New Technological Approaches to Orbital Debris Remediation
(1)
Executive Board, International Association for the Advancement of Space Safety (IAASS), Arlington, VA, USA
Introduction
Efforts to develop active space debris removal projects currently underway were described earlier in Chapter 2. Virtually all of these projects relied on the current state of space technologies. Many of these currently envisioned projects involve sending up a robotic spacecraft that can attach itself to a selected element of orbital debris such as a defunct spacecraft or upper stage rocket launcher and then deorbiting the debris along with the capturing spacecraft. Today, in some instances both the target and the capturing robotic spacecraft are launched as part of the effort to develop an active deorbiting capability and to avoid concerns related to liability claims. There are clear and apparent problems with this approach in that it is extremely expensive, slow and deliberate, and inefficient by almost any index of effectiveness. This chapter explores a number of technical approaches that have been identified as potentially viable that would ultimately be far more effective, achieve de-orbiting more rapidly and effectively, and thus logically cost far less than the one-by-one active de-orbit missions.
These technological approaches can be divided into a number of different categories as follows: (a) Ground based approaches to active debris removal or collision avoidance; (b) Passive de-orbit systems that can be deployed at end of life; (c) New types of active de-orbiting systems that could be mandated to be included that are separate from the regular positioning and orientation capabilities of spacecraft; (d) Innovative active de-orbiting systems that can assist with the removal of many debris elements in a single mission. These might also use different propulsion systems than conventional chemical rocket thrusters; (e) Improved technical means for locating orbital debris for removal by efficient proximity navigation and mating.
Key Trade-Off Considerations: Innovative Technology vs. Maturity, Reliability and Precision of De-Orbiting System Design
The conventional approach of one-by-one robotic capture of defunct satellites is one that has been demonstrated on a number of occasions and involves minimal risk of de-orbiting the wrong spacecraft. Ground-based systems might be more cost effective, but there are issues of high power particle beam or laser systems being considered space weapons systems and there are also concerns about the accuracy of their targeting systems and potential error. Many of the newer technological approaches discussed in this chapter are likely to be more cost effective, but these are new and largely unproven capabilities. It could take a number of years for these new methods to reach technological maturity. Such new systems need to be proven to be reliable and indeed able to accurately remove debris from orbit. The most secure way to achieve future debris removal might be to mandate active (or passive) removal systems that are separate and complementary to conventional positioning and orienting thruster systems for station-keeping. Such an approach would entail mandating separate and fail safe de-orbiting capabilities. One particularly challenging issue that will perhaps require the greatest amount of new technical capability is the problem of upper stage rocket launchers that transverse the geosynchronous orbital plane and threaten active or defunct satellites at accelerated relative velocity and dangerous relative angles of incidence.
Review of Alternative Technological Approaches
Ground-Based Systems
Ground-based systems can provide important capabilities in addressing orbital debris issues. The first type of capability involves irradiating a debris object that is threatening a collision with another orbiting element in such a way as to slightly change the debris object to avoid impact. This could be a laser-based tracking system or a particle beam projection system that is continuously focused on debris object prior to an impending collision. At orbital speeds even a change of an orbital period by a tiny fraction of second can be sufficient to avoid a collision. There have been innovative suggestions that the officially registered owner of the debris element could have their own operators control the beam projections to alleviate concerns that these systems would be deployed militarily against their own space assets. Such types of ground systems would not constitute active debris removal but simply debris collision avoidance. These temporary measures to avoid major collisions are important steps to undertake until more permanent solutions can be found and undertaken.
