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Space program for SETI
Acta Astronautica Vol. 42, No. 10±12, pp. 585±587, 1998 # 1994 International Astronautical Federation. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0094-5765/98 $19.00 + 0.00 PII: S0094-5765(98)00011-3
SPACE PROGRAM FOR SETI N. S. KARDASHEV{, S. F. LIKHACHEV and V. I. ZHURAVLEV Astro Space Center, Lebedev Physical Institute, Moscow, Russia AbstractÐTwo new space missions are now in preparation that can be used for the search for extraterrestrial intelligence (SETI). The project Millimetron is a millimeter and sub-millimeter space observatory with a 10 m diameter mirror, very sensitive receivers for single dish mode and will be used for orbiting VLBI (Very Long Base Interferometer). This telescope would be convenient for a very sensitive all sky survey with the possibility of constructing images of sources with a very high angular resolution. The mission will be useful for the search for astroengineering constructions in the universe. VLBI optical telescope (VOT) is another space system that consists of two or more optical telescopes to be placed in geostationary orbit for the interferometrical synthesis of images in UV, optical and near infrared ranges, which is also suitable for the identi®cation of astroengineering constructions. # 1994 International Astronautical Federation. Published by Elsevier Science Ltd. All rights reserved
The name SETI or CETI is closely related to the ®rst initiator and organizer of international meetings of IAA (International Astronautical Academy) and IAF (International Astronautical Federation), Professor Rudolph Peshek who was also one of the most in¯uential leaders in the ®eld. He proposed the acronym CETI in close connection with the similar pronunciation for t CetiÐone of the nearest stars apparently possessing a planetary system. According to these two acronyms and general scienti®c background, we would like to consider three main directions of the astronomical investigation [1]: 1. investigation of conditions for the existence of extraterrestrial intelligence (ETI); 2. search for astroengineering activity; 3. search for communication signals. Methods of investigation are connected inevitably with the assumption about the level of ETI technology. The highest probability of detection corresponds apparently with the highest level of development but this point is the most uncertain and unpredictable. This issue is probably directly related to all three directions of investigation. As a working model for evolved civilization, we approved the assumptionÐthe highest level of development corresponds to the highest level of utilizing solid-state matter and the highest level of energy consumption. It is very probable that considerable progress in astrophysics and a coordinated search in the ®rst two directions will be more successful in the beginning. As a result of the investigations we hope to ®nd objects whose emission or absorption characteristics resemble bulky solid state constructions. Then from these objects, one should {Author to whom correspondence should be addressed. 585
start to search for communication signalsÐthe third direction. Such a strategy seems the most promising and productive. Figure 1 shows the limits for the detection of the thermal emission of space constructions for dierent values of the energy consumption and distances from us. We estimate the temperature interval of such constructions to be T0 3±300 K, the consumption of energy L0 10ÿ6±1012 (LSun), the distance D 0 1±1010 pc without cosmological correction. Such constructions can be detected by their thermal emissions with a spectrum near to black body radiation [2±6]. For 3 K constructions, the maximum emission would occur in the mm wavelengths (1±3 mm), and the size of the constructions will be larger at a ®xed consumption energy. Their search is connected with the development of millimeter-wave astronomy. For the 300 K technology, the maximum emission would fall in the infrared band (10±30 mm) and the search is connected with observations on the infrared telescopes. The existing radio surveys of the sky (shortest wavelength is 6 cm on the ground and 3 mm for a COBE (Cosmic Background Explorier) satellite) enable one to ®nd an essential limitation on the 3 K technology of ETI (Extraterrestrial Intelligence) and their abundance. Also the analyses of the IRAS (Infrared Astronomical Satellite) catalog of the infrared satellite survey makes it possible to set limits on the 300 K technology. The expected spectral ¯ux density for the Raleigh±Jeans part of the Planckian spectrum of space constructions is Fn=2pkTR2/(l2D2), with their characteristical radius R = (L/2psT4)0.5, where k and s are Boltzmann and Stefan± Boltzmann constants. From these equations we have the relation L = aD2, where a = (s/k)T3l2Fn.
