(BEING CONTINUED FROM 30/06/18)
3 SPACE DEBRIS POPULATION
In general, the space debris environment, differently from the meteoroid one, is composed of manmade objects. Due to their origin, most of these debris have the following general properties : • their flight direction is almost parallel to Earth surface, • different altitudes and inclinations suffer of different impact probabilities (see Sec. 2), • the debris environment is in continuous time evolution (actually increasing in number), • many of them have almost circular orbits. This last characteristic is mainly due to the atmospheric drag that tends to decrease the orbital semimajor axis and reduces the orbital eccentricity . The most hazardous debris are not simply the largest ones, but the risk they represent also heavily depends from their specific orbit and their orbital lifetime. Of course, the longer their lifetime, the larger the impact probability, thus the larger is the risk of further growth in the debris orbital population. Moreover, there are specific orbits, like Sun-synchronous Orbits (SSO) and GEO, more crowded than others, thus an uncontrolled object into one of these regions represents a significant threat for any operative satellite, eventually producing further debris. Of course, the lower the altitude of these objects, the later their removal should be addressed and the same holds for the ones with smaller cross-sectional areas. These two situations are the ones limiting the hazards from random collisions due to their generally short lifetimes.
The goal of this section is to present the set of debris lists considered in this study in order to assess the performance of the deorbiting scenario proposed. In the following, besides introducing the three debris lists here considered, also a possible ranking of their dangerousness, according to the previous considerations on impact probability and lifetime is presented. Finally, this section aims also to identify a suitable set of target debris for the proposed foam-based active debris removal system, intended as a class with a given range of physical characteristics and particularly hazardous orbits. 3.1 Space Debris Lists Space debris lists rarely are open database and the exact number and nature of tracked objects is often covered by military intelligence. Space agencies and few other organizations worldwide have access to these lists and often reduced versions can be provided for research and educational purposes. In this study three lists are considered: • Filtered DISCOS list: This list is based on the ESA’s DISCOS database. The Database and Information System Characterizing Objects in Space (DISCOS) is a reference source for launches, orbits and general mission descriptions of more than 33500 tracked objects . The whole database includes more than 7.4 million orbit records in total, based on the US NORAD database. Furthermore, in the same database, the US Space Surveillance Network (SSN) constantly uploads orbital data of many tracked but unclassified objects. DISCOS can be used by means of both standard database queries and automatic generation reports (ESA Register of Space Objects, ESA GEO Log, ESA Fragmentation Events Log) . Based on this list, the ESA/ESOC orbital debris section, provided a filtered catalogue of space debris for this study. The list is composed of only 59 objects. These objects are the result of some sequential filters applied to the whole DISCOS list. It has been chosen, in this case, to query the database according to: o latest orbital element, considering only objects with perigee altitude above 700 km and apogee altitude below 900 km (at November 2010). Furthermore, also the eccentricity has been limited to 0.001 and no filter on inclination has been applied. o physical properties, considering only objects with an average cross sectional area larger than 1 m 2 and debris mass larger than 500 kg. o launch date, considering only objects launched before 2000 and rocket bodies (regardless of the launch date). Only one of the 78 Iridium satellites has been considered and it is worth saying that, as described in Sec. 10, this choice could lead to unfavourable results. Indeed, it gives an underperforming mission scenario as time and propellant mass required to move from one debris to the next depend on the distribution of debris considered.
The exclusion of many satellites, one close the other, increases these figures, reducing the number of debris deorbited per year and accordingly the average mass. The whole list of these objects resulting from the DISCOS database is summarized in Tab. 4. Here the average cross section (AREA, m 2 ), mass (MASS, kg), semi-major axis (SMA, km), eccentricity (ECC, -), inclination (INC, deg), right ascension of the ascending node (RAAN, deg), argument of perigee (ARGP, deg) are given. From these values also the areato-mass ratio (A/M m 2 /kg) and orbital altitude (ALT, km) are computed and shown in the two rightmost columns of Tab. 4.
Table 4: Orbital elements and physical properties of debris of the filtered DISCOS list.
