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New conceptions of unmanned planetary exploration
Acta Astronautica Vol. 9, No. 8, pp. 529-531, 1982 Printed in Great Britain.
0094-5765/82/080529-03503.00/0 Pergamon Press Ltd.
Academy Transactions Note NEW CONCEPTIONS OF UNMANNED PLANETARY EXPLORATION. EXTRA-SOLAR PLANETARY SYSTEMSt O. WOLCZEK~: Polish AstronauticalSociety, Palac Kultury i Nauki, pok. 2322 00-901 Warszawa, Poland (Received 9 March 1982)
Abstract--The supposed nature of planets belongingto extra-solar systems is considered.Then technical problems of foreign systems planetary probes are briefly outlined. Next, several exploration strategies of these systems are proposed and methods of scientificinvestigationdiscussedin detail. Conclusionsare drawn concerningthe value of direct investigationof extra-solar planets.
l. INTRODUCTION
Measurements of stellar rotational velocities seem to indicate that planetary systems are likely to exist around stars belonging to later spectral types starting from about FS. Taking into account the dynamic definition of planets[l], their size and composition as well as position of their orbits in the circumsteUar system, conclusion can be drawn that life may originate only on a terrestrial Earth-like body which reached the highest level of evolution revolving around stars of late F and G type, or perhaps some others belonging to K and M type[2]. According to recent estimations, the mass M of foreign terrestrial planets may be comprised between ~ 1-5 M e. A terrestrial planet with the mass of 5 M~ would have a radius somewhat smaller than 2 R~ and the gravitational acceleration on its surface would only slightly exceed 1 g. However, the dynamics of such a planet would be somewhat smaller than that of the Earth. It is not clear if plate tectonics can evolve on this body and if a magnetic field of sufficient strength may appear, especially in the case of absence of a suitable moon. It seems rather that the appearance of an Earth-like terrestrial planet is a rare event even on the scale of the whole Galaxy, and therefore the probability of life's rise is quite small. Yet, if an interstellar probe will be sent in order to investigate foreign planetary systems, suitable instrumentation should be included enabling search for life and signals from alien intelligent beings.
2. THE PROBE AND TRAVEL
Launching of interstellar probes is likely to occur in the next century. Present technical means are totally
tPaper presented at the XXXIInd Congress of the International Astronautical Federation, Rome, Italy, 7-12 September 1981. *Academy CorrespondingMember (Section 1).
inadequate for realization of such extremely difficult missions, therefore only little attention will be drawn here on some major problems. A fundamental question is that of the propulsion because of extreme distances separating even the nearest stars from the Solar System. Taking into account conclusions drawn on the ground of our knowledge of the vicinity of the Sun only a few targets can be selected situated >4-12 1.y. away from it. If propulsion will be developed enabling flights with velocities of the order of 10"-105 km/s (ion or photon propulsion) then such targets will be reached in reasonable time amounting to several tens of years. It is beyond the scope of this work to examine closely the propulsion problem and to consider constructional and functional details. The progress of technology during the next 30 or 40yr will cause surely all present engineering projects to become obsolate. However, it seems that the flight during the major part of the mission will proceed with relativistic velocities. It is well known that special effects will then manifest themselves but their real influence cannot be evaluated before the realization of appropriate experiments. Besides, impacts of interstellar gas and dust particles at relativistic speeds during long travels will be very hazardous to the probe and its instruments. To avoid mission failure, a solution of this problem must be found by means of shielding, divided and open-work construction, redundant elements and systems etc. The power will be connected with the propulsion system because for both purposes the same nuclear energy source will be utilized. A crucial problem will be that of communication with the Earth. The probe's signal travel times will increase to several years but the real troubles will be connected with signal attenuation and directionality. Transmitter power amounting to several hundred kW or even 1 MW seems to be indispensable for communication with the Earth receiving stations [3]. 529
530
O. WoLCZEK
The useful lifetime of the probe, its systems and instruments must be very long in comparison with recent ones, amounting to some 50yr or more. However, experience shows that even today a lifetime of over 15 yr can be achieved ("Pioneer 6"). Therefore a substantial extension of this period may be expected in the next 30, 40 yr, sufficient for the realization of missions to the nearest stars. 3. STELLARSYSTEM'SENCOUNTER
