(BEING CONTINUED FROM 16/10/14)
Active Removal of Space Debris
Expanding foam application for active debris removal
Controlling the amount of space debris is widely recognized as an important task to maintaining a sustainable space access for the decades to come. This is mainly due to the high risk of collisions that can easily invalidate both human and robotic mission. The topic is on the agenda of the Scientific and Technical Subcommittee and coordinated between space agencies in the Inter-Agency Space Debris Coordination Committee (www.iadc-online.org). Most current efforts focus on debris mitigation methods.
The Active Space Debris Removal System proposed in this study is based on an expanding foam system. The core idea of this method is to increase the area-to-mass ratio of these objects such that the atmospheric drag can cause their natural re-entry, thus “cleaning up” different regions in the near-Earth space. The drag augmentation system proposed does not require any docking system and just an uncontrolled re-entry can follow, thus it seems a short-term application free from the usual technological issues of these debris removal systems.
The drag augmentation is suggested to be performed exploiting the characteristics of the expanding foams that can nucleate almost spherical envelopes around the debris with very limited efforts of the spacecraft in charge that has the role to carry and spray the foam. Furthermore, the same method can also be conceived as a preventive system to be directly embedded in future space artificial satellites. The key technological aspect is the specific foam kind that has to be able, amongst other aspects, to significantly expand its original volume and has to be as light as possible.
This approach is demonstrated to be able to deorbit any kind of debris, but it has been proven to be particularly advantageous to deorbit up to 1 ton debris within 25 years from 900 km, of course the worst case. The actual scenario performance heavily depends on the specific foam considered and its characteristics. This study provides an approach to the drag augmentation methods identifying the foam ball radius assuring the best compromise between deorbiting time and impact probability for each debris. A brief review of the state of the art of foam technology for space and ground based
applications is presented to frame the scenario into realistic perspectives. From this, a low order foam expansion model is developed and implemented in order to provide the relevant foam characteristics to the mission analysis section.
In this study, conservative assumptions, rather close to the state of the art of ground based foams,have been considered. Polymeric foams are chosen as the most suitable candidate to implement the proposed method. The expansion of this kind of foam is modelled considering the internal-external pressure difference and foam viscosity. The time evolution of the radius in a single-bubble model is derived and implemented for a set of external pressures up to vacuum conditions. The main outcome of this investigation are the foam expansion model and its density.
Assuming a density of 1 kg/m3, the mission analysis section is implemented. A 5 kW Hall effect thruster is supposed to realize the transfers among different debris. Three debris lists covering a broad range of possible masses and initial altitudes are considered as set of targets to remove. The removal order is defined according to the minimum estimated low thrust velocity change to move from a debris to another in the list. In order to avoid assuming a precise mission starting date, a medium atmospheric density model is here used. It is a static model intended to be representative for long deorbiting times, like the ones resulting from many of the considered debris, where many solar cycles are completed. In this way the active removal missions designed are able to deorbit
about 3 tons per year corresponding to several space debris.
Together with the mission analysis, also the system configuration of the spacecraft in charge is addressed with special emphasis to the foam nucleating and ejection system. The spacecraft is sized according to the performance of a medium class launcher (like Soyuz) into a Sun-synchronous orbit. Its initial mass is close to 5 tons where approximately 1 ton is allocated for the on board equipment. This dry mass is a-posteriori verified by a rough mass and power budget for all the main subsystems. The remaining mass is allocated for the electric thruster propellant and for the foam in different proportion according with the specific mission.
Some foam nucleating options are proposed both based on a single and on multiple spacecrafts. A decision matrix is implemented in order to select the most suitable candidate among these. A foam ejection nozzle is considered as the baseline solution and it is roughly sized according to existing static mixers.
Some assumptions are applied in order to obtain preliminary results about the plausibility and reliability of the proposed approach. These are mainly about debris mass, acceptable deorbiting time, suitable foam characteristics and reasonable general mission architecture. Finally, also the main hazards related with this scenario are outlined in order to sketch a rough estimation of the complete mission.
Space debris are one of the main threats for an affordable and safe space exploration and
exploitation. Space debris are mostly concentrated in the near-Earth space region, in particular in the Low Earth Orbit (LEO) and Geostationary Earth Orbit (GEO) regions. This waste is composed of spent boost stages, collision fragments, ISS construction material, human discards and so on .
Between 1957 and 2008, approximately 4600 launches have placed some 6000 satellites into orbit.
Among these, about 400 were launched beyond Earth into interplanetary trajectories, but of the remaining ones only about 800 are operational. This means that roughly 85% of space objects belong to the uncontrolled satellite class, namely dead spacecrafts. To these, also launcher upper stages have to be added in order to have a rough idea of the large debris population. Furthermore,adding also smaller debris caused by explosions, fragmentations, collisions, accidental discharge and similar events, the whole debris population comprises millions of objects . Space debris are not uniformly distributed on the whole space, indeed they move into the more common launchtarget
regions, in particular in the LEO and GEO regions, as shown in Fig. 1.
Figure 1: Space debris population in the GEO (left) and LEO (right) regions.
Taking into account also very small objects, there are, besides paint flakes (particles with size below 1e-4 m), at least three non-fragmentation debris sources deserving particular attention .
These sources put in space small particles, but due to the high relative velocities, the threat they represent in case of a possible impact, although not catastrophic, is however not negligible:
• More than 1000 solid rocket motors release micrometre-sized dust and mm-/cm-sized slag
particles of aluminium oxide (Al2O3).
• At the end of operational life of the Russian RORSATs mission (Radar Ocean
Reconnaissance Satellites) in the 1980s, droplets of coolant liquid (a low-melting sodium
potassium alloy), used in the nuclear reactor cores, were released into space.
