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Chapter 7 AN EXPLORATIONTRATEGY S In this chapter we address the question of what sequence of missions should be followed as opportunities present themselves. We have identified several types of mission to Venus for the Planetary Explorers. These are listed together with their characteristics in Tables 2-6. in the determi- The following constraints are important and to a certain nation of what options are at our disposal ought to be followed. extent dictate the sequence that logically 1. No more than two launches are possible during each window. This is because of the number of launch pads avail- able--two--and the turn around time for the Delta vehicle-- greater than the length of the window. 2. Hybrid missions are impractical or impose severe pen- alties in compromises. Thus, to combine, say, orbiter and balloons in one payload is not feasible. 3. There is an economic advantage in preparing two iden- tical payloads for a given opportunity. Hence if there are scientifically valid reasons for launching two missions with identical payloads (though not necessarily identical objec- tives or targets at Venus) these ought to be exploited. 4. More than the 18 months between windows is required between mission definition and launch. Hence the results of one mission cannot be used to design for the next following window in any essential way. On the other hand, 36 months provides an adequate interval for digesting the results of a mission and using the information to design a succeeding one. 5. The system has a varying capability to perform a mission of a given type--say, an orbiter--during a me tonic cycle. Hence it is not practical to build and store space- craft to be used for all opportunities. Each mission must be tailored to fit a given window. 6. Certain missions depend for optimum design on the results obtained from others. In fact, the multiprobe and orbiter missions tend to provide data which are needed for proper definition of the various lander and balloon missions. For this reason it seems cleaL that the first two windows should be reserved for probes and orbiters. 73

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74 In choosing between probes and orbiters for the first opportunity we are moved by the following consideration to select the multiprobe mission. To a certain extent the sort of information an orbiter will provide is or will have been made available from previous Mariner missions (including Mercury/Venus) and from ground- based radar studies. On the other hand, the data that the probes will deliver concerning cloud composition, illumination conditions below the clouds and on the surface, magnetic fields in and below the ionosphere, seismic noise background (from microbarographs on the probes), atmospheric pressure, tempera- ture, and composition will all be novel and will be badly needed in planning both lander and balloon missions. Indeed, the multiprobe missions appear to offer the greatest return of in- formation which is essentially of a new class for each of our disciplines except perhaps for that of particles and fields. We have considered the argument that an orbiter should be selected for the first mission because placing a space- craft in orbit around another planet has already been performed, whereas landing probes has not. On the other hand there is ex- tensive experience in sending probes to the moon. We regard the scientific, technical, and logistical reasons for begin- ning with a probe mission as overriding the others. Hence, we recommend sending two multiprobe Planetary Explorers to Venus during the 1975 window. two probes rather than one because of the We recommend desirability, particularly for ionospheric, solar plasma, and aeronomic studies, of observing properties of the planetary environment on the days ide and nights ide of Venus at very nearly the same time. Thus we recommend that the first mis- sion target the bus and main probe on the days ide of the plan- et. Then, if the first entry is successful, a mid course cor- rection should redirect the second spacecraft to send the bus, at least, in on the nightside. If the first probe fails, the second should be allowed to follow the same trajectory as the first. This element of redundancy, making a successful mis- sion almost certain is, in any case, desirable for the first of a long series. Because of the requirement that the probe data be used in planning and designing balloon and lander missions and be- cause of the wide variety of information useful to many dis- ciplines available from successful orbiters, we recommend that during the 1976-1977 window an orbiter be sent to Venus. To take advantage of the payload bonus associated with a large apoapsis, we recommend that apoapsis be in the neighborhood

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75 of 36,000 km initially and the orbital period be 12 h in order to synchronize with earth rotation for maximum coordination with the Arecibo and Goldstone facilities. (We note that dur- ing the 1976-1977 opportunity Venus will enter the Arecibo beam just as the Explorer arrives and will remain in the beam all summer, i.e., during most of the nominal lifetime of the or- biter. This is true only for this window during the metonic cycle.) We also note the aeronomy requirement to study in situ the region between 150 and 400 km. We, therefore, recommend that periapsis be lowered during the course of these missions down to 150 km. The possibility of using periapsis drag to give an almost circular orbit should also be considered for this mission, but not until after substantial results have been obtained from the initial high-apoapsis condition. It has been suggested that the 1976-1977 orbiter mission should also be dual-launched. There are definite scientific advantages to be gained by so doing, but we believe them to be insufficient to justify an increase in cost of approximately 50 percent of the first orbiter. The question should, however, be kept under review by the Continuing Planning Group. It is less clear than for these first two missions what criteria should be used in selecting missions for subsequent opportunities. Other classes of orbiters, landers (composi- tion, seismic, and imaging), and balloons are candidates. However, because less information will have been provided by earlier studies concerning the solid planet than any other aspect of Venus it would seem most rewarding to reserve the 1978 window to a lander. The mission of this lander should be to studyc the composition of the crust with a gamma-ray spec- trometer and neutron source and to measure seismic properties with an active source and a seismic probe. The distance be- tween source and probes will be determined in part on the basis of information obtained during the 1975 probe mission. We note that it may be possible to use information obtained from the 1976-1977 orbiters concerning surface features of Venus to se- lect the exact site for the 1978 lander. Until some results have been obtained from the first two basic missions it would not be wise to fix too rigidly the scenario for launches in 1980 and beyond. It is conceivable that the results of the first mu1tiprobe experiment will call for another probe series at this time. Or it may have been that desirable experiments were excluded from the first or- biter and a .different type of orbiter ought to be scheduled. Nevertheless, from our present vantage it would appear most

