Active Debris Removal by Micron-Scale Dust Injection Gurudas Ganguli Plasma Phys. Div., Code 6756 Naval Research Laboratory, Washington DC 20375-5346 202-767-2401
[email protected]
Christopher Crabtree Plasma Phys. Div., Code 6756 Naval Research Laboratory, Washington DC 20375-5346 202-767-6644
[email protected]
Abstract — In — In response to the recent National Research Council report concerning orbital debris hazards and the National Aeronautics and Space Administration study suggesting active removal of small-scale (1mm – cm) ‘mission-ending’ orbital debris, we discuss concepts for small debris elimination through deployment of micron micron scale dust. Dust, which naturally fills the near-earth environment, can be deployed artificially artificially in a narrow altitude band to enhance drag on debris and force reentry. The injected dust dust will also reenter the earth’s atmosphere. atmosphere. Orbital and suborbital dust deployment deployment concepts for actively removing debris that (i) has uniformly spread around the earth or (ii) remains localized over a small volume, as well as the system risks and their possible mitigation are discussed.
TABLE OF CONTENTS 1. INTRODUCTION ................................................. 1 2. SALIENT PHYSICS ............................................. 2 3. UNIQUENESS OF DUST-BASED ARD SYSTEM.. 5 ISKS AND POSSIBLE MITIGATION ... 5 4. SYSTEM R ISKS 5. SUMMARY ..................................................... ......................................................... .... 7 EFERENCES..................................................... R EFERENCES ......................................................... .... 7 BIOGRAPHY ...................................................... .......................................................... .... 7 1. INTRODUCTION Hundreds of near-misses occur each year between orbital debris and operational satellites [1]. Recently the National Research Council (NRC) conducted an exhaustive study to assess NASA’s meteoroid and orbital debris program [2]. The NRC report concludes that we are at the ‘tipping point’ which is the threshold for collisional cascade [3]. Once the exponential rise in small debris sets in due to the cascade it will become increasingly risky to maintain space assets in Low Earth Orbit (LEO) without clearing the debris population. The NASA Orbital Debris Program Office O ffice has determined that satellite collisions with small orbital debris, with sizes in 0.5 mm – 1 cm range can be mission-ending [4]. Debris in this scale size is currently impossible to track individually and hence hence cannot be evaded. In addition, the population of smaller scale debris is orders of magnitude larger than larger debris. Consequently, the probability of collision of smaller debris with active satellites is proportionately higher. This makes the small scale debris 978-1-4577-0557-1/12/$26.00 ©2012 IEEE
Leonid Rudakov Icarus Research Inc. P.O. Box 30780 Bethesda, MD 20824
[email protected]
Scott Chappie Naval Center for Space Tech., Code 8232 Naval Research Laboratory, Washington DC 20375-5346 202-404-2620
[email protected]
particularly dangerous. The NASA study [4] recommends that as the larger debris objects are removed from orbit to stabilize the growth of the mission-ending smaller debris, it is also necessary to simultaneously remove the existing smaller debris from orbit. We discuss concepts to force reentry of such small scale orbital debris by deploying micron scale dust to artificially enhance drag. We investigate the possibility of dust deployment and drag enhancement [5] using dust sizes an order of magnitude below the NASA recognized definition (1 mm) [6] of harmful debris. Both Orbital and Suborbital operational concepts for Active Debris Removal (ADR) systems can be designed to address this issue of international concern. In both concepts, debris momentum is changed by dust impact such that fragmentation and damage to the debris at the macroscopic level does not occur. The sparse nature and small magnitude, of the individual dust impacts keeps the general character of an individual debris object unchanged; however, at the microscopic level, a multitude of hypervelocity impacts induce large change in debris momentum to force reentry. The key physics making these concepts practical is the large debris momentum change for individual hypervelocity impacts by a high mass density dust grain. Dust hypervelocity impacts create microcraters on debris surfaces. Because dust impacts the debris with velocity greater than the speed of sound in the debris material, a high pressure bow shock is created. High shock