The RemoveDEBRIS mission, launched in 2018, represents a significant milestone in the field of active debris removal (ADR) within low-Earth orbit (LEO). Managed by the Surrey Space Centre and a consortium of industrial partners including Airbus and SSTL, the mission utilized a modular microsatellite to validate multiple technologies for capturing and de-orbiting space junk. These technologies included a net capture system, a harpoon mechanism, and a vision-based navigation suite, all designed to address the increasing density of defunct hardware in critical orbital bands.
Deployed from the International Space Station (ISS) via the NanoRacks Kaber deployment system in June 2018, the satellite conducted a series of controlled experiments over several months. Each phase of the mission required rigorous ephemeris generation and tracking of orbital element changes to ensure that the experiments did not inadvertently create further debris or pose a collision risk to active assets. The successful deployment of a drag sail at the mission's conclusion provided a case study in the mechanics of passive atmospheric decay facilitated by increased surface area.
At a glance
- Mission Lead:Surrey Space Centre, University of Surrey.
- Launch Date:April 2, 2018 (via SpaceX CRS-14).
- Deployment Date:June 20, 2018 (from the ISS).
- Mass:Approximately 100 kilograms.
- Primary Payloads:Net capture system, Harpoon system, Vision-Based Navigation (VBN), and a Drag Sail.
- Target Objects:Two CubeSats (DS-1 and DS-2) deployed by the mother ship to serve as simulated debris.
- Collaborators:Airbus Defence and Space, ArianeGroup, new Solutions in Space (ISIS), CSEM, Inria, and Stellenbosch University.
Background
The accumulation of space debris in LEO has reached a threshold where the probability of cascading collisions, known as the Kessler Syndrome, threatens the long-term sustainability of space operations. Debris ranges from spent rocket stages and defunct satellites to small fragments resulting from previous impacts or explosions. Because these objects travel at orbital velocities—approximately 7.5 kilometers per second in LEO—even small fragments possess enough kinetic energy to destroy active spacecraft. The RemoveDEBRIS mission was conceived as a low-cost, high-return demonstration of the technical feasibility of capturing such objects using mechanical interventions.
Prior to the mission, theoretical models of debris remediation focused heavily on the use of robotic arms or electrodynamic tethers. However, the RemoveDEBRIS project sought to test "throwable" capture technologies. These methods are generally considered less complex in terms of docking requirements, as they allow the remediation satellite to maintain a safer distance from the target debris. The mission’s success depended on the precision of orbital mechanics and the ability to predict the behavior of composite materials in the vacuum of space.
Orbital Mechanics and Ephemeris Generation
Accurate ephemeris generation—the calculation of a celestial object's position and velocity over time—is the cornerstone of debris remediation. For the RemoveDEBRIS mission, tracking the mother ship and its released targets required constant updates to Two-Line Element (TLE) sets provided by the United States Space Command (USSPACECOM). Practitioners use these elements as inputs for propagation models like SGP4 (Simplified General Perturbations-4), which account for the Earth's non-spherical shape (oblateness) and gravitational perturbations from the Moon and Sun.
Atmospheric Drag and Thermospheric Modeling
In LEO, the primary non-conservative force acting on a satellite is atmospheric drag. This force is particularly difficult to model because the Earth's thermosphere expands and contracts in response to solar activity. The RemoveDEBRIS mission relied on advanced thermospheric models, such as NRLMSISE-00, to estimate the residual atmospheric density at altitudes between 300 and 600 kilometers. This model utilizes the F10.7 solar flux index and the Ap geomagnetic index to produce real-time density profiles.
Calculations for the orbital decay of Kevlar-composite materials, such as those used in the capture net, require a meticulous analysis of the ballistic coefficient. The ballistic coefficient is a function of the object's mass, its cross-sectional area, and its drag coefficient. Because a net is a porous structure, its aerodynamic behavior differs significantly from solid bodies. Practitioners must account for the changing orientation of the net as it interacts with the rarefied flow of the upper atmosphere, ensuring that the decay trajectory remains within safe margins.
