A COMPARISON OF THE RISKS CAUSED BY DEBRIS FROM SPACECRAFT RE- ENTRY AND METEORITE FALLS

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1 A COMPARISON OF THE RISKS CAUSED BY DEBRIS FROM SPACECRAFT RE- ENTRY AND METEORITE FALLS B.Lazare (1), F.Alby (2) (1) CNES, 18, Avenue Edouard Belin, TOULOUSE Cedex 9, France, (2) CNES, 18, Avenue Edouard Belin, TOULOUSE Cedex 9, France, ABSTRACT Thousands of tons of meteorites reach the Earth's surface every year, mainly in the form of dust. Larger meteorites between ten grams and one metric ton, comparable with space debris, only account for a small part of the total. In this range the annual flow is estimated by scientists to be around 50 metric tons, from the study of meteorites found on Earth and recordings by surveillance networks. The mass of space debris reaching the surface of the planet each year is on the same scale. Studies conducted using tools such as Drama and Orsat show that between 5 and 40 per cent of the mass of spacecraft re-entering annually (on average 170 metric tons) survives re-entry into the atmosphere. The similarity of the flow of meteorites and space debris encourages us to make a precise comparison of these two risks. The value of doing this link is to have a better appreciation of the recent risk associated with re-entries of space objects, using as a reference the risk from meteorites, which has been present since the origin of humanity. For an overall comparison of the two risks we are calculating the dangerous surface generated respectively by meteorites and re-entries. To do this we are using risk estimation methods associated with the re-entry of space debris developed over the last few years. The danger presented by a body is associated with its kinetic energy at the moment of impact. Bodies with a relatively low mass are slowed down by the atmosphere and hit the ground at moderate speeds of the order of a few dozen meters per second, which depend only on the ballistic coefficient of the body To be able to apply these methods, we therefore need statistical data (number, mass, density) for the population of debris reaching the ground and for meteorites. For this we are relying partly on the results of fragmentation studies published by the agencies, and partly on the characteristics of debris found and recorded in the Aerospace Corporation database. The characteristics of the meteorite population are available and have been determined as part of the scientific studies mentioned above. Because our objective is to compare the two risks and not to determine an absolute value for either, we will be using a simplified set of protection coefficients that are sufficient to take account of the specific features of the two populations. INTRODUCTION About 30 to tons of outer space matter are intercepted each year by our planet. Most of this material vaporises when it goes through the atmosphere. The meteorites (objects which reach the ground) are divided into two categories: micrometeorites between 30 and 500µm in size and meteorites. The annual mass of micro-meteorites which reach the surface of the Earth is today estimated at about [1] 6,000 tonnes. These micro-meteorites have a very low unit mass and are thus of no danger to people on the surface of the Earth. About 40,000 meteorites per year [2], in the range between10g and 1 tonne, hit the Earth. Meteorites of a mass greater than 1 tonne are even rarer: the frequency for meteorites of 10 tonnes reaching the Earth is about one per year. Beyond that size, large meteoroids can cause local or regional disasters. This type of phenomenon, given its probability and the serious consequences involved cannot be compared to the risks of debris from spacecraft re-entry. We have thus chosen to restrict our study to the risks caused by meteorites in the range of 10g to 1 tonne, to which humanity has been subject from its beginnings, by comparing them to the risks caused by debris from re-entering spacecraft reaching the ground, since the middle of the last century. Meteorites annual flow including masses between 10g and 1 tonne is estimated at 53.8 tonnes [2]. The fraction of debris reaching the ground following the re-entry of spacecraft has been estimated in different ways, using simulation software on the hand and the study of found fragments on the other hand. The published results range from 5 to 40 % of the initial mass of the space object, with the survival rate varying according to the characteristics of the object in question. Given an annual mean mass of uncontrolled re-entries of 170 tonnes, the mass of debris reaching the ground has the same order of magnitude as that for meteorites. The main danger caused by falling meteorites or debris is due to their kinetic energy and their impact s surface on the ground, which themselves depend on their number and characteristics: mass, dimension, shape.