Several strategies can be applied after approaching the stellar system. Due to deceleration of the probe during the second half of the mission the velocity of the probe at the distance of some 5 billion km from the central star may be reduced even to only few km/s. At such velocity, entering a distant near-circular circumstellar orbit would be easy, enabling reconaissance of the system and preliminary search of planets before the final decision will be made concerning the detailed investigation of individual bodies. Yet, a direct flight through the system immediately after approach from the interstellar space may be advisable taking into account the need to begin investigations and transmission of data as soon as possible. During this phase of travel observations of the system would be performed. However, the aim of such strategy would be to perform entering a circumstellar orbit at the distance of about 1 AU from the star and first of all to search for terrestrial planets if not discovered during the approach phase, especially those resembling the Earth. The flight through the stellar system could also proceed on a spiral orbit enabling more detailed investigation of the whole system. However, in any case the flight could be realized in the system in a plane very near to the star's equatorial plane. Therefore, the knowledge of this plane will be imperative. The probe's reflecting telescope with folded optics and electronic camera will serve this purpose, enabling adequate guidance. This telescope will be used also for observation of relatively bright celestial bodies situated in the vicinity of the star's equatorial plane. Their shifts will enable detection of some planets. It is a matter of course that in the meantime investigations of the system's central star will be realized. It can be expected that the Titius-Bode relation in a modified form holds in all planetary systems even if it does not reflect a true dynamical dependence postulated, e.g. by Ovenden [4]. In the last case planetary orbits around particular stars could be anticipated and their search facilitated. If not, then the discovery of a few planets and preliminary computation of their orbits may enable empirical finding of adequate Titius-Bode relation and an approximate calculation of orbits of the remaining planets. In the meantime remove investigation of hitherto discovered planets should proceed. Taking into account differences of distance from which pictures of planetary surfaces in the Solar System were obtained, an improvement of resolution by at least one order of magnitude during a period of 10 yr was achieved. One can safely accept that in 30-50 yr from now, when an inter-
stellar probe may be sent, undoubtedly a further considerable improvement of resolution by several orders of magnitude will be realized. Substantial progress will be achieved in performance of all instruments destined for planetary investigation. The interstellar probe will be equipped with such improved instruments. Its multispectral scanning devices will enable preparation of detailed maps of cloud covers and/or planetary surfaces in many intervals of a broad spectral range from UV to IR. Knowledge of detailed morphology of planetary surfaces even totally shadowed by cloud cover or night will be gained from distances of several tens to hundred thousand km due to the use of improved imaging radar. Important information will be collected with the aid of high gain antennae enabling reception of signals in a broad frequency range from several tens of MHz to several tens of GHz, especially in the water hole band. UV and IR spectrometers will serve principally as instruments for atmospheric research enabling investigations of constitution, structure and dynamics of gaseous planetary envelopes. Discovery of appreciable amounts of oxygen, an enhanced occurrence of ozone will be clue to the possibility of existence of life. Appropriate equipment should be available in the probe for measurement of polarization of radiofrequency radiation and its direction as well as of distribution of this radiation over planetary discs. Existence of trapped radiation belts could be thus discovered, presence of magnetic fields established and their strength evaluated. Though the presence of an advanced civilization on hypothetical planets of nearby stars seems highly improbable, active research means such as atmospheric probes, laser excitation etc. should be avoided. Therefore an overall improvement of sensitivity of measuring and imaging devices enabling investigations of planets from greater distances will be very advantageous. In this connection, utilization of the system's central star for atmospheric sounding seems to offer certain advantages. Observation of its radiation transmitted through the atmospheres of individual planets and possibly their moons may reveal presence of various important constituents and information concerning the structure and dynamics of these atmospheres. Such a method, used up to the present only once in the search of the Mercurian atmosphere and only in the EUV radiation band, may bring many valuable results in the case of a probe approaching the circumstellar system from the outside. However, if after prolonged remote investigations of discovered planets no signs of intelligent life is discovered, small atmospheric sounders and penetrators may be launched from the interstellar probe. They will transmit information on atmospheres, especially their lowest layers (tropospheres) and on the crust's external parts. Atmospheric sounding with radio and laser beams may serve as source of additional data. It seems also that at that time devices will be available enabling collection of ground samples and returning them to the interstellar probe for detailed analysis.
