No one likes trash—not in landfills and not in space. Yet the mounting problem of orbital debris continues to increase as space gets more congested, making orbital operations more hazardous and more costly. To address this growing problem, several companies are evaluating technologies for the capture and removal of space debris—and one Houston-based company, Airbus DS Space Systems, recently completed such testing on the International Space Station (ISS) U.S. National Laboratory.
Two investigations that Airbus conducted on the ISS National Lab in recent years provide new insight on how to stabilize and tow spinning space debris, whether the object carries fuel or solid mass. Research such as this is critically important as the problem of space debris continues to grow, posing a threat to working spacecraft and satellites in orbit. It is vital to address this increasing concern as the economic development of low Earth orbit expands and as we as a species venture farther into space to achieve the lofty goals of returning to the Moon with Artemis and embarking on the next journey beyond.
An Expanding Problem
Since the first artificial satellite, Russia’s Sputnik, first rocketed into orbit in 1957, some 8,900 satellites have followed suit, providing valuable tracking, Earth observation, and communications capabilities to the world. But that number is about to grow significantly with the wave of new high-throughput mega constellations of satellites heading into low Earth orbit.
“We can count more than 100,000 satellites that are projected to launch in the next decade,” said Ted Muelhaupt, principal director and manager of the Corporate Center for Orbital and Reentry Debris Studies (CORDS) at The Aerospace Corporation, a federally funded research and development center focused on national security space. “If we continue with business as usual, as we did for the first 50 years of the space age, then we’re going to create an untenable mess.”
Space debris puts working satellites at risk for collisions, exposing space companies and government agencies to losses easily in the hundreds of millions of dollars. From the perspective of another object in orbit, space debris might move many times faster than a bullet. Because of its relative velocity, space debris the size of a blueberry could create the impact of a falling anvil.
Today, the Space Surveillance Network (SSN), operated by the U.S. Space Force Command, tracks objects in space. Radar and optical sensors observe and track objects larger than a softball in low Earth orbit and objects basketball sized or larger in higher, geosynchronous orbits. Sensors determine which orbit the objects are in, and that information is used to predict close approaches, reentries, and the probability of a collision. Other nations also run space object tracking systems.
The U.S. Space Force Command estimates there may be more than half a million objects with a diameter larger than one centimeter orbiting the Earth and currently tracks more than 26,000 objects with a diameter of 10 centimeters or larger. Such space debris makes space operations hazardous and is becoming an increasing problem as additional companies and countries join the space economy.
In 1978, NASA scientist Donald J. Kessler predicted space overcrowding in low Earth orbit could lead to collisions between objects, which would trigger a cascade of more debris causing other collisions. Currently, geostationary satellites and those in low Earth orbit must carry extra fuel to perform maneuvers to enter a safe graveyard orbit at the end of their operational lifetime to avoid collisions with other satellites. If other means to avoid space debris became available, companies could use this fuel to extend their satellites’ operational lifetime.
You Capture Debris, Then What?
Several companies are tackling the problem of space debris by studying debris capture methods ranging from the use of harpoons to deploying giant nets or robotic arms. But once an object is captured, then what? The process of de-orbiting larger debris from space principally occurs in four phases: capture, stabilize, tow, and deorbit.
Hans-Jürgen Zachrau, senior project manager with Airbus Space Systems, led two ISS National Lab- investigations to explore the dynamics of stabilizing and towing larger space junk such as an uncontrolled satellite. “As objects are moving in space, there is velocity and trajectory,” Zachrau said. “After capturing an object, you need to stabilize it; you need to enter into a controlled movement. That’s what we did.”
These tests followed an earlier “NetCapture” experiment, in which a net deployed outside the ISS captured a spinning shoebox-sized object. Launched a safe distance from the ISS in open space, the NetCapture experiment had only one shot within a matter of seconds and no way to repeat it, Zachrau said.
However, performing more difficult maneuvers—to stabilize and tow an object in space—requires multiple attempts and positioning, making a test in open space impractical. The ISS served as the perfect test bed for research on such maneuvers, given that these types of experiments require several minutes and many repetitions to correct and optimize the maneuvers.
Airbus conducted its first ISS National Lab investigation in 2016, the “SPHERES Tether Demo” experiment, to assess the dynamics of satellites being tugged from different angles. In 2018, Airbus conducted a second investigation on the ISS National Lab, the “SPHERES Tether Slosh” experiment, to evaluate ways to actively steer a passive body that contains liquid in space, such as satellites holding fuel reserves.
