Harnessing sunlight to tackle space debris: A new paradigm for sustainable, multi-target orbital cleanup missions
‘There’s a problem brewing overhead.’ Few phrases capture the urgency of the space debris issue as effectively as this statement from the European Space Agency. Earth orbit, once an expansive and relatively empty domain, has become increasingly congested over the past decades. Today, more than 40,000 tracked objects larger than ten centimetres share orbital space with operational satellites, accompanied by hundreds of thousands of smaller fragments. These objects travel at orbital velocities of around 7 km/s, roughly 25,000 km/h, meaning that even a tiny fragment can cause catastrophic damage upon impact.
The consequences are already visible. Active satellites, including the International Space Station, must periodically perform avoidance manoeuvres to reduce the risk of collisions. Yet the growing density of debris introduces a far greater concern: the possibility of a cascading series of collisions, known as the Kessler Syndrome. If left unchecked, such a cascade could severely limit access to key orbital regions that modern society increasingly depends on, from GPS-based navigation in cars and smartphones and global telecommunications networks, to weather forecasting, disaster monitoring, and Earth observation data that supports climate research and environmental management.
It is therefore widely recognised that mitigation measures alone, such as stricter end-of-life disposal, will not be enough. To ensure long-term sustainability, the existing debris population must be actively reduced.
The fundamental limitation of current solutions
Active debris removal (ADR) is often compared to a garbage collection service in space: a spacecraft travels to a debris object, captures it, and ensures its safe removal, typically through atmospheric re-entry. Over the past decade, numerous techniques have been proposed to enable such missions, ranging from robotic arms and nets to tether-based systems.
However, despite this diversity of capture concepts, a key limitation persists. Most ADR mission proposals rely on conventional propulsion such as chemical or electric propulsion, meaning that every manoeuvre requires fuel. This constraint fundamentally limits mission duration and the number of objects that can be reached. As a result, many proposed missions effectively target only a single piece of debris before termination.
While these missions are valuable as demonstrations, they do not scale to the level required to stabilise the debris environment. Removing hundreds of objects over time demands a fundamentally different approach, one that is not limited by onboard propellant.
A new paradigm: Propulsion without fuel
This is precisely where the SWEEP project (Space Waste Elimination Around Earth by Photon Propulsion) introduces a paradigm shift. Rather than relying on chemical or electric propulsion, SWEEP explores the use of solar sailing, a method that harnesses the momentum of sunlight to generate thrust.
The principle is elegantly simple. A large, ultra-thin reflective membrane interacts with solar photons, each transferring a minute amount of momentum. Individually negligible, this force becomes significant over a large surface and effective over time due to its continuous nature. Unlike conventional propulsion systems, solar sails require no fuel, enabling sustained acceleration over long durations.
This characteristic fundamentally transforms mission design. A spacecraft equipped with a solar sail is no longer constrained by finite propellant reserves. Instead, it can continue manoeuvring for years, gradually adjusting its orbit and transferring between targets. In the context of debris removal, this opens up the possibility of a single spacecraft visiting multiple objects in sequence.
The aim of SWEEP
At its core, SWEEP seeks to determine whether solar sailing can provide a viable and scalable contribution to the debris problem. The central question is deceptively simple: Can a solar sail travel to a debris object, assist in its removal, and then proceed to the next, repeating this process multiple times?
Addressing this question requires significant advances in both scientific understanding and engineering capability. The project focuses on developing the tools and models needed to accurately predict solar sail behaviour in Earth orbit, design efficient paths from one location around Earth to the other, and plan multi-target active debris removal missions.
All of this work ultimately converges towards a single goal: the realisation of a technology demonstration mission. Such a mission would provide in-orbit validation of key capabilities, including solar sail manoeuvring in Earth orbit, propellant-free transfers between multiple targets, and the operational feasibility of sustained debris removal.
Solar sailing: Heritage and potential
Although solar sailing may appear futuristic, its foundations date back centuries. Astronomical observations of Halley’s comet by Johannes Kepler in the early 17th century, followed by theoretical and experimental work by Maxwell and Lebedev, laid the groundwork for understanding (solar) radiation pressure.
More recently, several missions have demonstrated the practical feasibility of solar sails. Missions such as the LightSail 1 and 2 missions of the Planetary Society, as well as NASA’s latest solar sail mission ACS3 (Advanced Composite Solar Sail Structure), have shown that sails can be deployed and controlled in space, providing confidence in the maturity of the technology.
Yet, despite this progress, solar sailing presents unique challenges. The force generated by sunlight is extremely small, on the order of micronewtons per square metre, which necessitates large sail areas and long mission durations to build up enough velocity to sail through space. Moreover, the direction and magnitude of thrust are coupled and constrained by the orientation of the sail relative to the Sun, limiting controllability compared to conventional propulsion.
