Space Is the World’s Highest Junkyard. Literally.
And the space technology up there might be destroyed from it.
When most people imagine space above the Earth, they probably imagine a pristine landscape with some satellites, rockets, and other space modules. What they don’t imagine is a danger zone full of space debris, ranging from the size of paint flecks to large satellites, whizzing around at bullet-like speeds with the capability to disable and destroy the space applications that almost everyone relies on.
What is the Kessler Syndrome?
Scientists have actually been aware of the dangers of space debris for decades. In 1978, astrophysicist and NASA scientist Donald J. Kessler proposed a hypothetical scenario in which, due to the increasing density of space debris in Earth orbits, a collision between two large space objects would result in a cascade of collisions thereafter, destroying operating and non-operating space objects.
For example, let’s say two large dead satellites crash into each other. The space debris generated from that collision would fly off in all directions and crash into other space objects. The same event would happen for those collisions, and eventually, most space applications would be destroyed regardless of their state of operation.
This is an extreme example, but it’s not out of our realm.
Space debris has become an increasingly worse problem since Kessler’s initial hypothesis. Most of the debris comes from remnants of previous space missions, explosions, anti-satellite tests, and collisions. In fact, most space debris was created up to 50 years ago. Space debris stays in orbit for decades because they deteriorate extremely slowly and are rarely removed from orbit. They can be a wide range of objects, such as paint flecks or parts of defunct satellites.
The quantity, mass, and overall area of space debris has been on the rise since the 1960s and is further fueled by in-orbit collisions.
The chart depicted above shows the steady increase in the total area taken up by space debris, which helps scientists predict the frequency of collisions in the future. According to ESA, “collisions between debris and working satellites [are] predicted to overtake explosions as the dominant source of debris” (ESA).
There’s also been a rise in fragmentation events.
Similar to the previous chart, there’s been an overall increasing trend in the number of fragmentation events per five years. With every fragmentation event, even more space debris is generated that may become a part of future collisions.
Has the cascade started yet?
In 2007, China performed an anti-satellite test by launching a ballistic missile from the Xichang Space Launch Center at the non-operating Chinese weather satellite, Fengyun-1C. The satellite was completely destroyed and generated a cloud of over 3,000 pieces of space debris.
In 2009, the operating Iridium 33 Communications Satellite collided with the defunct Russian 2251 Communications Satellite at an approximated 42,000 km/h. Similarly, the collision generated over 2,000 pieces of debris larger than 10 cm and thousands of smaller pieces.
These two impacts alone “have increased the large orbital debris population in LEO by approximately 70%, posing greater collision risks for spacecraft operating in low Earth orbit”(NASA). These two debris-generating events actually explain the peak in fragmentation events from 2005–2010 in the previous charts.
The Kessler Syndrome is not set in a specific time-frame, so many scientists believe that these events may have set the fire to a cascade of collisions in the future. By 2011, NASA was already debating whether or not Low Earth Orbit (LEO) had reached critical density. Critical density is the density of space debris where the chances of the Kessler Syndrome occurring are very high. If LEO is at critical density, then the space applications we rely on in LEO, including most satellites and climate monitoring technology, are at risk for being destroyed by the Kessler Syndrome.
What’s The State of Space Debris Today?
Today, it’s estimated that there are over 100,000,000 pieces of space debris in Earth orbits. Of these:
- 5,400 objects are larger than 1 m
- 34,000 objects are larger than 10 cm
- 900,000 objects are larger than 1 cm
- 130,000,000 objects are larger than 1 mm
This space debris orbits at speeds ranging from 18,000 km/h to 36,000 km/h. That’s about 7 times faster than a bullet. For reference, a 1 cm sized paint fleck at 36,000 km/h is equivalent to the damage inflicted by a 550-pound object traveling at 96 km/h on Earth.
NASA and the Department of Defense track space debris, but only pieces that are larger than a softball. Analysis and assessments of these space debris allow for collision avoidance maneuvers, which are effective against large objects. Objects that are smaller than a softball can’t be tracked, but are too large to shield against. They pose the greatest risk to spacecraft parts. Debris shields, however, are still effective for combatting space debris about a centimeter or smaller.
