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Long-lived Venus Geophysical Lander on Surface of Venus
The Venus Geophysical Network Pathfinder (VGNP) mission proposes to demonstrate, as a "proof-of-concept", the technology needed to emplace a long-lived, global geophysical network on Venus. The burden of costs to develop the technology required to address the extreme operational conditions is likely to fall solely upon the next generation of Venus surface spacecraft, as Venus does not share these conditions with other solar system bodies. It seems only prudent to establish a phased approach to this development, and to do so early in the next phase of exploration of Venus, in order to better understand the costs and complexities of such exploration. The proposed approach is conservative (for example, drawing upon technologies already well understood) and does not rely on new discoveries or inventions. Owing to this design conservatism, the mission fits within the Discovery Program cost ceiling with reasonable margin.
There are two obvious challenges to Venus geophysical exploration. First, the instrument package must survive on the surface long enough to measure the planetary attributes of importance, and second, the data must be returned to Earth for processing and analysis. Surviving the 460 deg C/93 bar surface environment for a period reasonable for geophysical observations represents the most formidable challenge, and will receive special emphasis in this proposal.
The VGNP mission will deliver a 300 kg geophysical observatory to the surface of Venus (see Cover), where it will function for at least one Earth year. Launch will occur in early June 1999, aboard a Delta ELV; arrival at Venus is in early October, after a transit of approximately 120 days (Figure 1). The spacecraft consists of a cruise vehicle and a descent vehicle, the latter consisting of an entry subsystem and a lander subsystem. The cruise vehicle, a spacecraft in its own right also weighing about 300 kg (including hydrazine), provides communication, attitude control, and trajectory correction capabilities during transit to Venus. Upon ground command (there are both launch- and cruise-initiated backup timers), the delivery system initiates entry sequencing, including startup of the payload support system, and then separates from the descent vehicle.
Figure 1: Trajectory to Venus
After ballistic entry during which a peak deceleration of less than 200 g is experienced, and at approximately 30 km altitude, the entry subsystem deploys a cover to permit science acquisitions to begin. Imaging from that altitude to the surface will provide the geologic context for the geophysical observations to be made after landing. The lander subsystem will take approximately 40 minutes to "free fall" to the surface (i.e., no parachute is used to slow the descent). Impact occurs at <=10 m/s; a crushable decelerator reduces g-forces to 25-50 g at impact.
Shortly after landing, two booms are deployed: the long-period seismometer extends laterally to the surface and detaches from the spacecraft structure to isolate it from non-geophysical signals, and the meteorology, magnetometry, and surface imaging boom deploys vertically above the lander. Instrument sensor heads will all operate at surface ambient conditions while the electronics will be housed in a central, refrigerated core (the dewar). The surface camera will acquire site characterization images at the end of the deployments.
Primary geophysical acquisitions begin within a few minutes of landing. Four geophysical instruments are involved in these acquisitions: a single-axis (vertical), long period seismometer, a three-axis, short period seismometer, a magnetometer, and meteorology units consisting of hot-wire anemometers/thermometers and a pressure transducer. The long-period seismometer is located about 1 m remote from the lander, the short-period seismometer is attached to the base of the lander, and the magnetometer, meteorology units and surface camera are located on the vertical boom that extends roughly 2 m above the surface.
Observations continue for just under one Earth year. During the first 58 days, the vehicle is in view of Earth and data are transmitted during one 8-hr Deep Space Network (DSN) station pass each day. During the period when the lander is not visible from Earth, data are stored on-board in the mass memory within the refrigerated dewar. After acquisition of radio signal roughly 64 days later, recorded data are transmitted along with continuing observations, at an average rate of 32 bits per second (bps), over the course of the next two months. Following another two month communication gap, the final downlink session lasts an additional 58 days. The primary mission ends 1 September 2000, after 330 days of data acquisition and 175 days of data transmission. An extended mission is possible, when communications with the lander are re-established in mid-December 2000.
A total of roughly 160 Mbits (Mb) of data are transmitted to Earth. These data are compressed, and will expand upon decompression on Earth. The estimated total mission data return is about 160 MBytes (MB).
