arXiv:2008.13032v1 [physics.geo-ph] 29 Aug 2020 hs udlnsi htpyodluce nolwEarth low into launched payload that is guidelines (see these standards those example guidelines accepting for introduced largely have requirements, NASA and and ESA like agencies tnad aebe nrdcdt iiieteamount the ( minimize debris to orbital of introduced been have standards rpitsbitdt dacsi pc Research Space in Advances to submitted Preprint anhdit ri rudErh hshsrie major raised ( has are debris This space objects about Earth. concerns more around in- orbit and the into more and launched commercialization, programs of space crease operating nations is evermore aerosols and atoms of atmosphere. form the into in injected matter prop- way, up chemical This and heat Earth physical erties. bodies their around reentry, on depending Upon orbit ablate and in atmosphere. the bodies enter objects rocket also anthropogenic and space, spacecraft of like exploration size the bod- any With differentiated even without mete- ies. and origin term asteroids, natural comets, the of from following, limit) natural objects the of to (in are refers meteoroids Most oroids space. e. i. from origin, objects various by ment Introduction 1. habitat. terrestrial Keywords: the and incre atmosphere atmosphere the Earth’s into on injectio aerosols effects the of prevails injection metals anthropogenic several the constellations of satellite injection future anthropogenic material but the origin, anthropogenic natural Today, of ablatio mass literature. injected and relevant composition, mass, the a the reviewing planned about considering information evaluated, Detailed asteroids is tions. like sources m objects mass the system anthropogenic solar increase of objects from continuously (anthropogenic) originating human-made will meteoroids, of and to influx future mass the the in compare debris a space stayin and While commercialization, about materia advancing adding space. programs, space and to with disintegrating sent countries mostly are atmosphere, objects the more reenter and more year, Every Abstract eeris bain eert composition Meteorite Ablation, Meteoroids, [email protected] ∗ nteAtrpgncadNtrlIjcino atrit Ea into Matter of Injection Natural and Anthropogenic the On ihtesed rwho pcflgtatvte with activities spaceflight of growth steady the With bombard- constant a to subject is atmosphere Earth’s mi addresses: Email author Corresponding a ntttfu epyi n xrtreticePyi,Te Physik, extraterrestrische und f¨ur Geophysik Institut topee aelt oseltos asiflx ua-aeinje Human-made influx, Mass constellations, Satellite Atmosphere, ESA , S eta Secretary Central ISO 2008 [email protected] Kr-en Glassmeier) (Karl-Heinz ; NASA b a-lnkIsiu ¨rSnessefrcug 77 G 37077 f¨ur Sonnensystemforschung, Max-Planck-Institut Klinkrad , 2019 LoadSchulz), (Leonard .Acneuneof consequence A ). enr Schulz Leonard , 2006 , 2019 .A result, a As ). n space and ) a, ∗ hiceUiesta rusheg 80 Braunschweig, 38106 Universit¨at Braunschweig, chnische alHizGlassmeier Karl-Heinz , in(Section separately tion (Section done injection is prod- This natural for ablation atmosphere. the resulting into its injected influx, the ucts mass and the on matter composition, focus of elemental We injection atmosphere. anthropogenic Earth’s and into natural the of ison detail. more the in needs Thus, considered constellations be satellite those to future. by the caused influx in mass future will atmosphere of lifetime amount Earth’s limited anthropogenic reentering their the with increase combined dramatically launched be to r eteotsmayo l h vial information. available the all of summary effort We best a information. try and studies, numbers published conflicting many ablation providing from partly the acquired and been has composition, processes elemental the influx, mass E rvdn lblitre n te telecommunica- other ( services and in tion internet spacecraft global small providing of thousands LEO to hundreds of stellations impacts itself. possible atmosphere about the and on caused meteoroids, of injection ablation natural the the by its to about comparison questions in raising significance at- atmosphere, material the the after anthropogenic into into years more injected reentry is and 25 by more achieved within Hence, is of This mosphere. disposed operation. be of to end has (LEO) orbit ssdsrprintl.Alti a aeytunknown yet have can this All case, disproportionately. this ases For %. 40 nearly to fraction this increase may nti td,w rvd rtoeve n compar- and overview first a provide we study, this In that find we Additionally, far. by sources natural by n eea opne aepooe ag aelt con- satellite large proposed have companies Several otentrlms u noErhsamshr due atmosphere Earth’s into flux mass natural the to darayprilyisaldlrestlieconstella- satellite large installed partially already nd of number increasing The atmosphere. upper the to l n oes h urn n erftr significance future near and current The comets. and btosstliecntlainpoet as concerns raise projects constellation satellite mbitious osmk paot28%cmae oteannual the to compared % 2.8 about up make does nobta ihattds bet tlwaltitudes low at objects altitudes, high at orbit in g fntrladatrpgncmtra r given, are material anthropogenic and natural of n s u noteamshr.I hssuy we study, this In atmosphere. the into flux ass 3 iue al. et Liou .Tencsayifraint ulf the qualify to information necessary The ). ¨ottingen, Germany a,b to,Atrpgnceffect, Anthropogenic ction, , 2018 .Teaon fspacecraft of amount The ). 2 t’ Atmosphere rth’s n nhooei injec- anthropogenic and ) etme ,2020 1, September Germany With this information we provide a review of the nat- quite low and decreases with increasing size. Bodies heav- ural injection and three different scenarios for the anthro- ier than hundreds of tons (larger than several meters in di- pogenic injection. These scenarios include a present day ameter) hit Earth once a year, while impacts with objects analysis as well as two near-future scenarios taking into several ten meters in diameter occur only once in a thou- account different planned projects for large satellite con- sand years (Chapman and Morrison, 1994; Zolensky et al., stellations. This allows to compare the relative contribu- 2006a). However, upon entering Earth’s atmosphere, large tions of human-made objects and natural objects entering impactors ablate and disintegrate, leaving behind a trail the atmosphere. of aerosols and particles of molecular size. Ablation ma- terial in the atmosphere in form of dust particles seems to sediment within several months (Klekociuk et al., 2005; 2. Natural injection Gorkavyi et al., 2013), while we can not rule out that ions Many meteoroids originating from asteroids, comets, and particles of molecular size remain for a longer time in material of planetary origin, interplanetary and even in- the upper atmosphere. Thus, we include large bodies that terstellar dust (see for example Jewitt, 2000; Plane et al., impact Earth at least every 10 years to account for such 2017) enter Earth’s atmosphere every day. In this section, ablation and disintegration processes. we look at the mass, composition and ablation of these me- Drolshagen et al. (2017) have calculated the mass in- −21 11 teoroids and estimate the resulting injection with respect flux of meteoroids in a size range of 10 to 10 kg. −2 to ablation products and the elemental composition. Their mean model for masses below 10 kg is based on the widely used interplanetary flux model of Gr¨un et al. 2.1. Yearly natural mass influx (1985), which is used by NASA (Moorhead, 2020) and is close to the newest ESA meteoroid flux model IMEM2 The knowledge of the total natural mass flux into Earth’s (Soja et al., 2019). It is derived from different spacecraft atmosphere is of high importance. The mass influx distri- in-situ measurements of meteoroids and zodiacal light as bution is sort of bimodal with a maximum at a particle − well as lunar impact measurements. The model provides mass of about 10 8 kg (e. g. Flynn, 2002; Carrillo-S´anchez an analytic function of the particle flux at 1 AU. For the et al., 2015; Plane et al., 2017) and a second maximum at − intermediate mass range, 10 2 to 106 kg, the power law high particle masses, although the mass influx increases model of Brown et al. (2002) is used, which is based on continuously for large objects. Objects in the size range of spacecraft fireball data. For large bodies heavier than 106, a few millimeters to meters, which are the main source of a similar power law adapted from Stokes et al. (2003) is meteorites found on Earth, only contribute a small frac- used. tion of the whole mass (Flynn, 2002). The mass influx Beside the named studies, Drolshagen et al. (2017) have distribution used in this study is displayed in Figure 1. included measurements from the Hubble Space Telescope The distribution has two mass ranges with rather high solar array impacts of meteoroids (McDonnell, 2005) as mass input, a small dust particle contribution in the sev- well as visual data from meteor entries (Koschny et al., eral microns to mm-size range and large meteoroids in the 2017) to verify the model of Gr¨un et al. (1985). Addition- tens of meters range. Differentiation of these ranges is ally, studies from Halliday et al. (1996) (fireball data) and crucial because of the different origin, composition and Suggs et al. (2014) (lunar impact flashes) were used to find ablation of those two groups (see Sections 2.2, 2.3). The the best way to connect the models of Gr¨un et al. (1985) peak at small meteoroid sizes of several microns to mil- and Brown et al. (2002). As Drolshagen et al. (2017) only limeters is caused by the large amount of interplanetary briefly reviewed other studies that also provide estimates dust particles (IDPs) in the solar system. The particles on the annual mass influx we shortly discuss these other themselves have very low masses, but are high in num- studies. ber. They have several sources, mainly various types of For the large impactor size range, we regard the models comets and the asteroid belt, whereas the contribution of by Brown et al. (2002) and Stokes et al. (2003) as the cur- interstellar material is negligible (Plane et al., 2017). rent best models. However, for the IDP mass range, there Dust particles are normally defined to be smaller than is a large difference between the various estimates pro- tenth of microns (Rubin and Grossman, 2010; Koschny posed. Plane (2012) reviews several studies, experiments, and Boroviˇcka, 2017). However, based on the analysis and models regarding the influx of IDPs into Earth’s at- and modelling of the observations of the zodiacal dust mosphere. The mass influx estimates vary from 1,800 to cloud by the Infrared Astronomical Satellite (IRAS) and 100,000t/yr for the respective mass range. Thus, the IDP ground based radars as well as various other observations mass range needs a more critical discussion to derive a suit- (Nesvorn´yet al., 2010, 2011) we adopt a cutoff size of able estimate for the purpose of our study. Four studies 2 mm. Therefore, the upper mass limit of the IPD popu- − are important here. lation is roughly 10 5 kg (Fig. 1). Nesvorn´yet al. (2010) modelled the zodiacal dust cloud In contrast, the mass flux peak at high impactor sizes using IRAS data. They estimated an influx of 100,000 t/yr. is caused by their high mass, while their impact rate is A later update (Nesvorn´yet al., 2011), using refined or- bital characteristics of the IPDs based on meteor radar 2 Meteoroid diameter (m) 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 100 101 102 3000 Grün et al. (1985) Brown et al. (2002) Stokes et al. (2003)