2 IDENTIFICATION OF THE POPULATION OF DANGEROUS METEORITES Meteorites of objects >10g >100g >1kg >10kg >100kg >1000kg Objects Mass Figure 1 Cumulative annual meteorite flux Document [2] gives an estimate of the frequency of meteorites as a function of their mass. These values are based on the analysis of meteorites found on accumulation sites on the one hand and on the analysis of data recorded by observation cameras on the other hand. While going through the atmosphere, meteoroids undergo mechanical and thermal stress, due to atmospheric drag, which most often leads to them breaking up. Reference [11] estimates that, on average, a re-entering object produces 5 fragments which are counted as meteorites. Meteorites are classified according to their composition: stone meteorites (chondrites) and metal meteorites (iron ferrites). For the scope of this study most of the meteorites (more than 96% for masses less than 10kg) are chondrites (stone meteorites) whose mass per unit volume is about 3,400 kg/m 3. Given the relatively low masses of meteorites in the range of 10g to 1 tonne, they are efficiently broken by the atmosphere and reach the ground at a speed corresponding to the equilibrium between gravity and air resistance. mg V f = 1. µ. S. Cx 2 With a kinetic energy of: 1 2 E = mv f 2 Vf = speed at which the object hits the ground in m/s M = mass of meteorite in kg g = gravitational acceleration µ = mass per unit volume of air S = cross-section of the meteorite C x = aerodynamic coefficient A projectile can seriously injure an unprotected person in the open if its energy is greater than 15 joules [3]. Given their mass per unit volume, stone meteorites are dangerous when their masses are greater than 20g. Based on this threshold we can calculate the characteristics of the annual population of meteorites which are dangerous for unsheltered people. mass (kg) Total mass in kg 20g<M<100g g<M<1kg kg<M<10kg kg<m<10 2 kg kg<m<10 3 kg Reference [4] shows that debris with a mass per unit volume of 544 kg/m 3 can break through a thin cover or the roof of a vehicle if its mass is greater than 0.4 kg, which corresponds to a kinetic energy of about 300 joules. Reference [5] estimates the risk

3 of a fragment with energy of 300 joules penetrating the roof of a house or a trailer at 5%. We may thus deduce that stone meteorites are dangerous for people inside a shelter when they have a mass of about 200g. This in turn gives the characteristics of the population of meteorites likely to hit a person in a shelter. mass (kg) Total mass in kg 200g<M<1kg kg<M<10kg kg<m<10 2 kg kg<m<10 3 kg EVALUATION OF RISKS RELATED TO METEORITES Reference [6] defines the concept of a casualty. This is the around the impact point of a projectile, within which any people who are present will be hit: D A ( 0, A ) 2 N i 1 6 = = + i D A = Total debris casualty in square meters (Add 0.3 man border to average cross section of object) N = of objects that survive re-entry A i = Calculated average cross-section of each object 0.6 = = Square root of cross-section of a standing individual viewed from above The risk of meteorites is thus proportional to the sum of the annual casualty s. We first calculate the casualty for people in the open by taking into account meteorites with a mass of more than 20g. mass (kg) /meteor (m 2 ) 20g<M<100g g<M<1kg kg<M<10kg kg<m<10 2 kg kg<m<10 3 kg Total mass (kg) /meteor (m 2 ) 200g<M<1kg kg<M<10kg kg<m<10 2 kg kg<m<10 3 kg Total IDENTIFICATION OF THE POPULATION OF DANGEROUS SPACECRAFT DEBRIS HITTING THE GROUND Just as for meteorites, we only take into account spacecraft debris whose energy at the moment it hits the ground is high enough to be of significant danger to people in the open. Given the widely varying shape and mass per unit volume of spacecraft debris, this energy cannot be related to a particular mass limit. The published results [7] however show that most debris of mass less than 50g is not likely to cause serious injury. However, for some dense debris (bolts, rods) there is risk if it has a mass of 10g or more. For sheltered people, [4] determines that light covers can resist the impact of spacecraft debris with a mass of about 400g (for a mass per unit volume of 544 kg /m 3 ). We shall use two sources to characterise the debris population: characteristics of spacecraft debris found on the ground, published by the Aerospace Corporation on its site [8] the study made at the request of the Italian Space Agency on the occasion of the reentry of the Bepposax [9] satellite. For both cases we have enough data to make a histogram N = F( m) (N is the number of fragments of mass greater than the mass m). The survival rate of launcher stages, which account for most of the uncontrolled re-entry debris, varies according to their design, from 10 to 40%. We shall thus take an average survival rate of 25%, with a degree of uncertainty of 10%. Given the mean annual flux of uncontrolled re-entries of 170 tonnes, the annual flux of spacecraft debris reaching the ground may then be estimated to be 42.5 tonnes. To facilitate the comparison, we drew two histograms corresponding to this annual flow, on the basis of the granulometry predicted for Bepposax and on that of the debris found. We also plotted the histogram corresponding to meteorites (53.8 tonnes /year) on the same sheet. We then determine the casualty for sheltered people who are protected from meteorites of mass less than 200g.