Solar Radiation Pressure (SRP)
Beyond atmospheric drag, solar radiation pressure (SRP) exerts a subtle but constant force on the satellite and its tethered components. SRP is the pressure exerted by photons from the sun. While small, its cumulative effect can shift a satellite's position by hundreds of meters over the course of several orbits. For precise capture maneuvers, such as the harpoon test, mission controllers had to calibrate thruster firings to compensate for these pressure gradients, particularly when the satellite moved from the Earth's shadow (penumbra) into direct sunlight.
Capture Mechanics: The Net and Harpoon
The net capture experiment involved the deployment of a small CubeSat (DS-1) which then inflated a balloon to increase its surface area and mimic a larger piece of debris. The mother craft utilized a cold-gas thruster system to position itself for the net deployment. The net itself was constructed from high-strength Kevlar-composite fibers, weighted at the edges with masses that caused it to expand upon ejection. The use of Kevlar is critical due to its high tensile strength and resistance to the extreme temperature fluctuations found in orbit.
The harpoon experiment followed a similar logic but focused on a more rigid capture method. A target plate was extended from the mother craft on a boom. The harpoon was fired at high velocity to penetrate the plate, demonstrating the ability to snag debris that might be tumbling or difficult to wrap in a net. Analyzing the transfer of momentum during these impacts is essential for maintaining the stability of the mother craft. Any unplanned change in velocity (delta-v) could lead to a loss of orbital altitude or a change in inclination that would jeopardize the mission timeline.
Propulsion and Delta-V Management
While the RemoveDEBRIS mission was largely a technology demonstrator, standard debris remediation satellites often use ion-thruster arrays for precise orbital maintenance. These thrusters commonly use xenon propellant due to its high atomic weight and relative ease of storage. Ion propulsion provides a high specific impulse, allowing for very efficient maneuvers over long periods. In the context of ephemeris refinement, the use of xenon allows for "micro-burns" that can adjust a satellite's orbit by centimeters, ensuring that it remains on a collision-free path with the target debris.
During the RemoveDEBRIS mission, the primary concern was minimizing the delta-v expenditure during the deployment of the drag sail. A drag sail is a large, ultra-thin membrane that increases the atmospheric drag acting on the satellite, forcing it to descend and eventually burn up in the atmosphere. The deployment of the sail must be timed to ensure that the re-entry window occurs over an uninhabited region, typically the South Pacific Ocean Uninhabited Area (SPOUA).
Data Validation and USSPACECOM Tracking
Throughout the mission, USSPACECOM monitored the orbital elements of the RemoveDEBRIS satellite. This external validation was critical for assessing the accuracy of the internal ephemeris generation. By comparing the predicted trajectory derived from the NRLMSISE-00 model with the actual TLE data, researchers could refine their understanding of how non-conservative forces affect complex, multi-body systems in space. This iterative refinement is necessary for future missions that will involve the capture of much larger and more volatile defunct payloads, such as the upper stages of Zenit or Ariane rockets.
The mission concluded with the deployment of the drag sail in early 2019. While the sail encountered some deployment issues, the data gathered regarding the increased drag coefficient provided invaluable insights into the feasibility of passive de-orbiting technologies. The interaction between the sail’s material and the atomic oxygen in the thermosphere was a primary area of study, as chemical erosion can significantly alter the reflective and aerodynamic properties of composite materials over time.
Re-entry and Risk Mitigation
The final phase of any debris remediation mission is the safe removal of the satellite from the operational environment. This requires calculating the re-entry trajectory to ensure complete thermal disintegration. For components that may survive re-entry, such as titanium propellant tanks or high-density composite joints, practitioners must predict the "footprint" of the debris field on the Earth's surface. This involves accounting for the oblateness of the Earth (the J2 perturbation) and the variation in atmospheric density during the descent through the mesosphere and stratosphere.
By successfully demonstrating net and harpoon capture, the RemoveDEBRIS mission validated the theoretical models that have underpinned orbital mechanics for decades. It proved that relatively simple mechanical systems, when guided by sophisticated ephemeris generation and thermospheric modeling, can effectively mitigate the risks posed by defunct hardware in the Earth's most crowded orbital bands.