Smith, T. and Benavides, J. V. (2025). Free-Flying Crew Cooperative Robots on the ISS: A Joint Review of Astrobee, CIMON, and Int-Ball Operations . International Conference on Space Robotics. Source
    BibTeX
    @inproceedings{yamaguchi2026free,
  title = {Free-Flying Crew Cooperative Robots on the ISS: A Joint Review of Astrobee, CIMON, and Int-Ball Operations},
  author = {Yamaguchi, Seiko Piotr and Mora Vargas, Andres and Eisenberg, Till and Rogon, Christian and Yamamoto, Tatsuya and Inoue, Shona and Kössl, Christoph and Coltin, Brian and Smith, Trey and Benavides, Jose V.},
  booktitle = {International Conference on Space Robotics},
  pages = {402-409},
  year = {2025},
  doi = {10.1109/isparo66239.2025.11436992},
  abstract = {Intra-vehicular free-flying robots are anticipated to support various work in human spaceflight while working side-by-side with astronauts. Such example of robots includes NASA’s Astrobee, DLR’s CIMON, and JAXA’s Int-Ball, which are deployed on the International Space Station. This paper presents the first joint analyses of these robot’s shared experiences, co-authored by their development and operation team members. Despite the different origins and design philosophies, the development and operations of these platforms encountered various convergences. Hence, this paper presents a detailed overview of these robots, presenting their objectives, design, and onboard operations. Hence, joint lessons learned across the lifecycle are presented, from design to on-orbit operations. These lessons learned are anticipated to serve for future development and research as design recommendations.}
}
  21. Grip, H. F., Lam, J., Bayard, D. S., Conway, D. T., Singh, G., Brockers, R., Delaune, J. H., Matthies, L. H., Malpica, C., Brown, T. L., Jain, A., San Martin, A. M. and Merewether, G. B. (2019). Flight Control System for NASA's Mars Helicopter . IEEE Aerospace Conference. Source
    BibTeX
    @inproceedings{grip2019flight,
  title = {Flight Control System for NASA's Mars Helicopter},
  author = {Grip, Håvard F. and Lam, Johnny and Bayard, David S. and Conway, Dylan T. and Singh, Gurkirpal and Brockers, Roland and Delaune, Jeff H. and Matthies, Larry H. and Malpica, Carlos and Brown, Travis L. and Jain, Abhinandan and San Martin, Alejandro M. and Merewether, Gene B.},
  booktitle = {IEEE Aerospace Conference},
  publisher = {American Institute of Aeronautics and Astronautics},
  year = {2019},
  doi = {10.2514/6.2019-1289}
}
  22. Hirano, D., Mitani, S., Watanabe, K., Nishishita, T., Yamamoto, T. and Yamaguchi, S. P. (2025). Int-Ball2: On-Orbit Demonstration of Autonomous Intravehicular Flight and Docking for Image Capturing and Recharging . IEEE Robotics & Automation Magazine, 3. Source
    BibTeX
    @article{hirano2025intball2,
  title = {{Int-Ball2}: On-Orbit Demonstration of Autonomous Intravehicular Flight and Docking for Image Capturing and Recharging},
  author = {Hirano, Daichi and Mitani, Shinji and Watanabe, Keisuke and Nishishita, Taisei and Yamamoto, Tatsuya and Yamaguchi, Seiko P.},
  journal = {IEEE Robotics & Automation Magazine},
  volume = {32},
  number = {3},
  pages = {76--87},
  year = {2025},
  doi = {10.1109/mra.2024.3505776},
  abstract = {This article presents the system architecture and the orbital demonstration results of the Int-Ball2, a free-flying camera robot developed by the Japan Aerospace Exploration Agency (JAXA). The purpose of the Int-Ball2 project is to assist astronauts and reduce their workload in the International Space Station (ISS). This robot is an upgrade from the first Int-Ball, enhancing the propulsion subsystem for greater maneuverability and adding a new docking station (DS) for autonomous battery recharging. This study performed comprehensive ground tests for autonomous maneuvering and docking, employing a combination of a fully software-based simulator, a hardware-in-the-loop (HIL) simulator, and a planar air-bearing facility. After a successful launch to the ISS, the Int-Ball2 demonstrated its ability to work in microgravity without relying on astronaut support. The results obtained from ground and orbital tests underscored the effectiveness of our system design and ground verification approach. Further, we present key technologies essential for the Int-Ball2’s successful implementation on board the ISS. We expect the insights from this project to be invaluable to future missions involving free-flying robots in microgravity.}
}
  23. Casoliva, J., Singh, G., Brugarolas, P. and Way, D. W. (2021). Reconstructed Flight Performance of the Powered Descent Guidance and Control System for the Mars 2020 Perseverance Mission . AAS/AIAA Astrodynamics Specialist Conference. Source