4 Meteorites of objects Debris upper estimation Debris based on recovered pieces Debris based on Bepposax study 10 1 >10g >100g >1kg >10kg >100kg Objects Mass Figure 2 Cumulative Meteorites and Space debris annual flux For masses above 10kg, the graph shows that the risk of meteorites reaching the ground is greater than for spacecraft debris. Beyond 10kg the two populations are equivalent. The change in slope of the cumulative curve may be related to the low probability of survival of objects of mass less than 1 kg, unless they are protected by the structure of the spacecraft object during re-entry. Finally the difference observed between the Bepposax type distribution and that of debris found, is logical. It is much easier to identify large debris, whereas small debris has only been found under specific circumstances (search for radioactive debris of Cosmos 954 which reentered in January 1978). EVALUATION OF RISKS RELATED TO SPACECRAFT DEBRIS We used the same approach for identified debris as for meteorites, using the values established on the basis of the Bepposax study: We first calculated the casualty in relation to people in the open, taking into account debris with a mass of more than 50g. (m 2 ) 50g<M<100g g<M<1kg kg<M<10kg kg<m<10 2 kg kg<m<10 3 kg Total We then calculated the casualty for sheltered people, who are protected from debris with a mass less than 400g (m 2 ) 400g<M<1kg kg<M<10kg kg<m<10 2 kg kg<m<10 3 kg Total ANALYSIS OF RESULTS If we assume that about 80% of the population at a given time is protected (inside a house, a vehicle or a shelter) we may then write: D = 0.2. D D AGlobal AUnsheltered ASheltered For meteorite showers we obtain a Global casualty of 6,394 m 2 and for spacecraft debris 1,814 m 2. The risk related to meteorites is thus estimated to be three times greater than that for spacecraft debris. Reference [6] establishes that a casualty of 8 m 2 corresponds approximately to a probability of that a person will be hit. The annual collective risk corresponding to meteorites showers may thus be estimated at 2 R = DAGlobal = 8.10 / year Which corresponds on average to one accident every 12.5 years somewhere in the world. Likewise we obtain

5 a value of /year for the probability of a risk related to spacecraft debris, which corresponds on average to one accident every 50 years. We have compared these estimates with available historical data. There are two documented occurrences [10] of people hit by meteorites in the second half of the twentieth century: On November 30, 1954 at Sylacauga, in Alabama. Annie Hodges was hit by a 3.86 kg meteorite. On August 14, 1992 a young boy of 13 years was hit but not wounded by a 3 g meteorite at Mbale, Uganda. Furthermore [11] estimates the risk, for meteorites, of one person being hit every nine years. For the risk caused by spacecraft debris we have no information of a case of wounding or death due to spacecraft debris, which is consistent with the probability of such an event occurring. We may however note that one person was hit, but not wounded, by light debris of about 10 cm in Turley (Oklahoma) on January 22, Another way of checking the relevance of risk evaluations is used in [11] by comparing the risk calculated for a building being hit, with damage effectively observed during meteorite showers. The calculated risk is of sixteen buildings in the world being hit per year. This forecast satisfactorily matches observations made in the USA (4.5% of the world population) for the period 1965 to 1985, during which buildings were damaged by meteorites nine times, which, when extrapolated to a world scale, corresponds to 10 buildings damaged per year. If we transpose this result to spacecraft debris likely to damage a building (4,535 debris items against 10,200 meteorites) we get a forecast of five buildings damaged per year. Whereas, no case of a building being damaged by a fragment has been recorded. This anomaly may be explained by the greater difficulty involved in identifying spacecraft debris, as compared to meteorites in the event of an impact, but also by a conservative evaluation of the amount of debris surviving re-entry. CONCLUSION The scope of correlated observations is limited by the fact that the available data is not complete: a significant proportion of meteorites or debris hitting buildings is not detected and wounds caused by meteorites may have been attributed to other causes. Most of the observations, however, confirm the order of magnitude for risks that have been evaluated. The two main lessons that may be drawn from the study are: the methods for analysing risks, used for reentry of spacecraft objects, are satisfactorily verified when applied to meteorite falls. the danger caused by meteorites with a mass of 10g to 1ton is greater (by a factor of 3 no doubt) to that created by spacecraft debris. Finally, the lack of any observations of damage done to buildings by spacecraft debris, although we have no proof of this, makes us confident versus the relevant calculation. ACKNOWLEDGEMENTS We would like to thank Jean Duprat for his suggestions which enabled us to improve this manuscript. REFERENCES 1. J. Duprat, C. Engrand, M. Maurette, F. Naulin, G. Kurat, M.Gounelle, Le flux de micro-meteorites sur terre, P.A. Bland, T.B. Smith, A.J.T.Jull, F.J. Berry, A.W.R.Bevan, S.Cloudt and C.T. Pilinger, The flux of meteorites over the last years, A Common Risk Criteria for National Test Ranges Standard Range Commander Council White Sands Missile Range. 4. Erik W. F. Larson, Large Region Population Sheltering Models for Space Debris Risk Analysis, ACTA Inc, Jeffrey Tooley, Trent Habiger and K. Rolf Bohman, A First-Order, Simplified, Conservative Methodology for Computing Re-entry Expectation, The Aerospace Corporation, Guidelines and Assessment Procedures for Limiting Orbital Debris, NASA Safety Standard NSS Office of Safety and Mission Assurance. 7. Determination of Debris Risk to the Public due to the Columbia Break-up During Re-entry, CAIB Report Volume II Appendix D Summary of Recovered Re-entry Debris, The Aerospace Corporation Center of Orbital Debris studies, 9. C. Portelli, L. Salotti, L. Anselmo, T. Lips, A. Tramutola, BEPPOSAX equatorial uncontrolled reentry, ELSEVIER, Walter Branch, Ph.D., Chronological Listing of Meteorites That Have Struck Humans, Animals and Man-Made Objects (HAMs), International Meteorite Collectors Association Halliday I., Blackwell A.T., Griffin A.A., Meteorite impacts on humans and on buildings, Nature Vol 318, Nov 1985.