Are solar power satellites sitting ducks for orbital debris?
by Al Anzaldua, David Dunlop, and Brad Blair
|Within two LEO altitude bands, the density needed to initiate the “Kessler Syndrome,” i.e., a cascading chain-reaction of collisions leading to uncontrollable growth of debris, may have already been reached.|
Individuals and governments around the globe are becoming aware of the danger that orbital debris presents both to our modern life and to future plans for the utilization of space. According to NASA, there are over 21,000 Earth-orbiting objects larger than a softball (10 centimeters) and 500,000 shrapnel fragments between 1 and 10 centimeters. The number of shrapnel smaller than 1 centimeters exceeds 100 million.2
Because of their high relative velocity on impact, typically 10 kilometers per second in low Earth orbit (LEO), orbiting shrapnel as small as 5 millimeters can disable a spacecraft.3 The debris is an ever-growing hazard to the International Space Station, future space flights, and the approximately 1,100 operational satellites.4 Indeed, Jer-Chyi Liou, Chief Scientist for NASA’s Orbital Debris Program Office, using estimates drawn from six space agencies, recently declared that even without a new catastrophic collision or explosion in orbit, and with 90% compliance with the 25-year deorbiting-after-use guideline, debris will continue to grow over the next 200 years.5 Moreover, it seems reasonable to expect that the increase in debris, by knocking out stationkeeping capabilities of impacted satellites, will worsen Liou’s current estimate6 that there will likely be a major catastrophic collision every five to nine years.
Although most of the debris is in LEO, with the greatest concentration found at altitudes of 750–1000 kilometers, there is a considerable amount in medium Earth orbit (MEO) and geostationary Earth orbit (GEO).7 Within two LEO altitude bands, the density needed to initiate the “Kessler Syndrome,” i.e., a cascading chain-reaction of collisions leading to uncontrollable growth of debris, may have already been reached.8 High debris density LEO bands should therefore be some of the first targets for remediation and parallel mitigation efforts.
Orbital debris, by threatening our satellites and related spacecraft, is also threatening to shred the very fabric of modern life. Satellites are intimately involved with our everyday activities. Anyone using Google maps, checking the weather forecast, watching TV, listening to the radio, flying on a plane, using an ATM while traveling, accessing certain Internet sites, taking a cruise, or calling on a cell phone makes use of satellite technology.
Worse yet, future space technologies and missions are threatened. For example, Solar Power Satellites (SPS) for terrestrial use, an energy technology with enormous potential to improve lives, is also at stake. In 2009, retired astrophysicist Donald Kessler, who started NASA’s work on orbital debris more than 30 years ago, stated, “large structures such as those considered… for building solar power stations in Earth orbit could set up a situation where a single satellite failure could lead to cascading failures of many satellites.”9 Solar power satellites are not the only future spacecraft that will be threatened. Bigelow Aerospace plans to have its BA 330 habitats serve as crew habitats in orbit starting as early as 2016.10 Add to this the untold satellites and other spacecraft scheduled to go into Earth orbits well into the future.
But would a hyper-modular system, such as proposed by John C. Mankins, also be vulnerable? Mankins admits that micrometeoroids and orbital debris might impact the SPS and cause damage, but then he argues, “Fortunately, with a hyper-modular architecture such as SPS-ALPHA11 there are no ‘single’ points of failure. Impacts will cause damage, but it will be mostly inconsequential and will only occasionally require repairs.”12
This statement bears skeptical examination. Much shrapnel debris exists below current detection limits, so quantification of risk remains problematic. Further studies of risk and greater detection capacity are needed to reduce uncertainty and to encourage potential investors that the risks to capital invested in solar power satellites (SPS) are acceptable.
Admittedly, the hyper-modularity of the SPS-ALPHA system would mitigate damage from orbital debris. But Mankins proposes multiple SPS-ALPHAs to solve our energy concerns, each measuring approximately three by five kilometers.13 These structures would be very large targets—“sitting ducks,” in the case of a Kessler-type runaway debris growth in GEO—and the damage would likely go beyond “inconsequential.” Even if the satellite remained structurally intact, maintenance costs would sharply rise. Keep in mind also that to build such a large SPS in the first place, many SPS module-carrying spacecraft would have first to pass through shrapnel-cluttered LEO bands before carrying modules to GEO for construction by telerobotically operated spacecraft.14
Perhaps SPS-ALPHAs require, not only hyper-modularity, but hyper-permeability, such that the modular elements can each separately move to avoid debris. Ideally, the modules would describe an array of SPS-ALPHA elements flying in precise formation and with the ability to self-adjust to avoid danger, reminiscent of a school of fish avoiding the lunge of a predator.