2500

2000

Large impactors (impact frequency < 0.1/yr) 1500

1000 IDPs Mass influx (t / yr mass decade)

Meteorites & Large impactors 500

0 10-20 10-15 10-10 10-5 100 105 1010 Meteoroid mass (kg)

Figure 1: Variation of the yearly mass influx distribution of Earth’s atmosphere with meteoroid mass. The particle diameter is calculated by using a density of 2500 kg/m3 and assuming a spherical shape. The blue data, mainly covering the IDP mass range, are based on the flux law of Gr¨un et al. (1985). The orange data, covering the meteorite and large impactor mass range, are calculated using the power law from Brown et al. (2002) with a mean impact velocity of 20 km/s. The grey data is calculated using the power law of Stokes et al. (2003), given in Drolshagen et al. (2017), again using the same average velocity. It covers large impactors which impact Earth less than every 10 years. The influx distribution reflects the mean model from Drolshagen et al. (2017) and was calculated as shown in Appendix A. Exact values are given there, too. The annual mass input is 11,509 t for the Gr¨un et al. (1985) model, 863 t from Brown et al. (2002), and 4,349 t from Stokes et al. (2003). data, proposes a more realistic value of about 15,000 t/yr. is much higher than in other observations (Hughes, 1978, Hughes (1978, pp. 148–157) used satellite, radar and pp. 150–155; Gr¨un et al., 1985; Nesvorn´yet al., 2010, 2011; visual data of IDP and micrometeor entries to reach an Koschny et al., 2017). Such a large velocity raises ques- estimate. The IDP mass influx rate of 16,100t/yr in a tions as it implies that the majority of the dust particles mass range of 10−17 to 10−1 kg is widely accepted and moves retrograde in the solar system. This could be due to part of the mass influx distribution presented by Flynn HPLA radars being unable to detect very slow (<15 km/s) (2002). and small particles. The lack of detection of smaller parti- Mathews et al. (2001) analyzed observations of microm- cles could explain the quite low influx estimate. von Zahn eteor entries into the upper atmosphere measured by the (2005) discusses further possible shortcomings and biases. high power low aperture (HPLA) radar at the Arecibo ob- Here, we assume that Mathews et al. (2001) significantly servatory. They determine the mass and speed of entering underestimate the amount of incoming meteors. particles, reaching estimates of 1,600 and 2,700 t/yr for the Love and Brownlee (1993) measured IDP and mete- mass range 10−8 to 10−1 kg. These are the lowest annual oroid impacts on the Long Duration Exposure Facility influx values of all those studies reviewed in Plane (2012). (LDEF). Impact crater size, depth and number were de- Their mean geocentric velocity of 50km/s, endorsed by termined in order to obtain information about the mass of later measurements with Arecibo (Janches et al., 2006), each impactor. Assumptions had to be made concerning

3 the particle properties, velocity and impact angle. The 1982; Anders and Grevesse, 1989) and used for normaliza- integration of the derived mass distribution yields approx. tion. The only element with a significant mass fraction 27,000–40,000t/yr (Love and Brownlee, 1993; Taylor et al., not determined by the two above mentioned studies is hy- 1998; Mathews et al., 2001). drogen. Here, we use the 2.02 wt% value from Anders and The estimate of Love and Brownlee (1993) has several Grevesse (1989). The absolute elemental mass abundances uncertainties as it is based on a mean geocentric particle used in following discussions are listed in Table 1. For fur- velocity of 16.9 km/s. For further detail on the velocity ther considerations we use mean values. To summarize, we refer to Gr¨un et al. (1985); Taylor (1995, 1996); Taylor a significant fraction of the mass contribution are metal and Elford (1998); Brown et al. (2005); Drolshagen et al. (31 %) and metalloid elements (13 %), while the majority (2008); ECSS (2008); Nesvorn´yet al. (2010, 2011), and is nonmetallic (47 %). About 8 % of the mass could not be Carrillo-S´anchez et al. (2016). After reviewing all the men- assigned to an element. tioned literature, we use the higher geocentric mean veloc- ity (normalized on mass) of 20km/s for IDPs, as the eval- Table 1: Elemental composition of IDPs and meteorites. uations of Taylor and Elford (1998); Brown et al. (2005) seem to incorporate the best combination of relevance and Z El. Unit Mean Meteo- Arndt Schramm IDP rites experimental falsification. A similar velocity is also used et al. et al. by Drolshagen et al. (2017). (1996) (1989) Increasing the mean velocity implies a reduction of par- 1 H µg/g 20200a 186 ticle mass. This results in a shift of the mass distribu- 3 Li µg/g 2 4 Be ng/g 31 tion towards lower masses and reduces the mass influx 5 B ng/g 377 value. Also considering Borin et al. (2009); Cremonese 6 C wt% 9.7 9.7 0.3 et al. (2012), a realistic mass influx estimate is a value 7 N µg/g 69 below 20,000 t/yr. 8 O wt% 30.7 30.7 35.2 9 F µg/g 97 11 Na µg/g 3747 5503 4625 5936 2.1.1. Resulting mass influx and mass distribution 12 Mg wt% 9.3 11.0 10.1 13.6 Results of the studies discussed roughly agree with the 13 Al wt% 1.2 0.9 1.1 1.2 mass influx estimate by Drolshagen et al. (2017). The 14 Si wt% 12.9 12.9 17.1 15 P µg/g 1746 1746 1292 Gr¨un et al. (1985) flux model is still widely accepted, and 16 S wt% 3.6 5.3 4.4 2.1 we use it to determine an annual mass influx for the differ- 17 Cl µg/g 1229 1229 188 ent mass decades. For the higher mass ranges, the power 19 K µg/g 540 540 764 laws by Brown et al. (2002) and Stokes et al. (2003) are 20 Ca wt% 0.3 1.0 0.6 1.3 21 Sc µg/g 12 12 8 used (Figure 1). For further details see Appendix A. 22 Ti µg/g 549 549 676 Thus, considering the mass range of objects impact- 23 V µg/g 74 74 71 ing Earth less than every 10 years, 10−21 to 106 kg, we 24 Cr µg/g 2199 3590 2894 3466 yield an annual mass influx of 12,372 t with most of the 25 Mn µg/g 1644 1644 2355 26 Fe wt% 17.3 17.9 17.6 25.9 mass (11,509 t) caused by particles with masses lower than 27 Co µg/g 337 337 808 − 10 2 kg. The mass flux is dominated by the IDP contri- 28 Ni wt% 0.5 0.7 0.6 1.7 bution of 10,856t/yr. For all the mass ranges studied con- 29 Cu µg/g 186 186 91 siderable differences in the estimation of the mass influx 30 Zn µg/g 405 405 57 31 Ga µg/g 18 18 7 exist. With an error factor 2 we yield a range of 6,186 to 32 Ge µg/g 42 42 13 24,746 t/yr for the mass influx. 33 As µg/g 15 15 4 34 Se µg/g 32 32 8 2.2. Composition of the natural material 35 Br µg/g 81 81 1 36 Rb µg/g 6 6 2 IDPs and larger meteoroids have quite different compo- 37 Sr µg/g 16 16 10 sitions due to different origins. The major contribution to 39 Y µg/g 2 2 2 the IDP flux is thought to originate from Jupiter Family 40 Zr µg/g 18 18 7 >40 µg/g 16 Comets (JFCs) (Zolensky et al., 2006b; Nesvorn´yet al., 2010; Jenniskens, 2015, pp. 282–283; Yang and Ishiguro, Total (%) 46.4 78.0 91.3 100.0 2015; Carrillo-S´anchez et al., 2016). The composition of Column 4 to 6 are derived from Arndt et al. (1996) and Schramm et al. (1989) with the sixth column representing the arithmetic mean IDPs has been examined comprehensively in two studies: of both studies. Column 7 is the meteorite composition, taking into Schramm et al. (1989) examined 200 IDPs on their major account the meteorite portion of each group (Table 2) and the re- element composition and Arndt et al. (1996) gathered data spective meteorite group elemental mass abundances. For further on 89 IDPs covering elemental abundances also for mi- details on the derivation see the text and Appendix B. a The abundance of hydrogen is estimated as described in the text. nor elements. Both studies only give relative abundances, normalized to Si, Fe, and CI carbonaceous chondrite class abundance. The CI abundance is representative of the so- By contrast, large meteoroids are mostly of asteroidal lar system abundances of elements (Anders and Ebihara, 4 origin (Bottke et al., 2002; Binzel et al., 2015), only a Table 2: Frequencies of meteorite classes derived from finds and falls (Grady, 2000). small portion originates from comets (Binzel et al., 2004; Fern´andez et al., 2005; DeMeo and Binzel, 2008) or dif- Class Portion(%) ferentiated bodies (Grady, 2000; Boroviˇcka et al., 2015, p. 258; Russell et al., 2015, p. 419). A significant amount of Chondrites 91.95 Ordinary Chondrites 86.35 material can survive upon entry and reaches the ground H 42.27 as meteorites. Therefore, we estimate the average elemen- L 37.72 tal mass abundance of large meteoroids by calculating the LL 6.36 composition of meteorites found on Earth. Meteorites are Carbonaceous Chondrites 3.40 CH 0.08 divided into different classes, based on their mineralogy CI 0.04 and thus elemental composition. By weighting the com- CK 0.54 position of each meteorite class with their respective fre- CM 1.18 CO 0.62 quency of finds and falls on Earth, we calculate an aver- CR 0.57 age meteorite composition. Due to the large amount of CV 0.36 classified meteorites of more than 22,000, this statistical Enstatite Chondrites 1.22 approach is possible. We use the data given in Grady EH 0.93 EL 0.28 (2000) to yield the frequency of each meteorite class (see Other Chondrites 0.99 Table 2). For the average elemental mass abundances we K (Kakangari) 0.13 use data from Wasson (1974); Lodders and Fegley (1998); R (Rumurutiite) 0.85 Mittlefehldt et al. (1998); Demidova et al. (2007). The de- Achondrites 3.69 rived average composition of each meteorite class as well Acapulcoites 0.07 as details on the data are provided in Appendix B. By Angrites 0.02 Aubrites 0.28 weighting the elemental mass abundances with the class Brachinites 0.04 frequencies of Table 2, we yield the overall elemental mass Lodranites 0.09 abundance of meteorites listed in Table 1. Ureilites 0.56 Winonaites 0.07 The metal (45 %) and metalloid (17 %) elemental abun- a From Vesta 2.36 dance is higher than in IDPs, while the non-metallic por- Diogenites 0.57 tion (38 %) is lower. All in all, IDPs and meteorites show Eucrites 1.22 considerable differences, but are similar in the abundance Howardites 0.57 of some elements. Lunar 0.11 Lunaite 0.11 The above described method of determination of the Martian 0.09 composition of the large meteoroids is biased, as mete- Shergottites 0.03 oroids show different ablation rate and behaviour depend- Nakhlites 0.03 ing on their composition upon entry into the atmosphere. Chassignites 0.03 Thus, the amount of produced meteorites and the final StonyIrons 0.52 Mesosiderites 0.29 composition to some extend depends on the properties Pallasites 0.22 of the initial meteoroids. Additionally, some meteorite Irons 3.85 classes are easier to find, e. g. iron meteorites are easier IAB 0.76 to distinguish from the environment due to their metal- IC 0.06 lic look. Also, the number of meteorite finds and falls for IIAB 0.60 some groups is quite low and the statistical sample might IIC 0.05 IID 0.09 be insufficient in that case. Thus, there are uncertainties IIE 0.11 in our approach. IIF 0.03 IIIAB 1.34 2.3. Atmospheric processing of the natural material IIICD 0.24 IIIE 0.08 Results from various studies are used to derive ablation IIIF 0.04 products of IDPs and larger meteoroids. Three ablation IVA 0.37 products are thought to be important: material due to IVB 0.08 deposition in the atmosphere in form of atoms, ions or Total 100 molecules; material deposited as aerosols, e.g. particles of a Expected to originate from Vesta. microns to nm size; material directly reaching the ground, thus not contributing to atmospheric injection. due to insufficient heating of the particle. This cutoff size 2.3.1. Small meteoroid ablation ranges between meteoroid masses of 10−11 to 10−15 kg de- For small meteoroids, several studies suggest that there pending on the study (Jones and Kaiser, 1966; Nicol et al., is a cutoff size below which no ablation is taking place, 1985; Popova, 2004; Vondrak et al., 2008). Meteoroids be-

