Recently, I published a post entitled “The problem with space junk: What is the problem?“. Today’s post follows up this post, to talk about what we can do to combat the issue of space junk.
As mentioned in my previous post, efforts are made to keep track of all of the junk currently floating around in space. The output of this is NASA’s LEGEND model. This model tracks in three dimensions the current positions of objects in Low Earth Orbit (LEO), Medium Earth Orbit (MEO), and Geosynchronous Earth Orbit (GEO). Additionally, it contains historical data and future estimations (potentially for the next 100 years). This model is able to track the movements of objects as small as 1mm in diameter, due to its complexity. In an article published by J. C. Liou and N. L. Johnson in 2006, it was suggested that in some regions of LEO that the rate of creation of new debris (due to collisions) will exceed the rate of depletion of debris (due to orbital decay). This was verified using simulations of the LEGEND model, and was found to be the case even in the event that no new debris is launched from Earth. This highlights the need for a solution to the existing problem, and that we cannot just limit our future space littering behaviors as a solution to the problem. So, in this post, I am going to look at a few of the suggested solutions.
The laser broom is a proposed Earth-based system for handing the issue of space debris. This system would be designed to target pieces of debris between 1 and 10cm in diameter, which may be at risk of collisions with crafts such as the International Space Station (ISS). The intention is that the laser will be powerful enough to both penetrate Earth’s atmosphere, and to be able to ablate pieces of space junk to increase their drag. This will alter the object’s orbit, and so will hopefully cause it to re-enter the atmosphere and burn up. NASA estimates that it would take a day of firing the laser at a piece of debris to alter it’s course by 200m, making this a niche solution to the issue of debris collisions. It would not be a suitable large-scale solution to the problem of space junk altogether. A study in 2011 found that it would take a minimum of 3.7 years for a laser such the one proposed here to de-orbit an object weighing 1,000kg, making this solution non-viable for the removal of larger pieces of debris. There is also potentially the risk that the laser hits the object in such a fashion that induces the object to break up, creating even more debris. As such, no laser broom system is currently in development.
ElectroDynamic Debris Eliminator (EDDE) Vehicle
The proposed EDDE system consists of a fleet of small (weighing up to 100kg each) vehicles which “sail” through ionosphere. EDDE is described as both propellant-less, and persistently maneuverable. A large part of an EDDE’s body is made up of a several kilometer long metal tape. Electrons from the ambient ionosphere plasma are collected in one end of this tape, and are ejected back in to the plasma from the other end of this tape using hot-wire electron emitters. Thrust is generated by the current in the tape crossing electromagnetic field lines. NASA tested this concept in 1993 in a Plasma Motor-Generator test flight. A 0.3A current was induced in a wire of 500m in length, and another test in 1997 was able to induce 1A in a 20km wire. Because of this dependency on the ionosphere, EDDE would be limited to LEO. EDDE also makes use of flexible solar panels to generate power, rotating slowly to increase performance. This rotation also allows for variance in the angles at which electromagnetic fields are encountered, and allows for thrust in different directions. It is planned that EDDE will only generate thrust when it has access to sunlight, coasting at “night”. By only operating in sunlight, this will help EDDE to locate target objects for removal.
EDDE is intended to “catch” debris using a net. EDDE would contain ~100 expendable gossamer nets, each weighing 50g. EDDE would maneuver itself in such a way that the relative velocities between itself and the target object are quite low, and then launch a net rotating at a speed of ~2rpm on the end of tethers. As the object to be captured passes over the center of the net, the tethers can quickly be retracted to capture the object. It is estimated that a peak load of 20N during the snatch can be tolerated by this set-up. Via careful maneuvering of EDDE, the potential mass of objects to be caught should be quite large. Objects can then be dragged so that they then re-enter the atmosphere and burn-up. With an estimated capture rate of 1 object per day, after 7 years the largest 2500 LEO debris objects could be cleaned up. In the current design, EDDE net launch would be controlled by a human operator. However, there is the potential for this target identification/capture to become automated in the future. There is also the suggestion that the EDDE system could work along-side laser ablation methods such as laser broom, with laser ablation being used to slow objects for ease of capture by EDDE.
An issue experienced by EDDE is that it cannot easily drag objects much larger than itself to the orbit needed for targeted re-entry. Instead, it is expected for EDDE to deposit objects at around 330km orbits, where un-targeted re-entry should occur within a few months. It is unexpected that congestion within this space caused by multiple EDDEs working simultaneously should not cause major issues. It has been suggested that small rocket packages could be attached to netted debris when targeted re-entry is absolutely required, but the added weight of these rockets on EDDE makes it an impractical option. It would only be reserved for cases in which there is a real need for targeted re-entry, which should be few and far between. Another suggestion to counter the risks of un-targeted re-entry is the use of “tethered scrapyards” at less congested orbits (perhaps 660-730km). This would be useful as most space junk is within heavily congested bands, and so moving junk from within said bands would reduce the chance of object collisions. It is estimated that bringing junk from higher orbits down to these scrapyards would be quite possible for EDDEs. The main objects to be targeted for this method would be old Russian rocket bodies. After the creation of these scrapyards, there are two main options for their disposal: controlled re-entry of the mass (potentially using an alternative method of controlled re-entry), or the recycling of the mass. It is hypothesised that parts of the mass could be removed by EDDEs and delivered anywhere within LEO. Fuel tanks could be cut in to shingles to serve as radiation shielding on other projects, or even as feed-stock for other processes.