Large debris, i.e. larger than ten centimeters in diameter and one kilogram in mass, can range in size all the way up to nine-ton rocket bodies and five-ton satellites. These multi-ton bodies make up much of the mass of approximately 6,300 tons of orbital debris, with approximately 2,200 tons in Low Earth Orbit (LEO) alone, and collisions among them are the source of millions of shrapnel fragments.15 For example, China in 2007 intentionally destroyed its Fengyun-1C weather satellite, and in 2009 the non-functioning Russian Cosmos 2251 satellite collided with the American Iridium 33 satellite. One-third of all orbital shrapnel can be traced to just these two collisions.16 Worse yet, orbital shrapnel smaller than ten centimeters and one kilogram is currently untrackable, and because of the high collisional velocities, even shrapnel as small as five millimeters can take out a spacecraft.17
A consensus is building among persons studying the orbital debris problem that the greatest danger will come from inevitable catastrophic collisions between large debris objects, which will produce immediate and subsequent financial loss due to untrackable shrapnel. And because the subsequent financial loss will dwarf the immediate loss, Jerome Pearson and his colleagues Joe Carroll and Eugene Levin in a recent article argued strenuously for dealing with such large objects as soon as possible. 18
|Space-based solar power structures would be very large targets—“sitting ducks,” in the case of a Kessler-type runaway debris growth.|
But which large debris objects should be the priority? Launching countries are naturally sensitive about the nature of their satellites. Therefore, to induce international cooperation to remove, recycle, or rehabilitate large debris objects, it is best to start with the much less sensitive, but still dangerous, upper stages (i.e. basically aluminum tanks.) They make up about half of the debris mass in LEO. Capturing aluminum tanks would also be a lot less complicated than grabbing satellites with solar arrays, antennas, and nuclear reactors. Because most of the large debris is of Russian origin, a bilateral treaty with Russia would be a good place to start, as discussed below.
The DoD’s Defense Advanced Research Projects Agency (DARPA), under a demonstration project called Phoenix, is teaming up with the private sector to harvest and “repurpose” still functional components of nonworking satellites in GEO to create new space systems at greatly reduced cost. Beginning in 2016, the project proposes to attach nanosatellites to parts of retired US government and commercial satellites, making the debris a resource. In a process called, “cellularization,” nanospacecraft separately carrying out functions such as power, communications, and attitude control would be launched into orbit as secondary payloads. A service-tender spacecraft would then be telerobotically directed to attach such miniature devices to large antennas or other large parts of dead satellites to produce working satellites at a fraction of the cost of new ones launched from Earth.19
Another way that defunct satellites in GEO can be rehabilitated, if not already too damaged by orbital debris, is through refueling. The 2010 Space Infrastructure Services (SIS) project by Canadian company MacDonald, Dettwiler and Associates (MDA) envisioned both refueling and otherwise servicing satellites in orbit telerobotically. Although MDA and Intelsat in 2012 cancelled their collaborative agreement in which MDA was to develop a satellite capable of servicing Intelsat’s 50 operating satellites, MDA remains interested in the concept and is waiting for a possible DARPA contract. 20
In this connection, it is important to note that in May 2013, NASA carried out a series of telerobotically operated “propellant transfer experiments” on an exposed platform of the International Space Station (ISS).21 Although the ISS is in LEO, the refueling technology being developed is intended for use in GEO.
Various ideas and technologies are being developed potentially to remove, recycle, or reuse (through rehabilitation or repurposing) large debris objects in LEO as well. For example, three companies—Star Technology and Research, Inc., Tether Applications, Inc., and Electrodynamic Technologies, LLC—have been developing a technology called ElectroDynamic Debris Eliminator (EDDE), wherein a long conductor is energized using solar energy to thrust against the Earth’s magnetic field. Operating without propellant, EDDE can repeatedly change its altitude by hundreds of kilometers per day and its orbital plane by degrees per day.22
Assuming effective EDDE or other non-propellant debris remediation technologies23 are developed, which LEO orbits are ripe for remediation? About half of the mass of orbital debris in LEO is at inclinations of 71–74°, 81–83°, and sun-synchronous orbits. According to Jerome Pearson, President of Star Technology and Research, Inc., and Joe Carroll, President of Tether Applications, Inc., disposing of upper rocket stages in these inclinations, which would remove 79 percent of the collision-generated debris potential, is a crucial first step to stopping the growth of shrapnel.24
There are good reasons for testing and developing EDDE and other debris remediation technologies at the ISS. In the first place, the ISS generates ten tons of waste annually and money and effort is already being spent to remove it.25 The ISS also has features that can facilitate early demonstrations of debris removal technologies: its own electrical power supply, a redundant international supply chain, human extravehicular capabilities, robotic grappling and docking, a Ka-band microwave transmission antenna, and a potential for servicing and refueling other spacecraft. Joe Carroll maintains that EDDE vehicles could bring another 100 tons of orbital debris to the ISS for either de-orbiting or salvage.26 Testing and developing EDDE and other technologies, such as energy-beaming and solar electric propulsion (SEP), at the ISS could inform the space development community on techniques and technologies for capturing and handling orbital debris for subsequent de-orbiting, metal recycling, or repurposing.