5 low this size can be treated as part of the aerosol mass 1 Rogers et al. (2005) fraction as they are slowed down to cm/s velocities. It Love and Brownlee (1991) takes them weeks to years to reach the ground depending on their size (Kasten, 1968; Rietmeijer and Jenniskens, 0.8 1998; Rietmeijer, 2002). Looking at masses higher than the cutoff mass, there 0.6 are three models to be considered. Rogers et al. (2005) present a numerical model of the ablation of small mete- oroids in the mass range 10−3 to 10−13 kg for discrete ve- 0.4 locities and different meteoroid densities. With the knowl- edge of the velocity distribution of meteoroids in the re- Atomic mass fraction spective size range, an overall estimate of the ablation can 0.2 be made. Taylor (1996) state that the velocity distribu- tion of meteoroids from 10−2 to 10−15 kg is similar. We use the velocity distribution from Taylor (1995), tabulated 0 in ECSS (2008), and recalculate the distribution to an in- 10-20 10-15 10-10 10-5 cident height of 100 km, thereby taking into account the Meteoroid mass (kg) acceleration due to Earth’s gravity. Values from Rogers et al. (2005) are weighted with that velocity distribution. 3 Figure 2: Fraction of mass ablated in atomic form in dependence of To come closest to an IDP density of 2,200kg/m (Carrillo- meteoroid mass. The crosses depict the data points derived from the S´anchez et al., 2016), the mean of the ablated mass for studies of Rogers et al. (2005) and Love and Brownlee (1991), while two different particle densities of 1,000 and 3,300 kg/m3 the line is the interpolation. is taken. Thus, we yield a final value for the fraction of ablated mass for the different mass bins (see Figure 2). Small meteoroids with masses above 10−8 kg show more two other study interpolations yield a fraction of 85 % and than 90 % ablated mass. Towards lower masses, the frac- 65 %, respectively. Thus, differences to the other two mod- tion of ablated material decreases to almost zero. For this els are large, partly to be explained by the slower average model, we adopt a cutoff size of 10−14 kg and interpolate velocity and the different mass distribution. In the fol- the data (also shown in Figure 2). By weighting with the lowing we use the simulated values of Love and Brownlee Gr¨un et al. (1985) mass influx, an ablated mass fraction (1991). of 86 % is obtained. We assume that all of the ablated material enters the atmosphere in atomic form, as recon- 2.3.2. Large meteoroid ablation densation of the vaporized material to dust is unlikely due For large meteoroids, ablation largely reduces the mass to the small particle masses. of entering meteoroids. However, a substantial fraction Love and Brownlee (1991) have performed a similar can survive atmospheric entry (Rietmeijer, 2002, p. 236; study, simulating the atmospheric entry of over 50,000 Boroviˇcka et al., 2015, p. 258). The survival fraction, meteoroids. Using their data on the amount of vapor- the ratio of the meteoroid’s terminal and initial mass, is ized mass, equal to the mass of atoms ablated, and trans- highly dependent on velocity, density, composition and ini- forming the particle diameter to mass by using a den- tial mass itself. The dependence of the survival fraction sity of 3,000 kg/m3, we yield the values and interpolation on mass is displayed in Figure 3, which is based on results depicted in Figure 2. Here, we adopt a cutoff mass of of Halliday et al. (1996); Klekociuk et al. (2005); Popova 10−13 kg. Weighting with the Gr¨un et al. (1985) mass in- et al. (2011, 2013). Data are widely scattered due to the flux, an ablated mass fraction of 69 % is obtained, which various dependencies mentioned above. An average mass is considerably lower than the Rogers et al. (2005) esti- dependent survival fraction is derived by fitting a scaled mate. Both, Love and Brownlee (1991) and Rogers et al. Rayleigh distribution to all the given data. The starting −1 (2005) show a very small to zero survivability of particles point of the distribution is chosen to be at 10 kg with −2 −1 in the mass range 10−6 to 10−2 kg. This is supported by no mass survival from 10 to 10 kg considering Bald- Rietmeijer (2002). win and Sheaffer (1971). This fits the results by Love and A third study, Carrillo-S´anchez et al. (2016), utilizes Brownlee (1991); Rietmeijer (2002); Rogers et al. (2005) the chemical ablation model CABMOD, introduced by and also matches our model of the small meteoroid abla- Vondrak et al. (2008) incorporating differential ablation tion. The resulting survival fraction model S(m) with of different elements along with the model of the zodiacal cloud (Nesvorn´yet al., 2010, 2011). They use a different 0 , −2 ≤ log m< −1 mass distribution and a different velocity distribution with m 2 S(m)= log(m)+1 − log( )+1 ( 2·1.34 ) − ≤ ≤ a lower mean velocity than used in our study. For their ( 0.67 1.342 e , 1 log m 8 mass range of 10−6 kg to 10−12 kg, only 18.2 % of the ma- (1) terial are ablated atoms. Taking the same mass range, the 6 0.9

Average (fitted) 0.8 Halliday et al. (1996) Popova et al. (2011) 0.7 Klekociuk et al. (2005) Popova et al. (2013)

0.6

0.5

0.4 Survival fraction 0.3

0.2

0.1

0 10-2 10-1 100 101 102 103 104 105 106 107 108 Meteoroid mass (kg)

Figure 3: Survival rate of large meteoroids. Shown are fireball data from different studies. Errors are depicted if available. The black line depicts our fitted estimate; for further details see the text. where S denotes the survival fraction, and log m the decadic from the previous two sections. For nearly every meteoroid logarithm of the meteoroid initial mass, is displayed in Fig- mass, the material is injected into the atmosphere either ure 3. in atomic or aerosol form. Only for a very limited mass In a further step one needs to clarify how much of the range significant amounts of the entering meteoroids reach ablated material is deposited in the atmosphere in atomic the ground directly upon entry into the atmosphere. or aerosol form. Here, ablated material can recondense to dust and also dust particles can leave the fireball as it is 2.4. Overall natural injection ablated. Observations of dust clouds are very rare and in- With the estimates of the mass distribution, the com- corporate large errors. The Chelyabinsk object created a position, and the ablation of incoming meteoroids a com- dust cloud of roughly 24 % of the initial mass of approx. plete picture of the injection of natural matter into Earth’s 7 1.3 · 10 kg (Popova et al., 2013). Klekociuk et al. (2005) atmosphere is available. The following estimates emerge. provide observational results from an entry of a massive 12,325t natural material are entering Earth’s atmosphere 6 meteoroid (roughly 1.4·10 kg). The dust cloud amounted every year. Only 48t/yr of meteoroids are reaching the to roughly 79% of the total meteoroid mass, at least 47%. ground (0.4% of the whole mass) directly upon entry. The 4 TC3, a smaller bolide of around 5 · 10 kg produced a dust rest is injected into the atmosphere, 8,421t (68% of the cloud of about 20 % (at least 15 %) of the initial mete- whole mass) in atomic form, 3,904 t (32 %) as aerosols. oroid mass (Boroviˇcka and Charv´at, 2009). Detailed mod- Most of the material is non-metallic (5,674 t), but metals elling of a fireball entry by Boroviˇcka et al. (2019) indi- are also significant (4,047 t). Metalloids take the small- cates that more fragmentation leads to more dust being est portion (1,655t). The most abundant metals are iron released. This would point towards an increase in the dust (2,295 t) and magnesium (1,300 t), other metals only con- fraction for larger meteoroids as fragmentation events are tribute with minor fractions, e. g. aluminum (131 t), nickel more likely for larger meteoroids. Therefore, we assume a (90 t), calcium (88 t) and sodium (59 t). Non-metallic and linear increase in the aerosol fraction with increasing log- metalloid elements with high injection masses are oxygen −2 arithm of mass. For meteoroid masses of 10 kg, all the (3,851 t), silicon (1,654 t), carbon (1,054 t), sulfur (513 t) material is ablated in atomic form in accordance with the and hydrogen (220 t). findings in the previous section. The aerosol mass fraction increases to 50% for a mass of 107 kg. 3. Anthropogenic injection 2.3.3. Resulting mass dependent ablation Since the beginning of the space age, anthropogenic The mass dependent fraction of the three different abla- injection into the upper atmosphere occurs. Decommis- tion products is displayed in Figure 4, based on the results 7 1

0.9

0.8

0.7

0.6

0.5

0.4 Mass fraction 0.3

0.2

0.1

0 10-20 10-15 10-10 10-5 100 105 Meteoroid mass (kg) Aerosol Atomic Meteorites (Ground-reaching)

