A.V. Arkhipov

Institute of Radio astronomy, Kharkov 310002, Ukraine

Astrophysics and Space Science, 1997, vol 252, p. 67-71

It is shown that the Earth is a natural collector of extraterrestial nonsterile artefacts that could impact our planet. Artefacts from 1.2*106 nearby stars could have reached the Earth over its history, and could be agents for spontaneous interstellar panspermia, even if alien civilizations pollute space only at the current terrestrial rate.

1. Introduction.

The search for extraterrestial signals and alien radiation leakage reflects the habit of astronomers to study the emission from celestial bodies. That is why only a negligible part
( 3*10-9) of the Galaxy's lifetime is accessible to the current SETI experiment. However, the search for alien artefact-meteorites accumulated by the Earth could cover the entire history of the Galaxy.

Space activities lead to lasting pollution of the solar system. Light pressure, gravitational interaction with the planets, and collisions and explosions of artefacts in the outer parts of a planetary system (similar to spontaneous explosions of Earth satellites) can lead to the effective, inevitable leakage of interplanetary trash into the interstellar medium, even in the absence of interstellar flights (Arkhipov 1996a).

If there are alien artefacts between the stars, some of them are likely to fall to Earth at times. Thus, if 1% of asteroid material is transformed into interstellar 100g artefacts, and 1% of planetary systems generate such artefacts, then the Earth could be impacted by about four thousand artefacts over the course of 4.5*109 years (Arkhipov 1996a). The aim of this paper is to show that alien artefacts could reach the surface of the Earth, and that the consequences of this are worth discussing.

2. Artefact accumulation.

Interstellar artefacts could survive breaking in the atmosphere, at least in part. Thus, according to Fisher's equation
where m and m0 are the final and initial meteroid masses; s is the ablation coefficient; and is the initial geocentric velocity of the artefact), the surviving part of the artefact is m/m0 > 0.01 if g2 < 9.2/. Assuming that the heliocentric velocities of artefacts outside the solar system have random orientations and magnitudes equal to the typical heliocentric velocities of nearby stars (32.5 km/s; Arkhipov 1996a), we can estimate the probability of an artefact surviving

(where e=29.9 km/s is the Earth's orbital velocity):

Thus, artefacts could reach the Earth (P > 0) if < 1.5*10-8*s2/m2 (for typical meterites, 0 2*10-8*s2/m2 (Bronshten 1981). Moreover, there are materials for which the heat of destruction (Q) differs considerably from the usual meteorite value (Q0=8*106 J/kg; Bronshten 1981). For example, a boron artefact would have a heat of fusion and sublimation Q=5.53*107 J/kg (Martin 1978). Hence, = 0Q0/q= 2.9*10-9 s2/m2 and P=0.40. Therefore, the impact of alien artefacts onto the Earth is not excluded, even in the absence of interstellar missions to the solar system.

Of course, geocentric velocities and the ablation of debris from the interplanetary medium are more favorable for artefact survival. Alien space activity directly in the solar system (Foster 1972) could also lead to pollution of the interplanetary medium. There are certainly interesting candidates for alien artefacts in orbits (e.g., Steel 1995). Such space debris could fall on the Earth like our own satellites do. This is why searches for candidates to such events are worth discussing.

For example, the disintegration of artificial satellites and formation of debris of various chemical compositions appear as multicoloured bolides. Such phenomena were unknown in meteoric astronomy before 1957; however, rare multicoloured bolides had been observed, e.g., in 1926 (Flammarion 1927) and 1936 (Keppler 1936). There are also reports about some puzzling formations (``pseudometeorites'') falling from bolides before 1957. For example, the Eaton meteorite, seen to fall on May 10, 1931, was so hot upon falling that it burned the fingers of it's finder, and its composition corresponds to yellow brass (Buseck et al. 1969). There is a new well-recorded case of a similar impact, apparently of nonsatellite origin (Arkhipov 1995). Unusual debris are collected and kept by the Kharkov Astronomical Observatory.