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76 TABLE7 Proposed Missions Mission (wt/year) Description Bus Multiprobe Large probe (881 lb/197S) Three miniprobes Orbiter Aeronomy; particles and fields (746 Ib/1976) Imaging and radar Lander Crustal composition; seismicity; atmospheric pressure, temperature, (845 Ib/1978) and winds Balloon Two sets of three balloons at 50, 500, and 1200 mbar reasonable to plan tentatively in 1980 to launch a balloon mission. Otherwise it will be 1981 at least before many data are available concerning atmospheric dynamics. A summary of the resulting mission sequence is shown in Table 7. It is also clear that with the completion of this sequence of four launches we shall have acquired only the first round of basic information. We foresee the need to plan as a matter of policy to take advantage of every subsequent launch opportunity until l.,Je have adequately exploited the scientific potentiali- ties of these Explorer-type probes. Planning for the series beginning in 1981 should begin after the results from the 1975 and 1976-1977 experiments are available. We note that eventually this sequence of controlled and modest observations can lay the basis for a more ambitious series of probes of the orbiter-lander class. We endorse the concept which the Planetary Explorers express of preparing for such an elaborate venture with a well-thought-out series of preliminary observations carried out with moderate resources. The scientific requirements outlined in preceding sec- tions will most effectively be satisfied, we believe, with the payload assignments shown in Tables 8-10. The time available to the Study Group did not permit a complete evaluation of all the considerations needed to arrive at a firm opinion regard- ing the optimum payload. These tables give a first attempt at this evaluation, which must be reviewed by the Continuing

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77 TABLE 8 Multiprobe Payloads Priority Weight (lb) Experiment BUS 2 1 Dayglow photometer 2 6 Dayglow spectrometer 2 7 Solar wind 4.7 2 Magnetometer 2.5 3 ac electric field Neutral mass spectrometer 1 13 or ion mass spectrometer 1 4 Ion trap or Langmuir probe 2 5 Fluorescence MINIPROBES 1 1.0 Temperature 1 0.8 Pressure 1.2 1 Solar radiation 1 2.0 Surface approach 1 1.3 Magnetometer (unclean)a MAIN PROBES 1 1.2 Temperature 1 1.3 Pressure 1 4.0 Acceleration 1 10. Mass spectrometry (1-140 AMU) 1 4.0 Solar flux 1 3.0 Infrared flux 1 3.6 Transponder 1 6.0 Altimeter 1 1.3 Magnetometer (unclean) 1 4.0 Nephelometer 1 12. \.Jind drift radar 1 2.0 Condensimeter/evaporometer 1 5.0 Cloud particle-size distribution 1 2.0 Aureole 2 1.0 Hygrometer 2 20 Cloud-particle composition 1 1.4 Omniantenna 1 1.0 Miniseismometer aHigh priority is contingent upon further feasibility studies.

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78 TABLE 9 OrbiterPayloada \\feight (lb) Experiment 6 infrared/sounder Thermal 20 altimeter/bistatic/radio 3 Ion mass spectrometer 10 Neutral mass spectrometer 2 Electron temperature probe 7 Solar wind probe 5 Hagnetometer ac ric field 2.5 2.5 Geiger counter 7 Dual-frequency radio propagation 15 Tops sounder 5 Airglo\v 10 Spin-scan TV 15 Data storage aThe data transmission rate and the pmver available for experiments in the Planetary Explorer orbiter may be insuffi- cient to handle these experiments simultaneously. If this is the case) time-sharing of experiments would be required. TABLE 10 Lander Payload Experiment Weight (lb) 2S-45a Active seismic experiment Source Seismic instrument r\,lS Surface composition experiment Gamma-ray scintillation spectrometer Neutron source Pressure probes 2.5 Temperature probes aDepending on mission.

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79 Planning Group. We have satisfied ourselves that these pay- loads lie within the capabilities of the spacecraft proposed. We note that, in the case of the orbiter, periapsis probably will be behind the planet during the first one or two months of satellite life. Hence adequate data-storage capability will be needed. We note also that it is possible to reduce apoapsis drastically by taking advantage of the high drag in orbit at temporarily very low periapsis. By using the tech- nique of lowering apoapsis in this way we add an entirely new class of missions based on near-circular orbits. We envision taking advantage of this maneuver relatively in the life- time of the orbiter to ensure accomplishment of other major mission objectives before risking very low periapsis,