It is theorised that dust could be used to induce drag (and so re-entry) on objects. A rocket could be launched in to LEO, releasing ~20 to 40 tons of dust, orbiting in the opposite direction of the targeted object. The collision between the dust and the object would increase the drag on the object, causing its orbit to decay (and so for the object to re-enter). Via careful selection of dust properties (e.g. material, particle size), it would be possible to synchronise the rates of dust and junk object descent. There is also the added bonus that the dust would have a “sweeping” effect, causing drag on all objects encountered as it heads towards re-entry. This means that each dust rocket could in theory remove numerous objects in one go.
CleanSpace One is a system similar to the proposed EDDE system, in that it uses nets to capture objects for forced re-entry. Initial designs of the system proposed using a claw to grab objects, but a conical net was deemed to be more practical. CleanSpace One appears to be a one-shot system: it is intended to capture a single target (the target being SwissCube, a satellite garnering 5-6 collision warnings a year), and then re-enter with the target causing both objects to be destroyed.
Adding small “sails” to satellites which can be deployed at the ends of their life-spans could provide an easy way of increasing drag and so inducing re-entry. It has also been suggested that solar sails could be developed which could then be deployed on to existing space junk, causing the drag needed for re-entry. Solar sail systems would be designed to automatically orient in such a way that maximum drag can be achieved, for faster orbital decay. Sails are also planned to be made of a reflective metallic material, allowing for the use of solar radiation pressure to maneuver the object.
Sling-Sat is a novel solution to the power issues associated with having vehicles rendezvous with space junk objects in an effort to capture them; Sling-Sat aims to capture and release objects through plastic collisions. It is proposed that the momentum gained from such collisions could be used to send the Sling-Sat on its way towards its next targeted object. Objects would be ensnared on the end of the spinning satellite, and released using precise calculations to ensure that both objects are sent in the correct directions (i.e. the space junk towards re-entry and the Sling-Sat towards its next target). It is unclear at this point what methods would be used to capture the target object. Some systems intend to use a “harpoon” to impale objects. This does not seem like the best of solutions to the problem however, as such a capturing would surely spawn further pieces of debris.
Tractor beams are a staple of science fiction, and would certainly be an elegant solution to the problem of space junk. Researchers have been able to create a traction beam for tiny objects using ultrasound. It has been proposed that this technology could be used in the creation of “electronic sonotweezers” which allow for the careful maneuvering of particles. However, there are two glaring issues with the use of such technologies to handle the issue of space junk: 1. Space junk tends to be quite large, and 2. In space, no-one can hear you scream (that is to say, the sound waves used in this technology cannot propagate in space).
A different potential type of tractor beam has appeared in an unusual place: the Hendo Hoverboard (by Arx Pax). These boards are able to hover via “electromagnetic repulsion” generated by an engine known as the “Magnetic Field Architecture”. Elon Musk of SpaceX has expressed an interest in the technology for the Hyperloop, an envisioned high-speed transportation system. NASA has begun collaboration with Arx Pax in the hopes of creating a traction beam suitable for the maneuvering of defunct satellites, the theory behind this being that the satellite is both pulled and pushed away with equal force, causing it to be captured and held stationary relative to the beam’s source. This method of capturing an object without having to physically contact it could be very useful as it has little chance of creating debris, but unfortunately it is still a way off of being a viable technological solution.
The e.DeOrbit system is a large vehicle (weighing in at around 1600kg), designed to capture space junk objects and induce their re-entry/burn-up. Once again, there is some debate over the method of capture: a net has been suggested, but a harpoon would make it easier to capture objects. The use of robotic arms have also been proposed, and the outputs of similar trials are to be investigated to assist in the decision making process. It has also been suggested in a similar vein to robotic arms that robotic tentacles could be used to ensnare objects. As with CleanSpace One, this system is designed to de-orbit itself alongside the captured object.
Developed by the Japan Aerospace Exploration Agency (JAXA) the electrodynamic tether system is designed to generate electricity to slow down space junk orbits, causing re-entry. This is done using similar principles to those seen in the EDDE system, whereby a current can be induced in a wire via traveling through the Earth’s electromagnetic field. These tethers are to be attached to pieces of space junk, and should generate drag to slow the orbit of the objects. This system is currently in testing to see how much electricity is generated, and if it is indeed sufficient to alter orbits.
Space Debris Elimination (SpaDE) system
The SpaDE system aims to remove space junk from via firing controlled pulses of atmospheric gasses in to the paths of orbiting debris, increasing drag and de-orbiting the junk using a method similar to that seen in the dust system mentioned previously in this post. The gasses will safely dissipate after this, meaning that this method could be considered fairly fail-safe. It is also improbable that this method would create any additional debris, as the dust method potentially could. This idea is currently under investigation by NASA and Raytheon, and results so far are promising. It is proposed that this technology could be used to remove objects which have the potential to collide with the ISS. With each evasive maneuver by the ISS costing around $2million, it seems viable that this method could go ahead as a cheaper alternative.
So far, it seems there’s no single solution that wins out as the obvious best choice. A lot of the above mentioned methods are still in the testing stages, and so hopefully within the next 10 years or so we’ll see which ones are actually most viable!