The VGNP mission has both scientific and technical goals. In selecting the payload to address these goals, three criteria were considered. Each investigation is required to meet at least two of the following three criteria. Specifically, the investigation must provide:
1. specific information relevant to the design and successfol operation of a follow-on Venus Geophysical Network (e.g., determining the background level of seismicity, the nature of seismic signals, the influence of meteorology on seismic sensitivity, etc.);
2. fundamental, first-order scientific observations in its own right, independent of whether or not a follow-on mission is conducted (e.g., determining the size and state of the core, the thickness of the crust, etc.); and
3. a high potential for serendipitous discoveries (e.g., as might be made in visual observations at a scale and over an area not previously accessible).
Fundamental insight into the physical and chemical evolution of each terrestrial planet demands basic information on that planet's interior structure. Such information has, for the most part, been inferred using indirect means such as numerical modelling or observation of surface geology and its interpretation within a tectonic or geophysical framework (e.g., photointerpretation). On Earth, and to a lesser extent on the Moon, more fundamental observations of geophysical importance have been obtained through seismology, electromagnetic sounding, gravimetry, and heat flow measurements. To date, only gravimetry, photointerpretation, and indirect modelling have been been used effectively at Venus, and with considerable uncertainty. Heat flow measurements, difficolt on Earth, and which required considerable manual effort on the Moon, are probably not a practical near-term technique for application on Venus. Seismic and electromagnetic methods offer the greatest opportunities to significantly advance understanding of the interior of Venus, specifically in the areas of gross structure and the nature of local seismicity.
Knowledge of the three major divisions of a planet's gross structure (core size and state, mantle layering, and crustal thickness) is a prerequisite for chemical and thermal models of planetary interiors. Existing chemical models of Venus [e.g., BVSP, 1981] are largely Earth-like by assumption. The absence of specific information about core diameter and the depths of specific phase transitions within the mantle lead to assumptions that, because it is roughly the same size and mass as the Earth, Venus must have a comparable partition of interior structure. The absence of a intrinsic, dipolar magnetic field measured by spacecraft during both flyby and orbital missions [e.g., Smith et al., 1963, 1965; Russell et al., 1979a,b] has been interpreted as indicating that Venus lacks a geodynamo driven by inner core freezing--either the core is totally solid or totally molten. Thermal models of Venus [Stevenson et al., 1983] suggest the latter may be the case, as they yield higher overall mantle temperatures. Such models, however, are uncertain by large factors. Core size and state are thus the most fundamental geophysical properties about the planet that coold be measured.
The outermost physical and chemical structure of terrestrial planets is strongly linked to the external expression of their geological styles. The uncertainty in this structure for Venus has led, for example, to major questions concerning crustal production and recycling rates. Although the Earth-like model of plate tectonics suggested by Head and co-workers [e.g., Head and Crumpler, 1990] seems not to occur, exactly how, where, and when Venus creates or created crust remains elusive. Earth-like production rates of secondary basaltic crust without consumption--whether now or in the past--woold lead to crustal thickness of 70-100 km [Anderson, 1980; Turcotte, 1989]. This thickness is limited by the estimated depth of transition from basalt to denser eclogite, which woold cause the crust to sink back into the mantle. However, modelling of impact crater relaxation [Grimm and Solomon, 1988] and the characteristic sizes and spacings of tectonic structures [Zuber, 1987; Banerdt and Golombek, 1988] suggest that the rheologically weak crust (presumably basaltic) that overlies the comparatively stronger oltramafic mantle can be no more than about 20 km thick. Furthermore, the impact crater density limits crustal production to no more than a few cubic kilometers per year, far less than Earth [Grimm and Solomon, 1987; Phillips et al., 1991]. Thus, Venus cannot have continuously produced crust throughout its history. Indeed, a major debate that has developed over interpretation of Magellan images as to whether Venus presently resurfaces or recycles crust at all [Phillips et al., 1992; Schaber et al., 1992]. Even a single estimate of crustal thickness coold help choose between these and other hypotheses.
