ASSESSMENT OF THE ON-GROUND RISK DURING RE-ENTRIES H. Klinkrad Mission Analysis Section, ESA/ESOC, D-64293 Darmstadt, Germany ABSTRACT they produce a significant mass percentage of break-up fragments which can withstand the aerothermal heat flux and structural loads during re-entry, and which can gen- The present paper outlines methods which allow to quan- erate ground impacts with significant risk levels to the tify and monitor the on-ground population risk in the population within the debris swath. Historic examples course of a hazardous re-entry event. The risk poten- of this category were the re-entries of Skylab (on 11-Jul- tial will be derived from an object related casualty cross- 1979, with a mass of 74 t), and Salyut-7 (on 07-Feb-1991, section, the endangered ground swath, and the underlying with a mass of 40 t). The latest, most massive object in population density. Means will be described to perform a space history to re-enter was Mir (on 23-Mar-2001, with long-term risk assessment for a given latitude band, and a a mass of 135 t). The re-entry of Skylab was analysed in short-term risk assessment for a given nodal longitude of detail by Dreher [2] et al., while the Salyut-7 event was the final re-entry orbit. In case of residual manoeuvring well documented in the proceedings of an ESOC work- capabilities strategies will be outlined to manage and re- shop [5]. Results of re-entry simulations with satellite duce the risk potential by controlling the nodal longitude break-up models were compiled by Fritsche [3] et al., of the final orbit, and the impact footprint. The usefulness and by Bouslog [1] et al. In context with launch events, of these concepts will be demonstrated for the historic re- Kompaniets [8] et al., and Smith [11] investigated possi- entries of Skylab and Salyut-7, and for the recent re-entry ble debris sources, and derived related on ground risks. of the Mir space station. Based on available technical information, NASA [10], ESA [6], and NASDA compiled debris mitigation hand- books, which also address risk assessment and mitigation 1. INTRODUCTION procedures for uncontrolled re-entries. Of the 26,500 Earth orbiting objects which USSPACE- In the present paper the risk associated with the re-entry COM has tracked since 1957 more that 18,000 have re- of a risk object will be analysed, based on empirically entered into the atmosphere by the year 2001. Most of justified and computationally verified assumptions on the these objects disintegrated and burnt up, posing only a break-up altitude, and on the cross track and along track minor risk on ground. In the year 2001 the number of un- extension of an impact probability distribution. The most classified, correlated objects in the USSPACECOM cat- prominent historic re-entries of massive objects, i.e. the alog was on the order of 8,500. Due to the limited sen- decays of Skylab, Salyut-7, and Mir, will be used to dis- sor sensitivities, all of these objects have diameters larger cuss results of the risk analysis methods which are devel- than 10 to 30 cm. Out of the catalog population, about oped in this paper. 1 object re-enters each day, and 1 to 2 objects with a radar cross-section (RCS) larger than 1 m2 re-enter each week. The latter ones, which have a radius of about 1 m, 2. RE-ENTRY GROUND SWATH have an increased survival potential, and they are gener- ally followed more closely by USSPACECOM until their Some 80% of the currently catalog space object popula- final entry. A NASA [4] analysis of several months of tion have near-circular orbits of e 0 ¡ 05. This is partly such re-entry monitoring and prediction exercises indi- due to operational practices, and partly due to the natural cated that the global distribution of entry locations, for circularisation of orbits under the effect of airdrag, con- the given inclination distribution of the catalog orbits, is centrated around perigee passes. Those orbits which due rather uniform, with no evident clustering on the North- not fall under this category are mostly highly eccentric ern or Southern hemisphere. throughout their orbital lifetime, with their decays driven by luni-solar perturbations acting on the perigee altitude. Occasionally, at rates of one in several years orbital struc- Such re-entries can be forecast with good precision, and tures re-enter, which have geometric cross-sections of they will not be dealt with in this paper. The dominant 100 m2 or more, and masses of several 10 tons. Such class of near-circular orbits shows clusters at certain in- spacecraft can be classified as high-risk objects, since clination bands. The orbital inclinations are very stable Proceedings of the 3rd European