pressures fragment, liquefy, or vaporize material depending on the energy of the collision. Shock pressure ejects the fragments, liquid, and gas from the microcrater into vacuum as a recoil jet that imparts thrust to the debris mass. The induced thrust is the key feature of the dust based ADR concepts; the debris momentum change is much larger than that of inelastic or elastic collisions and is not very sensitive to impact angle. The dust-induced drag force on the debris is magnified by a factor kappa, κ = Δ p/ p p0, where Δ p and p0 and are debris momentum change and dust grain momentum respectively. Hypervelocity impacts by high mass density micron-scale dust can make κ >>1. The efficiency and economy of the dust based ADR system is directly proportional to κ . From our study so far, tungsten appears to be a prime candidate for the dust because of its i ts high density densit y 3 (19.3 g/cm ), relative abundance, availability in powdered
form, and modest cost. However, other dust materials can also be considered. Two scenarios are discussed. discussed. The first is where satellite fragmentation has occurred long ago and the debris has spread around the earth almost uniformly. This is the more difficult case since the volume occupied by the debris is large and filling this entire volume with dust is impractical. To make the procedure practical, we suggest a snow-plowlike technique in which dust fills only a limited range in altitude (~ 30 – 50 km) and is resident at any particular point in space for a limited duration. As the t he dust naturally descends due to drag, it has a sweeping effect on the debris situated at altitudes altitudes below it. The narrow dust layer also allows for maneuvering of active satellites around the layer to avoid any adverse effect of the injected dust for the duration when the dust layer is resident at the satellite altitude. The second case applies to the immediate aftermath of a fragmentation or removal of larger trackable objects. This is a relatively easier easier case since the volume of the debris cluster is small. In addition to lower dust mass necessary for this case, there are other advantages as well. For example, dust can be deployed sub-orbitally so that dust is not injected into the orbit and remains in space for only a few minutes.
(a)
x1
dust cloud in polar orbit
x2 R
R
N
R
x0
S
2. SALIENT PHYSICS Consider the case where debris has spread around the earth, as would be within months after satellite fragmentation [7]. As shown schematically in Fig. 1a debris is localized between altitudes X1 and X2. We target small debris with ballistic coefficient, defined as the ratio of the debris mass to its area, B ~ 5 kg/m2 or less. Orbital lifetime of debris can be reduced by lowering the orbit altitudes to X0, below which the natural drag will force reentry of the debris within a desired time. Drag enhancement on the debris necessary for lowering their orbit altitudes can be achieved by injecting dust in counter rotating orbits. Due to perturbations caused by the Earth’s irregular gravitational field, the debris and dust orbits will precess. However, injection in polar orbits minimizes dust precession. Dust would distribute over latitude due to spread in the injection velocity and form a partial shell slowly expanding in azimuth, as shown in Fig. 1b. Differential precession of debris orbits creates a nearly homogeneous shell with a distribution in right ascension; bringing debris into counterrotation with dust over time. At a given time half of the debris population will counter-rotate with the dust orbits while crossing the dust shell and experience enhanced drag. The change in debris momentum is the sum of effects from dust impact and atmospheric drag [8] and may be expressed as; B
dV dt
2
= − κ nd md (V − vd ) −
Dust Impact
C D
ρ A (V − vA )
2
2
(1)
Atmosphere
where B = M / A A is the ballistic coefficient; M and V are
Figure 1. (a) Meridional view showing debris location between altitudes X1 and X2 (blue). A dust cloud of thickness R is deployed in polar orbit at the upper edge of the debris band (beige). The dust orbit decays due to natural drag and sweeps the debris population below it. The descent rates of the dust and debris can be synchronized. Below X0 natural atmospheric drag is sufficient to force debris reentry. (b) Polar view of debris orbits spread around the earth (Source: NASA). Partial dust shell in polar orbit is shown schematically in red. White arrows show typical debris orbits whose Right Ascension of Ascending Node (RAAN) differs by 180 degrees. Half of the debris population counter-rotates with respect to dust while crossing the dust shell.