    BibTeX
    @inproceedings{casoliva2021reconstructed,
  title = {Reconstructed Flight Performance of the Powered Descent Guidance and Control System for the Mars 2020 Perseverance Mission},
  author = {Casoliva, Jordi and Singh, Gurkirpal and Brugarolas, Paul and Way, David W.},
  booktitle = {AAS/AIAA Astrodynamics Specialist Conference},
  publisher = {JPL Open Repository},
  year = {2021},
  url = {https://dataverse.jpl.nasa.gov/dataset.xhtml?persistentId=hdl:2014/55198}
}
  24. Brugarolas, P. (2017). Guidance, Navigation and Control for the Entry, Descent, and Landing of the Mars 2020 Mission . Annual Guidance and Control Conference. Source
    BibTeX
    @inproceedings{brugarolas2017guidance,
  title = {Guidance, Navigation and Control for the Entry, Descent, and Landing of the Mars 2020 Mission},
  author = {Brugarolas, Paul},
  booktitle = {Annual Guidance and Control Conference},
  publisher = {JPL Open Repository},
  year = {2017},
  url = {https://dataverse.jpl.nasa.gov/dataset.xhtml?persistentId=hdl:2014/46389}
}
  25. Mazouz, R., Quadrelli, M. and Mooij, E. (2021). Convex Optimization Guidance for Precision Landing on Titan . AIAA SCITECH Forum. Source
    BibTeX
    @inproceedings{mazouz2021convex,
  title = {Convex Optimization Guidance for Precision Landing on Titan},
  author = {Mazouz, Rayan and Quadrelli, Marco and Mooij, Edwin},
  booktitle = {AIAA SCITECH Forum},
  publisher = {American Institute of Aeronautics and Astronautics},
  year = {2021},
  doi = {10.2514/6.2021-1345},
  abstract = {View Video Presentation: https://doi.org/10.2514/6.2021-1345.vid Precision landing is an anticipated technology for future interplanetary missions. Autonomous spacecraft Entry, Descent and Landing (EDL) on the surface of a planetary body with a degree of precision in the order of meters is highly challenging. In this paper, a successive convexification guidance algorithm is utilized to simulate autonomous precision landing sequences on Saturn’s moon Titan. Due to its unique geophysical features, studying the science of matter within Titan’s atmosphere and beneath its surface is one of NASA’s most important planetary science objectives. As part of the Space Exploration Technology Directorate, a parafoil is proposed for landing on Titan due to its cost effectiveness, ease of deployment, low mass compared to the prospective payload and capabilities of precise autonomous delivery. This paper focuses on path optimization and guidance law development for high-fidelity dynamics parafoil tuning in the dense and adverse wind atmosphere of Titan, defined as a nonlinear and nonconvex optimal control problem. The powerful successive convexification method is used to solve the problem accordingly. The algorithm is designed such that the converged solution adheres to the nonlinear dynamics and kinematics in accordance with the original formulation, while respecting the state and control constraints. The six-degree-of-freedom (6DoF) simulations results show that this robust method is suitable for autonomous interplanetary applications.}
}
  26. Ramezani, M., Alandihallaj, M. A., Yalçın, B. C., Mendez, M. A. O. and Voos, H. (2025). MPC-based Deep Reinforcement Learning Method for Space Robotic Control with Fuel Sloshing Mitigation . IEEE/RSJ International Conference on Intelligent Robots and Systems. Source
    BibTeX
    @inproceedings{ramezani2025mpc,
  title = {MPC-based Deep Reinforcement Learning Method for Space Robotic Control with Fuel Sloshing Mitigation},
  author = {Ramezani, Mahya and Alandihallaj, M. Amin and Yalçın, Barış Can and Mendez, Miguel Angel Olivares and Voos, Holger},
  booktitle = {IEEE/RSJ International Conference on Intelligent Robots and Systems},
  pages = {1503-1510},
  year = {2025},
  doi = {10.1109/iros60139.2025.11247300},
  abstract = {This paper presents an integrated Reinforcement Learning (RL) and Model Predictive Control (MPC) framework for autonomous satellite docking with a partially filled fuel tank. Traditional docking control faces challenges due to fuel sloshing in microgravity, which induces unpredictable forces affecting stability. To address this, we integrate Proximal Policy Optimization (PPO) and Soft Actor-Critic (SAC) RL algorithms with MPC, leveraging MPC’s predictive capabilities to accelerate RL training and improve control robustness. The proposed approach is validated through Zero-G Lab of SnT experiments for planar stabilization and high-fidelity numerical simulations for 6-DOF docking with fuel sloshing dynamics. Simulation results demonstrate that SAC-MPC achieves superior docking accuracy, higher success rates, and lower control effort, outperforming standalone RL and PPO-MPC methods. This study advances fuel-efficient and disturbance-resilient satellite docking, enhancing the feasibility of on-orbit refueling and servicing missions.}