New conceptionsof unmanned explanetary exploration.Extra-solar planetary systems By and large it seems that a more general investigation strategy--an integrated statistical analysis of planetary media-should be applied. In each such medium in its various magnetospheric, atmospheric, hydrospheric and global (intraplanetary) phases, specific changes occur reflecting the phases' composition and dynamics. These changes characterize therefore the medium. That is why many simultaneous measurements of selected parameters at its adequate levels for a sufficiently long time will bring enough information for its identification. At an advanced stage of investigation, in several dozen years from now such measurements will be performed with the aid of small sounders inserted in a given medium and/or by collecting information from its appropriate levels transmitted in various electromagnetic radiation lines and bands. An automatic comparison of collected data with information stored in the probe's computer will give the final answer.
4. CONCLUSIONS (i) The remoteness of even the nearest stars and relative scarcity of those which may have planetary systems renders, due to extreme technical problems, an early
531
realization of interstellar probes questionable. They may be sent in 30-50 yr from now at the earliest. (ii) However, the realization of such probes will be of greatest scientific importance. For the first time drawing of comparison between a foreign circumstellar system and the Solar one will be possible, bringing invaluable information on the formation of planets, their structure, composition, dynamics and evolution. (iii) Investigations with the aid of advanced instruments of interstellar probes will throw light on the matter of the rise and development of life. There is some but very small chance that on this occasion foreign intelligent beings will be discovered. REFERENCES 1. Wolczek O. Dynamic systems as factors determining the structure, dynamicsand evolutionof planets exemplifiedby the terrestrial planets.XXIIICOSPARPlenary Meeting,Budapest. Paper No. 1.8.6. Adv. Space Res. l, 217-221 (1981). 2. Wolczek O. The influence of evolution of extraterrestrial environments on the possibilityof CETI. XXXIIAF Congress. Paper No. 80-IAA-53.Tokyo (1980). 3. MartinA. R. (Editor)Project Daedalus.J.B.I.S. 31, Supplement (1980). 4. OvendenM. W. Bode's law and the missingplanet. Nature 239, 508-509.
0094-5765/82/080529-03503.00/0 Pergamon Press Ltd.
Academy Transactions Note NEW CONCEPTIONS OF UNMANNED PLANETARY EXPLORATION. EXTRA-SOLAR PLANETARY SYSTEMSt O. WOLCZEK~: Polish AstronauticalSociety, Palac Kultury i Nauki, pok. 2322 00-901 Warszawa, Poland (Received 9 March 1982)
Abstract--The supposed nature of planets belongingto extra-solar systems is considered.Then technical problems of foreign systems planetary probes are briefly outlined. Next, several exploration strategies of these systems are proposed and methods of scientificinvestigationdiscussedin detail. Conclusionsare drawn concerningthe value of direct investigationof extra-solar planets.
l. INTRODUCTION
Measurements of stellar rotational velocities seem to indicate that planetary systems are likely to exist around stars belonging to later spectral types starting from about FS. Taking into account the dynamic definition of planets[l], their size and composition as well as position of their orbits in the circumsteUar system, conclusion can be drawn that life may originate only on a terrestrial Earth-like body which reached the highest level of evolution revolving around stars of late F and G type, or perhaps some others belonging to K and M type[2]. According to recent estimations, the mass M of foreign terrestrial planets may be comprised between ~ 1-5 M e. A terrestrial planet with the mass of 5 M~ would have a radius somewhat smaller than 2 R~ and the gravitational acceleration on its surface would only slightly exceed 1 g. However, the dynamics of such a planet would be somewhat smaller than that of the Earth. It is not clear if plate tectonics can evolve on this body and if a magnetic field of sufficient strength may appear, especially in the case of absence of a suitable moon. It seems rather that the appearance of an Earth-like terrestrial planet is a rare event even on the scale of the whole Galaxy, and therefore the probability of life's rise is quite small. Yet, if an interstellar probe will be sent in order to investigate foreign planetary systems, suitable instrumentation should be included enabling search for life and signals from alien intelligent beings.