It is worth noting that the cross sectional area of some objects was not available. For these (the ones with the red AREA value in Tab. 4), it has been computed weighing the average area of all objects with the object mass with respect to the average list mass. A further filter on this list can be easily imposed ranking the objects according to their mass. As shown in Fig. 10, imposing a filter at 5000 kg allows to discharge only 9 objects, but it limits the actual maximum mass to slightly more than 3000 kg. In this way the heavier objects are not considered as we are assuming that dedicated missions or different approaches could be implemented. It is interesting to observe that these heavier objects actually are launcher stages, a class of space junk deserving particular attention. Finally, it is worth stressing that these objects are not the most dangerous ones as they are enough large and well tracked. Indeed, an operational satellite can always foresee a collision avoidance manoeuvre or design a non-interception trajectory with the tank. Thus, it is reasonable to assume that a foam-based method is actually not the best choice for such debris given their high mass and well known nature that can lead to more tailored solutions for their removal.
Figure 10: Mass distribution of the DISCOS list. The black horizontal line shows the additional filter applied at 5 tons.
• Proprietary SSO list: While the first list aims to have a broad coverage of relatively large space debris on the LEO region, the second one is based on more stringent filters in terms of orbital parameters. According to , there are at least three critical regions (see Sec. 1) to be cleaned up before others. The region here considered is the one around SSO orbits. This region contains already several uncontrolled objects and it is still today one of the most important regions for commercial and scientific purposes. Accordingly, we will focus on the Sun-synchronous region limiting the orbital parameters of the objects as follows :
o Orbital altitude between 600 and 900 km
o Inclination between 97 and 100 deg
o Eccentricity smaller than 0.035
Besides these constraints, only objects with masses larger than 1 kg are considered. In this way, the object list results composed of 140 objects (covered by non-disclosure agreement) with masses up to 8225 kg. It is worth stressing, one more time, that the heaviest objects are launcher tank. These, although lighter than the ones found in the DISCOS list, are here considered. On the contrary, the lightest objects of this list are small CubeSats (4 objects) and also these are kept into account. This list is more crowded than the previous one in terms of number of objects and also the mass distribution is broader.
It is worth recalling, at this point, that the initial debris mass heavily influences the foamed area-to-mass ratio, thus the heavier the object, the larger the foam mass required to deorbit it. In the considered list, half of the objects has a mass smaller than 500 kg, the 21.4% is in the range of 500-1000 kg and the remaining are mostly concentrated in 1500-2000 kg with just 12 objects above 2000 kg. This analysis is summarized in Fig. 11.
From these considerations, it seems that, in this SSO region, the active deorbiting system proposed should address masses below 1000 kg as in this case the total number of objects would be significantly reduced, thus decreasing the number of possible impacts causing a cascade of debris.
• UCS Active Object list: In order to have an even broader description of the real space debris environment, the last list here considered as set of targets, is based on currently tracked objects. The Union of Concerned Scientists (UCS)  makes available a list of hundreds of tracked objects (downloaded at November 2010). These objects are mainly active spacecrafts, but some wise filters can be applied in order to have a list of possible candidate debris. The list contains the object launch date and its expected lifetime. The first filter here applied is to consider an object as debris if it is still in the UCS list after the end of its lifetime. This assumes that no mission extension has been foreseen for the given object. Furthermore, in this way only objects large enough to be tracked are taken into account. Further filters on the whole list are imposed in terms of mass and orbital parameters:
o The debris mass has been arbitrarily limited to 2000 kg. The dry mass of the UCS objects has been considered when available, otherwise the launch mass (much more conservative situation) is assumed.
o The orbital altitude has been limited to 1000 km, in order to focus again on the LEO region, the more crowded one. No filters on eccentricity and inclination have been assumed.
In this way the full UCS list  returns 237 objects, covering a possible future scenario of debris objects in LEO. Instead providing a full table of these objects, Fig. 12 shows the mass, altitude, inclination and eccentricity of these. It is worth stressing that these objects are not, at least today, space debris and the list can change according to future update of the UCS database.
In Fig. 12, the long horizontal plateau at approximately 670 kg (at an altitude of 777 km) is due to the whole Iridium satellite series. The smaller one, around 45 kg (at approximately 820 km), corresponds to the 28 satellites of the ORBCOMM-FM series and the last one, very small, around 450 kg (at 920 km) is due to the Globstar FM satellites (9 in this list). The eccentricity plot shows that these orbits are almost circular, as more than the 80% of debris has an eccentricity below 0.003. Finally, the inclination plot shows that the orbits are mainly crowded into highly inclined orbits, around 80 and 100 deg, i.e. the Sun-synchronous region (already well described by the previous list). It is worth stressing since now that the presence of few debris out from this range causes a very large mission ∆V in order to change inclination and reach just these few debris. This means that considering dedicated missions, for instance operating at different inclination ranges, would significantly reduce the time required moving from a given debris to the next and accordingly the propellant mass required.
(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