Due to growing space debris, most space agencies need to perform maneuvers on their space applications in order to avoid collisions. Just this year, the ISS had to perform 3 emergency avoidance maneuvers due to space debris.
Is Current Action Enough?
Although scientists in many fields are well aware of this issue, there’s been little action to control it. The United Nations Committee for Peaceful Uses of Space (CUPUOS) follows mitigation guidelines created by the Inter-Agency Space Debris Coordination Committee (IADC). However, they are only mandatory regulations for major space powers and only apply to new debris. Old debris, which is still flying around and colliding with each other to generate more debris, is receiving very little attention.
In 2017, the ESA Space Debris Conference ultimately concluded that these guidelines were inadequate for controlling space debris. Simulations by NASA predict that in order to fully combat the space debris crisis, at least 90% of the satellites currently in LEO must be disposed of post-mission and at least 5 of the largest pieces of space debris need to be removed from orbit annually. Ironically, no space debris has been removed.
So, How Do We Combat Space Debris?
Combatting and ultimately controlling space debris will require an international effort. Both political and technological advancements are the key to resolving space debris, such as clarifying how to claim space debris from different states, as well as making efforts to remove space debris from orbits or repair damaged satellites to make them functional again. In 2013, even Kessler said that the only way to fix this crisis is to “bring back the larger objects […] If you want to stop this collisional cascading process, you have to bring back satellites” (HuffPost).
Most existing proposals to actively remove space debris from Earth orbits are based on methods of contactless retrieval, stiff-connection retrieval, or flexible-connection retrieval. I’ll cover a few of the proposals to remove space debris through each of these methods.
Contactless Retrieval
Contactless retrieval describes methods of space debris removal that avoid the complication of attaching or docking to debris whose physical state and attitude are unknown. Instead, contactless solutions use weaker particles or magnetism to deorbit debris. However, since these techniques are weaker than using the transfer of momentum that stiff or flexible-connection retrieval methods use, the deorbiting process takes longer. This can be dangerous since other flying debris can disturb the process.
The use of lasers through vaporization or ablation processes and magnetism have been proposed as ways to control space debris. If you need a quick brief on lasers…
Lasers 101
Laser actually stands for Light Amplification by Stimulated Emission of Radiation. So how do they work?
Electrons usually have different amounts of energy, which can be thought of in levels. When you excite electrons with a form of energy, like heat, they get excited and their energy level increases. When their energy levels go back down, they emit photons, which are particles of light.
However, photons are produced differently in Stimulated Emission. In Stimulated Emission, an already excited electron is hit by a photon with a specific wavelength that causes the electron to emit a daughter photon. When you perform this process with a bunch of electrons, you get Light Amplification, hence the laser acronym.
Most lasers have two reflectors on each side of a cavity. The cavity has a gain medium, which is anything that has excitable atoms, like carbon dioxide. A pump excites the medium with energy and photons are emitted and bounce throughout the cavity. Photons that bounce off the reflectors bounce back through other electrons, producing more photons.
One of the reflectors is only partially reflective, so some photons escape. Those photons come out as the light we see in a laser. Since the photons are moving in the same direction, we get a concentrated line of light of the same wavelength. This intense concentration of light has many uses, but in the case of space debris removal, it can be used to vaporize the surfaces of space debris.
Laser Vaporization
Laser vaporization is a contactless retrieval method where a short wavelength (ideally UV) laser stationed in orbit is used to vaporize small pieces of space debris.
The short wavelength laser, pumped by solar energy, scans, identifies, positions, and illuminates its target. The laser detonates the space debris at 10⁵ m/s. However, to vaporize a 10 cm³ aluminum target in 3 minutes with a continuous 5.38 MW laser beam at 5% pumping efficiency and 9% laser absorption by the target, at least 108 MW are required to power it. In other words, that’s a lot of energy for a not-very-efficient laser to vaporize a small bit of space debris.
We could make the process faster, but the higher the power for vaporization, the greater the strain on the solar panels.
The laser, optics, monitoring, tracking, and steering optics technology for laser vaporization are already available, but this proposal isn’t really practical. The goal is to simply control space debris. The space debris doesn’t need to be completely vaporized. The laser just needs to slow down the attitude and orbital velocity of the object enough to let it fall back into the Earth’s atmosphere, where it’ll burn up. Laser vaporization uses a lot of energy to completely turn the space debris into plasma when it doesn’t have to. Thus, laser vaporization is unnecessary. That’s why laser ablation was proposed.
