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Chapter 6 SCIENTIFIC CAPABILITIES OF THE PLANETARY XPLORER E Chapters 3, 4, and 5 of this report define the principal sci- entific questions and measurements which we feel should be per- formed on, and near, Venus. This study was primarily concerned with how these questions should be answered and was prepared to consider all types of missions. We were, however, presented with a series of studies of the Planetary Explorer concept made by NASA.* This concept includes a relatively low-cost, univer- sal bus, which can carry entry probes, orbiters, balloons, or landers to the planet. The studies of entry probes and orbit- ers were extensive, and we concluded that weight and cost fig- ures were firm; balloon capability was based on extrapolation from thorough studies and was thought to be relatively secure but not in the same category as the probe and orbiter data; lander studies were only sufficient to demonstrate that a ca- pability existed. We reviewed these studies and considered that the Plane- tary Explorer was almost ideally suited to answering the sci- ence questions posed, and one of our most important conclusions is that the Planetary Explorer should be the prime vehicle for exploration of Venus in the next decade. We have, therefore, *The most important documents are: Comp~ehensive Study of Venus by Means of a Lo~-Cost Ent~y P~obe and Orbiter Mission Series, by J. E. Ainsworth, GSFC Rep. X-625-70-203 (June 1970). Planetary Explore~J Phase A Repoyt~ Technical Plans, GSFC Rep. 1969) (Oct. [deals with orbiters only]. Final Project Report for Delta-Class Venus Pyobe Mission Study, AVCO Government Products Division Rep. AVSD-0433-69-RR (Oct. 1969) [available from GSFC]. Final Pyoject Repoyt foy Planetary Explore~ Unive~saZ Bus Study, AVCO Government Products Division Rep. AVSD-0146- 70-RR (Oct. 1970) [available from GSFC]. Delta-Class Balloon and Lande~ Missions foy the Exploration of Venus, Martin Marietta Corp. Rep. MCR-70-211. 66
67 TABLE 2 Instrumented Probes--1975 Mission BUS Science weight 25 lb Science power 19 W Lifetime (from 15 radii to loss of communications) 4h Communications loss altitude 130 km Telemetry rate '\)350bps Spin rate 30-85 rpm MAIN PROBE Descent time 90 min Science weight 68 lb Science power 90 W Telemetry rate 80 bps (p < 1 atm) 40 bps (p > 1 atm) Landing site (with 40-bps telemetry rate) Limited to a 350 circle from subearth point Impact velocity 17 m/sec Spin rate Variable Lifetime Survival after impact not currently planned SMALL PROBES Descent time 95 min Science weight 4 lb Science power 3W Telemetry rate 1 bps Landing sites Limited to annular region from '\)200to '\)600from the sub- earth point Impact velocity 6 m/sec Spin rate Variable Lifetime Survival after impact not currently planned
68 TABLE 3 Planetary Explorer Orbiter Lifetime >6 months Science power 50 W (sunlight) 25 W (shadow) Shadow duration ~30 min (12-h orbit period) Orbit inclination 20-90Â° Velocity at periapsis 8-10 km/sec TABLE 4 Planetary Explorer Orbiter--1976-l977 Mission (Launch Date: December 1976; Travel Time: 169 Days) Periapsis of Orbit 400 km 1000 km Periapsis change capability (km) 0 1500 low 26,000 22,000 Apoapsis high (km) 61,000 52,000 100 SO Experiment weight (lb) 170 130 8.2 6.9 Orbital period (h) 21.8 17.9 Communication distance at arrival (AU) 0.54 Maximum communication distance (AU) 1.64 Bit rate at arrival to 85-ft dish ts/sec) 440 Minimum bit rate to 85-ft dish its) 25 Bit rate at to 210-ft its) 4600 bit rate to 210-ft dish (bits ) 550
69 TABLE 5 Atmospheric Balloon Mission Number of separate packages 2 Number of balloons per package 3 Float levels (typical) 50, 500, 1200 mbar Float altitude 70, 57, 51 km Initial target sites lOoN, 400N latitude 20Â° on dark side of terminator Lifetime 30 interrogations Duration of interrogation 5 min Tracking accuracy :t200 km Float Level 50mb ar 500 mbar 1200 mbar Science weight 4.3 lb 4.8 lb 4.8 lb Science power 22 IN 23 IN 23 IN Telemetry rate Tracking only 20 bps 20 bps plus plus tracking tracking TABLE 6 Planetary Explorer Landera Impact site Subearth point Lifetime 2 h minimum Lander weight 245 1b Total power 127 H average 220 1\1peak Science weight 55 Ib Telemetry rate rv2000 bps aValues given depend on payload and number of landers derived from the primary vehicle.