Figure 4: Estimated fractional mass ablation of meteoroids in dependence of the meteoroid mass. Values are given for each mass decade. For details on the derivation, see the text. Note that all of those values are rough estimates. sioned spacecraft, rocket bodies, and other debris are en- of 450 km, roughly matching calculations by Braun et al. tering Earth’s atmosphere. This is due to the aerodynamic (2013). As a result, the annual mass influx amounts to drag of the atmosphere, which is reducing the speed of or- about 190 t/yr, a value comparable to that one estimated biting objects even at altitudes as high as 1000 km. Most by Pardini and Anselmo (2013). About 60% of the mass of the space components are made of metals. are spacecraft, 40% rocket bodies. This mass influx value With the ongoing use of space, more and more space will increase in the future as the amount of mass in orbit is debris is inserted into orbits around Earth. Space debris rising continuously (Liou et al., 2020). It should be noted has become a serious problem as it is a hazard to operat- that space debris is large in numbers but contributes only ing spacecraft and even the International Space Station. a negligible part to the anthropogenic mass influx. All the debris in LEO can lead to a cascade effect of de- In a further step, the core stages of launch vehicles bris impacting satellites causing even more debris and so need to be considered. Although these are accelerated to forth, possibly rendering whole orbits unusable for decades considerable velocities, they remain suborbital and reen- to hundreds of years. In order to reduce the amount of fu- ter the atmosphere right after liftoff. These objects are ture space debris and ensure safety in space, guidelines not tracked by space agencies and therefore are not in- have been introduced (ESA, 2008; ISO Central Secretary, cluded in the studies by Pardini and Anselmo (2013) and 2019; NASA, 2019). For LEO, satellites and upper stages Liou et al. (2018). To estimate the mass contribution by have to be de-orbited within 25 years after their end of core stages, we consider the launch history of 2019. From lifetime. Due to the increasing use of space, these require- each orbital launch, we consider every rocket stage that ments might tighten in the following years and decades. is jettisoned into a suborbital trajectory. Using available Thus, nearly everything launched into LEO nowadays will data on launch profiles, the approximate entry velocity of burn up in the atmosphere, eventually. each stage can be calculated. Only stages with entry ve- locities higher than 3.8 km/s are taken into account as we 3.1. Today’s annual anthropogenic mass influx consider lower entry velocities to be insufficient for signif- We derive an estimate of the annual mass influx from icant ablation and contribution to the injection rate into the altitude depending mass distribution around Earth as the atmosphere. Additionally, we neglect launch vehicles provided by Liou et al. (2018). Due to the atmospheric with a payload mass lower than 1 t as well as suborbital drag, there is an altitude below which, on average, all ob- rocket launches. A complete list of the data and sources jects reenter the atmosphere within one year. With that, is given in Appendix C. Using the mass of each stage, the mass entering every year is the sum of the mass of all we estimate about 702t of rocket stage mass reentering objects below this altitude. Using Boykin and Mc Nair Earth’s atmosphere in 2019, with speeds from 3.8 to km/s (1966) together with data from Bowman (2002); Saunders to 7.6 km/s. This mass value will increase in the near fu- et al. (2012), we come up with an average reentry altitude ture, too. Summarizing, today’s (2019) annual mass influx

8 from anthropogenic sources amounts to about 890t. The projects from Table 3 with bold and normal font type num- largest contribution (87 %) are rocket bodies. ber of satellites). We expect all satellites in LEO to reen- ter Earth’s atmosphere. However, we do not expect the 3.2. Satellite constellations satellites of OneWeb’s 8,500 km height constellation (see With evermore companies engaging in commercial space- Table 3) to enter the atmosphere; only their upper and flight, satellite constellation projects have been proposed core stages will reenter and are taken into account. All in and some of them already started. Mostly, these projects all, in 5 years additional 19,411 satellites as well as 585 aim at providing global telecommunication services for the upper stages and 443 core stages will be brought to orbit. global internet (Liou et al., 2018). Therefore, hundreds to With the above mentioned lifetime, failure rate and upper thousands of satellites used as relays will be brought to stage and core stage per payload mass, every year, 960t of LEO and eventually, after reaching their end of lifetime, satellites, 291 t upper stages, and 1,491 t core stages will burn up in the atmosphere. In Table 3, proposed and reenter the atmosphere. All in all, the annual mass influx (partially) realized large satellite constellation projects are amounts to 2,742 t/yr. However, a significant portion of listed with additional information such as characteristics that material may reach the ground, as discussed further of the satellites or the current status. In total, constella- down. tions of nearly 110,000 satellites have been proposed. The two constellations of SpaceX and OneWeb are about to op- 3.2.2. Future Scenario 2 — Maximum influx erate soon (Arianespace, 2020; Krebs, 2020d), others will This scenario assumes a doubling of the 2019 mass in- most certainly follow. flux. To this influx all of the planned satellite constellation Constellation satellites are launched into relatively low projects are added. However, from the projected addi- orbits because of the effective range of antennas and la- tional 30,000 Starlink and nearly 48,000 OneWeb satel- tency. Some satellites even have to raise and retain their lites we merely assume 50 % of them being realized. Thus, orbit by themselves using on-board propulsion. Most likely, within 5 years, nearly 75,000 additional satellites, 1,984 due to the large number of satellites, some of the satel- upper stages, and 1,503 core stages are considered in this lites will fail in orbit due to electronic failure or propul- scenario. The total mass influx is estimated at 8,114 t/yr, sion problems. From the first Starlink launch, 3 out of consisting of 3,153 t satellites, 880 t upper stages, and 4,081 t 60 satellites did not seem to work properly. Thus, we as- core stages. sume a failure rate of 5 % for the satellites, which have to be replaced. This increases the mass estimate. Ad- 3.3. Composition of the anthropogenic material ditionally, we expect most of the satellites at high orbits The composition of satellites and rocket bodies differs (around 1,000km) to be de-orbited after service, as many largely from the composition of meteoroids. Generally, the companies have already vowed to do so. metal abundance is way higher as most structural compo- In order to estimate the total mass influx caused by nents are made of alloys. We distinguish between rocket constellations, upper stages and core stages of the launch- bodies (core stages that reenter right after launch and up- ing rockets have to be incorporated, too. Depending on per stages which reach orbit) and spacecraft. the launch vehicle, the upper stage mass relative to the Rocket bodies mainly consist of propulsion tanks and payload mass is different. Typical launch vehicles are the rocket engines. For composition information of the propul- Falcon 9 and the Soyuz 2.1 Fregat rockets. The Falcon 9 sion tanks we use the information provided by Henson payload is 15.6 t (derived from Krebs, 2020d), that one of (2018). Tanks have to withstand the pressure of the loaded the Soyuz 2.1 Fregat 5 t (Arianespace, 2020). With their fuel and thus are made out of durable alloys. Most com- respective upper stage and core stage mass (Kyle, 2018; mon is AA2219 aluminum alloy but newer rockets like Arianespace, 2012) one can estimate a typical ratio of pay- SpaceX’ Falcon 9 are also made out of Al-Li alloys like load to upper (core) stage mass of 0.24t (0.89t) per ton AA2198 (Wanhill, 2014). On the other hand, the Centaur launched and reentering within years. This is included into tank is manufactured out of 301 stainless steel. Those the subsequent calculations. Based on statements from rockets using solid rocket motors (e.g. Antares or Vega some companies, we expect the lifetime for each satellite launch vehicles) use D6AC steel or similar for their tanks. to be around 5 years. This means, every 5 years a whole Other parts like feedlines, other pressured vessels, and un- constellation has to be replaced. pressurized structure are made out of steels, Al-alloys and In the following, we consider two scenarios for a possi- Ti-alloys. Non-metallic materials are rare. They are, for ble future mass influx: example, found in thermal insulation and adapters. Based on the available information and respecting the large vari- 3.2.1. Future Scenario 1 — Probable influx ety of materials we assume the following relative composi- This scenario assumes today’s, 2019 based mass influx. tion values as a first estimate: 80 % AA2219 Al-alloy, 5 % To this influx we add that one due to satellite constella- AA2198 Al-Li-alloy, 5 % D6AC steel, 5 % 301 steel, 5 % tions most probably installed in the future (constellation others.

9 Table 3: Satellite constellation projects with more than 100 satellites and satellite masses greater than 10 kg.

Project Country Satellite Proposed Height Project Status Sources mass (kg) sat. number (km)

Starlink USA 260 7518 335–346 538 launched, of that 60 a (SpaceX) 1548+2824 540–570 prototypes (Krebs, 2020c,d), FCC (2018a,b, 2019); 30000 300–600 filing for 30000 more Space Exploration satellites to FCC prepared Technologies (Koksal, 2019) Corporation (2020) OneWeb UK 147 720b 1200 74 satellites launched 1280 8500 (Arianespace, 2020), LEO FCC (2017, 2020); c 1280 8500 extension to 47844 satellites WorldVu Satellites 47128 1200 requested in 2020 Limited (2018, 2017, 2020) Telesat LEO Canada 168d 117+292 1000 1 prototype launched (Telesat) Grant (2019); FCC (2018c) Kepler Canada 12–15 140 500–600 2 prototypes (3U Cubesats) (Kepler Com- launched FCC (2018c); Grant munications) (2019) Project USA 3236 590–630 FCC filing submitted in 2019 Kuiper Kuiper Systems LLC (Amazon) (2019) Hongyun China 247d 320 1000 1 prototype launched (CASC) Grant (2019) Xingyun China 93 156 570 2 satellites launched in 2020 (CASIC) Grant (2019) Satellogic Argentina 37 300 500 8 satellites launched since 2016 Lal et al. (2017) Boeing USA 3000 1200 Boeing withdrew several FCC filings and Grant (2019); Lal et al. (2017) Samsung South 4600 300–2000 No perceptible action since Korea 2015 Khan (2015) MCSat France 800 +4000 Beside filings at the (Thales) International Grant (2019) Telecommunication Union (ITU), no information available 3ECOM-1 Lichtenstein 288 Beside ITU filings, no information available Grant (2019)

The list might not be complete as companies in this area emerge and disappear rapidly. Reliable information are hard to gather. The font used for the proposed satellite number (column 4) indicates the probability of realization: an italic font means that the realization is not safe or improbable, normal font implies a high probability for the project realization, and a bold font means that the constellation is granted by the FCC of the USA and will therefore most likely be materialized or is already in process. Additional information is acquired from company websites and satellite launch data. The projects are ordered after their probability of realization and the number of spacecraft. a SpaceX currently has approval for 1,548 satellites at 550 km altitude and 2,825 satellites at 1,110–1,325 km altitude. They have filed for a modification of the orbit altitude to 540–570 km and reduction to 2,824 satellites. As earlier modifications of this kind were successful, it is likely this gets granted. b OneWeb has authorization of the launch of 720 satellites at this altitude but company statements suggest only 648 are needed. c OneWeb filed at the FCC for a doubling of that number, but has withdrawn that request. d Prototype mass.