2. THE PROBE AND TRAVEL
Launching of interstellar probes is likely to occur in the next century. Present technical means are totally
tPaper presented at the XXXIInd Congress of the International Astronautical Federation, Rome, Italy, 7-12 September 1981. *Academy CorrespondingMember (Section 1).
inadequate for realization of such extremely difficult missions, therefore only little attention will be drawn here on some major problems. A fundamental question is that of the propulsion because of extreme distances separating even the nearest stars from the Solar System. Taking into account conclusions drawn on the ground of our knowledge of the vicinity of the Sun only a few targets can be selected situated >4-12 1.y. away from it. If propulsion will be developed enabling flights with velocities of the order of 10"-105 km/s (ion or photon propulsion) then such targets will be reached in reasonable time amounting to several tens of years. It is beyond the scope of this work to examine closely the propulsion problem and to consider constructional and functional details. The progress of technology during the next 30 or 40yr will cause surely all present engineering projects to become obsolate. However, it seems that the flight during the major part of the mission will proceed with relativistic velocities. It is well known that special effects will then manifest themselves but their real influence cannot be evaluated before the realization of appropriate experiments. Besides, impacts of interstellar gas and dust particles at relativistic speeds during long travels will be very hazardous to the probe and its instruments. To avoid mission failure, a solution of this problem must be found by means of shielding, divided and open-work construction, redundant elements and systems etc. The power will be connected with the propulsion system because for both purposes the same nuclear energy source will be utilized. A crucial problem will be that of communication with the Earth. The probe's signal travel times will increase to several years but the real troubles will be connected with signal attenuation and directionality. Transmitter power amounting to several hundred kW or even 1 MW seems to be indispensable for communication with the Earth receiving stations [3]. 529
530
O. WoLCZEK
The useful lifetime of the probe, its systems and instruments must be very long in comparison with recent ones, amounting to some 50yr or more. However, experience shows that even today a lifetime of over 15 yr can be achieved ("Pioneer 6"). Therefore a substantial extension of this period may be expected in the next 30, 40 yr, sufficient for the realization of missions to the nearest stars. 3. STELLARSYSTEM'SENCOUNTER
Several strategies can be applied after approaching the stellar system. Due to deceleration of the probe during the second half of the mission the velocity of the probe at the distance of some 5 billion km from the central star may be reduced even to only few km/s. At such velocity, entering a distant near-circular circumstellar orbit would be easy, enabling reconaissance of the system and preliminary search of planets before the final decision will be made concerning the detailed investigation of individual bodies. Yet, a direct flight through the system immediately after approach from the interstellar space may be advisable taking into account the need to begin investigations and transmission of data as soon as possible. During this phase of travel observations of the system would be performed. However, the aim of such strategy would be to perform entering a circumstellar orbit at the distance of about 1 AU from the star and first of all to search for terrestrial planets if not discovered during the approach phase, especially those resembling the Earth. The flight through the stellar system could also proceed on a spiral orbit enabling more detailed investigation of the whole system. However, in any case the flight could be realized in the system in a plane very near to the star's equatorial plane. Therefore, the knowledge of this plane will be imperative. The probe's reflecting telescope with folded optics and electronic camera will serve this purpose, enabling adequate guidance. This telescope will be used also for observation of relatively bright celestial bodies situated in the vicinity of the star's equatorial plane. Their shifts will enable detection of some planets. It is a matter of course that in the meantime investigations of the system's central star will be realized. It can be expected that the Titius-Bode relation in a modified form holds in all planetary systems even if it does not reflect a true dynamical dependence postulated, e.g. by Ovenden [4]. In the last case planetary orbits around particular stars could be anticipated and their search facilitated. If not, then the discovery of a few planets and preliminary computation of their orbits may enable empirical finding of adequate Titius-Bode relation and an approximate calculation of orbits of the remaining planets. In the meantime remove investigation of hitherto discovered planets should proceed. Taking into account differences of distance from which pictures of planetary surfaces in the Solar System were obtained, an improvement of resolution by at least one order of magnitude during a period of 10 yr was achieved. One can safely accept that in 30-50 yr from now, when an inter-
