We rely on Earth’s satellite infrastructure every day, whether it be GPS, watching a football game, or checking the weather. However, our most popular satellite orbits, namely Low Earth Orbit (LEO), have become overpopulated and filled with debris, which presents a large threat to satellite infrastructure. When we imagine such a risk, we imagine large satellites colliding into each other, but that is just the source of the problem. The real issue is the multitude of small debris pieces that form in the aftermath of this collision. All of these small debris fragments can then go on to perform their own collision, leading to a scenario called Kessler Syndrome [1], [2]. If such a situation were to arise, which is ever more likely as we launch more satellites into orbit, then we could lose our satellite infrastructure and the ability to launch anything else into space.

To prevent this imminent disaster, aerospace engineers are developing a solution which is called Active Debris Removal (ADR). The idea is we can send a spacecraft into orbit with the goal of removing as much debris as possible from orbit. It is kind of like an orbital garbage truck, and after it has taken on the debris, it can push it into the atmosphere for it to burn up or into a less crowded orbit. Furthermore, instead of sending up garbage men, the ADR spacecraft is autonomous, typically housing a robotic arm or harpoon. However, performing this type of operation is incredibly risky, as it puts this spacecraft into contact with an uncontrolled object that, if interacted with poorly, could crash into the ADR spacecraft, causing the exact type of collision it was sent up to avoid. Because of the high risk involved in an ADR mission, it is more essential than ever to perform extensive testing.

Due to the high cost of launching something into space, we must test all sensors, algorithms, and capture mechanisms on Earth. This presents unique engineering challenges, such as simulating micro-gravity (also known as zero gravity), including the unique tumbling motion that objects experience in space. By understanding the innovative ways researchers have solved these testing challenges, we can be more confident in their ability to quickly and cost-effectively create capable spacecraft for cleaning our orbit.

Simulating Micro-Gravity

The largest hurdle that researchers have to overcome when testing ADR missions on Earth is simulating micro-gravity [3], [4]. Orbital debris often tumbles in chaotic 3d rotation, which can be very difficult to replicate. To test whether an autonomous spacecraft can match the tumbling motion of the debris, which is necessary for safe capture, we need facilities that replicate frictionless zero-gravity environments [5]. Historically, aerospace engineers utilize dynamic facilities, which means that they focus on replicating the forces but not the actual motion. The most popular example of this is the air-bearing testbed [6].

The idea behind the air-bearing testbed is quite simple. It is essentially a massive air hockey table with a replica spacecraft on top, which allows it to float, thus eliminating friction. This successfully simulates the ‘floating’ effect that spacecraft experience; however, it has one major downside… Air hockey tables are flat. Because of the geometry of the testbed, it can only replicate 2D rotation, which is very different from the complex, unconstrained, 3D tumbling that debris often experiences [6], [7].

To overcome the flat limitation, researchers are now utilizing large robotic arms, referred to as a six-degree-of-freedom (6DOF) testbed. A mock satellite is attached to the end of the robotic arm, which allows it to spin and tumble exactly how it would in space [4], [5], [6]. These robotic arms are perfect for simulating the most dangerous part of the mission, which is the final moment where the two bodies actually come into contact. However, robotic arms are limited by a very short range. Much like a human arm, the robotic arm can only extend its arm so far, leading to a mechanical failure that researchers call a “shoulder” or “elbow” singularity [8].

To overcome this challenge and to have adequate, full-motion, and force-accurate testing centers, researchers have placed robotic arms on large rails that allow them to simulate the incoming approach as well as the final moment of contact [8]. Other facilities, like the INVERITAS facility, use a suspended cable system to fly the robotic arm around the room [9]. Another interesting facility is the ORION simulator, which mounts rotational mechanisms on top of an air-bearing testbed to get the best of both worlds [3].

Robotic testbed utilizing six-degree-of-freedom mechanical arms to recreate three-dimensional tumbling of space debris (From Ali et al. 2024 [6])

How to Catch an Uncontrolled Spacecraft

As I have mentioned, the most difficult part of an ADR mission is the moment of contact. Unlike docking with the International Space Station, where both vehicles are coordinating together, capturing debris is a chaotic process by nature. As a result of this chaos, the debris capture mechanism used in ADR missions has to be highly adaptable, which entails a diverse and rigorous testing suite [3], [10].
ADR missions are generally designed with one of two distinct capture methods: rigid systems, such as a robotic arm with a claw, or flexible systems, such as a tethered harpoon. Both require their own unique testing suites.

Firing a harpoon, for instance, requires a great deal of precision, and thus it is necessary to have a testing suite involving targeting lasers to measure deviation from the target, and a high-speed camera for moment-to-moment tracking [10]. Ground testing is essential for an operation like this, not only to ensure accuracy but also to protect the ADR spacecraft from the potential of the harpoon ricocheting. That is why it is also important that the target is a realistic body, such as a honeycomb, which is very common amongst spacecraft [10].

Blinded By the Sun

Validating capture mechanisms is certainly the most difficult aspect of an ADR mission to validate due to the difficulties in simulating microgravity; however, even seemingly simple things like the effect of sunlight on the vision system can prove to be a nightmare for validation. ADR spacecraft rely on a control system referred to as Visual-Based Navigation (VBN), where their sensors act like eyes, to guide them to the point of contact with the debris. Here on Earth, our atmosphere protects us from the sun by scattering light; however, in space, that obfuscation layer is not there. The lighting conditions in orbit are extreme; one moment, a spacecraft may be completely blinded by direct sunlight, and at other times it may be in near complete darkness [8], [11].

Testing the sensor arrays of the spacecraft requires a combination of computer simulations and physical testing facilities. Often, VBN algorithms are tested using Software-in-the-loop simulators, where no hardware is involved. These simulators can be used to calculate the sun's angle and to ensure the spacecraft does not aim its sensors directly at the sun [11], [12]. However, sunlight reflecting and refracting against the lens of a camera can be very difficult to simulate, and thus, physical testing is necessary to ensure a mission's success.

Sensors are physically tested using what is known as a Hardware-in-the-Loop test bench [8], [10]. In these tests, the sensors are subject to bright moving spotlights. Through this process, researchers have found that when the lens flares, it can send the navigation algorithm into a panic, leading to drastic 180-degree flips [8]. This proves the need for physical testing.

Sun Glinting off the Hubble Telescope (Adapted from wikimedia.com)

Securing Our Future in Space

An Active Debris Removal mission is a massive undertaking, given that it is already quite the engineering challenge; replicating the extreme conditions and environment to validate the mission's success on Earth is that much harder. Even with this immense difficulty, aerospace researchers prove that it is entirely possible to safely validate these missions before ever touching the launchpad.

Understanding these rigorous testing procedures is critical because the stakes of a failed mission are incredibly high. If a multimillion-dollar ADR mission fails, it not only wastes money but also worsens the very problem that it was sent out to fix. This failure could be catastrophic if, on impact, it not only becomes debris but breaks apart into many tiny fragments of debris, instantly accelerating Kessler Syndrome. These debris fragments could then go on to interrupt GPS, weather tracking, or our access to the internet.

When we think about the space junk problem and the risk of Kessler Syndrome, we imagine an uncontrollable disaster. But through the effort of researchers and engineers utilizing massive air hockey tables, robotic arms, and complex simulations here on Earth, we can not only prevent our orbits from becoming more of a minefield, but we can also take steps to remove the debris.