debris mass and velocity; C D is the coefficient of atmospheric drag; vA is neutral atmosphere velocity; ρ A is the mass density of the atmosphere; and md , vd , and nd , are the mass, velocity, and number density of dust grains. A is the average debris area exposed to dust drag and A represents the characteristic dimension. Since the debris mass M = ρ Al , where ρ and l are debris mass density and
thickness, B = ρ l l is independent of the characteristic debris dimension. Small debris may be a 10 x10 cm broken piece of aluminum satellite structure ~2 mm thick with ρ ~ 2.7 g/cc weighing weighing about 50 g with B ~ 5 kg/m2. The kinetic energy of such debris at 10–15 km/s is 2.5–5 MJ, which is comparable to the explosive power of 1 kg of TNT. The collision of a satellite with such small debris could be fatal and a source for secondary small debris [5]. Since there are at least an order of magnitude more small debris objects than satellites, the collision frequency of small debris with a satellite is proportionately higher than collisions between satellites. Thus, even even smaller debris population can be a source for collisional cascade or the “Kessler syndrome” [3]. The drag magnification factor in Eq. (1) is κ = (1 + 1 + f )
, where f = – 1 implies inelastic collision, f = 0 implies elastic collision, and f > 0 implies loss of debris mass as ejecta resulting from hypervelocity impacts. Maximum drag is achieved when the relative velocity between dust and debris, V – vd = 2V where V ~ 7.5 km/s is the orbital speed. At such high relative speeds the impact of tungsten grains will generate Mbar (10 11 Pa) range shock waves in the debris, resulting in evaporation, melting, fragmentation of the debris material in microcraters, and formation of ejecta from its surface. This increases the drag force by a factor of κ . The debris mass that evaporates and melts is fmd . The specific kinetic energy of tungsten grains at 15 km/s is 110 kJ/g. Assuming the debris is aluminum, the specific heat of melting is ~0.35 kJ/g. An ejecta mass of 300md can be formed by melting, corresponding to f = 300 and κ ~ 18. A similar estimate with 7.5 km/s impact velocity gives κ ~ 10. These are representative estimates of the range of possible values of κ , since micro fragmentation around the microcrater can lead to additional ejecta mass not considered here. It is necessary to conduct laboratory experiments to determine accurate values of κ . Orbital Concept
At altitudes of 900–1100 km and at higher inclinations, where the debris population is large, atmospheric drag on the debris is negligible and their orbital lifetime is long. To reduce their lifetime we artificially enhance the drag on the debris by dust injection. However, the atmospheric drag on 30–70 μm diameter dust grains is not negligible, so the dust orbit will naturally decay. The dust orbit decay rate depends on the grain size and mass density (see Eq. (3)) and, to a certain extent, can be controlled. We exploit this by injecting a narrow dust layer of width Δ R, which is smaller than the altitude interval δ R (Fig. 1a) to be cleared, and synchronizing the rate of descent of the debris and the dust as described below. R is the debris distance from the center of Earth. As the dust descends in altitude due to atmospheric drag, it sweeps the small debris until a sufficiently low altitude (X0) is reached, below which the natural drag is R, the volume enough to force debris reentry. Since Δ R << δ of dust is much less than the volume to be cleared of debris,
and the dust mass to be transported to orbit can be kept to a minimum. A small Δ R ~ 30–50 km allows for the option to maneuver active satellites to avoid prolonged contact with the injected dust. Neglecting the second term from Eq. (1), the order of magnitude of total dust mass M d d may be estimated as; Δ Rδ R
M d =
4κ NC
B=
ΔRT
4κ C
⎛ δ R ⎞ ⎟ ⎝ NT ⎠
B⎜
(2)
where N is the number of revolutions of the debris fragments in orbit before reentry, T is the period of one R / NT ) is the rate of debris revolution (~ 90 min in LEO), (δ descent due to induced drag. C ~ 0.5–1.0 is a correction factor due to orbital geometry and is assumed to be 0.5 for R / 2 R. this estimate. In deriving (2) we have used ΔV / V = δ Eq. (2) indicates that M d d is a ‘trade’ between various parameters to be chosen as warranted by the mission objectives. For example, an estimated M d d of 20 tons (about one cubic meter of tungsten) is necessary to lower the orbit altitudes of all B ≤ 5 kg/m2 debris from 1100 km to below 900 km in 10 years by releasing tungsten dust in a layer of width Δ R ~ 30 km at 1100 km. The dust may be injected in one or several installments using excess launch capacity to be cost effective. Based on analysis discussed earlier we used κ = 18. From Eq. (2) it can be gleaned that M d d depends sensitively on κ . Determination of κ and its scaling with relative velocity through laboratory measurements is necessary. The rate of dust orbit decay, assuming vd >> v A, can be obtained from Eq. (1) by neglecting the first term as dR dt
=
2 R ρ A ( R) vd
ρ d d
(3)
where ρ d d and d are dust mass density and dimension and C D d R NT NT = 2. By choosing appropriate ρ d and we can make / δ d ~ dR/dt which synchronizes the decay rates of the debris and dust orbits to realize the dust sweeping ‘snow plow’ effect. Dust Orbit Analysis