Laser Ablation
Laser ablation, similarly to laser vaporization, removes material from a solid surface by irradiating it with a short wavelength laser beam. The material that’s removed is converted to plasma.
What’s better about laser ablation is that it sends pulses of laser beams instead of one continuous beam, reducing its energy requirements. The amount of material removed from the space debris’ surface depends on the laser wavelength, pulse duration, pulse energy, and material absorptivity. A shorter wavelength, like UV, is ideal for laser-material interaction but is less efficient than longer wavelengths.
The pulse width after compression of laser pulse energy directly affects the amount of power that the laser delivers for laser-material interaction. A compressed pulse power is enough to melt and vaporize the material, turning it into plasma.
The diagram above depicts the process of laser ablation on a piece of space debris, with plasma plumes (due to laser-material interaction) ejecting at 10⁵ m/s, decelerating the space debris. Laser pulses occur every 300 nanoseconds on a 1–2 mm diameter focal point.
An ideal laser that would be used for laser ablation is a CO2 laser, which has a carbon dioxide medium. The wavelength would be 10.6 μm (about the length of infrared light) and have 100 J of energy and a 40 ms pulsewidth. It’s basically a laser with a shorter wavelength and compressed pulsewidth, which is what we want.
Since this is a developing proposal, there are errors that still need to be addressed. For example, the ablation spot might not always be the same and may hit the debris off from its own center of momentum, causing the debris to spin off. However, in comparison to laser vaporization, laser ablation is much more favorable due to its lower energy requirements and efficiency.
What’s the practicality of lasers in space?
Lasers are actually a really efficient and attenuation free contactless method of controlling space debris. Unfortunately, using very short wavelength lasers, such as UV lasers, isn’t practical due to their large energy requirements. This is because producing UV wavelengths requires a high level transition from electrons’ ground state (low energy) to an upper state (higher energy). In space, it’s more likely that infrared lasers will be used. Pumping the lasers for laser ablation is achievable since the proposal just uses solar panels for energy. Overall, laser ablation is a promising method of controlling small space debris.
The Magnetic Tug
As the name implies, the proposal of a magnetic tug uses a chaser satellite with a steerable magnetic dipole to gradually slow the orbit of a target by constantly exerting a magnetic force on it. The magnetic dipole allows the chaser satellite to create forces and torques on the magnetic torque rods the target carries.
Many satellites in Low Earth Orbit (LEO) have magnetic torque bars (MTQ), which are usually used for attitude control. MTQ create strong magnetic fields that allow a chaser satellite equipped with a magnetic dipole to create forces and torques on the target.
The chaser satellites would generate a strong magnetic field by cooling superconducting wires to cryogenic temperatures. Not only can this magnetism be used on the target but the chaser satellite, but it can also be used to maintain formations with multiple chaser satellites in cases of capturing large amounts of debris.
Finn Ankersen, an ESA Automatic Control Systems Analyst, says the magnetic tugs would be able to remove space debris with immense precision. It could work as far as 10–15 meters away with positioning precision within 10 cm and attitude precision within 1–2 degrees. The large distance is ideal, since it reduces the risk of direct contact and damage between the chaser satellite and debris.
However, there are some complications with this proposal. For example, studies assume that the target is able to control its own magnetic dipole. Yet in many cases, the targets are completely non-functional, so the chaser satellite can’t utilize the target’s magnetic dipole. Creating magnetic forces also creates torques between the interacting dipoles, which risks the chance for one of the objects to spin uncontrollably. Existing studies also assume that the magnetic dipole is located in the center of the target, whereas in reality, many satellites have magnetic dipole located elsewhere. The final obstacle is how Earth’s magnetic field and orbital distances vary in orbit, which interfere with the capture process.
Many researchers find that magnetic tugs are a promising method of large space debris retrieval, but so far, it only works on paper. Further research and testing needs to be conducted to make it a viable control method.