stellar probe may be sent, undoubtedly a further considerable improvement of resolution by several orders of magnitude will be realized. Substantial progress will be achieved in performance of all instruments destined for planetary investigation. The interstellar probe will be equipped with such improved instruments. Its multispectral scanning devices will enable preparation of detailed maps of cloud covers and/or planetary surfaces in many intervals of a broad spectral range from UV to IR. Knowledge of detailed morphology of planetary surfaces even totally shadowed by cloud cover or night will be gained from distances of several tens to hundred thousand km due to the use of improved imaging radar. Important information will be collected with the aid of high gain antennae enabling reception of signals in a broad frequency range from several tens of MHz to several tens of GHz, especially in the water hole band. UV and IR spectrometers will serve principally as instruments for atmospheric research enabling investigations of constitution, structure and dynamics of gaseous planetary envelopes. Discovery of appreciable amounts of oxygen, an enhanced occurrence of ozone will be clue to the possibility of existence of life. Appropriate equipment should be available in the probe for measurement of polarization of radiofrequency radiation and its direction as well as of distribution of this radiation over planetary discs. Existence of trapped radiation belts could be thus discovered, presence of magnetic fields established and their strength evaluated. Though the presence of an advanced civilization on hypothetical planets of nearby stars seems highly improbable, active research means such as atmospheric probes, laser excitation etc. should be avoided. Therefore an overall improvement of sensitivity of measuring and imaging devices enabling investigations of planets from greater distances will be very advantageous. In this connection, utilization of the system's central star for atmospheric sounding seems to offer certain advantages. Observation of its radiation transmitted through the atmospheres of individual planets and possibly their moons may reveal presence of various important constituents and information concerning the structure and dynamics of these atmospheres. Such a method, used up to the present only once in the search of the Mercurian atmosphere and only in the EUV radiation band, may bring many valuable results in the case of a probe approaching the circumstellar system from the outside. However, if after prolonged remote investigations of discovered planets no signs of intelligent life is discovered, small atmospheric sounders and penetrators may be launched from the interstellar probe. They will transmit information on atmospheres, especially their lowest layers (tropospheres) and on the crust's external parts. Atmospheric sounding with radio and laser beams may serve as source of additional data. It seems also that at that time devices will be available enabling collection of ground samples and returning them to the interstellar probe for detailed analysis.
New conceptionsof unmanned explanetary exploration.Extra-solar planetary systems By and large it seems that a more general investigation strategy--an integrated statistical analysis of planetary media-should be applied. In each such medium in its various magnetospheric, atmospheric, hydrospheric and global (intraplanetary) phases, specific changes occur reflecting the phases' composition and dynamics. These changes characterize therefore the medium. That is why many simultaneous measurements of selected parameters at its adequate levels for a sufficiently long time will bring enough information for its identification. At an advanced stage of investigation, in several dozen years from now such measurements will be performed with the aid of small sounders inserted in a given medium and/or by collecting information from its appropriate levels transmitted in various electromagnetic radiation lines and bands. An automatic comparison of collected data with information stored in the probe's computer will give the final answer.
4. CONCLUSIONS (i) The remoteness of even the nearest stars and relative scarcity of those which may have planetary systems renders, due to extreme technical problems, an early
531
realization of interstellar probes questionable. They may be sent in 30-50 yr from now at the earliest. (ii) However, the realization of such probes will be of greatest scientific importance. For the first time drawing of comparison between a foreign circumstellar system and the Solar one will be possible, bringing invaluable information on the formation of planets, their structure, composition, dynamics and evolution. (iii) Investigations with the aid of advanced instruments of interstellar probes will throw light on the matter of the rise and development of life. There is some but very small chance that on this occasion foreign intelligent beings will be discovered. REFERENCES 1. Wolczek O. Dynamic systems as factors determining the structure, dynamicsand evolutionof planets exemplifiedby the terrestrial planets.XXIIICOSPARPlenary Meeting,Budapest. Paper No. 1.8.6. Adv. Space Res. l, 217-221 (1981). 2. Wolczek O. The influence of evolution of extraterrestrial environments on the possibilityof CETI. XXXIIAF Congress. Paper No. 80-IAA-53.Tokyo (1980). 3. MartinA. R. (Editor)Project Daedalus.J.B.I.S. 31, Supplement (1980). 4. OvendenM. W. Bode's law and the missingplanet. Nature 239, 508-509.