Once we have learned to deal with this smaller amount of debris in connection with the ISS, we will be better prepared to deal with the estimated 2,200 tons of dangerous large debris objects in LEO and elsewhere. The ISS occasionally has to dodge space debris, and this involves moving its million-pound mass with rocket engines using chemical propellants. Perhaps the ISS-connected debris remediation demonstrations, done with free flyers operating within power beaming-distances,27 could evolve into technologies specifically to protect the ISS and thus obviate the need to burn precious chemical propellant.
Assuming that SpaceX does indeed manage to get the payload price to LEO down to $2,200/kilogram28 using the Falcon Heavy and eventually half that cost with routine first-stage reuse, debris remediation at LEO and higher using only rockets would remain prohibitively expensive. Fortunately, using EDDE and other propellant-less vehicles to carry out the actual removal of at least a thousand tons of large debris from LEO will make a noticeable difference at a more reasonable cost. In this regard, Jerome Pearson, et al., in considering an orbital debris removal campaign removing only upper stages from LEO, estimate that in seven years of operation “1,000 tons of upper stages and 79% of the collision-generated debris potential can be removed at an average cost of less than $500 per kg and an average annual cost of about $70 million.”29
To the above considerations, we must add the salvage value of 2,000 metric tons of refined metal. Aluminum scrap on Earth is currently around $1,730 per metric ton.30 So, at a minimum, large debris in LEO represents at least $3,460,000 in raw materials. Finished products would have many multiples of that value in orbit. However, as shown below, getting to finished products would involve heavy production costs.
Salvaged metal can only be worth something to a company ready and able to process it into new tools, devices, or spacecraft for a profit. To get that profit, the potential buying company will have to figure in capitalization costs necessary to transform the metal into final products. Then, the buyer must either sell the new tools, devices, or spacecraft, or use them to provide a service for which there is demand. All these actions within the cislunar market will determine the actual value of the salvaged metal to the first buyer. Also keep in mind that it is unlikely that all those tons of salvaged metal will be bought for space construction in the foreseeable future; a good number of the smaller upper stages and “zombie” satellites will likely be deorbited.
Beyond these preliminary market figures and considerations, on-orbit recycling of materials for construction and manufacturing would counteract the throwaway culture that has made space operations largely beyond the reach of the commercial economy, with the exception of commercial communications and GPS satellites.
|There are good reasons for testing and developing debris remediation technologies at the ISS.|
On the other hand, lowering or removing the odds of large debris collisions, by whatever means, which threaten a satellite industry grossing over $200 billion annually, is a valuable service that must be quantified. The community of satellite users must remove large debris objects safely and thus lower the risk of catastrophic collisions or face customer anger and loss, coupled with much higher costs for satellite replacement. Retiring this risk of collision will avoid subsequent much larger losses. Market-based insurance and salvage quantification-models could be used to provide economic incentives to remove, reuse, or recycle space debris, and thus save this industry.
Serious thought should be given to where orbiting scrapyards would best be located and what sorts of vehicles should emplace them. Most orbital debris resides at altitudes of less than 1,500 kilometers, although there is a significant band of debris around GEO. The orbits of scrapyards below 600 kilometers would degrade, depending on the particular altitude, within a few years or months because of atmospheric drag and deorbit.
At around an altitude 650 kilometers, however, orbital debris is relatively sparse and scrapyards there would need only infrequent boosting to maintain altitude. EDDE vehicles could therefore carry large debris objects to cross-truss scrapyards at that altitude. Also, carrying defunct upper stages to 650 kilometers for collection would make the raw aluminum more accessible for subsequent construction in LEO31 and would be quicker than carrying them to deorbiting altitudes.
Orbiting scrapyards could also be located within other sparse debris bands in higher orbits or even around Earth-Moon Lagrange points. Scrapyards embedded in cross-frames in meta-stable halo orbits near Earth-Moon L1/L2 (EML1/ L2), with a little stationkeeping, could serve as a metal-resource site and a nexus for cislunar infrastructure, facilitating the later growth of staging sites, fuel depots, spacecraft construction sites, communications satellites, and habitats with telerobotic capabilities.
Keep in mind that it takes a bit less chemical propellant from LEO to reach EML1/2 than to reach and circularize on orbit in GEO.32 On the other hand, in comparison to going to GEO, reaching EML4 or L5 would take a little more chemical propellant. Scrapyards in these latter locations, however, could remain in stable bean-shaped orbits without stationkeeping for many years. When dealing with low-thrust SEP from LEO to Lagrange orbits in comparison to GEO, the propellant cost is not as favorable.33 With SEP, however, we would be dealing with much less propellant in the first place.
Of course, every proposed salvage operation should entail reducing the risk of orbital debris collisions, not increasing it. Moreover, the act of grappling, controlling, or moving debris should not generate more of the material. Any international system monitoring such salvage operations should operate transparently and give notice of voluntary space "clean up" activities by sovereign nations or parties registered with those countries to do business. Opportunities for third-party review, comments, filing of objections, and unilateral “holds” should all be part of the process. Finally, liability assignment under various scenarios will have to be agreed upon by all parties before orbital remediation can begin.