}
  27. Nenchev, D. N., Umetani, Y. and Yoshida, K. (1992). Analysis of a redundant free-flying spacecraft/manipulator system . IEEE Transactions on Robotics and Automation, 1. Source
    BibTeX
    @article{nenchev1992analysis,
  title = {Analysis of a redundant free-flying spacecraft/manipulator system},
  author = {Nenchev, Dragomir N. and Umetani, Yoji and Yoshida, Kazuya},
  journal = {IEEE Transactions on Robotics and Automation},
  volume = {8},
  number = {1},
  pages = {1-6},
  publisher = {Institute of Electrical and Electronics Engineers (IEEE)},
  year = {1992},
  doi = {10.1109/70.127234},
  abstract = {An analysis of the momentum conservation equations of a redundant free-flying spacecraft/manipulator system acting in a zero-gravity environment is presented. In order to follow a predefined end-effector path, the inverse kinematics at velocity level is considered. The redundancy is solved alternatively in terms of pseudoinverses and null-space components of the manipulator inertia matrix, the manipulator Jacobian matrix, and the generalized Jacobian matrix. A general manipulation task is defined as end-effector continuous path tracking with simultaneous attitude control of the spacecraft. Three subtasks of the general task are considered. The case of manipulator motions that yield no spacecraft attitude disturbance is analyzed in more detail and a special 'fixed-attitude-restricted' (FAR) Jacobian is defined. Through singular-value decomposition of this Jacobian, corresponding FAR dexterity measures (FAR manipulability and FAR condition number) are derived.>}
}
  28. Woicke, S., Jipp, J., Steimle, C., Pokrupa, N. and Metrailler, L. (2025). ADRIOS ClearSpace-1: In orbit demonstration of the removal of a non-cooperative spacecraft . European Conference on Space Debris. Source
    BibTeX
    @inproceedings{woicke2025adrios,
  title = {ADRIOS ClearSpace-1: In orbit demonstration of the removal of a non-cooperative spacecraft},
  author = {Woicke, Svenja and Jipp, Jimmy and Steimle, Christian and Pokrupa, Nils and Metrailler, Lionel},
  booktitle = {European Conference on Space Debris},
  organization = {ESA Space Debris Office},
  year = {2025},
  url = {https://conference.sdo.esoc.esa.int/proceedings/sdc9/paper/104}
}
  29. Giordano, A. M., Dietrich, A., Ott, C. and Albu-Schäffer, A. (2020). Coordination of thrusters, reaction wheels, and arm in orbital robots . Robotics and Autonomous Systems. Source
    BibTeX
    @article{giordano2020coordination,
  title = {Coordination of thrusters, reaction wheels, and arm in orbital robots},
  author = {Giordano, Alessandro M. and Dietrich, Alexander and Ott, Christian and Albu-Schäffer, Alin},
  journal = {Robotics and Autonomous Systems},
  volume = {131},
  pages = {103564},
  publisher = {Elsevier BV},
  year = {2020},
  doi = {10.1016/j.robot.2020.103564},
  abstract = {A fuel-efficient control strategy for a manipulator-equipped spacecraft is presented. The strategy uses the thrusters , the reaction wheels, and the arm drives in a coordinated way to limit the use of the thrusters and achieve ideally zero fuel consumption in contact-free maneuvering. The thrusters are activated automatically only after contact, to stabilize the inertial motion of the system. The controller regulates the translation of the center-of-mass (CoM) of the whole space robot, the rotation of the spacecraft, and the pose of the end-effector (EE) in a decoupled way, utilizing the thrusters to control the CoM translation only and the remaining actuators to control the rotation and end-effector coordinately. The method is validated experimentally using a hardware-in-the-loop simulator composed of a seven degrees-of-freedom (DOF) arm mounted on a 6DOF simulated spacecraft. Numerical simulations with discrete thrusters assess the fuel efficiency of the proposed strategy.}
}
  30. Li, Q., Yuan, J., Zhang, B. and Gao, C. (2017). Model predictive control for autonomous rendezvous and docking with a tumbling target . Aerospace Science and Technology. Source
    BibTeX
    @article{li2017model,
  title = {Model predictive control for autonomous rendezvous and docking with a tumbling target},
  author = {Li, Qi and Yuan, Jianping and Zhang, Bo and Gao, Chen},
  journal = {Aerospace Science and Technology},
  volume = {69},
  pages = {700-711},
  year = {2017},
  doi = {10.1016/j.ast.2017.07.022}
}
  31. 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.}
}