How to Clean Space: Disposal and Active Debris Removal
Cleanup of the space environment is possible if postmission disposal tactics are built into future space systems. Active debris removal techniques are also a means of mitigation.
First published Fall 2015, Crosslink® magazine.
One of the goals of space debris research is to determine how to prevent debris in Earth orbit from becoming so populous that it adversely affects operational satellites. Research conducted at The Aerospace Corporation and NASA, and for other organizations including the Inter-Agency Space Debris Coordination Committee (IADC) show that the major contributor to growth of the future debris environment is collisions, particularly in low Earth orbit (LEO), where there is the highest density of debris. The larger the colliding objects, the more debris generated.
The amount of mass from nonoperational objects left in orbit must be limited to prevent the generation of increasing amounts of debris and to slow or stop that debris from creating cascading collisions (the Kessler effect or syndrome). This is especially true in the most populated regions of space, including the 800–1000 kilometer altitude in LEO.
Postmission disposal (PMD) is a method used for limiting the amount of unused mass in orbit. One PMD technique is controlled reentry, which is performed when an object is placed on a trajectory that causes it to reenter Earth’s atmosphere and impact in a particular region. This approach removes the object from orbit and limits hazards on the ground, but it may require a significant amount of fuel to complete the orbit change necessary for reentry. Controlled reentry is useful for launch vehicle upper stages because they have short mission time frames and may have enough remaining propellant to perform the required maneuvers.
If a satellite or upper stage is not capable of a controlled reentry, a limited lifetime disposal orbit may be used. In this scenario, the object is placed in a postmission orbit that will cause it to reenter Earth’s atmosphere over time from natural perturbations.
The most common rule for disposal time in LEO states that an object should not remain in orbit for more than 25 years beyond its end of mission. This is part of U.S. space policy and IADC debris mitigation guidelines. The rule attempts to limit the orbital lifetime of objects and lower their placement so that atmospheric drag eventually causes reentry.
Another means of disposal is through the use of a drag enhancement device that increases the cross-sectional area of an object. This technique employs inflatable or extendable spheres or large flat surface tethers that may use electrodynamic drag with Earth’s magnetic field to increase the rate of orbital decay.
If neither controlled reentry nor limited lifetime disposal orbits are an option because of fuel expense, which may be the case for higher altitude orbits, a long-term disposal orbit may be used. The strategy here is to remove satellites and upper stages from heavily used orbits and move them to less congested regions of space. Although this does not remove the mass from orbit, it does remove it from areas with the most operational satellites. Geosynchronous orbit (GEO), with its narrowly defined range of altitudes and inclinations, is where this approach is most often used.
The IADC guidelines define a disposal region sufficiently high above GEO so that even under the conditions of orbital perturbations, the disposed satellites will not recross the GEO region for at least 100 years. During the last ten years, the use of GEO long-term disposal orbits has significantly increased. In fact, most GEO satellites are now moved to long-term disposal orbits at the end of their missions.
Each of these PMD techniques is in use with today’s operational systems. Controlled reentry has been used to dispose of at least six evolved expendable launch vehicle (EELV) upper stages. The small satellite MSTI-3 used a controlled deorbit in 1997. Other larger satellites, such as NASA’s Compton Gamma Ray Observatory, have undergone controlled reentries. The use of drag enhancement devices have also been tested on the ORS-3 mission upper stage and satellite using deployable membranes.
Mission orbits are often chosen where natural atmospheric drag will cause the satellite to reenter within 25 years. This is especially important for satellites that do not have maneuvering capabilities.
The use of PMD can be highly effective at reducing the buildup of mass in orbit and growth in the debris environment. It employs the existing capabilities of satellites and upper stages to remove them as possible sources of debris, but it must be conducted by all of the users of space to be truly effective at inhibiting future debris growth. Widespread use of PMD will control the future deposition of mass in orbit, but it will not address the existing debris problem.
Active Debris Removal
Another method for addressing existing large debris objects is active debris removal (ADR), which is similar in concept to PMD. One difference, though, is that in ADR an external vehicle is supplying the mechanism by which the disposal is performed. Another difference between PMD and ADR is that ADR can be applied to any objects that are floating in space, even ones that have been aloft for many years. PMD, on the other hand, can only be applied to missions that have capability for such acts built into them during the planning stage or through residual available capacity.
One example of ADR is a “space tug,” which can be used to rendezvous and grapple with a large object such as an upper stage or inactive satellite. The object can then be boosted into a lower orbit that allows for a reentry compliant with the 25-year rule, or into a long-term disposal orbit. Another possibility is to attach a drag enhancement device to the object.
However, there are drawbacks to these techniques. For one, the cost of launch and operations of an ADR system only make it economical if it can service multiple objects during a single mission. This is possible at GEO, where many old objects are residing in similar orbits, making multiple rendezvous from a single ADR vehicle viable, but it is much more difficult to do in LEO.