Conference on Space Debris, ESOC, Darmstadt, Germany, 19 - 21 March 2001 (ESA SP-473, August 2001) during the entire orbital phase, and they can be used for Tab.2 summarises related results for the Mir ground- a coarse risk analysis, focused on the constrained latitude track drift (inclination 51.6 , with decay to an altitude φ §©¨ band of ¢¤£¦¥ i i . of 200 km at the end of the propagation). In all anal- ysed cases the application of the groundtrack correction impact probability 1σ 2σ 3σ scheme to previously predicted COIW locations led to an adjusted cross-track position well within 5 km of the elliptic corridor 0.39347 0.86466 0.98889 most recent prediction. rectangular corridor 0.46606 0.91107 0.99987 Tab. 1: Integrated impact probability for elliptic and rect- 3. CASUALTY CROSS-SECTION angular fragment dispersion areas of 1σ, 2σ, and 3σ ex- tensions along track and cross track. One of the key issues in re-entry risk analyses is the iden- The prediction of the orbital decay of a spacecraft, and of tification of components of a spacecraft which are likely its final re-entry, aerothermal break-up, and ground dis- to survive the deceleration and heating peaks following a persion of fragments is affected by uncertainties of the break-up event at altitudes of typically 75 to 80 km. Stud- initial orbit and attitude state, of the ambient atmosphere, ies sponsored by NASA [1] and ESA [3] have addressed of the aerodynamic characteristics, of the structural and this challenging task, which involves a simultaneous so- thermal response of the materials, and of the resulting lution of 6 degrees-of-freedom (DoF) flight dynamics break-up altitude and fragment spread. Depending on equations, with aerodynamic, aerothermodynamic, ther- the entry angle and break-up altitude, the ground disper- mal, and structural loads acting on a complicated geom- sion of fragments can extend over more than 1500 km etry, composed of many different materials. Computa- along track and 80 km cross track, assuming a sus- tions predict that certain materials (e.g. stainless steel), tained lift-to-drag ratio of up to L D 1, and a mean and certain shapes (e.g. tanks) have a particularly good break-up altitude of 80 km. During the orbital decay potential of re-entry survival, particularly if high melting phase, the final re-entry location can only be predicted temperatures are combined with low area-to-mass ratios. with considerable uncertainties, which can be cumulated The retrieval of such objects at several re-entry sites has into an impact probability density function (PDF). This supported these assumptions [2, 4]. PDF can be formally produced by propagating the overall error covariance matrix from the last available orbit de- In order to assess the on-ground risk due to surviving termination to the most probable re-entry location, which debris of a single re-entry event, the NASA safety stan- shall be denoted as the centre of impact window (COIW). dard [10] NSS 1740.14 introduces an equivalent casualty Based on the analysis of 15 historic re-entry events, we cross-section Ac, which is composed of the cross-sections shall assume that the 2σ extension of the fragment dis- Ai of individual fragments, augmented by a projected hu- 2 ¡ persion area on ground is an ellipse, which is symmetric man risk cross-section of Ah 0 36m (corresponding to ¡ to the predicted COIW location, both in the along and Ah 0 6m, as used by NASA). cross track direction. We shall further assume that the n 2 along track extension corresponds to a 20% error in the ¨ Ac ∑ Ah Ai (1) COIW time, and that the cross track extension is 80 km, relative to a drift corrected ground track. For implemen- i 1 tation purposes, we shall analyse a rectangular fragment This quantity Ac is a simple, yet very efficient way to con- dispersion area which is tangent to the 2σ dispersion el- centrate the entire knowledge on the break-up process of lipse. The resulting probabilities of an impact within a a re-entering spacecraft into a single figure. It would al- 1σ, 2σ, and 3σ ellipse and rectangle are listed in Tab.1. low a spacecraft manufacturer to provide means for a reli- For the adopted conditions, the total impact probability able re-entry risk assessment without disclosing sensitive within a 2σ rectangular ground swath is 91.1% (as com- information on construction details. As such, the casualty pared to 86.5% for the elliptic integration limits). cross-section could become a common international stan- dard of data exchange in the course of re-entry prediction ∆ n ∆ n ∆λ n ∆ campaigns (e.g. in the frame of the Inter-Agency Space ∑ ∑ ∑ t d t sec s km pred i N i N i u Debris Coordination Committee, IADC).