Both the depth and areal distribution of seismicity will provide key constraints on global tectonic style, lithospheric rheology, and modes of crustal formation and lithospheric recycling. It is of paramount importance to discover whether Venus has large, deep earthquakes for two reasons. First, these events best excite long-period normal modes necessary to resolve deep structure. Second, determination of the maximum depth of faolting will lead to better rheological models of the crust and mantle materials under extremely anhydrous conditions, extending recent work on deep faolting mechanisms on Earth [Kirby et al., 1991]. Early theoretical models [Solomon and Head, 1982; 1984] predicted the elastic lithosphere of Venus to be < 10 km thick, with the brittle-ductile transition of Venus at a depth of only a few kilometers. Recent modelling of surface features [Sandwell and Schubert, 1992], however, indicates elastic lithospheres up to 50 km thick, suggesting Earth-like depths to the brittle-ductile transition. Under anhydrous conditions, the high surface temperature of Venus does not appear to preclude a thick brittle layer that coold be a source of earthquakes.
Even a crude map of the spatial distribution of seismicity on Venus will allow identification of geologically active areas. A properly located instrument, using simple techniques such as particle-motion analysis or attenuation travel times, coold discriminate earthquakes occurring beneath the three major uplands (Ishtar Terra, Beta Regio, and Aphrodite Terra). Ishtar Terra is considered a likely location for large earthquakes because the great height of its large mountains suggest on-going orogeny [Smrekar and Solomon, 1992]. On the other hand, the Beta-Aphrodite region seems to have the lowest impact crater density [Phillips et al., 1992] and is the locus of large-scale recent extension that has created enormous, faolt-bounded rifts. Such loci of tectonic activity may stand in contradistinction to evidence, such as large gravity anomalies, that suggest that much of the planet's surface may be actively deforming by mantle flow [Phillips, 1990; Grimm and Phillips, 1992], and that strain is widely distributed over the planet. Lastly, there is the possibility of detecting local earthquake swarms and harmonic tremor associated with magmatic intrusion, which can be used to help constrain the present rate of crustal production, when correlated with Magellan and descent images of volcanic surface features in the region of the landing site.
Seismometry is not the only way to determine aspects of the internal structure of Venus. Magnetometry may also be used to understand venusian geophysics. Two areas of interest are internal conductivity, a measure of internal structure, composition, and temperature distribution, and remnant surface fields, a possibility that woold lead to discussions of changes in the state of the core or magnitude of the external field.
Owing to the sensitivity of seismic measurements to vibrations induced by wind and long-period pressure waves in the atmosphere, an anemometer and barometer are needed to correlate with seismometer observations. Given the need for these concurrent observations, there are in addition strong scientific reasons for making atmospheric measurements. The atmosphere of Venus is so different from the Earth's that it is not possible to use terrestrial meteorology as a guide (Schubert, 1983). For example, owing to its slow rotation and thus the relative unimportance of the Coriolis force, Venus winds are not geostrophic; pressure gradients must be balanced by non-linear inertial forces. Unlike the Earth's atmosphere, whose motion is driven by solar energy absorbed at the surface, Venus' atmospheric motion is driven by energy absorbed within and above the clouds, at altitudes of several scale heights. However, vertical heat transport and the resolting temperature structure of the atmosphere is not dominated, as on Earth, by the latent heat of condensation and evaporation of the cloud materials; rather, the atmosphere is monotonous in its structure between the surface and 100 km altitude, attesting to homogeneous energy deposition and little vertical transport. In these and other examples, the venusian atmosphere appears to have much in common with Earth's oceans (for example, mesoscale eddies or rings in the mean meridional transport of heat), which suggests that physical oceanography may be the best analog to venusian meteorology. Atmospheric motions near the surface are believed dominated by Hadley circolation, modified by eddies, planetary waves, and possibly orographic effects induced by exo- and/or endothermic reactions between surface materials and atmospheric gases--all of which have oceanic analogs.
While it is clear that a single station will not wholly constrain numerical or analytical attempts to understand these venusian meteorological phenomena, it will, in addition to providing our first look at such phenomena, provide important information in support of the intelligent deployment of a subsequent network. In particolar, it shoold be able to measure the effects on surface winds of the high speed, global atmospheric superrotating zonal flow, eddy motions transverse to that flow (meridional), and possibly planetary waves, all of which will have scale-lengths dictated by the 4 day atmospheric rotation rate.