10 Liquid fuelled engines make up the other part of a 3.4. Atmospheric processing of the anthropogenic material rocket body. They need even more resistant alloys, es- Spacecraft and upper stages entering the atmosphere pecially due to the high temperature load in combustion have a much longer interaction time than meteoroids due chambers. According to Halchak et al. (2018), different to their shallower entry angle and small entry velocity of groups of alloys are used. For the nozzle, the most heavy about 7km/s. This is due to these anthropogenic ob- part of an engine, Ni-alloys like Inconel 718 (used in the jects approaching almost circular orbits during the reentry Space Shuttle main engine RS-25) or Inconel 600 (Vulcain phase. Therefore, their ablation is largely different from 2) are the most common alloys. The much used RD-107 that one of meteoroids. Anthropogenic material reaches and RD-108 engines of the Soyuz launch vehicle incorpo- temperatures of 850 to 1950K (Rochelle et al., 1997; Ailor rate Copper-Chromium alloys (3 % Cr). A similar alloy et al., 2005; Lips et al., 2017) while meteoroids and fireballs called Narloy-Z has been used in the combustion chamber can reach temperatures above 3000K (Boroviˇcka, 1993; of the RS-25. RD-107 and RD-108 nozzles are additionally Jenniskens, 2004). This strong difference in temperature made of stainless steel, nickel-based alloys or other metals. and the different materials imply different ratios of the Other components of engines like propellant pumps, com- atomic and aerosol ablation for meteoroids/fireballs and ponents of turbopumps, valves, and feed lines are made of anthropogenic material. In need of better data, we as- different alloys, typically Al-, Ti-, or Ni-based due to the sume a higher aerosol fraction (75%) for the anthropogenic required durability. Only a minor portion is made of non- material while that one of meteoroids is only about 30 % metallic components like silicon carbide. Again, based on (compare with Figure 4). This is due to higher tempera- the variety of different materials being in use, we use the tures causing transition into the gas and/or plasma phase. following estimate for the composition of the rockets’ en- Due to the lower ablation temperature and the high gines: Inconel 718 Ni-alloy, Inconel 600 Ni-alloy, and A286 mass (in the order of tons) of the anthropogenic material, alloy each 25 %, 10 % Cu-Cr-alloy (alloy with 3 % Cr), and a significant fraction of their mass reaches the ground. To- 5 % Ti, 5 % Al, and 5 % Ni. day, survival rates of human-made objects are expected to We additionally differentiate between core stages and range between 5 to 40% (Ailor et al., 2005; Anselmo and upper stages. For upper stages, the engine mass portion Pardini, 2005; Pardini and Anselmo, 2019). Simulations of the whole rocket body normally is lower than for core with reentry software indicate values in this range, too stage. Looking at the Ariane 5, Soyuz 2.1 Fregat, and (Anselmo and Pardini, 2005; Klinkrad et al., 2006; Kelley Atlas V launch vehicles, we estimate 8 % mass (dry mass) et al., 2010). The thermal ablation of spacecraft and up- of upper stages taken up by the engine, while for core per stages differs as well due to the different structure. For stages it is 18 %. spacecraft, we assume an average survivability of 20%, for Satellites differ largely in composition and size depend- upper stages 35%, and for core stages 70%. Large constel- ing on their field of use, mission lifetime, etc. Finckenor lation satellites are estimated to burn up completely in the (2018) provides some insight into materials used in space- atmosphere (Space Exploration Technologies Corporation, craft. The structure mainly consists of Al-alloys, Ti-alloys, 2016, 2018; WorldVu Satellites Limited, 2016). or stainless steel. Even Ni-alloys are in use, if more dura- bility is needed. Aluminum is a preferred material as it 3.5. Overall anthropogenic injection is lightweight. Generally, most structures are made out of Combining all the information about the annual an- metals. However, some parts of the spacecraft, e. g. the thropogenic mass influx, composition, and atmospheric outer hull exposed to the sun, are constructed out of ma- processing provides the following estimates for today’s in- terial capable of withstanding thermal expansion. These jection and the two different future scenarios emerge. materials are mostly non-metallic, for example polyimide, graphite or fiberglass. Non-metallic materials can also be 3.5.1. Today’s influx found in orbital debris and meteoroid shielding, although Currently, 892t of anthropogenic material enters Earth’s Al-sheets are common. As many spacecraft in LEO are Earth observation satellites, optical materials like quartz atmosphere every year of which 88t are injected in atomic or mirror material also play a role in the overall mass com- form; aerosols make up 263 t. The remaining material (541 t) reaches the ground. From the injected elements, position. A very important mass contributor of satellites are solar arrays. We estimate their mass contribution to be aluminum is most abundant with 211 t, followed by iron (36 t), nickel (23 t), and copper (15 t). Metals make up at 16%. We assume half of the solar arrays to be old-fashion least 86 % of the injected material. Si solar arrays, whereas the other half use newer technol- ogy with multijunction solar cells made of layers of Ge, 3.5.2. Future Scenario 1 Ga-As, and Ga-In-P. All in all, we assume the following mass composition: 40 % Al, 5 % Ni, 5 % Ti, 10 % Fe, 8 % For Scenario 1, the annual anthropogenic mass influx Si, 4 % Ge, and Ga, In, P, and As each 1 %. The remaining increases drastically to 2,742 t. 1,573 t are injected into 24 % are other materials. We expect the large constella- the atmosphere, 1,180 t as aerosols, 393 t in atomic form. tion satellites to also match this composition reasonably Again, aluminum is the largest part of the injection with well. 807 t, followed by iron (159 t), nickel (89 t), and silicon 11 Table 4: Anthropogenic and natural injection for the different ablation products. Masses are given in t/yr. Numbers in parenthesis are the percentage compared to the value of the natural material in the respective column.

Atomic Aerosol Total injection Ground-reaching Today 88 (1.0) 263 (6.7) 351 (2.8) 541 (1,138) Anthropogenic Scenario 1 393 (4.7) 1,180 (30.2) 1,573 (12.8) 1,168 (2,458) Scenario 2 1,226 (14.6) 3,678 (94.2) 4,904 (39.8) 3,210 (6,753) Natural 8,421 3,904 12,325 48

Table 5: Anthropogenic and natural injection per element group. Masses are given in t/yr. The numbers in parenthesis depict the percentage compared to the natural injection value in the respective column.

Metals Metalloids Non-metals Not assignable Total injection Today 305 (7.5) 12 (0.7) 1 (0.02) 33 351 (2.8) Anthropogenic Scenario 1 1,189 (29.4) 123 (7.4) 10 (0.2) 252 1,573 (12.8) Scenario 2 3,643 (90.0) 406 (24.6) 32 (0.6) 822 4,904 (39.8) Natural injection 4,047 1,655 5,674 949 12,325

(76 t). Again, most of the injected material is metal (at with 29.4 % (Scenario 1) and even 90.0 % (Scenario 2) least 75 %). of the natural metal injection, respectively. Additionally, there are more elements for which the anthropogenic in- 3.5.3. Future Scenario 2 jection surpasses the natural injection: for example tita- For Scenario 2, the annual anthropogenic mass influx nium (2459 %), chromium (131 %), and nickel (304 %) for increases even more to 8,114 t from which 4,904 t are in- Scenario 2. Satellite constellations also lead to a massive jected into the atmosphere. Aerosols contribute 3,678 t, enhancement of the injection of aluminum and copper. material of atomic form 1,226t. The order of the most The anthropogenic injection also increases the injection injected elements is the same as in Scenario 1 with alu- of aerosols disproportionally as we estimate the entry of minum (2,467 t), iron (496 t), nickel (272 t), and silicon human-made objects to produce more aerosols than atoms. (251 t). The metal portion is at least 74 %. Today, human-made bodies make up 6.7 % compared to the natural injection, while atomic material is only at 1.0 %. For future satellite constellations, the aerosol frac- 4. Final results and comparison tion increases to 30.2 % and 94.2 % for the two scenarios.

With all the available information, the natural and Table 6: Anthropogenic and natural injection of some selected ele- anthropogenic injection can be tabulated and compared. ments. Masses are given in t/yr. The numbers in parenthesis depict Three aspects have been evaluated in this study: the in- the percentage compared to the natural injection value in the respec- jection by ablation products (Table 4), by element group tive row. Note that percentages larger than 100 % indicate that these elements are mainly of anthropogenic origin. For some elements, no (Table 5), and the injection of selected elements (Table 6). anthropogenic abundances were calculated. Today, the injection into the atmosphere is dominated by natural material. About 2.8 % of the mass is of human Anthropogenic Natural El. origin. Although metals are highly abundant in space- Today Scenario 1 Scenario 2 injection craft and rocket bodies, the anthropogenic metal injection H 220 is also well below the natural metal injection. However, C 0.1 (0) 0.2 (0) 0.5 (0) 1,054 there are elements which are injected mainly by human- O 3,851 made objects, for example aluminum or copper. The an- Mg 0.04 (0) 0.1 (0) 0.3 (0) 1,300 thropogenic injection can also prevail the natural injection Al 211 (161) 807 (614) 2,467 (1,877) 131 for some specific elements that are not very abundant in Si 8 (0) 76 (5) 251 (15) 1,654 the solar system and therefore in meteoroids, e.g. germa- S 513 nium. Ti 7 (100) 52 (754) 171 (2,459) 7 With the incorporation of large satellite constellations, Cr 7 (20) 17 (47) 48 (131) 37 the injection situation changes strongly. The near future Fe 36 (2) 160 (7) 496 (22) 2,295 Scenario 1 predicts 1,573 t of anthropogenic material in- Ni 23 (25) 89 (99) 272 (304) 90 jected into the atmosphere, which is already 12.8 % of the Cu 15 (720) 38 (1,747) 106 (4,923) 2 Ge 4 (776) 37 (7,973) 124 (26,435) 0.5 natural injection. For the extreme Scenario 2 we infer an anthropogenic mass injection rate of about 39.8% of the natural rate. For metals, the injection is even higher