Orbit analysis using silicon and tungsten dust of a variety of sizes from 1 to 100 μ m has been performed. The solar radiation pressure introduces a spatial spread to the dust orbit. These calculations suggest that 30–70 μ m tungsten dust is optimal for small debris elimination. The orbital lifetime of 60 μ m diameter tungsten grains (Fig. 2a) in circular polar orbit injected at an altitude of 1100 km is about 15 years. The average radial spread of the dust orbit due to radiation pressure pressure is about 30–50 km. Above 600 km the rate of decay of the dust is about 20 km/year so that dust is resident on a given given altitude for about two years. years. Dust released in a polar circular orbit will remain in an approximately circular polar orbit during its descent and will deviate from its initial inertial longitude by only a few degrees (Fig. 2b). A more detailed analysis analysis of the geometry
(a)
(b)
maintained on orbit. By using anti-ballistic missile technology, it is already possible to rapidly respond to orbital debris incidents using ground-based rockets. The major advantage of rapid response is that debris fragments will still be localized over a small volume, and a much lower dust mass can eliminate them from orbit with just one pass through a dust cloud. Another advantage is that since the dust is deployed on a ballistic trajectory, it is resident in space for only a few minutes. Use of tungsten dust offers the benefit of large κ on hypervelocity impact and hence enhanced efficiency over other suggested forms of drag enhancing agents, such as water mist. Large κ and high mass density of tungsten implies lower mass and volume to be transported. When matured, this technique may be used to prevent collisional cascade in the aftermath of collision between two large objects, thereby preventing the onset of the Kessler Syndrome. This concept is schematically shown in Fig. 3. For the suborbital deployment the mass of dust is given by M d =
δ R Ad B V R 2κ V rel
(4)
where Ad is the cross sectional area of the dust cloud to be
Figure 2. (a) Altitude versus time for a 60 m tungsten dust grain in a circular polar orbit initiated at an alitude of 1100 km . (b) Altitude versus the longitude of the ascending node. The dust layer remains confined in altitude and azimuth through out its orbital lifetime. li fetime.
of the dust cloud will be investigated in the future. To analyze the orbits of dust grains in LEO we consider the following accelerations: (1) gravity including the Earth’s oblateness up to the term J2, (2) solar radiation pressure including the shadow of the Earth, (3) atmospheric drag using a spherically symmetric, rigidly rotating US standard atmospheric model, and using a dimensionless drag coefficient Cd=2. We integrate the equations of motion numerically using a standard Runge-Kutta method. In the future we will investigate the effects of higher order gravity terms, lunar and solar gravitational perturbations, electrodynamic forces due to charge accumulation on the dust grains, and more accurate atmospheric models including solar cycle effects. Suborbital Concept
It is conceivable that improvements in space situational awareness will allow anticipation of satellite fragmentation or detection soon after the event. Dust may be deployed in such cases by either ready-to-launch ballistic rockets or dust-carrying satellites with high ΔV capability strategically
deployed which must be large enough to engulf the debris cluster, δ R is the desired altitude change of the debris, V is the orbital velocity of the debris and V rel is the relative velocity between the debris and the dust. We will estimate the dust mass, Md, necessary to force debris reentry within one revolution by assuming that the fragmentation has occurred occurred at an altitude of about 1000 km and δ R ~ 400 km. Below 600 km atmospheric drag is sufficiently strong to decay the debris orbit rapidly. 2 Ballistically deployed dust will accelerate at g eff eff = g – v h / R 2 where g is the gravitational acceleration and v h / R is the centripetal acceleration due to the horizontal velocity vh of the dust imparted by the rocket. By designing release with large vh the linger time of the dust cloud in space can be made long enough to allow all of the targeted debris to fly through the falling dust cloud. In addition, the larger vh results in larger κ and higher efficiency. For example, considering a low energy fragmentation [5] at 900 km, with fragment velocity spread of ΔV ~ 10 m/s, one revolution after a collision the debris field would spread out 180 km along track, 300 meters in altitude, and 100 meters out of the orbital plane. If vh = 4 km/s the dust cloud would interact with the whole debris field for about 16 seconds at a relative velocity of 11.5 km/s, and the resulting free-fall distance would correspond to the radial spread of the debris field. The necessary dust mass to change B < 6 debris altitude by > 400 km is only 180 kg for κ = 18. However, with subsequent revolutions the size of the debris field increases both along track and radially during its passage through the fragmentation point; the required dust mass is therefore larger if deployed long after fragmentation.
