Advances Space Debris Removal with Safe, Close-Range Orbital Robot Rendezvous Techniques

Safely approaching and capturing tumbling objects in space presents a significant hurdle for future robotic missions designed to remove debris or service satellites, and researchers are now addressing this challenge with innovative trajectory planning techniques. Kenta Iizuka, Akiyoshi Uchida, Kentaro Uno, and Kazuya Yoshida, all from the Space Robotics Lab at Tohoku University, demonstrate a new method for guiding a robotic arm towards a free-floating, rotating target, bringing it within reach for capture. Their approach utilises a dynamic ‘keep-out sphere’ that adjusts to the specific conditions of each approach, enabling closer and safer proximity than previously possible. The team also developed a practical control system, using simple thrusters, to accurately follow the calculated trajectory, representing a crucial step towards fully autonomous orbital robotics.

Space Debris Rendezvous and Capture Strategies

This research paper focuses on the problem of space debris and the need for active debris removal (ADR). In particular, it studies the rendezvous and capture of non-cooperative targets, such as tumbling objects with unknown motion. The main challenges include accurate trajectory planning, reliable control in microgravity, and safe close-range operations around complex-shaped targets.

Proposed Approach

The paper presents a two-stage method.
First, an ideal trajectory is generated using nonlinear optimization, assuming continuous control. This stage aims to find a collision-free path while meeting constraints such as thrust limits and alignment requirements.
Second, the planned trajectory is tracked using realistic ON/OFF thrusters based on pulse-width modulation, which better reflects real spacecraft hardware where continuous thrust is not always possible.

To improve safety, the method uses dynamically updated restricted regions around the target to avoid collisions during the final approach and alignment.

Methodology and Tools

The approach is tested through 2D simulations using the MuJoCo physics engine. Nonlinear optimization is performed using Casadi. The authors also mention future plans to validate the method using a real-world air-bearing microgravity testbed.

Results from Case Studies

The results show that the trajectory planning remains feasible for target angular velocities of up to about 2.0 rad/s. However, the final position error increases significantly when the chaser approaches the target from the rear, around 180 degrees, indicating a more difficult control scenario. Despite this, the chaser’s final attitude is controlled very accurately, with errors on the order of 10⁻³³.

Future Work

Future work includes extending the method to fully 3D tumbling targets and conducting hardware experiments using the air-bearing testbed.

Dynamic Keep-Out Sphere for Safe Approach

Scientists developed a novel trajectory planning method to address the critical challenge of safely approaching tumbling targets during robotic space debris removal missions. The work focuses on close-range rendezvous, bringing free-floating, rotating objects into the workspace of a robotic manipulator, a crucial preliminary step for capture. Researchers employed nonlinear optimization to plan trajectories in a two-dimensional plane, enabling the servicing satellite to approach the debris object without collision. This approach differs from existing methods by introducing a dynamic keep-out sphere, which adjusts its size based on the specific approach conditions, allowing for significantly closer and safer access to the target., The team engineered a system that accurately predicts the motion of the tumbling target and incorporates this prediction into the optimization framework, generating feasible trajectories while respecting safety constraints.

Experiments utilized a simulated environment to test the effectiveness of the trajectory planning, demonstrating the ability to navigate complex rotational movements of the target object. To address practical limitations of spacecraft systems, scientists also developed a control strategy that reproduces the optimized trajectory using discrete ON/OFF thrusters, rather than continuous thrust, a common configuration for orbital maneuvering. This involved careful consideration of actuator limitations and the need for precise control despite the pulsed nature of the thrusters., The methodology achieves a high degree of precision in positioning the chaser satellite relative to the tumbling target, as demonstrated through simulations showing successful pose adjustments prior to capture. This innovative approach overcomes the difficulties of simultaneously handling nonlinear dynamics, actuator constraints, and collision avoidance, issues that often plague traditional optimization-based methods. By focusing on practical implementation with discrete thrusters, the study pioneers a solution that bridges the gap between theoretical trajectory planning and real-world space operations, paving the way for more effective and reliable debris removal missions and future on-orbit servicing applications.

Safe Capture of Tumbling Space Objects

Scientists have developed a trajectory planning method for robotic spacecraft to safely approach and capture tumbling objects in space, a critical capability for debris removal missions. The work focuses on close-range rendezvous, bringing a free-floating, rotating object into the range of a robotic manipulator for subsequent capture. A key innovation is a dynamic “keep-out sphere” that adjusts its size based on the approach conditions, enabling closer and safer access to the target object., Experiments demonstrate the method’s effectiveness in a high-fidelity simulation environment, incorporating realistic imperfections such as discrepancies between the modeled and actual system and the discrete nature of thruster activation. The team quantitatively evaluated performance under varying conditions, including target attitude, angular velocity, and maximum thruster output.

Results show the system successfully generates trajectories that account for the target’s motion and the spacecraft’s limitations., The research involved modeling the chaser and target spacecraft in a two-dimensional plane, defining the chaser’s state with a six-dimensional vector encompassing position, orientation, velocity, and angular velocity. The chaser’s motion is governed by Newton-Euler equations, driven by eight thrusters controlled in an on/off manner. Tests confirm that the thruster configuration, mirroring those used in planar air-floating testbeds, accurately reproduces both translational and rotational motion. The optimization model, solved using IPOPT, minimizes a cost function considering the distance to the goal, kinetic energy, and control effort. The breakthrough delivers a robust framework for trajectory generation and tracking, paving the way for more efficient debris capture strategies in future on-orbit servicing missions.

Safe Trajectory Control for Rotating Space Objects

This research presents a novel trajectory planning and control method for spacecraft approaching and aligning with free-floating, rotating objects in microgravity environments, a crucial capability for future debris removal missions. The team successfully developed a two-stage process, beginning with nonlinear optimization to generate safe and accurate approach trajectories, followed by a control strategy utilizing practical ON/OFF thrusters to execute those trajectories. A key innovation lies in the implementation of a dynamic keep-out sphere, which adjusts based on the relative motion between spacecraft and target, enabling closer proximity and reducing collision risk during final alignment., Evaluations using simulations demonstrate the method’s ability to maintain high accuracy and safety across a range of conditions, with the optimized trajectories consistently achieving minimal attitude error. Detailed case studies revealed the system performs well with target angular velocities up to approximately 2.0 rad/s, though the research acknowledges that tracking accuracy diminishes when approaching the target from its rear side, specifically between 150 and 210 degrees. The team intends to extend this work to three-dimensional scenarios involving tumbling targets and validate the approach with hardware experiments on a two-dimensional air-floating test platform, promising further advancements in robotic space servicing technologies.