12 5. Conclusion at 1AU given as

− The extensive review, analysis, and estimates presented F (m)= (2.2 · 103 m0.306 + 15) 4.38 in this study provide an overview on the natural and an- − − +1.3 · 10 9 (m + 1011 m2 + 1027 m4) 0.36 thropogenic injection of matter into Earth’s atmosphere. −16 6 2 −0.85 At the present time, the anthropogenic injection already +1.3 · 10 (m + 10 m ) (A.1) contributes a non-negligible amount of mass to the injec- tion. With large satellite constellations, proposed and is used. The mass influx in a mass range between the m m started from companies all over the world, the anthro- masses 1 and 2 can be calculated by integrating over F m S pogenic injection will become significant compared to the the flux ( ) and multiplying with Earth’s surface E = π · . · 3 m 2 natural injection. Although many of the values used to es- 4 (6 471 10 ) and the gravity enhancement factor G . timate the injection inhibit uncertainties due to different =1 445 (see Drolshagen et al., 2017). Here, an incident scientific results on many topics or insufficient data, the atmospheric altitude of 100 km is used. Additionally, the T . · 7 results of this study should raise attention and also con- number of seconds in a year = 3 1536 10 has to be cern towards the alteration of Earth’s atmosphere due to multiplied to yield the annual mass influx, then given as the reentry of human-made spacecraft and rocket bodies. m2 Especially looking at metals, the anthropogenic injection MGr¨un = SE · G · T · F (m) dm. (A.2) may well exceed 30 % of the whole material deposited in Zm1 the upper atmosphere every year. Overall, in the near In the mass range from 10−2 to 106 kg and 106 to future we need to be prepared that the injection of an- 1011 kg, the power laws from Brown et al. (2002) and thropogenic material will increase to 12.8 % – 39.8 % of Stokes et al. (2003) the natural injection. Those values clearly show that the −0.9 anthropogenic injection is not negligible in the near future NBrown(E)=3.7 E (A.3) and requires further consideration with respect to their − N (E)=2.4 E 0.79 (A.4) impact on Earth’s atmosphere. Stokes The uncertainties involved demonstrate that more re- are used, respectively. The latter one is obtained from search needs to be done to clarify the significance of the Drolshagen et al. (2017). N(E) represents the cumulative effects of the human use of space on Earth’s habitat. There number of meteoroids with a kinetic energy greater than are many different possible effects on the atmosphere that E impacting Earth every year, where E is in units of kt may be caused by an increased injection. For example, TNT equivalent. With an average meteoroid velocity of the large amount of aerosols injected by the ablation of 20km/s and 1kt TNT equivalent = 4.184 · 1012 J, the en- anthropogenic material may have an effect on Earth’s cli- ergy dependence can be transformed to a dependence of mate as aerosols in the high-altitude atmosphere have a mass: negative radiative forcing effect Lawrence et al. (2018). 4 −0.9 Beside the intensively discussed problem of space de- NBrown(m)=2.86 · 10 m (A.5) bris (e. g. Klinkrad, 2006) we conclude that the re-entry of − N (m)=6.22 · 104 m 0.79 (A.6) human-made objects into the upper atmosphere may have Stokes a significant effect on our habitat and needs more attention with m the mass in kg. So N(m) represents the number of in future studies. Advances in technology and a stronger meteoroids of a mass greater than m hitting Earth every and stronger use of Earth’s environment always have side year. effects that are most often not perceived at the beginning To yield the annual mass influx from these power laws, of innovation and progress. further calculations are necessary (e.g. compare with Bland et al., 1996, Appendix A). The number of meteoroids im- Acknowledgements pacting per year in a mass range from m1 to m2 can be expressed by The authors thank Carsten Wiedemann, Martin Sippel, m Sven Stappert, Gerhard Drolshagen, and J¨urgen Blum for 2 dN(m) N(m1) − N(m2)= − dm. (A.7) helpful discussions. dm Zm1 To yield the annual mass influx M in the respective mass Appendix A. range, the mass has to be incorporated in the integral by multiplication: The annual mass influx per mass decade shown in Fig- ure 1 is derived separately for the three different mass m2 dN(m) ranges. For masses between 10−21 to 10−2 kg, the inter- M = − m dm. (A.8) m dm planetary flux model from Gr¨un et al. (1985) with the flux Z 1 This way, we yield the annual mass influx for both models

13 Table A.7: Annual mass influx of meteoroids into Earth’s atmosphere (Eucr), Howardites (How), Lodranites (Lodr), Shergot- for each mass decade (values of Figure 1). tites (Sher), Nakhlatites (Nak), Chassignites (Chas), Ure- ilites (Ur), and Winonaites (Wino) from Lodders and Fe- m M m M log (kg) (t/yr) log (kg) (t/yr) gley (1998, Tables 16.11, 16.17, and 16.18) and Mittle- -21 0.02 -5 384 fehldt et al. (1998, Tables 6, 8(4), 19, 21, 22, 26, 34, 35, -20 0.03 -4 184 and 40); lunar achondrites, so called Lunaites (Luna) from -19 0.04 -3 86 Demidova et al. (2007); stony irons, namely Mesosiderites (Meso) and Pallasites (Pal) from Mittlefehldt et al. (1998, -18 0.09 -2 42 Tables 13 (main group), 14 (main group), 15 (main group), -17 0.26 -1 53 16 (all except Eagle Station), 17 (all except Eagle Station), -16 1.0 0 67 45(1), 46 (all except the last three)) and Wasson (1974, Ta- -15 4.3 1 84 bles II-5 and II-7(Ni)); and for Irons (IAB to IVB) from -14 18 2 106 Mittlefehldt et al. (1998, Tables 3 and 8 (1 & 9)) and Was- -13 85 3 133 son (1974, Table II-5). In a few cases, some abundances -12 367 4 168 were estimated (mainly oxygen) considering similar mete- -11 1,125 5 211 orite subgroups to complement the data. The total mass -10 2,193 6 (264) abundance of all meteorites is shown in the last column. -9 2,671 7 (429) It is the product of the meteorite abundance with the re- spective elemental composition normalized to 100 % of the -8 2,213 8 (696) mass. Thus, it represents the overall elemental composi- -7 1,410 9 (1,129) tion of meteorites found on Earth. This is used as the -6 768 10 (1,831) average elemental composition of meteorites given in the The mass influx M is given for the interval of the object mass log m last column of Table 1. to log (m) + 1. For example, 384 t of meteoroids in the mass range from 10−5 to 10−4 kg impact Earth every year. The values derived from Stokes et al. (2003) are in parenthesis as they are not included in the annual mass influx in this study.

in the mass range from m1 to m2

m2 4 −0.9 MBrown = 2.57 · 10 m dm (A.9) Zm1 m2 3 −0.79 MStokes = 4.9 · 10 m dm. (A.10) Zm1 Taking the respective valid mass ranges of each model (given above and in Figure 1), the integration yields MGr¨un = 11, 509 t/yr, MBrown = 863t/yr, and MStokes =4, 349 t/yr. The values for each mass decade, depicted in Figure 1, are given in Table A.7. Remember that only the mass values −21 from MGr¨un and MBrown (so masses in a range from 10 to 106 kg) are counted to the annual mass influx used in this paper.

Appendix B.

The elemental mass abundances of meteorites are given in Table B.8. Compositions are derived from the follow- ing sources: Chondrites, namely ordinary chondrites (H, L, and LL), carbonaceous chondrites (CH, CI, CK, CO, CR), Kakangari and Rumurutiites chondrites (K and R), and enstatite chondrites (EH and EL) from Lodders and Fegley (1998, Tables 16.10 and 16.11); most achondrites, namely Acapulcoites (Acap), Angrites (Angr), Aubrites (Aubr), Brachinites (Brac), Diogenites (Diog), Eucrites