In both Orbital and Suborbital dust based ADR options, the
to sufficiently enhance drag on the debris with the minimum amount of deployed dust and on the ability to dispense the dust in space with a large dust/debris relative speed. Knowledge of κ and its dependence on relative velocity is critical for designing and analyzing such missions.
3. UNIQUENESS OF DUST-BASED ADR S ADR SYSTEM For the Orbital dust ADR system, it is possible to selectively remove small debris by orbital dust injection to artificially enhance the natural drag process, for a limited duration over a limited region of space (Fig. 1). Since the ballistic coefficient is independent of the characteristic debris size a small value of B ~ 5 kg/m2 represents a majority of small debris in a variety of sizes. Thus the injected dust mass can selectively remove the small debris without significantly perturbing the orbits of active satellites with much larger B. The natural drag on small debris with low ballistic coefficients, e.g., B ~ 5 kg/m2, is negligible above 900 km, but atmospheric drag on micron scale dust is sufficient to decay the orbit of dust at a significant rate. This offers a unique opportunity to synchronize the rates of orbit decay of the injected dust and debris to create a sweeping effect on the debris by a descending narrow dust layer. Consequently, the necessary dust mass is significantly lowered making the dust based ADR technique practical and cost effective. The suborbital dust dust ADR system system is capable of removing small (as well as larger) debris very rapidly with less dust mass and does not inject dust in orbit. Neither of the concepts require exotic technology development or expensive orbital infrastructure, can be implemented with off-the-shelf-type technology, and are achievable in the near-term. ISKS AND POSSIBLE MITIGATION 4. SYSTEM R ISKS
Natural & Artificial Artificial Dust Environment
Fig. 3 Schematic illustration of the ballistic dust deployment. (a) A ballistic rocket is used to release dust in the path of a debris fragments. fragments. (b) The debris population is engulfed by the dust cloud. Debris experiences enhanced drag due to the dust. (c) This results in the loss of debris altitude. The debris population descends to an altitude of x0 km within one debris revolution time below which earth’s natural drag is sufficient to force reentry within a desired time. The dust cloud cloud also descends under gravity and reenters atmosphere.
efficiency and economy of the system depends on the ability
About 100 tons of cosmic dust is introduced daily in the earth’s environment naturally in the form of micrometeorites [9-10]. Average micrometeorite micrometeorite velocity in near earth space is 20 km/s [11] and the mass distribution peaks at 1.5 x 10-5 grams (200 μm in diameter) [9]. In addition to this natural source, human space activity also introduces large large quantities of dust in space space regularly. For example, more than 60 solid rocket motors (SRMs) were fired in orbit by the US from 2000 to 2009. This number only includes solid rocket motors that were already in earth orbit when activated. More than 30 percent of SRM exhaust is in the form of Aluminum Oxide (Al2O3) dust. These dust particles are generally thought to be in t he 0.1 to 100 micron size range [12 - 14]. The flux of the Al2O3 dust resulting from just one SRM burn can exceed the natural micrometeoroid flux in LEO [13]. The total mass of aluminum oxide dust created on orbit during 2000 to 2009 by the US alo ne is approximately 43 metric tons. Hence, the deployment of 20-40 tons of micron sized dust (for an ADR system) in orbit would not be unprecedented. Considering the routine and accepted use of SRMs on orbit over the course of the space age, a one-time deployment of a narrow
band of dust to remediate the deadly small orbital debris may not be unreasonable considering the substantial benefits. Spacecraft are already designed to operate in the dusty space environment. There are a number of possible mitigations for this artificial dust flux, starting with operational concepts for active spacecraft to completely avoid it. Operational Mitigations
In the suborbital deployment concept, no dust will be injected in orbit. The dust will follow a ballistic trajectory and will descend back to earth under gravity within minutes. Hence, there is an extremely low probability of conjunction between the ballistic dust cloud and operational spacecraft and there are no long term risks whatsoever to space assets. While this is a major advantage of the suborbital dust deployment for ADR, the procedure is not practical for eliminating debris that has already spread around the earth. To mitigate this category of debris, dust must be deployed in orbit albeit in a limited region of space and for a limited duration. The NRL ADR concept involves deploying a tungsten dust layer of a limited thickness, perhaps 30 to 50 km. The most optimal mitigation for the possible effects of an orbital dust deployment is to simply maneuver operational spacecraft above or below the descending dust layer and avoid the artificial dust flux altogether. While not every LEO satellite currently has onboard propulsion capability, according to background information that was published as part of a House of Representatives Hearing on 28 April 2009 concerning orbital debris, the vast majority of current operational spacecraft are maneuverable. The Congressional record states that there are approximately 900 operational spacecraft currently in orbit and of those, approximately 800 are maneuverable [15]. A 50 km orbit raise in LEO LEO using hydrazine can be accomplished for a few percent in increase in spacecraft mass. Any new space system would require years to