586
N. S. Kardashev et al.
Fig. 1. Limits on the detection of 3 K technology: a is the existing radio astronomy data; b and c are from the ground radio telescopes, 3 and 30 m diameters; d is the space radio telescope Millimetron, l = 2 mm.
This relation is plotted in Fig. 1 for dierent values of the limit of detection Fn. The duration of the half-hemispheric survey is t5 =DtA/(2l2n), where Dt is the integration time for each receiver (detector), n is their numbers, A is the eective area of the telescope. For a radio astronomical receiver Fn=2akTN/ [A(DnDt)0.5], a is the signal to noise ratio, TN is the system noise temperature of the radio telescope, Dn is the bandwidth. For bolometrical detection Fn=ad/ADn(Dt)0.5, where d is the root mean square (RMS) of the power sensitivity for one s of integration time. In Fig. 1, we plot the limits for detection for a 6 cm survey (upper left shaded part), for 3 and 30 m on the ground antennas (dashed lines) and for a 10 m space radio telescopeÐthe Millimetron project [7]. We adopted ts=107s, l = 2 mm, a = 10. For the ground antennas A = 3.5 and 350 m2, respectively, Dn 00.1, n 0 15 GHz, TN=100 K. For Millimetron, Dn 00.1, n 0 150 GHz and A = 80 m2, d = 3.10ÿ18Ws0.5 (solid line). The 300 K ETI technology corresponds to optimal detection in the infrared band. In Fig. 2 we plot similar L/D relations for IRAS results (limit 100 Jy for l = 20 mm) and for a space IR telescope with cooling. Here also ts=107s, l = 20 mm, a = 10, A = 0.8 m2. The dashed lines indicate the limits for the surveys with n = 102 and 104 pixels. Figure 3 shows the histogram of color temperatures, determined for sources of the IRAS catalog by approximating the shape of the spectra by Planck's function. For each direction (galactic cen-
Fig. 2. Limits on the detection of 300 K technology: a is existing IRAS satellite data; b and c are future IR missions with a 1 m diameter telescope and 102 or 104 pixels of very sensitive bolometers; l = 20 mm.
ter, anticenter, northern and southern galactic pole) we have selected 3000 of the brightest stars at 100 mm sources. The histograms reveal a strong maximum for all types of sources at very low temperatures (R30 K), but the sources identi®ed with stars and unidenti®ed sources show very long wings of distribution in the region of higher temperatures. The histograms are continued up to 1000 K for stellar envelopes and show a second maximum in range of 300±400 K for unidenti®ed sources. Special observations of similar objects of both types would be necessary in the mm band. The IRAS catalog was also analyzed in connection with SETI in Refs. [8,9].
Space program for SETI
587
Another new opportunity is connected with the proposal to construct optical VLBI systems in space. This mission, named VOT (VLBI Optical Telescope [7]), would include two (or more) free ¯ying satellites with 0.5 m optical telescopes providing angular resolution as good as 1 ms of arc, and a spectral range of 0.1±2 mm. This mission can be used also to detect a big construction illuminated by its own star. As was shown before [10], a similar interferometer with a larger collecting area (2±10 m2) is needed for direct detection of planets around the nearest stars. As an example, a Dyson-sphere around a star similar to t Ceti (scattering isotropically nearly all of the light from its own star) would be detected by a VOT system from distances up to 200 kpc, i.e. in the whole galaxy and its satellites. Finally, problems of hidden mass and solid state matter in the universe are of considerable importance for SETI [11,12]. This work was supported by the Russian Space Agency.
REFERENCES
Fig. 3. Histograms of color temperatures for 3000 IRAS sources compiled as the brightest on 100 mm: the top picture is a general histogram; the bottom one is the ampli®ed vertical scale. a is the center of the galaxy; b is the anti-center; c and d are northern and southern galactic poles, respectively.