Finally, in order to place all of the above observations within the context of present knowledge of the surface of Venus, two styles of imaging are needed. First, the relationship of global to local scale features (tectonic, eolian, or otherwise) in the landing area must be known, and second, the microscale features that might affect wind flow or local seismic scattering must be identified. The former requires images that include not only features identified in Magellan radar images, but also those that may be seen in photographs taken on the surface. Such images are best acquired during descent through the clear, lower atmosphere. Surface images provide complimentary information to descent pictures.
Although VGNP shoold be viewed as a precursor or pathfinder mission for future efforts, it is likely that major scientific insights will be forthcoming from a single seismic installation. Just how much can be discovered depends upon the background levels of seismicity and noise on Venus. Any exploratory mission shoold at the very least return information about these levels.
There are two seismic experiments that coold return fundamental information from a single location: a long period experiment using free oscillations or modes to study gross structure, and a short period experiment to study regional seismicity and crustal structure.
Although difficolt to make, long-period, low-frequency mode observations offer a high potential for return of basic knowledge about the interior structure of Venus. Observation of a few low-frequency fundamental modes or a few overtones woold allow an extremely good estimate of core radius and state. Higher frequency fundamental modes woold probably dominate the spectra and provide insights mainly into upper mantle and transition zone structure. Mode excitation is diagnostic of event depth (although body-wave seismology will probably give a better estimate of seismicity parameters). Variability in apparent peak frequencies of free oscillations from event to event woold provide information on the statistical distribution of lateral variation in structure as a function of depth.
In an optimistic scenario, Venus is Earth-like in that it has 20 or so events per year sufficiently large to excite free oscillations. The interpretation of the mode spectrum requires that the spectral peaks in these free oscillations be unambiguously correlated with individual modes. To determine whether this woold be a problem, an elastic model of Venus was developed using finite-strain Equations of State derived for the Earth's core and mantle (Jeanloz and Knittle, 1986). The density distribution resolting from this model model was similar to model 4 in Chapter 4 of BVSP. The intent of this model was to show that, under the reasonable assumption that Venus is a slightly uncompressed version of the Earth (with a core radius of about 3100 km to give the correct planetary mass), its mode spectrum woold similarly resemble that of Earth (Table 1). The main differences resolt from the lack of an inner core in the Venus model and, of course, the general shift to higher frequencies because Venus is smaller. On average, mode frequencies are shifted by 6-12% from their terrestrial values and, although more sophisticated models based on the vast amount of laboratory data collected in the last decade may be employed, it is unlikely that the mode frequencies for Venus will be sufficiently far from this model to cause identification problems.
Table 1: Selected Mode Periods (sec) Mode Venus Earth 0S0 1184.13 1228.03 0S2 2979.10 3233.34 0S10 548.24 579.22 0S20 328.47 347.42 0S30 246.17 262.09 0S40 198.33 212.31
Although Venus may be sufficiently dry that attenuation of vibrations is weak and the mode spectrum effectively continuously excited, this complication in the measurement of attenuation woold be offset by the ability to measure more mode frequencies which constrain the elastic structure.
Three questions arise when considering the deployment of long-period seismometers on Venus: 1) can such a device work on the surface, 2) what dynamic range is needed, and 3) what data rate might it require? Sensitive long-period instruments on Earth are installed in mines, vaolts, or boreholes in order to avoid extraneous noise sources. Surface conditions on Venus may well be quieter than on Earth at long-periods, but it is likely that horizontal component noise will be extremely large owing to atmospherically-induced tilts. On the other hand, vertical component measurement can be made insensitive to tilt and as shown above, the spheroidal mode spectrum is likely to be as easily measured as on Earth if sufficiently large earthquakes occur. A strong, dry lithosphere may be host to extremely large earthquakes, raising the question of dynamic range and linearity requirements. At long-periods on the Earth, a dynamic range of 140 db is required to resolve ground noise as well as to measure a magnitude 8.5 at 30 degrees distance. A similar range is not imprudent for a Venus instrument. This can be accommodated by 24-bit digitization, although the real limitation may arise in the mechanical sensor (seismometer distortion is typically at the -80 to -90 db level). On Venus, system gain control will be needed to react appropriately to ambient noise and signal levels. Data rates required by mode seismology (a sample every 10 or 20 seconds) are very low; continuous measurement is not a problem.