be programmed, developed, and launched. This T his is a key fact in the concern over adversely affecting functional satellites with the proposed ADR. While some GEO satellites have design lives of a decade or more, this is not common for LEO spacecraft. Thus, it is not the current generation of satellites that would be affected by an ADR based on the NRL drag enhancement concept, rather it would be the next generation of LEO satellites that would potentially be affected by the injected tungsten dust. The next generation of LEO satellites could be designed and built with additional resistance to micron sized impacts and/or features to avoid the tungsten dust flux such as onboard propulsion. It should also be noted that the injected tungsten dust is no longer a significant concern to operational spacecraft below an altitude of about 600 km because once at that altitude, the tungsten dust orbital lifetime would be very brief (see Fig. 2a) and any interaction time with operational spacecraft below 600 km (including the International Space Station) would be minimal. Avoidance of the injected dust flux
through operational implementation is the most desirable mitigation strategy for possible effects on operational spacecraft. If this cannot be achieved, then there are a number of spacecraft design features that can be considered to mitigate possible negative effects. Spacecraft Design Mitigations
Satellites have been redesigned to protect critical components from micrometeoroids and orbital debris damage by moving critical components from exterior surfaces to deep inside a satellites structure [2]. The primary technique t echnique for meteoroid protection is placement of multi-layer insulation (MLI) blankets on critical areas of spacecraft [16]. The primary purpose for MLI is thermal control, but in its function as a micrometeoroid shield, it also serves to break up a projectile before it strikes the exterior structural wall wall of a spacecraft. spacecraft. For very very thin materials such as the films that are used for MLI, hypervelocity particles tend to cause perforations in the outer layer(s) that are only slightly larger than the impactor diameter [17]. Results from the NASA Long Duration Exposure Facility (LDEF) that spent 69 months in LEO indicate that the surface damage from micrometeoroids/space debris does not significantly affect the overall surface optical thermal physical properties [18]. Some parts of spacecraft cannot be moved deep inside structure or completely covered with shielding. Optical sensors, solar arrays, and thermal radiators are the most obvious features that cannot simply be covered up to preclude potential damage from dust. d ust. Optical sensors (both payloads and spacecraft bus attitude control sensors) are typically baffled and shielded against light and their detectors located deep within the device. The baffles function as debris shields and it should be noted that most instruments on LEO spacecraft either point toward the earth, or generally toward zenith. The highest flux of injected tungsten dust would approach an active spacecraft from ram, a direction that sensors seldom point. Previous studies of hypervelocity impacts on optical substrates and mirrors has found that transmissivity and reflectivity are relatively unaffected by the impacts [17]. Thermal radiator surfaces on a spacecraft are typically structurally robust due to the requirement for them to conduct as well as dissipate waste heat. The potential for degradation of surface properties on thermal radiators can be addressed by the typical method of providing area margin and biasing toward the lower end of temperature operating range at beginning of life. LDEF results show that hypervelocity impacts did not compromise the thermal-optical properties of the silver-Teflon material that is commonly used on the exterior surface of thermal radiators in LEO [17]. Solar arrays are a particular concern for dust impacts because of their large and exposed area. However, micrometeroids and orbital debris impacts appear to minimally affect solar cell electrical electrical properties. [19]. LDEF results show that the standard coverglass on solar cells provides good protection against micrometeoroid/space
debris. Microcraters, cracking, and even penetrations of the coverglass only have a local effect [18]. Solar arrays were returned to earth after years of exposure to the LEO debris environment on the Hubble Space Telescope. Despite 2,700 impact sites in the 100 micron and above size range, HST solar cells continued to function without impact attributed power loss [20]. Solar array cell-to-cell interconnects and wire harnesses are particularly vulnerable to micrometeoroids and orbital debris, because unlike the vast majority of other wiring on a spacecraft, they are not located inside a structure. However, the physically separated redundant electrical paths that are a standard feature of interconnect design can all but eliminate local damage failures [18]. Thus we can can see that that good design practices result in spacecraft that appear to be tolerant to micron sized dust impacts, even for long duration exposures in LEO. How effective these practices will be for tungsten dust at the flux levels required for the ADR system needs evaluation. Most importantly however, when collision with debris becomes inevitable due to a collisional cascade, the alternative to LEO debris mitigation is loss of operational use of the debris contaminated altitudes. In this situation, the small one-time modifications that may be required for future operational spacecraft to accommodate and/or avoid the artificial dust cloud during ADR would be minor compared to complete loss of satellite functionality due to debris impact.