Much high powered particle beam weapons or even very high powered laser systems, however, could, in fact, provide active debris removal. It has been proposed that a beamed or directed energy system could be positioned on the International Space Station and from this location it could systematically remove debris from low earth orbit through the use of such a strategically advantageous location. [Lubin and Hughes 2014]
Directed energy systems might well be developed for a range of other capabilities. The logic of developing a super intense beamed energy system could well be used to change the orbit of a potentially hazardous asteroid so it could be captured by the gravity of the sun or perhaps over time entirely break up and disintegrate into harmless pieces of a potentially hazardous asteroid. Such a beamed energy system or super intense laser beam system might also be used for strategic purposes or even to power a spaceship drive system. [Lubin and Hughes 2014]
These ground based systems are clearly most effective to deploy in the case of low earth orbit spacecraft and especially for debris which are orbiting only a few hundred kilometers from the Earth’s surface. The ability of ground based systems to address the removal of debris from medium earth orbit, from geosynchronous orbit or defunct upper stage launch vehicles in 12 h transfer orbit is technically challenging and probably not financially viable nor practical under the current terms of the Outer Space Treaty and the Liability Convention. Finally, the use of ground-based systems to create orbital changes for such debris elements, rather than saving the situation, could increase the risk of a collision and not be able to effectuate removal. Currently these ground-based approaches have not been practically demonstrated to be reliable and effective.
Passive De-Orbit Systems
The deployment of passive de-orbit systems at end of life represents the most economical means to ensure the longer-term deorbit of low earth orbit satellites. A number of different concepts have been conceived and tested that at the end of life for a small satellite that could be deployed to create a significant amount of atmospheric drag and thus hasten de-orbit. These concepts include inflatable balloons, inflatable tube membranes (ITMs), suspendable tethers, or solar sails. Essentially these are all rather simple and easily deployable drag systems that are designed to increase the rate at which the de-orbiting process occurs. Such systems are really appropriate and effective for small satellites at relatively low orbits, i.e. under 800 km or so. Such mechanisms can accelerate the rate of de-orbit and allow small LEO satellites to meet the current standard of de-orbit within 25 years. [Rasse]
Satellites that larger in size with deployable solar sails of a larger cross section could use their arrays to assist de-orbit at the end of life. Solar sails have been used to accelerate the de-orbit of the NASA Fastrac satellite. This approach has also been utilized in the CANX-Drag Sail which is a project of the Canadian Government. [Grant Bonin et al.]. This approach of deploying a reasonably large, but very low mass solar sail was also utilized in the case of the European Union Protec 1-2015 program [“Passive Means.”]. A large number of university programs in the United States, Europe and other parts of the world have also developed similar capabilities. These are typically designed for low earth orbit and quite small satellites. When these passive systems are deployed the cross section that creates atmospheric drag can be significantly increased and thus accelerate the de-orbit time and thus make de-orbit two to three times more rapid. There are today some new chemical and electronic thrusters of sufficiently small size and mass that they could be used to assist in the de-orbit of some small satellites or to work in tandem with passive de-orbit systems. The smallest cube satellites, nano-satellites, or so-called femto-satellites will for the most part decay simply due to gravitational effects, especially if they are deployed at 400 km altitude or below.
There are sufficient numbers of these very small satellites now being deployed—and in some cases without formal registration and at altitudes above 400 km. In these instances their deployment can be considered a problem. Solutions to this problem might include flying experiments on the International Space Station and thus not becoming free-flyers. Another option would be to designing “consolidator” satellites that could be the host for a number of small experiments and then deorbit in a controlled burn. In the case of the “host” or “consolidator” satellite they might not only provide thrusters for de-orbit but could also provide a common power supply and perhaps other services common to the small experimental packages that fly in common. National action that provides very clear registration procedures and perhaps imposes fines for not registering small satellites might also be considered. Ultimately there will need to be a review of the 25 year de-orbit rule to see if that is adequate to depopulate low earth orbit at a sufficiently rapid rate. Certainly international agreement to require a 20 year rule for removal of spacecraft from the protected LEO and GEO orbits would be a step forward in seeking to reduce debris in orbit.
It needs to be particularly noted that these passive systems work well for low earth orbit satellites, particularly when Solar Max activities serves to balloon the Earth’s atmosphere to higher altitudes but that these systems are not as effective in higher LEO orbits and do not work in any way for medium earth orbit or geosynchronous orbit since they are well above the Earth’s atmosphere.