1. Likhachev, S. F. and Kardashev, N. S., Foundation and Strategy of SETI: Intuitionistic Approach, Astronomical and Astrophysical Transactions, 1997, 14, 225±231. 2. Dyson, F. J., in Interstellar Communication, ed. A. G. W. Cameron. Benjamin, New York, 1963, pp. 111± 114. 3. Buyakas, V. I., Danilov, Yu. I., Dolgopolov, G. A., Feoktistov, K. P., Gorshkov, L. A., Gvamichava, A. S., Kardashev, V. I., Klimashin, V. I., Komarov, V. I., Melnikov, N. P., Narimanov, G. S., Prilutsky, O. F., Pshennikov, A. S., Rodin, V. G., Rudakov, V. A., Sagdeev, R. Z., Savin, A. I., Semenov, Yu. P., Shklovsky, I. S., Sokolov, A. G., Tsarevsky, G. S., Usyukin, V. I. and Zakson, M. B., Acta Astronautica, 1979, 6, 175±201. 4. Kardashev, N. S., Acta Astronautica, 1979, 6, 33±46. 5. Slysh, V. I., in The Search for Extraterrestrial Life: Recent Developments, ed. M. D. Papagiannis, 1985, pp. 315±319. 6. Kardashev, N. S., in The Search for Extraterrestrial Life: Recent Developments, ed. M. D. Papagiannis, 1995, pp. 497±504. 7. Kardashev, N. S.et al., Acta Astronautica, 1995, 37, 271. 8. Backman, D. E. and Gillett, F. C., in BioastronomyÐ The Next Steps, Vol. 93, ed. G. Marx. Kluwer, Dordrecht, 1988. 9. Jugaku, J. and Nishimura, S., in Bioastronomy Proc. of 3rd International Conf. on Bioastronomy, ed. J. Heidmann and M. J. Klein. Springer, Berlin, 1991, pp. 295±301. 10. Burke, B. F., in BioastronomyÐThe Next Steps, ed. G. Marx. Kluwer, Dordrecht, 1988, pp. 139±152. 11. Kardashev, N. S. and Strelnitksky, V. S., in Proc of the 3rd International Conf. on Bioastronomy, ed. J. Heidmann and M. J. Klein. Springer, Berlin, 1991, pp. 433±441. 12. Kardashev, N. S., Astrophysics and Space Science, 1997, 252, 25±40.
SPACE PROGRAM FOR SETI N. S. KARDASHEV{, S. F. LIKHACHEV and V. I. ZHURAVLEV Astro Space Center, Lebedev Physical Institute, Moscow, Russia AbstractÐTwo new space missions are now in preparation that can be used for the search for extraterrestrial intelligence (SETI). The project Millimetron is a millimeter and sub-millimeter space observatory with a 10 m diameter mirror, very sensitive receivers for single dish mode and will be used for orbiting VLBI (Very Long Base Interferometer). This telescope would be convenient for a very sensitive all sky survey with the possibility of constructing images of sources with a very high angular resolution. The mission will be useful for the search for astroengineering constructions in the universe. VLBI optical telescope (VOT) is another space system that consists of two or more optical telescopes to be placed in geostationary orbit for the interferometrical synthesis of images in UV, optical and near infrared ranges, which is also suitable for the identi®cation of astroengineering constructions. # 1994 International Astronautical Federation. Published by Elsevier Science Ltd. All rights reserved
The name SETI or CETI is closely related to the ®rst initiator and organizer of international meetings of IAA (International Astronautical Academy) and IAF (International Astronautical Federation), Professor Rudolph Peshek who was also one of the most in¯uential leaders in the ®eld. He proposed the acronym CETI in close connection with the similar pronunciation for t CetiÐone of the nearest stars apparently possessing a planetary system. According to these two acronyms and general scienti®c background, we would like to consider three main directions of the astronomical investigation [1]: 1. investigation of conditions for the existence of extraterrestrial intelligence (ETI); 2. search for astroengineering activity; 3. search for communication signals. Methods of investigation are connected inevitably with the assumption about the level of ETI technology. The highest probability of detection corresponds apparently with the highest level of development but this point is the most uncertain and unpredictable. This issue is probably directly related to all three directions of investigation. As a working model for evolved civilization, we approved the assumptionÐthe highest level of development corresponds to the highest level of utilizing solid-state matter and the highest level of energy consumption. It is very probable that considerable progress in astrophysics and a coordinated search in the ®rst two directions will be more successful in the beginning. As a result of the investigations we hope to ®nd objects whose emission or absorption characteristics resemble bulky solid state constructions. Then from these objects, one should {Author to whom correspondence should be addressed. 585