• Finally, also the release of thin copper wires from radio communication experiment during the MIDAS missions in the 1960s contributes to the increasing of these very small, but hazardous, particles .
It is worth stressing that these objects are too small to be deorbited with the conceived foam-based method assessed in this study.
It is estimated that approximately 50% of all traceable objects are due to in-orbit explosions or collisions . Moreover, only statistical break-up models are available for these events and the actual debris resulting from such an event can cover a quite spread region. The threat represented by these objects is further increased by their high velocities. Up to 52000 km/h can be reached and at this velocity even a nail could cause significant damages, even catastrophic, to operation satellites.
To give an idea of the threat represented by space debris, it is sufficient to think that the ISS has to perform occasionally collision avoidance manoeuvres and over 80 windows of the Space Shuttle had been replaced during the program lifetime.
1.1 Active Debris Removal
Recent studies demonstrated that the problem of space debris is slowly becoming more and more important for future use of the outer space . Many simulations suggest that the number of objects in orbit might grow, even when no further objects are added to space, due to collisions caused by fragments generated by other collisions [5,6,7]. This collisional cascading may potentially lead to a chain reaction situation, with no further possibility of human intervention and with a substantial increase of the hazard level for space operations .
This feedback collision effect has been highlighted for the first time in 1978 by Kessler and Cour-Palais  and has become popular as Kessler syndrome even without ever having had a strict definition. Recently, Kessler itself has concluded that there is little doubt that the so called Kessler syndrome is a significant source of future debris, stressing at the same time that, even if the growth of orbital debris has slowed, still we are not capable of preventing the growth in the debris population from random collisions .
Figure 2  shows the catalogued population of object in space in the last 54 years. It can be noticed the high presence of fragmentation debris, compared to the number of spacecrafts or rocket bodies. In fact, several studies found that “derelict spacecraft and orbital stages now outnumber active spacecrafts by more than 5 to 1” .
It is possible to classify the growth evolution into three main phases :
• 1960-1996 during which the growth is almost linear at a rate of 260 debris per years,
• 1996-2006 during which the growth is still almost linear, probably due to implementation of debris mitigation guidelines,• 2006-2010 during which two impact events created more than 1250 debris per year.
At this point even an almost full compliance with IADC (Inter-Agency Space Debris Coordinating Committee) Guidelines  should not be sufficient forcing space agencies to agree about the retrieval of a number of objects that are already in orbit. In order to solve this problem, a number of active debris removal concepts have been described, such as: electromagnetic methods, momentum exchange methods, remote methods, capture methods and modification of material properties or change of material state .
In order to face this situation, a good understanding of the orbital debris problem is binding. As a matter of fact, the distribution of the present debris in terms of mass and spatial density is an important factor to decide in which way their removal should be addressed. In order to estimate the population of debris, i.e. their position and physical characteristics, and to have an idea of the future environment, several modelling program were implemented in the 1970s . The EVOLVE modelling series  is the first of its kind exploiting Monte Carlo processing to estimate future fragmentations. Furthermore, EVOLVE models include also useful classification for type of impact:
intact-on-intact, intact-on-fragment, fragment-on-fragment.
Another model, implemented in the end of 80s, was the NASA90  able to derive curve fits of debris environment. The principal characteristic of this model is that it can be implemented with semi-analytical relations, see Sec. 2. The last NASA series are LEGEND and ORDEM .
The ORDEM96, besides implementing curve fit methods, is the first model able to characterize the debris population by altitude, eccentricity, inclination, and size.
The spatial density distribution for relevant debris is shown in Fig. 3  at altitudes between LEO and GEO. It is possible to divide the spatial density in percentages. More than 70% of the objects,in fact, are in LEO, about 20% are in intermediate highly eccentric and Medium Earth Orbits (MEO from 12846 km to 33786 km) and less than 10% are in near-geostationary orbits . It is estimated that the peak of spatial density of objects is in LEO, from 500 up to 2000 km of altitude. A second smaller peak is recognizable in the GEO region around 36000 km altitude.
More in detail, as we can notice in Fig. 3, there are two significant peaks in LEO at altitudes of 800-1000 km, and around 1400 km. Furthermore, peaks in the latitude distribution can be observed between 65 and 82 deg . In the LEO region, the probability of collision with traceable debris,according to NASA ORDEM model, is 8e-3 per year for a satellite with 10 m2 cross-section. This relatively high probability is due to the high debris density and their significant average velocities.
The probability of collisions in GEO, instead, is smaller, between 3e-6 and 3e-7. This smaller probability is due to the limited number of the debris, their large spatial distribution and the lower average relative velocities. In GEO there are many critical and commercial payloads, generally larger and more expensive than LEO satellites. This region is much harder to access than the LEO one and there are not energy dissipating orbital perturbations, like atmospheric drag. For this reason, there is a high priority to remove debris in GEO. It is since now worth mentioning,however, that the proposed method rely on the drag augmentation idea, thus it can not be applied in the GEO region.
An object can be tracked only if its size is larger than a given threshold. In order to define this threshold it is possible to classify space debris in three categories: small, medium and large. Debris less than 5 mm are catalogued as small and are considered non-traceable, debris between 5 mm and 10 cm are medium, again non-traceable and debris larger than 10 cm are catalogued as large. The large debris are usually traceable . In Tab. 1 the number and the dangerousness of space debris tracked in LEO is summarized :
In this catastrophic scenario, it is clear that active removal is a necessary way to control and reduce
debris growth, in order to permit easier future space activities.
(TO BE CONTINUED)
Authors: M. Andrenucci, P. Pergola, A. Ruggiero /2011
Affiliation: University of Pisa – Aerospace Engineering Department – Italy
ACT researcher(s): J. Olympio, L. Summerer
University of Pisa
Advanced Concepts Team