These constraints mean that solar sailing is not suited for rapid manoeuvres. Instead, its strength lies in continuous, gradual orbit shaping, making it particularly attractive for long-duration missions such as multi-target debris removal.
Understanding solar sail motion around Earth
One of the major challenges identified at the start of SWEEP was the lack of accurate models describing solar sail motion in the Earth’s environment. Unlike interplanetary space, Earth orbit is characterised by a complex interplay of forces, including those from atmospheric drag, gravitational perturbations, eclipses, and planetary radiation effects.
To address this lack, SWEEP has developed high-fidelity dynamical models that capture these effects in detail. These models identify which physical phenomena must be included to accurately model trajectories and which uncertainties dominate system behaviour. In particular, the effects of atmospheric variability and planetary radiation pressure have proven essential and exploitable for realistic mission design.
From manoeuvres to mission design
With accurate models in place, the next step is understanding how solar sails can manoeuvre within Earth orbit. Research conducted within SWEEP has shown that, despite their low thrust, solar sails can achieve a wide range of orbital changes. These include raising or lowering orbital altitude (at several kilometres a day for an ACS3 type sail), adjusting orbital inclinations, and performing rendezvous with target objects. However, because these manoeuvres are inherently achieved gradually and slowly, unfolding over many orbital revolutions, there is a strong need to optimise them to be completed as efficiently and as quickly as possible. This requirement, combined with the unique constraints of solar sailing described earlier, leads to a particularly challenging problem: a huge-dimensional, highly constrained optimisation problem.
To tackle this optimisation problem, a variety of optimisation techniques have been explored within SWEEP, ranging from direct and indirect optimal control methods to differential dynamic programming, Q-law-based approaches, and semi-analytical formulations. Each of these methods presents its own advantages and limitations, and each finds its own applicability within different aspects of the SWEEP mission concept. Despite their differences, these approaches consistently highlight a key insight: both the speed of a transfer and, in some cases, whether a transfer is feasible at all, depend strongly on the geometry of the orbit with respect to the Sun. In favourable conditions, performance can be dramatically higher than in less favourable orientations, highlighting the importance of selecting appropriate mission conditions.
Designing multi-target missions
Perhaps the most distinctive challenge within SWEEP is determining how to visit multiple debris objects in the most efficient manner. This problem resembles the classical travelling salesperson problem, where the goal is to determine the shortest (or quickest) route visiting a set of locations.
In the orbital context, however, the problem becomes significantly more complex. Transfer times depend on orbital mechanics rather than distance alone, and both the spacecraft and targets are constantly in motion. This presents another huge-dimensional combinatorial optimisation problem, which SWEEP addresses by combining key insights from the manoeuvre design with advanced optimisation techniques, including reinforcement learning.
These methods enable the identification of near-optimal sequences of targets, allowing mission planners to design efficient multi-target trajectories.
Bridging theory and operations
While dynamical modelling and trajectory design are essential, practical implementation requires addressing operational realities. Solar sails introduce unique challenges in this regard. Unlike conventional spacecraft, solar sails cannot simply shut off their propulsion. This necessitates new approaches to proximity operations and rendezvous, which are developed within SWEEP, including the use of controlled trajectories to approach target objects without compromising on safety.
Additionally, collision avoidance must be carefully considered. The large surface area of a solar sail increases exposure to debris, requiring robust strategies to assess risks and perform avoidance manoeuvres. Research within SWEEP has demonstrated that such manoeuvres are feasible using environmental forces alone, within lead times currently available for collision avoidance manoeuvres.
Broader implications
Although SWEEP focuses on debris removal, the underlying technologies have far-reaching implications. The ability to perform sustained, propellant-free manoeuvres could transform in-orbit servicing, enabling spacecraft to inspect and maintain multiple satellites over extended periods. Similarly, solar sails offer a pathway for small satellites to escape Earth orbit and undertake interplanetary missions without onboard propulsion systems. At an even larger scale, concepts such as solar radiation management for countering the effects of climate change using large reflective structures further highlight the potential societal impact of solar sail propulsion.
Towards a sustainable space environment
The SWEEP project represents more than a technological innovation; it embodies a shift towards sustainability in space mission design and operations. By replacing finite resources with an effectively limitless propulsion mechanism that exploits natural resources, it enables a new class of missions that is sustainable and can operate continuously to serve multiple objectives.
The next step is clear. A technology demonstration mission will be required to validate the concepts developed within SWEEP and to bridge the gap between theory and practice. Such a mission would mark a significant milestone, proving that solar sailing can be used not only for space exploration but also for the maintenance of the space environment.
Looking ahead, one can envision a future in which fleets of solar sail spacecraft continuously operate in Earth orbit, removing debris, supporting infrastructure, and ensuring safe access to space.
In that vision, sunlight itself becomes the enabler of sustainability, powering a system that not only explores space but also actively preserves it.
Project SWEEP is funded by the Domain Applied and Engineering Sciences (AES) of the Dutch Research Council (NWO)
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