start to search for communication signalsÐthe third direction. Such a strategy seems the most promising and productive. Figure 1 shows the limits for the detection of the thermal emission of space constructions for dierent values of the energy consumption and distances from us. We estimate the temperature interval of such constructions to be T0 3±300 K, the consumption of energy L0 10ÿ6±1012 (LSun), the distance D 0 1±1010 pc without cosmological correction. Such constructions can be detected by their thermal emissions with a spectrum near to black body radiation [2±6]. For 3 K constructions, the maximum emission would occur in the mm wavelengths (1±3 mm), and the size of the constructions will be larger at a ®xed consumption energy. Their search is connected with the development of millimeter-wave astronomy. For the 300 K technology, the maximum emission would fall in the infrared band (10±30 mm) and the search is connected with observations on the infrared telescopes. The existing radio surveys of the sky (shortest wavelength is 6 cm on the ground and 3 mm for a COBE (Cosmic Background Explorier) satellite) enable one to ®nd an essential limitation on the 3 K technology of ETI (Extraterrestrial Intelligence) and their abundance. Also the analyses of the IRAS (Infrared Astronomical Satellite) catalog of the infrared satellite survey makes it possible to set limits on the 300 K technology. The expected spectral ¯ux density for the Raleigh±Jeans part of the Planckian spectrum of space constructions is Fn=2pkTR2/(l2D2), with their characteristical radius R = (L/2psT4)0.5, where k and s are Boltzmann and Stefan± Boltzmann constants. From these equations we have the relation L = aD2, where a = (s/k)T3l2Fn.
586
N. S. Kardashev et al.
Fig. 1. Limits on the detection of 3 K technology: a is the existing radio astronomy data; b and c are from the ground radio telescopes, 3 and 30 m diameters; d is the space radio telescope Millimetron, l = 2 mm.
This relation is plotted in Fig. 1 for dierent values of the limit of detection Fn. The duration of the half-hemispheric survey is t5 =DtA/(2l2n), where Dt is the integration time for each receiver (detector), n is their numbers, A is the eective area of the telescope. For a radio astronomical receiver Fn=2akTN/ [A(DnDt)0.5], a is the signal to noise ratio, TN is the system noise temperature of the radio telescope, Dn is the bandwidth. For bolometrical detection Fn=ad/ADn(Dt)0.5, where d is the root mean square (RMS) of the power sensitivity for one s of integration time. In Fig. 1, we plot the limits for detection for a 6 cm survey (upper left shaded part), for 3 and 30 m on the ground antennas (dashed lines) and for a 10 m space radio telescopeÐthe Millimetron project [7]. We adopted ts=107s, l = 2 mm, a = 10. For the ground antennas A = 3.5 and 350 m2, respectively, Dn 00.1, n 0 15 GHz, TN=100 K. For Millimetron, Dn 00.1, n 0 150 GHz and A = 80 m2, d = 3.10ÿ18Ws0.5 (solid line). The 300 K ETI technology corresponds to optimal detection in the infrared band. In Fig. 2 we plot similar L/D relations for IRAS results (limit 100 Jy for l = 20 mm) and for a space IR telescope with cooling. Here also ts=107s, l = 20 mm, a = 10, A = 0.8 m2. The dashed lines indicate the limits for the surveys with n = 102 and 104 pixels. Figure 3 shows the histogram of color temperatures, determined for sources of the IRAS catalog by approximating the shape of the spectra by Planck's function. For each direction (galactic cen-
Fig. 2. Limits on the detection of 300 K technology: a is existing IRAS satellite data; b and c are future IR missions with a 1 m diameter telescope and 102 or 104 pixels of very sensitive bolometers; l = 20 mm.
ter, anticenter, northern and southern galactic pole) we have selected 3000 of the brightest stars at 100 mm sources. The histograms reveal a strong maximum for all types of sources at very low temperatures (R30 K), but the sources identi®ed with stars and unidenti®ed sources show very long wings of distribution in the region of higher temperatures. The histograms are continued up to 1000 K for stellar envelopes and show a second maximum in range of 300±400 K for unidenti®ed sources. Special observations of similar objects of both types would be necessary in the mm band. The IRAS catalog was also analyzed in connection with SETI in Refs. [8,9].