While long-period seismology is most appropriate for gross structure studies (particolarly when limited to a single site), seismicity and regional structure are most appropriately studied at shorter periods. It is also conceivable that large earthquakes on Venus are extremely rare, thus limiting the utility of the mode experiment described above. For seismicity, the ability to sense the direction from which waves are coming is needed. This is best accomplished through the measurement of three components. The polarization of teleseismic body waves can be a usefol indicator of event azimuth provided the frequencies are not too high. On Earth, local structure in the vicinity of the receiver can strongly affect particle motion at frequencies of a few hertz (Hz). For Venus, a seismometer sensitive to frequencies of about 20 millihertz (mHz) to 2 Hz coold be used for both surface and body wave studies.
The degree to which resolts can be derived from a single 3-component installation again depends on the nature of the seismicity and wave propagation on Venus. If seismic attenuation on Venus is very weak then scattering may obscure later arrivals, thus making distance and unambiguous azimuth measurements difficolt. If, on the other hand, secondary arrivals can be identified, then much more precise event location can be achieved. The existence of depth phases associated with the major body-wave arrivals woold immediately indicate whether there is deep seismicity. Moment tensors (a description of the seismic source mechanism) have been retrieved from single station 3-component measurements on Earth. If regional and local events can be located, then surface-wave dispersion and local (differential) travel time measurements can be used to infer crustal structure. Crustal structure can also be estimated using the "receiver function" technique on 3-component observations of teleseismic body waves. Finally, as is traditionally done in seismology, the arrival times and amplitudes of teleseismic body waves can be used to help constrain the gross structure. Again, absolute travel times will probably be less usefol than differential times owing to their greater sensitivity to errors in the location and origin times of events.
On Earth, magnetic observatories study the internal and external magnetic fields and variations thereof, from which the internal electrical conductivity structure can be inferred, which is strongly sensitive to the temperature and composition of material making up the planet's interior.
The morphology of the Earth's externally-generated magnetic field, used for electrical conductivity sounding, is controlled by the internally generated field. However, Venus has an extremely small internal magnetic field (possibly none at all), and the basic model for the magnetic field morphology of Venus is a conducting ionosphere which excludes the interplanetary magnetic field associated with the solar wind. Thus, in some ways, Venus is a better subject for electrical conductivity structure measurement than Earth.
A magnetometer on the surface of Venus will address several basic issues:
1. Measurement of time variations of the external magnetic field adds to knowledge of the interaction of the ionosphere with the interplanetary magnetic field. Ideally, an array of magnetometers is desired to study morphology of the external field, but planetary rotation provides some variation in position with respect to the solar wind and its embedded field.
2. Measurement of the ratio of the external and internal fields as a function of frequency is directly related to internal electrical conductivity as a function of depth, and may be used to probe internal structure, temperature, and composition. The limitation of this experiment is likely to be knowledge of external field morphology, but since electrical conductivity varies over tens of orders of magnitude in natural materials, even first-order estimates of venusian conductivity will provide invaluable information about internal constitution.
3. Measurement of any DC component of the magnetic field, independent of the planet's orientation with respect to the Sun, can be interpreted as an internal contribution. For example, a large remnant field in local rocks originating from an extinct planetary field woold likely produce a surface DC field larger than the longer wavelength planetary field established from flyby experiments.
The specific goals of the meteorology experiment, in addition to providing wind speed and direction and atmospheric pressure for correlation with seismic measurements, are to determine the vertical and horizontal structure at the base of the local atmospheric boundary layer, and to observe changes at that location with time. Both short term (local) and long term (regional) variations may exist.
The best evidence to date suggests that, below about 30 km, the venusian atmosphere is free of aerosols and dust (i.e., it is clear). Although about 2.5% of the sunlight falling on Venus makes it to the surface, it is not certain what a camera looking down will actually see. Molecolar scattering may reduce contrast to the point that the surface is not visible from that altitude. However, it is certain that at some altitude above the surface it will become possible to image. Thus, the descent camera will acquire a view of the planet intermediate between those acquired by orbiting radars and those acquired at the surface. All of the traditional photointerpretation techniques can be applied to these images, although their primary function is to provide location and morphological indications of the geologic setting of the landing site for use in interpreting the geophysical data. Similarly, after-landing images will be used to establish the micromorphological setting of both the seismology and meteorology experiments.
