Luciano Anselmo

Collisional evolution of the Earth's orbital debris cloud

We have developed a numerical algorithm to model the future collisional evolution of the low-orbiting Earth debris population, accounting for both the wide spectrum of masses (or sizes) of the orbiting objects, and their different altitudes, which result in a variable efficiency of the drag-induced decay. The evolution process has been assumed to be caused by a number of source and sink mechanisms, such as launches, explosions, atmospheric drag, and mutual collisions. The collisional outcomes have been described through a semiempirical model for the fragment mass distributions, consistent with the available experimental evidence. A runaway exponential growth of collision fragments is always found in our model. Although its timing and pace are sensitive to some poorly known parameters, fairly plausible parameter choices predict that the runaway growth will occur within the next century, starting in the crowded shells between 700 and 1000 km of altitude and, somewhat later, between 1400 and 1500 km. The runaway growth is delayed until a few centuries in the future only if the catastrophic breakup threshold in specific impact energy for orbiting objects exceeds that for natural rocky bodies by at least a factor of 10. Our simulations show that the sensitivity of the results to future launch and/or deorbiting and removal policies is rather weak, so that drastic measures will need to be taken soon in order to significantly avoid or delay a catastrophic outcome.

Modelling the evolution of the space debris population

Abstract The current space activities are already disturbed and jeopardized by the growing number of orbiting debris. Those planned for the near future, such as the launch of large satellite constellations and the construction of the international space station, are even more sensitive to the evolution of the space environment. Therefore, a clear picture of the present situation in Earth orbit and its future evolution is needed. In this paper we describe in some details the work we have carried out on this problem in the last several years. Starting from the current population and simulating a reasonable scenario for the space activities in the next decades, we have obtained plausible quantitative models of the possible future space environment. We summarize some results concerning the effectiveness of possible mitigation measures and assess the robustness of these results, by checking how sensitively they depend upon the initial conditions and the choice of some model parameters. We also analyze the effect of the launch of a number of satellite constellations, showing the importance of the adoption of some debris prevention measures in their launch policies. Finally, we study the possible problems arising from the recent discovery of a new family of debris composed by drops of NaK coolant, that leaked outside the nuclear reactors of the Soviet RORSAT-class satellites. Our preliminary results indicate that these drops are going to cause an increasing number of small-scale, possible satellite-damaging impacts but, due to their small size, no additional catastrophic collisions; therefore their influence on the long-term evolution of the overall debris population is limited.

Testing the gravitational interaction in the field of the Earth via satellite laser ranging and the Laser Ranged Satellites Experiment (LARASE)

In this work, the Laser Ranged Satellites Experiment (LARASE) is presented. This is a research program that aims to perform new refined tests and measurements of gravitation in the field of the Earth in the weak field and slow motion (WFSM) limit of general relativity (GR). For this objective we use the free available data relative to geodetic passive satellite lasers tracked from a network of ground stations by means of the satellite laser ranging (SLR) technique. After a brief introduction to GR and its WFSM limit, which aims to contextualize the physical background of the tests and measurements that LARASE will carry out, we focus on the current limits of validation of GR and on current constraints on the alternative theories of gravity that have been obtained with the precise SLR measurements of the two LAGEOS satellites performed so far. Afterward, we present the scientific goals of LARASE in terms of upcoming measurements and tests of relativistic physics. Finally, we introduce our activities and we give a number of new results regarding the improvements to the modelling of both gravitational and non-gravitational perturbations to the orbit of the satellites. These activities are a needed prerequisite to improve the forthcoming new measurements of gravitation. An innovation with respect to the past is the specialization of the models to the LARES satellite, especially for what concerns the modelling of its spin evolution, the neutral drag perturbation and the impact of Earths solid tides on the satellite orbit.

Collision Risk Mitigation in Geostationary Orbit

The short- and long-term effects of spacecraft explosions, as a function of the end-of-life re-orbit altitude above the geostationary orbit (GEO), were analyzed in terms of their additional contribution to the debris flux in the GEO ring. The simulated debris clouds were propagated for 72 yrs, taking into account all the relevant orbital perturbations.The results obtained show that 6–7 additional explosions in GEO would be sufficient, in the long term, to double the current collision risk with sizable objects in GEO. Unfortunately, even if spacecraft were to re-orbit between 300 and 500 km above GEO, this would not significantly improve the situation. In fact, an altitude increase of at least 2000 km would have to be adopted to reduce by one order of magnitude the long-term risk of collision among geostationary satellites and explosion fragments. The optimal debris mitigation strategy should be a compromise between the reliability and effectiveness of spacecraft end-of-life passivation, the re-orbit altitude and the acceptable debris background in the GEO ring. However, for as long as the re-orbit altitudes currently used are less than 500 km above GEO, new spacecraft explosions must be avoided in order to preserve the geostationary environment over the long term.