There are also technical challenges to removing large debris via ADR. Rendezvous and grappling is difficult from both mission design and mechanical perspectives and requires extensive planning and the ability to perform sophisticated guidance and control during operations. The targeted object may also be tumbling, which can make attachment and stabilization difficult. A generic ADR system would also have to be robust enough to handle many different target object physical designs, including the presence of extended structures such as antennas and solar panels.
ADR can also be used for smaller objects, but the techniques are quite different. Unlike large objects, small ones cannot be tracked from the ground, nor targeted individually for collection from space. The objects that are most likely to disable a satellite are small at approximately 1 centimeter in size. It is estimated that there are hundreds of thousands of these objects in orbit, so any ADR method expected to have a significant impact on collision rates would have to remove much of that debris. A single collision could generate enough debris to repopulate the environment, making small debris removal an ongoing effort.
One fix proposed for small objects involves ground-based lasers to either use pressure from photons or vaporize a small amount of material to “bump” the objects slowly over time into orbits where reentry can occur much earlier than within their existing orbits. Aerogels and other low-density materials have been proposed to “catch” small debris objects, in essence sweeping space clean. However, the benefits that
accrue by removing these particles must be balanced with the technique’s potential interference with operational satellites. To have any significant effect, this technique would also require many sweeper satellites operating at once.
Larger objects—intact satellites and upper stages—are much less likely to hit an active satellite, but studies at Aerospace and other organizations show collisions between large objects, infrequent though they may be, are likely to be the primary source of future debris. This debris may then collide with other medium- or large-size debris and go on to incapacitate other active satellites, generating even more debris. By targeting large satellites and upper stages now, ADR can prevent the generation of hundreds of thousands of mission-ending debris in the future.
Additional studies performed at Aerospace show that while ADR is effective at lowering the overall growth rate of future debris production, there is a limit to its cost/benefit effectiveness. Mission designers and space debris specialists do not know exactly which objects will collide in the future. Therefore, target objects are chosen based on their likelihood of causing future debris growth, rather than any certainty that the specific object will increase the debris environment.
A number of techniques have been proposed to identify which objects are best to remove in ADR scenarios. These typically involve using probability to conduct a severity assessment where a combination of the chance of a collision occurring and the amount of debris generated (the severity) is determined. Probability is determined from the number of objects crossing a given target’s orbit and the area that target presents for a possible collision. The severity calculation is mainly a function of the target’s mass, which determines how much material is available to generate new fragments.
Both PMD and ADR are designed to control the growth of the debris environment by limiting the amount of mass in space that may cause future collisions. PMD has the advantage of being significantly less expensive than ADR. If space missions are designed with PMD as a requirement, the cost to the mission can often be small to none. The widespread use of PMD built into future missions could nearly eliminate the buildup of debris in orbit and is a necessary component of any effective debris mitigation effort.
Although ADR is potentially much more expensive, it may become necessary if PMD is not performed with a sufficiently high percentage of objects and within a short enough timeframe. The longer PMD is not widely performed, the larger the buildup of mass in orbit and the more difficult it will be to remove. The most effective long-term ADR strategy is to focus on the larger objects, which will prevent the creation of future debris.
However, there are several issues with ADR as a debris-control option. The technique used must be cost-effective (i.e., the cost of removing the large object cannot be greater than the benefit it accrues to the space community). A legal and policy framework must also be established to effectively deal with international treaty–related ownership issues, as well as liability in the event of mishaps.
Earth orbit is a shared resource, so what one user does in it affects all other users. This is especially true with debris since there are no borders to keep it confined. As such, it is critical that all users of space follow best practices for maintaining the Earth orbit environment, such as PMD, particularly in the heavily used orbits of LEO and GEO. Organizations such as the IADC are attempting to bring the international community together to share best practices and encourage good stewardship of space.
About the Authors
Marlon E. Sorge, Senior Project Engineer, Space Innovation Directorate, joined Aerospace in 1989. He has worked on space debris issues for more than 25 years, including fragmentation modeling, risk assessments, debris environment projection, mitigation techniques, and policy development. He also coordinates Aerospace’s debris research program. He has a B.S. in physics and an M.S. in aeronautical and astronautical engineering from Purdue University.
Glenn E. Peterson, Senior Engineering Specialist, System Analysis and Simulation Subdivision, joined Aerospace in 1997. He works on a variety of topics including space debris, collision analysis, meteor showers, and satellite disposal. He has a B.S. and an M.S. in aerospace engineering from San Diego State University, and a Ph.D. from the University of Texas at Austin.
Back to the Fall 2015 Table of Contents
Go to side bar: Legal Issues for Active Debris Removal