Stiff-Connection Retrieval
Stiff-connection describes methods of space debris removal that use physical grippers to firmly attach to space debris, allowing them to apply force and slow down the orbital velocity of the space debris. However, the phase in which a chaser satellite must match the target’s velocity, attitude, and rotation is extremely difficult and complex. If errors are made, the chaser satellite could bounce off and potentially be lost. What’s worse is that, since most space debris is defunct, the target’s mass, rotational dynamics, and attitude are often unknown.
Stiff-connection retrieval methods have had the most research out of the 3 main retrieval methods since they can be tested on the ground. The use of a chaser satellite with robotic arms and Cubesats are two methods of stiff-connection retrieval that are in testing and may be used in the future.
ClearSpace-1, The First Retrieval Mission
ClearSpace-1 is the name of the first-ever retrieval mission that is set to occur in 2025. The mission, funded by ESA at $129 million USD, is starting out small by retrieving a remnant of ESA’s Vega rocket launch from 2013. Using four robotic arms, the chaser satellite will drag the rocket debris out of orbit and burn in the atmosphere. Unfortunately, the chaser satellite will burn with it. The goal of the mission is to not only remove space debris, but to also demonstrate the effectiveness of the retrieval method.
Cubesat Capture
A newer proposal that’s different from traditional tethers or robotic arms is the use of Cubesats to capture space debris. Cubesats are small 10 cm x 10 cm 10 cm satellites used for space research. The proposal uses Cubesats in an end to end multi-robot system to capture damaged spacecraft for salvaging, repairing, or deorbiting.
In the model, 3 fully autonomous 1U Cubesats are linked together by tethers and equipped with a microgripper spine, 3D vision, a navigation system, propulsion system, solar panels and a 3-axis attitude determination and control system. The microgrippers are very versatile, increasing the range of the types of objects the Cubesats can capture.
The tethers are efficient, flexible bodies with a series of point masses connected by springs and dampers. The material can be described as viscoelastic due to its properties of both elasticity and viscosity.
Inside each Cubesat is the attitude determination and control system, which has 3-axis reaction wheels and magnetorquers. There are additional lithium batteries to power the hardware. The added computer board holds a UHF transceiver with a dual-core ARM processor and FPGA subsystem for 3D vision and navigation.
Each Cubesat has a propulsion system, whose main goal is to align that Cubesats into the proper star configuration used for capture. The propulsion system is sublimate based, meaning a medium is sublimated to produce gas that is then ejected from the thrusters. Sublimates have low chamber pressure, allowing for a simpler propulsion system design and more propellant mass. A benefit of a sublimate-based propulsion system is that as the temperature increases, vapor pressure increases as well. This way, the sublimate serves as a pressure regulator for itself. Traditional cold gas systems, on the other hand, need to be tougher in order to withstand higher storage pressure.
Each Cubesat also has a 1 m toroidal inflatable deorbit device that’s used to protect spacecraft brought back through the atmosphere, allowing them to be repaired or reused. The inflatable uses a sublimate, such as benzoic acid, that turns into gas in a vacuum to inflate itself. The inflatable is highly durable, being able to withstand small punctures while remaining inflated for months or years. This inflatable protects the Cubesats and debris when it re-enters the atmosphere, allowing the debris to be salvaged and the Cubesats to be reused.
The chaser satellites are equipped with a 3U P-POD deployer. When released from a 3U P-POD deployer, the 3 1U Cubesats form a star configuration to wrap around the spacecraft or debris. The modules then attach to the target using their microgrippers. Once the target is captured, the modules begin analyzing it, looking for pieces that can be repaired, salvaged, or reused. From then on, each Cubesat deploys its inflatable and tracking beacon for deorbiting.
Scientists are still validating the components of this proposal so that they are at the acceptable technology readiness level. Experiments are already being conducted in Low Earth Orbit. Studies expect that this system will be of low-cost and easily disposable, making it ready for mass production in space.
Flexible-Connection Retrieval
Flexible-connection describes methods of space debris removal that apply forces and torques through flexible materials, such as tethers and nets, to slow down and deorbit space debris. The method can be used to retrieve a variety of objects since the debris tumbling rates don’t have the match the chaser satellite. Some proposals also don’t require a connection to a specific location on the target. However, flexible-connection methods need more control than stiff-connection methods and there’s a greater risk that parts of the chaser satellite or debris may break off during the capture phase.