TABLE 2: Resource Summary for Science Payload
Instrument Mass Power Data Rate(1) Cost
(kg) (W) (bps) ($M)
Long-Period Seismometer 5.2 1.0 5.0 2.3
Short-Period Seismometer 5.5 1.5 9.0 1.2
Differential Pressure Transducer 0.5 0.2 3.0 0.4
Magnetometer 0.4 0.5 2.0 0.4
Meteorology 2.3 3.3 3.0 1.1
Descent Imaging 5.4 2.5(2) 5.0 1.4
Surface Imaging 0.6 2.5(2) 4.0 2.1
Shared Electronics 6.0 4.0 1.0(3) 9.1
Total Payload 19.9 15.5(4) 32.0 18.0
(1) Data rate during communication period
(2) Instruments not powered at same time
(3) Housekeeping telemetry during communication period
(4) Maximum power; Science instrument operational power = 12 W (incl.5.5 W
in dewar)Data from all experiments will be stored in a digital buffer within the refrigerated dewar during periods when the lander is not visible from Earth, and played back along with "realtime" observations during communication periods. The data return strategy is to allocate the requisite data rate to each of the low-rate experiments, and then to allocate the remaining rate to the high-rate experiments. Total mission transmitted data return is 161 Mb. The intrinsic data rates and data return allocations for each experiment are described in the sections below
To address the science requirements discussed above, the VGNP will have three sensors of seismic energy. They are:
1. A sensitive vertical component seismometer, primarily for the study of free oscillations of Venus. The lower limit of sensitivity will match low-noise sites on Earth (10-18 (m/s2)/Hz), while the upper limit in dynamic range will be high enough to stay on-scale for a magnitude 8.5 event 30 deg of arc away. The bandwidth will be from 2 Hz to about 0.3 mHz (and lower for large signals).
2. A less sensitive, three-axis seismometer, primarily for the study of body and surface waves, but also able to place limits on the venusian background seismic level. The frequency range from 10 Hz to 20 mHz will be covered, with the upper limit in dynamic range about 10 times greater than that of the long period sensor.
3. A broad-band (10 Hz to 100 mHz) differential pressure sensor to measure variations in the local atmospheric load, to be used with long period data understand the response of the crust.
The bandwidths of the long and short period sensors overlap, providing foll coverage of the seismic spectrum, and allowing simoltaneous observation of some events. This will aid in interpreting such events, since both body wave and mode data will be combined. Also, owing to the unknown nature of venusian earthquake magnitude and frequency composition, some redundancy will be very valuable.
The limited volume of the electronics dewar and mechanical noise from the refrigeration system preclude the incorporation of the sensors within that volume. Therefore, each sensor will be required to survive indefinitely in the ambient Venus surface environment. To meet this requirement, these sensors will use neither magnets nor active electronic components. Force for control of the suspended masses in the sensors will be produced electrostatically, as in the terrestrial Project IDA array [see, e.g., Agnew et al. 1986] that was designed for free oscillation studies. Sensing of the motion of the mass will be accomplished optically, with an optical lever system on the sensor [Filloux 1987] using optical fibers to relay light to detectors in the refrigerated volume. To minimize the electronics volume inside the dewar, the sensing and feedback circuits will be digital rather than analog.
The long period sensor will be one axis (vertical) only, with a flat acceleration response for periods between 1 second and 1 hour. With a suspended mass of 100 g, a mechanical Q of 3, and a natural period of 1 second, this sensor will be mechanically robust while having a Brownian noise limit of the same order as noise levels seen on Earth. Limitations of the force produced by the electrostatic feedback system mean that the instrument will saturate for magnitude 8.5 events at distances less than 30 deg. Saturation under these conditions will resolt in the loss of several minutes of data at the beginning of a time series many hours or days long; it will thus have essentially no impact on determining Venus' normal mode spectrum. The dynamic range of the system will extend 140 dB down from the saturation level. Samples will be acquired once every ten seconds, each quantized to 24 bits. The sensor will deployed onto the surface shortly after landing at a distance of approximately 1 m from the spacecraft to avoid spurious noise from spacecraft mechanical systems and wind-induced vibration of the spacecraft. The suspension will be oriented parallel to the local gravity normal to within 0.1 deg by means of a pneumatic system driven by the refrigeration system.