5. SUMMARY We described a technique for selectively eliminating small debris from the low earth orbits. We studied the case where where satellite fragmentations have occurred long before dust is deployed and the debris fragments have already spread around the earth as well as the case of immediate aftermath of a satellite satellite fragmentation. The system efficiency depends on the value of the momentum boost factor κ . Further research is necessary to accurately determine the value of κ . In addition, more detailed orbit analysis for both dust and debris is required. The debris fragment fragment created by collision of objects is likely to rotate. Hence the effective area area exposed to the dust flow flow is averaged. averaged. This will result in correction to the ballistic coefficient. Here we ignored this correction for simplicity. We also analyzed the possible risks of injecting dust and suggested their mitigation. Our preliminary analysis indicates that the dust based ADR systems could be a cost effective means to clear the deadly small debris and the risks are are manageable. An operational system based on the concept described here can be developed with off-the-self type technologies in the near term.
Acknowledgements This work is supported by the Naval Research Laboratory base program. Stimulating discussions with Sasha Velikovych and Mihaly Hornyi are acknowledged.
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Gurudas Ganguli is the Head of the Space Analysis and Application Section at the Plasma Physics Division, Naval Research Laboratory, Washington DC. He received his B.Sc. B.Sc. in physics from St. Xavier's College, Ahmedabad, India in 1974 and Ph.D. in Physics from Boston College, Boston, MA in 1980. His research interests concern the study of turbulent and coherent processes in collisional and collisionless plasmas, dusty plasmas, and their applications to to both space space and laboratory. laboratory. One of his current research focuses is mitigation of small orbital debris. He is a Fellow of the American Physical Society and member of the American Physical Society, American Geophysical Union, and International Union of Radio Science. Chris Crabtree received his Ph.D. in theoretical plasma physics from the University of Texas at Austin in 2003. From there he went to the University of California in Irvine as a Department of Energy Fusion Postdoctoral Fellow to study the plasma physics of the near earth space environment. After two years in Irvine he moved to the Laboratory for Nuclear Science at the Massachusetts Institute of Technology to study magnetically confined fusion relevant plasmas plasmas and astrophysical astrophysical plasmas. plasmas. Finally he came to the Naval Research Laboratory in 2009 to work on many topics including the physics of the radiation belts, solar wind, and orbital debris. Leonid Rudakov received his Masters degree from Moscow Institute of Physics and Engineering in 1956 and began his career in Kurchatov Institute of Atomic Energy, where he remained till 1998. He received his Ph.D. in physics in 1961. He was the Head of the Applied Physics Department of the Russian Research Center “Kurchatov Institute” for more than 25 years, as well as professor of physics of the Moscow Institute of Physics and Technology. He is recipient of the Russian State Prize in 1981 and 1987, and the Einstein Fellowship in Weizmann Institute, Israel. He has been a consultant in the development of the bright X-ray sources for the inertial confinement fusion program at Univ. of Nevada, Reno and Sandia National Lab. His current research interest is the physics of the plasma turbulence in laboratory and space.
Scott Chappie received a B.S in Mechanical Engineering from the University of Maryland in 1991, and an M.E. in Mechanical Engineering from Johns Hopkins University in 1996. He is the head of the Mission Integration and Development Section. He served in the Spacecraft Engineering Department at the Naval Naval Research Laboratory since 1991 in a variety of positions including Integration & Test Lead Engineer, System Engineer, Mechanical Systems Lead Engineer, and Program Manager.