70 continually referred to this specific concept in foregoing chapters, for our ideas were clarified by considering the sci- ence within a framework of known practicability. The Planetary Explorer is a small, 850-lb, spin-stabilized, spacecraft launched by a Delta rocket. For a typical mission designed to send instrumented probes through the atmosphere to the surface of Venus, the spacecraft consists of a bus that will operate down to 130 km altitude, a main probe to carry ~70 lb of instruments to the surface, and three smaller probes, each carrying 4 lb of instruments, which are separated from the bus and provide measurements from three widely separated regions of Venus. Some of the important characteristics of the spacecraft are given in Table 2. A second mode of operation of the Planetary Explorer is to place a 470-lb spacecraft in orbi~ about Venus. The total weight for scientific experiments depends on the desired orbit (or orbits) during the mission lifetime, but representative figures on this, and other mission parameters, are given in Tables 3 and 4. A third mode of operation which can be accommodated with- in the Planetary Explorer framework is a mission to place con- stant-level balloons in the atmosphere of the planet. Pre- liminary study of a typical mission shows that six balloons, two each floating at levels of 50, 500, and 1200 mbar, corre- sponding to altitudes of 70, 57, and 51 km, respectively, are feasible. One set of balloons can be placed at 40oN and the other at lOoN latitude and 20Â° on the dark side of the termi- nator. Representative parameters are given in Table 5. Finally, the Planetary Explorer can be utilized to soft- land an experiment package on the surface of Venus. This as- pect of the Planetary Explorer concept has been studied less than the others, but typical mission parameters are given in Table 6. If desired, such as for a seismic experiment, se- veral small stations can be soft-landed on the surface by re- leasing more than one lander from the bus. The numbers given in Tables 2-6 should be taken as rep- resentative of the potential of the Planetary Explorer space- craft, bearing in mind that some values will be perturbed by particular constraints imposed by the dynamics of a specific Venus opportunity and each scientific payload. In any event, we conclude, without reservation, that most important scien- tific measurements of Venus fall within the capability of the Planetary Explorer spacecraft as currently envisioned. Our study has pinpointed several ways in which the sci- entific utility of the spacecraft and its systems can be im- proved. These can be summarized as follows:
71 1. The data-taking lifetime of the main bus in the upper atmosphere can be significantly improved by targeting it to have a low angle of arrival. This increases its useful life and provides an increase in its total telemetry capability. 2. The atmospheric probes, both the main probe and the small probes, are greatly enhanced in value if they can be de- signed to operate after impact on the surface. This is true even if their useful surface life is only a minute or two. We anticipate that, with normal contingencies, a probe designed to reach the surface will have a capability for brief survival. If it does not inflate the cost or complexity of the mission, this feature should be incorporated. 3. All instrument packages designed to operate, even briefly, on the surface of Venus pay severe penalty in terms of scientific payload or lifetime or both because of the high- temperature environment. The primary concern appears to be the power supply, usually a battery, but all equipment must be provided with extensive thermal shielding. A significant increase in payload or lifetime could be obtained if space- craft and experiment components could be developed to with- stand temperatures of ~)800 K. It is recognized that this may, and probably will, require extensive (and expensive) develop- ment, but there should be many auxiliary applications for such components and materials both within and outside the space program. 4. Near-circular orbits, which are desirable for many experiments, can be achieved by using atmospheric drag. The low periapsis required is also useful for measurements of the upper atmosphere. '~en the orbit has decayed sufficiently, the periapsis can be raised out of the sensible atmosphere by means of the orbit-changing motor. The concept of the Planetary Explorer, as a lower-cost alternative to Mariner, was the basis for recommendations made by this study. We were presented with cost estimates for a three-mission sequence involving bus, probe, and orbiter devel- opment costs and new instrumentation on each mission. The se- quence consisted of a dual-launch multiprobe, a single multi- probe, and a single orbiter and was estimated to cost $130 million, for an average cost of $33 million per launch. Sub- sequent probe or orbiter missions, with substantial new in- strumentation, were estimated to cost close to $20 million. These costs are for outside contractors but with a significant manpower contribution by Goddard Space Flight Center.
72 The cost of a Mariner launch, averaged over the whole series, is considerably higher than the cost of a Planetary Explorer. Estimates prepared by NASA for the Mariner 1971 Mars orbiter compared to a 1975 Planetary Explorer orbiter to Venus show that, including all development costs, the Mari- ner costs at least twice as much as the Planetary Explorer. Although elaborate equipment for the Planetary Explorer could greatly increase its cost, it is clear that the Plane- tary Explorer is fundamentally cheaper, because, as compared to Mariner, it has about half the capacity, a booster costing about 40 percent as much, and a simpler stabilization system. Even on the basis of a fixed cost per pound, a costs factor of 2 lower can be anticipated. In practice the cost saving should be greater than this. The smaller and simpler system may well allow economies in the number of development models, less stringent controls during construction, and less paper work. The savings on these accounts can be substantial. If the concept of a stan- dard bus is strictly adhered to, further savings are possible. Thus the important features of the Planetary Explorer are that it is adequate, but minimally so, for the explora- tion of Venus, and that we recommend a policy of maximum stan- dardization of hardware and restraint in instrumental com- plexity. This represents a novel approach to planetary ex- ploration with a flexibility and possibility for quick reac- tion that make it most attractive for the coming decade.