I’ll begin with a brief on a proposed tether deployment mechanism. Most flexible-connection retrieval proposals use a tether in order reach their target. Then I’ll go over a tether-net proposal that may be used to retrieve space debris.
Tethered Control Mechanism
One tethered control mechanism uses a tether deployment module to carefully reel-in and reel-out a tether.
The tether deployment module has a Cold Gas Actuator Propulsive Unit, two nozzles, a tank, a relative fluidic system, and a reel-in/reel-out subsystem. The subsystem is made of a rotating wheel, a system of pulleys, and a motor used to brake the tether during deployment and actuate it during reel-in processes. An internal mechanism allows the pulley systems to decouple tether dynamics outside of the module, making a safer deployment.
The ideal length for the tether would be from 500 m to 1 km. It can be made of tape tether or round wire, although tape tether is more durable in space environments.
The deployment process begins by releasing a tether tip mass from the chaser satellite. The help reel the tether out, the tip mass is given an initial momentum. In this case, the deployment module serves as the tip mass. The momentum and pulley systems send the tether out tens of meters. After that, the gravity gradient can pull the tether to the target. During this phase, tape velocity isn’t constant, since the brake is used to prevent the tether from unwinding inside the module. It is essential that there is continuous tip control during deployment.This process may take up to an hour, depending on how far the target is.
In the rewind process, the motors and control system reel-in the tether, instead of the pulley system.
Other tethered deployment mechanisms are influenced by transient events, like tethered tension spikes that cause lateral oscillations that affect the target’s attitude, which can be dangerous for the tether. The primary benefit of this deployment method is that, unlike other existing tether deployment mechanisms, it smooths load transmissions and dampens oscillations. Overall, other space agencies and companies can use this method in their retrieval proposals to avoid complications that other tethered deployment mechanisms may have.
Tether-Net Proposal
The final space debris capture proposal I’ll cover is the tether-net method, which uses a net released from a tether to capture and deorbit a large variety of space debris. This proposal avoids the complications of needing to know the target’s torque and orbital speed since it handles rotating debris relatively well.
In the design, the chaser satellite is equipped with thrusters that can counteract the torque applied to the tether from the rotating debris. Once the debris’ been captured, the attitude can be controlled and the debris can be deorbited using forces applied by the tether. Careful control over the tether is required to prevent the chaser satellite and debris from colliding.
The net is attached at the end of the tether and has four point masses at each corner. When the net is launched, it spreads open through centrifugal forces. The point masses propels the net to the debris. Once the point masses contact the debris, bullet inertia allows them to wrap around the debris. A suggestion that is still undergoing approval is the use of maneuverable robots as point masses. Controlling robots at the corners of the net would have far more accuracy than relying on centrifugal forces and bullet inertia. Tether tension, thrusters, and reaction wheels can all be used to control the robots through gyroscopic tension and attitude control. However, the process is much more complicated and consumes more fuel.
Momentum wheels are used for power and control. Most spacecraft already use momentum wheels to control attitude by rotating the satellite through Newton’s First Law and the conservation of momentum. In the tether-net method, three momentum wheels are aligned with the three axes. The momentum wheels can also be used for energy storage and release.
One tether-net mission has the capability to deorbit 5 large pieces of space debris without generating more space debris. The chaser satellite is launched not far behind the target and releases the tether-net at the target. A momentum wheel applies forces to the tether to detumble the debris. Once the debris is detumbled, the initial chaser satellite detaches the net. The net attaches to a second satellite that deorbits the debris. The initial satellite then repeats this process four more times to deorbit a total of five pieces of space debris. Once it finishes the five pieces, it deorbits itself and burns up in the atmosphere.
This proposal is promising because it avoids the complications that come with latching onto a rapidly spinning target and is able to deorbit multiple pieces of space debris in one mission.
What Can We Expect In the Future?
Many of these proposals are currently in experimental stages, with some farther ahead than others. It’ll be a while before any of them are actually implemented. The current space debris crisis must receive international attention for it to be properly addressed and controlled. Action towards combatting space debris is only just starting, but in the long term, it will prevent the collapse of space-based applications that we rely on.
Have questions? Send me an email at chloewang.lv@gmail.com and I’ll be happy to respond!