14 Table B.8: Elemental compositions of meteorite groups.

Z El. Unit H L LL CH CI CK CM CO CR CV EH EL K R Acap Fraction → wt% 42.27 37.72 6.36 0.08 0.04 0.54 1.18 0.62 0.57 0.36 0.93 0.28 0.13 0.85 0.07 1 H µg/g 20200 14000 700 2800 3 Li µg/g 1.7 1.85 1.8 1.5 1.4 1.5 1.8 1.7 1.9 0.7 4 Be ng/g 30 40 45 25 40 50 21 5 B ng/g 400 400 700 870 480 300 1000 6 C µg/g 2100 2500 3100 7800 34500 2200 22000 4400 20000 5300 3900 4300 580 7 N µg/g 48 43 70 190 3180 1520 90 620 80 420 240 8 O wt% 35.7 37.7 40 30 46.4 38 43.2 37 38 37 28 31 30 34 35.6 9 F µg/g 125 100 70 60 20 38 30 24 155 140 11 Na µg/g 6110 6900 6840 1800 5000 3100 3900 4200 3300 3400 6880 5770 6800 6630 6619 12 Mg wt% 14.1 14.9 15.3 11.3 9.7 14.7 11.5 14.5 13.7 14.3 10.73 13.75 15.4 12.9 15.8 13 Al wt% 1.06 1.16 1.18 1.05 0.865 1.47 1.13 1.4 1.15 1.68 0.82 1 1.3 1.06 1.24 14 Si wt% 17.1 18.6 18.9 13.5 10.64 15.8 12.7 15.8 15 15.7 16.6 18.8 16.9 18 18.0 15 P µg/g 1200 1030 910 950 1100 1030 1210 1030 1120 2130 1250 1400 1706 16 S wt% 2 2.2 2.1 0.35 5.41 1.7 2.7 2.2 1.9 2.2 5.6 3.1 5.5 4.07 2.76 17 Cl µg/g 140 270 200 700 260 430 280 250 570 230 100 19 K µg/g 780 920 880 200 550 290 370 360 315 360 840 700 710 780 516 20 Ca wt% 1.22 1.33 1.32 1.3 0.926 1.7 1.29 1.58 1.29 1.84 0.85 1.02 1.22 0.914 1.20 21 Sc µg/g 7.8 8.1 8 7.5 5.9 11 8.2 9.5 7.8 10.2 6.1 7.7 7.9 7.75 8.63 22 Ti µg/g 630 670 680 650 440 940 550 730 540 870 460 550 700 900 505 23 V µg/g 73 75 76 63 55 96 75 95 74 97 56 64 73 70 87 24 Cr µg/g 3500 3690 3680 3100 2650 3530 3050 3520 3415 3480 3300 3030 3600 3640 4229 25 Mn µg/g 2340 2590 2600 1020 1940 1440 1650 1620 1660 1520 2120 1580 2400 2960 2852 26 Fe wt% 27.2 21.75 19.8 38 18.2 23 21.3 25 23.8 23.5 30.5 24.8 24.7 24.4 22.8 27 Co µg/g 830 580 480 1100 505 620 560 680 640 640 870 720 750 610 789 28 Ni wt% 1.71 1.24 1.06 2.57 1.1 1.31 1.23 1.42 1.31 1.32 1.84 1.47 1.46 1.44 1.44 29 Cu µg/g 94 90 85 120 125 90 130 130 100 104 215 120 110 30 Zn µg/g 47 57 56 40 315 80 180 110 100 110 290 18 145 150 205 31 Ga µg/g 6 5.4 5.3 4.8 9.8 5.2 7.6 7.1 6 6.1 16.7 11 8.2 8.1 8.99 32 Ge µg/g 10 10 10 33 14 26 20 18 16 38 30 16 33 As µg/g 2.2 1.36 1.3 2.3 1.85 1.4 1.8 2 1.5 1.5 3.5 2.2 2.4 1.9 2.19 34 Se µg/g 8 8.5 9 3.9 21 8 12 8 8.2 8.7 25 15 20 14.1 9.75 35 Br µg/g 0.5 0.5 1 1.4 3.5 0.6 3 1.4 1 1.6 2.7 0.8 0.9 0.55 0.2 37 Rb µg/g 2.3 2.8 2.2 2.3 1.6 1.3 1.1 1.2 3.1 2.3 1.7 0.2 38 Sr µg/g 8.8 11 13 7.3 15 10 13 10 14.8 7 9.4 39 Y µg/g 2 1.8 2 1.56 2.7 2 2.4 2.6 1.2 40 Zr µg/g 7.3 6.4 7.4 3.9 8 7 9 5.4 8.9 6.6 7.2 41 Nb ng/g 400 400 250 400 400 500 500 42 Mo µg/g 1.4 1.2 1.1 2 0.92 0.38 1.4 1.7 1.4 1.8 0.9 44 Ru ng/g 1100 750 1600 710 1100 870 1080 970 1200 930 770 850 960 670 45 Rh ng/g 210 155 140 180 160 170 46 Pd ng/g 845 620 560 560 580 630 710 690 710 820 730 47 Ag ng/g 45 50 75 200 160 100 95 100 280 85 50 48 Cd ng/g 5.5 30 40 690 420 8 300 350 705 35 30 20 49 In ng/g 0.8 10 10.5 80 50 25 30 32 85 4 3 4 50 Sn ng/g 350 540 1700 490 790 890 730 680 1360 51 Sb ng/g 66 78 75 90 135 60 130 110 80 85 190 90 150 72 83 52 Te ng/g 520 460 380 2300 800 1300 950 1000 1000 2400 930 2000 1100 53 I ng/g 60 70 430 200 270 200 160 210 80 55 Cs ng/g 98 240 150 190 110 80 84 90 210 125 50 56 Ba µg/g 4.4 4.1 4 3 2.35 4.7 3.1 4.3 3.4 4.55 2.4 2.8 57 La ng/g 301 318 330 290 235 460 320 380 310 469 240 196 320 310 468 59 Ce ng/g 763 970 880 870 620 1270 940 1140 750 1190 650 580 830 59 Pr ng/g 120 140 130 94 137 140 174 100 70 60 Nd ng/g 581 700 650 460 990 626 850 790 919 440 370 62 Sm ng/g 194 203 205 185 150 290 204 250 230 294 140 149 200 180 223 63 Eu ng/g 74 80 78 76 57 110 78 96 80 105 52 54 80 72 96 64 Gd ng/g 275 317 290 290 200 440 290 390 320 405 210 196 65 Tb ng/g 49 59 54 50 37 51 60 50 71 34 32 58 66 Dy ng/g 305 372 360 310 250 490 332 420 280 454 230 245 29 468 67 Ho ng/g 74 89 82 70 56 100 77 96 100 97 50 51 59 68 Er ng/g 213 252 240 160 350 221 305 277 160 160 69 Tm ng/g 33 38 35 40 25 35 40 48 24 23 70 Yb ng/g 203 226 230 210 160 320 215 270 220 312 154 157 215 216 241 71 Lu ng/g 33 34 34 30 25 46 33 39 32 46 25 25 33 32 36 72 Hf ng/g 150 170 170 140 105 250 180 220 150 230 140 210 150 161 73 Ta ng/g 21 21 14 19 74 W ng/g 164 138 115 150 93 180 160 150 110 160 140 140 180 75 Re ng/g 78 47 32 73 38 60 50 58 50 57 55 57 43 60 76 Os ng/g 835 530 410 1150 490 815 670 805 710 800 660 670 550 690 693 77 Ir ng/g 770 490 380 1070 465 760 580 740 670 730 570 560 550 610 798 78 Pt µg/g 1.58 1.09 0.88 1.7 1 1.3 1.1 1.24 0.98 1.25 1.29 1.25 1 1.3 79 Au ng/g 220 156 146 250 145 120 150 190 160 153 330 240 220 183 217 80 Hg ng/g 30 22 310 60 81 Tl ng/g 0.5 2.4 15.5 142 92 40 60 58 100 7 3 20 82 Pb ng/g 240 40 2500 800 1600 2150 1100 1500 240 83 Bi ng/g 5 14 12.5 110 20 71 35 40 54 90 13 25 27 90 Th ng/g 38 42 47 29 58 41 80 42 58 30 38 50 92 U ng/g 13 15 15 8 15 12 18 13 17 9.2 7 25 Total % 101.9 100.8 101.6 99.7 100.4 99.1 100.0 100.7 99.3 99.6 97.2 96.8 98.1 98.4 100.6

15 Table B.8 (continued)

Z El. Unit Angr Aubr Brac Diog Eucr How Lodr Luna Sher Nak Chas Ur Wino Meso Pal Fraction → wt% 0.02 0.28 0.04 0.57 1.22 0.57 0.09 0.11 0.03 0.03 0.03 0.56 0.07 0.29 0.22 1 H µg/g 3 Li µg/g 5.4 3.5 3.7 3.9 1.4 4 Be ng/g 5 B ng/g 6 C µg/g 3000 500 265 200 850 7 N µg/g 8 O wt% 40.9 46.4 36 43.3 42.5 43.0 32.0 43.8 40 39 37 39.4 35 23.5 20.9 9 F µg/g 43.5 57 15 11 Na µg/g 179.2 3692 2805 896 3095 1568 760 2595 9700 3200 920 743 3720 1327 113 12 Mg wt% 7.84 22.7 17.3 14.6 4.8 9.2 16.8 3.8 6.19 7.55 19.2 20.6 14.6 5.2 12.6 13 Al wt% 5.66 0.58 0.60 0.97 6.84 4.50 0.23 11.60 3.42 1.1 0.42 0.26 0.7 2.43 0.024 14 Si wt% 18.8 26.8 17.4 24.4 22.9 23.3 15.1 21.0 23.8 22.3 17.5 18.2 15 12.4 8.3 15 P µg/g 556 1200 724 720 1787 1484 370 2715 1230 275 1463 10355 16 S wt% 0.59 0.61 1.52 0.12 0.138 0.077 0.62 0.16 0.0335 0.026 2 1.78 1.31 17 Cl µg/g 23 19 122.5 72.5 34 19 K µg/g 179 370 29.5 331 138 64 536 1305 985 300 66 337 166 5.4 20 Ca wt% 11.5 0.62 1.68 1.43 7.27 4.72 1.34 10.46 7.18 10.05 0.47 0.93 0.00082 2.22 0.26 21 Sc µg/g 43.8 10.9 17.5 51.8 20.7 7.2 24.5 53.5 54.5 5.3 8.1 8.1 16.5 0.64 22 Ti µg/g 7209 644 1160 862 3432 2775 320 5687 4810 2280 480 600 859 15 23 V µg/g 93 115 75 117 78 23 300 180 40 118 55 40 24 Cr µg/g 1078 182 4045 8889 2657 5490 4536 1221 1805 1525 5240 8391 1950 5992 5622 25 Mn µg/g 1433 480 2565 4294 4079 4002 2739 1042 3945 3850 4120 3024 2070 2478 1887 26 Fe wt% 13.9 1.0 22.8 13.3 12.6 13.4 31.1 7.7 14.6 16.4 21.2 13.8 19.8 46.3 46.2 27 Co µg/g 23.5 299 20.4 7.0 23.4 783 25.6 38 45.5 123 102 761 1199 3.5 28 Ni wt% 0.0047 0.41 0.0054 0.0012 0.0244 1.3 0.0119 0.00635 0.0093 0.05 1.02 1.21 4.52 10.00 29 Cu µg/g 2.5 8 16 12 2.6 30 Zn µg/g 1.47 239 0.71 1.24 26 120 64.5 66 72 230 131 3.0 31 Ga µg/g 4.9 0.18 1.7 0.75 6.0 3.6 15 3 0.7 4.9 10.9 32 Ge µg/g 0.020 0.14 0.765 2.75 0.01 37.5 33 As µg/g 0.37 0.197 1.5 0.0375 0.0825 0.008 269 2.57 34 Se µg/g 8.0 0.4 0.23 0.25 6.3 0.35 0.075 0.04 1.6 11.2 9.6 35 Br µg/g 0.41 0.10 0.21 0.855 2.435 0.088 0.27 37 Rb µg/g 2 0.1 0.13 0.32 3.1 6 3.3 0.73 38 Sr µg/g 15 1.7 65.9 31 143 46.5 67 7.2 39 Y µg/g 1.2 17.8 19 3.85 0.6 40 Zr µg/g 2.7 3 30 17 80 65 9.1 2.1 41 Nb ng/g 2700 5050 1530 340 42 Mo µg/g 0.015 0.37 0.086 44 Ru ng/g 45 Rh ng/g 46 Pd ng/g 2 0.4 10 1.75 15.85 0.15 47 Ag ng/g 11 30.0 19 49 2.6 48 Cd ng/g 21 13 47 94 14 19 49 In ng/g 3.2 0.92 25 20 4 50 Sn ng/g 10 600 51 Sb ng/g 56 11 7.2 62 45 7.1 40 0.9 121 52 Te ng/g 5 5.3 2.45 4.75 50 53 I ng/g 25 40 97 24 140 10 55 Cs ng/g 200 1.1 3.85 20 400 355 37 56 Ba µg/g 12 34.1 14 65 30 28 7.6 57 La ng/g 3545 365 154 2373 1214 80 5373 1835 1960 530 69 190 1366 59 Ce ng/g 10350 1600 315 7185 2668 14065 4600 5345 1120 4698 59 Pr ng/g 970 280 810 735 130 60 Nd ng/g 860 110 4960 1400 9228 3475 3160 620 62 Sm ng/g 2828 135 147 1440 680 58 2785 1290 805 140 26 90 308 63 Eu ng/g 956 57 41 560 276 27 925 537.5 230 45 10 48 173 64 Gd ng/g 240 2348 905 744 2540 890 110 65 Tb ng/g 803 65.5 409 200 23 653 380 120 30 82 66 Dy ng/g 175 2990 893 313 930 2850 860 200 67 Ho ng/g 48 758.75 230 211 710 162.5 44 68 Er ng/g 140 1740 590 1740 385 90 69 Tm ng/g 50 20 280 1000 300 52 70 Yb ng/g 2590 154 281 1526 790 173 2343 1450 360 110 73 157 408 71 Lu ng/g 380 43 23 228 124 30 336 227.5 53.5 15 12 24 61 72 Hf ng/g 1800 205 1317 1760 80 2044 1850 275 100 255 73 Ta ng/g 240 29 193 84 148 225 40045 20 74 W ng/g 6.5 30 558 535 260 46 75 Re ng/g 0.06 0.0053 0.0375 0.033 0.06 99 76 Os ng/g 0.7 0.008 460 0.26 0.3035 1.6 1220 77 Ir ng/g 2.4 117 3.6 4.57 14.9 194 5.1 0.045 0.155 2.1 247 1150 35 78 Pt µg/g 0.0017 0.0027 0.0053 79 Au ng/g 38 0.9 2.7 169 2.8 1.46 0.36 0.73 30 265 33 80 Hg ng/g 81 Tl ng/g 12.5 5.25 3.7 82 Pb ng/g 83 Bi ng/g 0.2 2.1 1.2 2.9 0.4 90 Th ng/g 447 130 303 110 872 375 175 57 92 U ng/g 4.4 109 33 211 112.5 49 18 Total % 100.2 99.2 99.0 100.0 98.7 99.8 99.6 99.6 97.9 97.8 97.1 95.6 89.2 99.8 101.3