The Prospect of Mandating New Types of Fail Safe End-of-Life De-Orbiting Systems
Another new concept that has been suggested to address the orbital debris removal issue is not so much a new technology, but a new approach to end-of-life processes. This is the proposal that there should be a de-orbit thrusters system that is separate from a spacecraft’s regular orientation and station-keeping systems that could also be separately commanded. This capability would, in effect, provide a fail-safe de-orbit system. This idea is not likely to be greeted with enthusiasm by spacecraft owners and operators in that it could involve a separate telemetry and command system, an additional fuel tank, and additional fuel. Conceivably this de-orbit capability could be an ion thrusters system that would make the system lighter in mass. Nevertheless this sort of fail-safe deorbit system might add 5 % or more to the mass budget for a spacecraft. This would initially be for low earth orbit satellites, but the additional capabilities related to MEO and GEO satellites and there redeployment to graveyard parking orbits might presumably come into play at a future date.
To accomplish this “guaranteed de-orbit” D-Orbit of Italy has developed and is now promoting the future use of a new product which they have designated as a Decommissioning Device (DD). This is a unit which as now designed includes a solid propellant motor and a control/command unit. The advantages of this product would be that it is completely autonomous even if the satellite is defunct, and that it is fully compliant with ESA and NASA safety standards. D-Orbit claims that there would no single point of failure except for the solid fuel motor and that it would be guaranteed to be reliable for more than the lifetime of the satellite and that it would be scalable to adapt to different types of missions. This guaranteed de-orbit system could be designed with a timer set for a period of time well passed the planned operational life to provide additional margin against failure. It could also use a chemical thruster or even an ion thruster either to make this system “cleaner” or to reduce the mass of the fail-safe system. [Antonetti et al.]
As interesting as this proposal is from the perspective of likely limiting the buildup of space debris there are a number of factors to consider. These factors include: (1) this would be a partial solution and as now designed would only be for the de-orbit of low earth orbit satellites. There could, of course, be similar systems designed to raise the orbit of geosynchronous satellites; (2) this type of program would not assist with upper stage rocket motors and other debris elements unless this program was expanded in scope; (3) it would be too large of a system to assist with nanosatellites; (4) it would be a very “expensive” program for commercial satellite operators in terms of a major lost operational capacity and the associated opportunity costs—even if this were just an orbit raising system to deploy to graveyard orbit and used a separate ion thruster; and (5) solid fuel rocket motors although they are quite reliable, are also environmentally more polluting than liquid fuelled rockets. Further the potential future use of electric ion systems, although slower and with less thrust, could be more efficient in terms of reduced overall mass penalties that would be added to the mission and certainly would be less polluting. In short the design of fail-safe systems to raise geosynchronous satellites to super GEO might well find ion-thrusters optimum in terms of imposing the minimum mass penalty.
New Technical Concepts for Active Removal Systems for Orbital Debris
Robotic Capture and De-Orbit
The range of technical approaches that might be used to remove orbital debris are quite diverse and the innovative concepts continue to grow and diversify. The main-line approach which a number of aerospace companies and space agencies are now proceeding involves a basic strategy of sending up a robotic satellite to attach to a debris element and then de-orbiting the composite system. The various projects that are being developed with this type of capability were reviewed in Chapter 2. These developments are on one hand conceived as a way to remove major debris elements from low earth orbit and on the other hand they are seen as a possible mechanism for capturing operational satellites and servicing them by providing new batteries and fuel. The most exotic concept in this regard is the idea that grappling robotic spacecraft with the ability to capture defunct satellites might “harvest” antennas or other re-usable components in space and redeploy them on a new space system. This “harvesting” spacecraft concept is unique in that it is primarily designed to operate at GEO altitudes and thus be able to rendezvous with application satellites in geosynchronous orbit.
The one at a time approach to active debris removal, which is currently the prime approach under development, has the major disadvantage of being extremely expensive, time consuming, and ultimately inefficient. The only exception at this time is the Electro-Dynamic Debris Eliminator which has been provided funding by NASA for prototype development by Space Technology and Research (STAR) Inc. Thus this innovative approach is addressed in both Chapters 2 (existing programs) and Chapter 5 (Future technology).