It is not excluded that such phenomena occurred millions of years ago. The problem of ``fossil artefacts'' found in prehuman layers has been described in the scientific literature (Corliss 1978).

Of course, these cannot be regarded as concrete evidence, but are rather illustrative of search possibilities.

3. Astroinfection threshold.

As noted previously (Arkhipov 1996a), freeze-dried microbial spores in microartefacts (e.g., from drops of rocket fuel or human fecal material, etc.) are a potential danger for our biosphere. Moreover, an astroinfection principle has been formulated (even moderate pollution of the interplanetary medium of a planetary system can spontaneously infect earth-like planets in a great number of other systems). Hence, it is important to estimate the minimum pollution threshold for such panspermia.

Since there are microbes of diameter <0.4 m (e.g., M.aquatilis, M.minimus, M.subtilis; Fedorova 1970) and they must be protected from UV radiation by a shield of thickness l (Fedorova 1970; Weber and Greenberg 1985), the minimum radius of artefact that could be associated with interstellar panspermia would be a=2 m. The lifetime of such a microsphere ejected near the orbit of Jupiter orbit is 35 000 yr, due to the Poynting-Robertson effect (Allen 1973). This time is sufficient for close interaction with Jupiter and for leakage from the Solar System. In our computer simulations, the number of ejected subjovian artefacts increased with time scale 3000 yr.

Cosmic rays are the main limiting factor for interstellar panspermia. The level of Galactic cosmic radiation is 4*10-7 rad/s (Murphy 1981). Some microorganisms (e.g., Microcoleus, Phomidium, Synechococcus; Imshenetskiy 1975; Clostridium botulinum; Vashkov 1970) can survive radiation doses up to 2.5*106 rad. The limit for the microorganism Micrococcus radiodurans is about 7*106 rad (Rubenchik 1983). According to numerous experiments (Imshenetskiy 1975), the limiting dose could be increased by a factor of 10 for microorganisms frozen in vacuum. Thus, sterilization can not be guaranteed in the interstellar medium for doses of <107 rad (formally, this corresponds to the sterilization criterion for space probes; Jaffe 1962). The maximum exposure time is t107 rad / 4*10-7 rad s-1 = 2.5*1013 s. For an artefact injection velocity of *=10 km/s (as for the Pioneer and Voyager probes), a nonsterile zone with a radius of R *l=2.5*1019 cm=8.1 pc will surround the star.

For uniform and isotropic ejection, the artefact space density at distance d is M/4 mRd2, and the average number of artefact falls to the Earth during passage through a non-sterile zone is:

where =1.87*10-9 is the probability of a fall onto the Earth for an artefact inside the Earth's orbit (Arkhipov 1996a); M is the total mass of non-sterile zone artefacts; and m=3.4*10-11 g is the average artefact mass, which corresponds to a sphere of a=2 m radius and density =1 g/cm3. Here, A is the effective radius of the Earth's orbit corrected for the gravitational focusing of artefacts:

where K=(1.7*104 cm/g) a is the ratio of the solar gravity and the solar radiation pressure for the microartefact (Allen 1973); V=42.1 km/s is the escape velocity 1 a.u. from the Sun; and =32.5 km/s is the typical heliocentric velocity of nearby stars (Arkhipov 1996a).

According to experiments (Hoyle et al. 1986), microartefact impacts on the Earth's surface can occur without thermal sterilization if the geocentric velocity is lower than 40 km/s. If the orientation of the artefact velocity is random, the probability of the artefact having such a geocentric velocity has been estimated to be W=0.14 (Arkhipov 1996b). However, the solar radiation pressure reduces the artefact escape velocity 1 a.u. from the Sun: (1-K-1)V=35.4 km/s. The revised estimate taking this into account is W=0.20.