Space program for SETI
587
Another new opportunity is connected with the proposal to construct optical VLBI systems in space. This mission, named VOT (VLBI Optical Telescope [7]), would include two (or more) free ¯ying satellites with 0.5 m optical telescopes providing angular resolution as good as 1 ms of arc, and a spectral range of 0.1±2 mm. This mission can be used also to detect a big construction illuminated by its own star. As was shown before [10], a similar interferometer with a larger collecting area (2±10 m2) is needed for direct detection of planets around the nearest stars. As an example, a Dyson-sphere around a star similar to t Ceti (scattering isotropically nearly all of the light from its own star) would be detected by a VOT system from distances up to 200 kpc, i.e. in the whole galaxy and its satellites. Finally, problems of hidden mass and solid state matter in the universe are of considerable importance for SETI [11,12]. This work was supported by the Russian Space Agency.
REFERENCES
Fig. 3. Histograms of color temperatures for 3000 IRAS sources compiled as the brightest on 100 mm: the top picture is a general histogram; the bottom one is the ampli®ed vertical scale. a is the center of the galaxy; b is the anti-center; c and d are northern and southern galactic poles, respectively.
1. Likhachev, S. F. and Kardashev, N. S., Foundation and Strategy of SETI: Intuitionistic Approach, Astronomical and Astrophysical Transactions, 1997, 14, 225±231. 2. Dyson, F. J., in Interstellar Communication, ed. A. G. W. Cameron. Benjamin, New York, 1963, pp. 111± 114. 3. Buyakas, V. I., Danilov, Yu. I., Dolgopolov, G. A., Feoktistov, K. P., Gorshkov, L. A., Gvamichava, A. S., Kardashev, V. I., Klimashin, V. I., Komarov, V. I., Melnikov, N. P., Narimanov, G. S., Prilutsky, O. F., Pshennikov, A. S., Rodin, V. G., Rudakov, V. A., Sagdeev, R. Z., Savin, A. I., Semenov, Yu. P., Shklovsky, I. S., Sokolov, A. G., Tsarevsky, G. S., Usyukin, V. I. and Zakson, M. B., Acta Astronautica, 1979, 6, 175±201. 4. Kardashev, N. S., Acta Astronautica, 1979, 6, 33±46. 5. Slysh, V. I., in The Search for Extraterrestrial Life: Recent Developments, ed. M. D. Papagiannis, 1985, pp. 315±319. 6. Kardashev, N. S., in The Search for Extraterrestrial Life: Recent Developments, ed. M. D. Papagiannis, 1995, pp. 497±504. 7. Kardashev, N. S.et al., Acta Astronautica, 1995, 37, 271. 8. Backman, D. E. and Gillett, F. C., in BioastronomyÐ The Next Steps, Vol. 93, ed. G. Marx. Kluwer, Dordrecht, 1988. 9. Jugaku, J. and Nishimura, S., in Bioastronomy Proc. of 3rd International Conf. on Bioastronomy, ed. J. Heidmann and M. J. Klein. Springer, Berlin, 1991, pp. 295±301. 10. Burke, B. F., in BioastronomyÐThe Next Steps, ed. G. Marx. Kluwer, Dordrecht, 1988, pp. 139±152. 11. Kardashev, N. S. and Strelnitksky, V. S., in Proc of the 3rd International Conf. on Bioastronomy, ed. J. Heidmann and M. J. Klein. Springer, Berlin, 1991, pp. 433±441. 12. Kardashev, N. S., Astrophysics and Space Science, 1997, 252, 25±40.