Both investigations used Synchronized Position Hold, Engage, Reorient, Experimental Satellites (SPHERES), free-flying satellites that maneuver using small puffs of compressed gas. The experiments were done in collaboration with NASA’s Ames Research Center, the managers of the SPHERES system, and the Massachusetts Institute of Technology, the developers of the SPHERES system. For the SPHERES Tether Slosh experiment, NASA’s Kennedy Space Center loaned Airbus a sloshing tank, having previously performed related studies on liquid sloshing in space.
The Dynamics of Tugging Debris
The SPHERES Tether Demo investigation included a series of tests to explore the flight dynamics as one tug satellite pulled a target satellite under different starting and control conditions. The ISS crew also tested the tension and torque of two different tethers—a nylon monofilament and a Kevlar thread—in 31 maneuvers. Airbus augmented its findings from these tests with data from accelerometers, gyroscopes, and positioning sensors on each SPHERES satellite.
Airbus performed all the baseline and most of the optionally defined tests using both tether materials—the Kevlar (a more rigid material) and nylon monofilament (more elastic). Although the SPHERES operational envelope only allowed for a 40-cm tether length, the Kevlar tether was preferred for follow-on experiments because it provided better predictability of its properties. With the results obtained from the SHPERES Tether Demo experiment, Airbus adapted its dedicated software tools to analyze the stability and dynamics of flexibly coupled systems. Furthermore, the data were used to validate a simulation environment for tethered Active Debris Removal (ADR) missions.
Building on this first experiment, Airbus conducted the SPHERES Tether Slosh investigation, which was done in four separate sessions on the ISS from January to September 2018. The goal was to evaluate methods to actively steer a passive body that contains liquid—such as a disabled spacecraft with fuel in its tank—in space, when it is unknown whether the remaining fuel is liquid or frozen. The tests included both a liquid and a solid mass tank with otherwise similar mass properties. The ISS crew ran test profiles with both tanks to allow comparison of the same maneuvers.
The Airbus team was unsure how easy it would be to safely steer an object containing liquid in space. The liquid could slosh around in unpredictable ways, potentially making space maneuvers difficult. However, the results surprised them.
“The liquid mass tank was pretty tame and easier to steer, while the solid mass tank had a much more aggressive dynamic—the force was so powerful that it could actually affect the direction of the satellite,” Zachrau said. “The crew had to secure the equipment because, otherwise, the tank would crash into the SPHERES.”
Philipp Behruzi first envisioned the sloshing experiment as the senior expert in the Fluid Mechanics group at Airbus Defence and Space before becoming part of ArianeGroup GmbH, an Airbus and Safran joint venture. According to Behruzi, this type of experiment had never been done, and what they found was not what they were expecting. “We discovered that a liquid-filled object behaves much easier when we tow it away than a rigid body,” Behruzi said. “That’s because some of the energy goes into the sloshing of the liquid, and in that context, the liquid behaves like a dampener.”
Behruzi went on to explain, “In space, if you want to brake, you can’t—and that’s where you get into problems. Without the dampening effect, all the energy is kept as kinetic energy—there is no braking of the system.”
Another key finding was that the rope must be attached through the center of gravity of both the towing object and of the object being towed, because otherwise, the momentum produced causes the path of the towing object to deviate.
“By aligning both the towing and the controlling SPHERES vehicles through the center of gravity of the object being towed, we obtained good control over the dynamic movements of the overall system throughout the variety of our experimental profiles,” Behruzi said.
Toward a Cleaner Future in Low Earth Orbit
Researchers hope that the Airbus experiments will fuel further research in the area of space debris cleanup. “It will take a concerted effort,” Zachrau said. “No one can do it alone, so we must work as a worldwide community to address the space debris problem.”
Muelhaupt added, “A clean space environment is in everyone’s self-interest,” noting that the Airbus experiments are advancing the technology readiness needed to safely remove existing space debris. Before the world has a viable cleanup mechanism, Muelhaupt contends that space firms and innovators like Airbus will have to go through several generations of testing technologies.
As the world gets serious about active debris removal missions, the ISS will play a significant role. Given the diverse nature of space debris and the need to more accurately track a growing debris population, there is a critical need for additional research and testing of debris removal technology, and the ISS is an ideal platform for both. Although there is still a long way to go in developing reliable scenarios to securely de-orbit objects from space, the Airbus investigations are an important initial step toward understanding the complexity of towing objects in space.