The astroinfection threshold can be found from the condition nW > 1 for the number of nonsterile artefact impacts:

Since the artefacts fill the non-sterile zone during a time t R/ *, the astroinfection threshold for the interplanetary pollution rate is:

where =0.2 is the probability of gravitational ejection for small interplanetary bodies of the solar system indicated by computer simulations (Duncan et al. 1987; Farinella et al. 1994; Ipatov 1995).

The current average increase of the total mass of space debris is 100 tons/yr=3.2 g/s (Heysmann 1993). Thus, even one percent of the current flux of space debris (if it occurred in the region of the giant planets and continued for t 8*105 yr) could spontaneously infect the planets of N 4 *R3/3=290 stars (where *= 4.43*10-57 cm-3 is the stellar density near the Sun; Allen 1973). Analogously, about * R2T=1.2*106 stars could infect the Earth during T=4.5*109 yr.

4. Conclusions.

From our analysis, it follows that:

  1. There could be alien artefacts on the Earth even in the absence of alien expeditions to the Earth, or of interstellar missions at all.
  2. Therefore, it seems reasonable to revise the a priori negative attitude of meteorite experts to some pseudometeorites and artefact-like finds in prehuman geological layers. Isotopic analysis could reveal an extrasolar origin for some of them.
  3. Nonsterile artefacts from as many as 1.2*106 nearby stars could be agents for spontaneous interstellar panspermia, even if alien civilizations pollute space at the modern terrestrial rate (in fact, the interplanetary medium could be polluted by biological material even without technical activity -- by asteroid bombardment of planets possessing biospheres). This possibility must be taken into consideration in space policy and discussions about the origin of life.


Allen, C.W. Astrophysical Quantities, 1973, The Athlone Press: London
Arkhipov, A.V. Spaceflight 1995,37,94
Arkhipov, A.V. Observatory 1996a,116,175
Arkhipov, A.V. Observatory 1996b,116,No 1138 in press
Bronshten, V.A. Physics of Meteoric Phenomena, 31, Nauka: Moscow,1981
Buseck, P.R. Holdsworth, E.F., and Scott, G.R. Meteoritics, 1969, 4, No 4, 267
Corliss, W.R. Ancient Man: A Handlbook of Puzzling Artifacts Glen Arm, Sourcebook Project 652,1978
Duncan, M., T. Quinn, T., and Tremaine, S. Astron. J. 94,1330 1987
Farinella, P. et. al. Nat. 371,314 1994
Fedorova, R.I., ed. A.A. Imshenetski Extraterrestrial Life and Its Detection Methods NASA TT F-710 Nauka Press: Moscow 125 1970
Flammarion, G.C. l'Astronomie 41,230 1927
Foster, G.V. Spaceflight 14,447 1978
Heysmann, H. Spaceflight 35,182 1993
Hoyle, F., Wickramasinghe, N.C., and Al-Mufty S. Earth, Moon, and Planets 35 No. 1, 79 1986
Imshenetsky, A.A., eds. M. Calvin and O. Gazenko Foundations of Space Biology and Medicin 1 Nauka: Moscow 271 1975
Ipatov, S.I. Astron. Vestn. 29,304 1995
Jaffe, L.D. Aeronaut. Aerosp. Eng. 1,22 1962
Keppler, F.W. Pop. Astronomy 44,569 1936
Martin, A.R., Project Daedalus, JBIS Supl. 116 1978
Murphy, J.R. JBIS 34,470 1981
Rubenchik, L.I. Poisk microorganizmov v kosmose Naukova Dumka: Kiev 44 1983
Steel, D., Observatory 115,78 1995
Tutukov, A.V. Astron. Zhur. 72,400 1995
Vaskov, B.I. A.A.Imshenetskiy Extraterestrial Life and its Detection Methods NASA TT F-710 (english translation) Nauka Press: Moscow 169 1970
Weber, P. and Greenberg, J.M. Nat. 316,403 1985