16 Table B.8 (continued)

Z El. Unit IAB IC IIAB IIC IID IIE IIF IIIAB IIICD IIIE IIIF IVA IVB Mean Fraction → wt% 0.76 0.06 0.60 0.05 0.09 0.11 0.03 1.34 0.24 0.08 0.04 0.37 0.08 100.00 1 H µg/g 185.6 3 Li µg/g 1.66 4 Be ng/g 31 5 B ng/g 377 6 C µg/g 2100 2000 140 2100 200 40 2566 7 N µg/g 69 8 O wt% 7.2 7.2 35.2 9 F µg/g 97 11 Na µg/g 2151 2151 5936 12 Mg wt% 3.1 3.1 13.6 13 Al wt% 0.28 0.28 1.16 14 Si wt% 4.0 4.0 17.1 15 P µg/g 3400 4300 12000 5300 9800 2600 7450 3400 5600 2200 2193 1000 1292 16 S wt% 0.60 8.6 4.1 0.60 0.9 0.03 2.10 17 Cl µg/g 188 19 K µg/g 166 166 764 20 Ca wt% 0.24 0.24 1.32 21 Sc µg/g 11.8 11.8 8.4 22 Ti µg/g 120 120 676 23 V µg/g 22 22 71 24 Cr µg/g 111 70 38 87 31 40 111 40 210 140 87.7 3466 25 Mn µg/g 232 232 2355 26 Fe wt% 74.5 90 82 86 85 90 84 84 74.5 88 89 88 80 25.9 27 Co µg/g 3050 4600 5300 6500 4700 4700 7000 5000 3050 5000 3600 4000 7400 808 28 Ni wt% 7.74 7.1 5.8 10.8 11.2 9.13 12.9 8.14 7.74 8.49 7.95 8.2 17.1 1.67 29 Cu µg/g 132 160 129 260 280 416 300 160 132 160 170 150 12 91 30 Zn µg/g 170 170 57 31 Ga µg/g 52 51.7 58.3 37.3 76.2 24.4 9.8 19.1 52 17.6 6.82 2.15 0.0221 6.7 32 Ge µg/g 208 230 119 94.6 87 68.3 140 37.5 208 35.6 0.91 0.13 0.053 13.4 33 As µg/g 11 11 9.9 8.2 10 16 16 10.5 11 10.5 11 7.6 1.1 3.5 34 Se µg/g 7.9 35 Br µg/g 0.56 37 Rb µg/g 2.2 38 Sr µg/g 10.3 39 Y µg/g 1.9 40 Zr µg/g 6.8 41 Nb ng/g 363 42 Mo µg/g 8.2 7.7 6.9 8.4 9.4 6.8 7.2 8.2 7.2 7.2 5.9 27 1.4 44 Ru ng/g 795 45 Rh ng/g 149 46 Pd ng/g 3500 3500 2600 6000 5300 5300 3500 3500 3500 4400 4600 3000 787 47 Ag ng/g 49 48 Cd ng/g 31 49 In ng/g 7 50 Sn ng/g 385 51 Sb ng/g 270 98 201 150 220 300 250 265 270 265 86 9 1.5 77 52 Te ng/g 477 53 I ng/g 61 55 Cs ng/g 146 56 Ba µg/g 4.4 57 La ng/g 550 550 338 59 Ce ng/g 924 59 Pr ng/g 129 60 Nd ng/g 657 62 Sm ng/g 550 550 215 63 Eu ng/g 120 120 82 64 Gd ng/g 302 65 Tb ng/g 56 66 Dy ng/g 350 67 Ho ng/g 85 68 Er ng/g 233 69 Tm ng/g 41 70 Yb ng/g 410 410 231 71 Lu ng/g 45 45 35 72 Hf ng/g 178 73 Ta ng/g 32 74 W ng/g 1600 1300 2100 2400 780 1000 1000 1600 1000 1200 600 3000 188 75 Re ng/g 55 76 Os ng/g 614 77 Ir ng/g 1470 380 7433 6400 9900 4100 6200 4200 1470 4100 3200 1900 22000 728 78 Pt µg/g 1.2 79 Au ng/g 855 1140 1050 1100 1100 1600 1500 1013 855 1200 910 1550 150 211 80 Hg ng/g 13 81 Tl ng/g 5 82 Pb ng/g 171 83 Bi ng/g 11 90 Th ng/g 43 92 U ng/g 15 Total % 98.8 98.0 98.3 98.0 97.7 99.7 97.9 97.6 98.8 97.6 97.6 97.8 98.0 100.0

17 Appendix C.

Table C.9 provides an overview over the rocket launches of 2019 and the respective reentering core stages.

18 Table C.9: Rocket launches of 2019 with the mass and velocity of reentering core stages. Only stages with a significant velocity are considered.

Launch vehicle Launches Stage Stage 2019 Reentry Sources (2019) number mass (t) mass (t) speed (km/s) Soyuz 2.1a,b/Soyuz-FG 7 2 6.55 45.8 4.3 SciNews (2018); Arianespace (2012) Soyuz 2.1a,b/Soyuz-FG 2 6.55 58.9 3.9 arianespace (2019b); 9 with Fregat upper stage 3 2.36 21.2 7.0 Arianespace (2012) Soyuz 2.1v Volga 2 2 2.36 4.7 Krebs (2020b); Arianespace (2012) Falcon 9a 11 0 2 11.0 55.0 4.6 International Launch Services Proton 5 3 3.5 17.5 7.3 (2009) 2 5.4 27.0 4.0 Bergin and Graham (2018); PSLV 5 3 0.7 3.5 6.1 Kyle (2019c) Ariane 5 4 2 14.7 58.8 6.9 Arianespace (2016); arianespace (2019a) Falcon Heavya 2 0 Atlas 5 2 2b 21.1 42.2 4.9 United Launch Alliance (2010, 2018) Delta4Medium 2 2b 27.5 55.0 4.9 United Launch Alliance (2013); Kyle (2019b) Antares 2 1 20.6 41.2 3.8 Spaceflight101 (2020, 2016) Rokot 2 2 1.5 3.0 5.9 Kyle (2019d) 2 2.5 5.0 4.0 Vega 2 (1) Arianespace (2014) 3 1.4 2.8 7.6 Delta4Heavy 1 2 27.5 27.5 6.0 United Launch Alliance (2013); Kyle (2019b) H-2B 1 2 24.2 24.2 5.5 JAXA — (2019); Japan Aerospace Explo- ration Agency (2003) GSLVMK3 1 2 9 9 4.6 Indian Space Research Organisation (2017) CZ-2C 1 1 10 10 Kyle (2020a); Krebs (2020a) CZ-2D 1 1 10 10 Kyle (2020a); Krebs (2020a) 2 10 80 CZ-3B, 4 stage version 8 Kyle (2020a); Krebs (2020a) 3 4 32 3 4 12 CZ-3B, 5 stage version 3 Kyle (2020a); Krebs (2020a) 4 2.8 8.4 2 10 10 CZ-3C 1 Kyle (2020a); Krebs (2020a) 3 4 4 CZ-4B/C 7 (1) 2 4 28 Kyle (2020a); Krebs (2020a) CZ-5 1 2 3.52 3.52 Kyle (2020b) CZ-6 1 2 1.49 1.49 Kyle (2020b) Sum 81(2) 702

Small rockets (launch payload <1t)/Suborbital launches Smallrockets 21(3) Ballistic missile test 11 Other suborbital launches 3 The number of launches of each launch vehicle is retrieved from Kyle (2019a), for the chinese rockets (CZ-X) also from Krebs (2020a). Numbers in parenthesis in column 2 depict the number of failures. The stage numbering in column 3 considers booster stages as first stages. The stages’ masses are retrieved mostly from data sheets and information available online. The reentry velocity is calculated from the velocity and altitude at stage separation (if data is available). Small rockets and suborbital flights are given but neglected in the calculations as the reentering mass is comparably low. a First stage (for Falcon Heavy also the second stages) perform a controlled landing, thus do not ablate in the atmosphere. b Given that the rocket is configured with boosters, otherwise this is stage 1.

19 References

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