Effect of mitigation measures on the long-term evolution of the debris population

Abstract The relative effectiveness of mitigation measures on the long-term evolution of the orbital debris population was investigated in detail by using a new version of the Space Debris Mitigation (SDM) analysis tool, developed under ESA/ESOC contract. Starting from new initial conditions, updated to 1999, and from a future traffic model, the influence of the selective adoption of mitigation practices was determined over a 100-year time span. The analysis included the suppression of the release of mission-related objects, the on-orbit explosion avoidance and the de-orbiting of upper stages in low earth orbit. A particular effort was devoted to study the long-term effect of different strategies of spacecraft disposal at the end of the operational life. The end-of-life disposal of low earth satellites in orbits of given residual lifetime, in between 0 and 50 years, was simulated to assess its potential long-term benefits for the debris environment below 2000 km. Moreover, for geostationary and low earth satellites above 1400 km, the re-orbiting to higher altitudes was considered as well. The results show clearly that the explosion avoidance in orbit is quite effective and should be strictly applied. However, some form of de-orbiting will be needed to roughly stabilize the long-term debris collision risk in low earth orbit.

Updated results on the long-term evolution of the space debris environment

Abstract The long-term evolution of artificial debris in earth orbit has been analyzed, taking into account a detailed traffic model, explosions, collisions and the effects of air drag. Several scenarios, most of them implementing mitigation measures discussed at international level, have been simulated over a 200-year time span. Moreover, the sensitivity of the results to different collision model assumptions has been assessed. The simulations confirm the importance of spacecraft and rocket bodies passivation to avoid in-orbit explosions, but the de-orbiting of upper stages is needed as well to curb the debris and collision rate increase and to avert the onset of an exponential growth of artificial objects in the near earth space. The additional removal of end-of-life spacecraft does not improve the outcome dramatically, but may be able to reduce the collision rates in low earth orbit, reversing the historical trend of the last four decades. Of course, the fragmentation models and the simulation assumptions are still affected by a certain degree of uncertainty, but the results of the sensitivity analysis show that our conclusions are consistent and reliable, at least for the first century.

Interaction of the Satellite Constellations with the Low Earth Orbit Debris Environment

The effect on the orbital debris environment of several satellite constellations, to be launched and maintained in LEO over the next decades, has been analyzed with the SDM software system, including updated initial conditions and traffic model scenarios.

Long term evolution of the space debris population

Abstract In the last few years two different computer models have been developed in Pisa to study the evolution of the debris population. With them it is possible to simulate different scenarios for the future launch policies. We have defined a reference case which is a reasonable forecasting of the future space activities. Based upon it we have performed a number of simulations to study the influence of the initial population on the long-term evolution of the debris environment. Moreover, we have analyzed the sensitivity of the models upon the different parametrizations of the physical processes involved in order to highlight the main areas of uncertainty which may affect the long term predictions.

Influence of the spacecraft end-of-life re-orbiting altitude on the long-term collision risk in the geostationary ring

A novel approach was developed to assess the contribution of satellite explosions to the object density in the geostationary ring. A low intensity explosion of a typical operational spacecraft was simulated at eight different altitudes, in between 0 and 2000 km above the geostationary orbit (GEO). The fragments produced were propagated for 72 years, taking into account all the relevant perturbations, and their contribution to the average object density in the GEO ring, both short and long-term, was analyzed as a function of the end-of-life re-orbiting altitude. The explosions in geostationary orbit are the most detrimental for the GEO ring environment. However, the average fragment density in the ring is never higher than 15 of the current background, decreasing to less than 1100 of the existent environment after 4 years, apart for the density rebound, about five decades later, due to the luni-solar perturbations. The spacecraft end-of-life re-orbiting is a possible mitigation solution, but the long-term situation improves quite slowly, as a function of the altitude above GEO, if the explosions continue to occur. A re-orbiting 300 km above the geostationary altitude seems adequate to guarantee, after 2 – 4 years, a long-term average density of fragments in the GEO ring at least two orders of magnitude below the present-day background, even during the density rebound observed after 54 years. However, at least 1000 km of re-orbiting are needed to stay below that 1100 threshold also in the short-term.

Satellite Re-Entry Prediction Products for Civil Protection Applications

In order to meet the specific requirements of civil protection authorities, since 2003 a set of tailored products has been developed and applied in Italy to define, a few days ahead of re-entry and in wide areas of interest, risk zones and corresponding alert time windows in the event of an uncontrolled satellite decay leading to undue debris impact hazard on the ground and in the overlying airspace. Based on the general properties of re-entries from nearly circular decaying orbits, on the results of detailed fragmentation analyses, when available, on standard re-entry prediction outputs, on specific simulations of endo-atmospheric debris dynamics, and on the basics of orbital motion with respect to the Earth, accurate re-entry tracks over the region(s) of interest are determined, with sufficiently conservative ground safety swaths, accounting for the sources of cross-track debris dispersion, and associated risk time windows, depending on debris flight time dispersion and residual trajectory along-track uncertainties. With the approaching re-entry and the consequent shrinking of the global uncertainty window, some of the risk zones and time windows identified in advance can be progressively discarded, leaving at most, until the end, one re-entry opportunity, but more often none, over a region the size of Italy. These products are easy to understand and are timely, accurate, unambiguous and remarkably stable, all qualities that render them particularly suitable for civil protection applications.