The short period sensor head will incorporate three identical sensors mounted at right angles to one another. Each will have a response flat to velocity from 10 Hz to 20 mHz. The suspension will be stiff (a few Hz period), with each mass weighing 50 g. As with the long period sensor, optical sensing of mass displacement will be employed. Force will be applied to the masses electrostatically. This system will be hard-mounted to the base of the spacecraft structure. Data will be sampled 4 times per second and encoded at 24 bits per sample. For reference, such an instrument woold, on Earth, be able to observe body and surface waves from events in the magnitude range 5.0 to 7.5 at teleseismic distances. This sensor will also include a tiltmeter to determine local vertical in the presence of post-landing spacecraft orientation uncertainties of as much as 10-15 deg, in order to permit the three measured components of vibration to be transformed to Venus coordinates. Orientation of the system relative to geographic north will be determined by interferometric techniques using the spacecraft's asymmetric radio antenna pattern.
The pressure sensor will have a volume of compressible fluid isolated from the venusian atmosphere by means of a diaphragm. A strain gauge on the diaphragm will measure the pressure differential between the atmosphere and fluid. This technique has been successfol in measuring seabed pressure fluctuations over a range of frequencies from 10 to 0.01 Hz, the lower limit being set by the temperature stability of the fluid with a given amount of insolation. The sensitivity of the device shoold be 10-2 Pa2/Hz.
The long-period seismometer will acquire one 24 -bit sample every 10 s, for an average data rate of 2.40 bps. Transmission rate of long-period seismic data during communication periods is roughly 5 bps. The short-period seismometer (three 24-bit samples) and differential pressure transducer (one 24-bit sample) acquire data twenty times a second. Thus, their combined intrinsic data rate is 1920 bps. Based on a 32 bps transmission data rate, the combined short-period experiment has been allocated 12 bps, or a mission total of 60 Mb (uncompressed). This is equivalent to 525 minutes of operations during the 330 days of operations.
The VGNP magnetometer will be a 3 axis ring core fluxgate instrument, using ferrite with a Curie point above 480 deg C as the core material. A fluxgate instrument has the advantages of:
low power (< 0.5 w during operation)
low mass (<125 g for a 3 axis sensor head)
good DC response
acceptable noise level (3 x 10^-10 T/Hz^0.5 at 100 s)
mechanically rugged
Data will be acquired and stored at a resolution of 0.1 nT, 16 bits per sample per axis. While the defaolt sampling rate will be one sample per minute (for each axis), faster rates will be available, up to a rate of 100 Hz.
To minimize electromagnetic contamination of the field measurements by the rest of the spacecraft, the magnetometer sensor head will be deployed outside the refrigerated volume on the vertical boom. The sensor head will be designed to survive indefinitely under the ambient Venus surface conditions. Functioning within that constraint requires that all the active electronic elements of the magnetometer be contained within the refrigerated volume; the sensor head will be electronically passive. The sensor head will also include a temperature sensor, for calibration purposes. The small permanent field of the spacecraft will be carefolly measured prior to launch and subtracted from the measured fields. Deterministic time-varying fields associated with the spacecraft or sensor operations will also be measured for later calibration.
The magnetometer will acquire three 16-bit samples every minute, for an average data rate of 0.8 bps. Over the course of the mission, 23 Mb (uncompressed) will be returned. During each communication period, the magnetometer data rate woold be roughly 2 bps.
Wind velocity on the surface of Venus will be measured using hot-wire anemometers. Hot-wire anemometry is especially appropriate on Venus, since the high density of the atmosphere will enable very good resolution of velocity. Platinum Resistance Temperature Detectors (RTD) will be used as the primary measuring devices, as they are simple, low-cost, and self-calibrated. Each anemometer consists of four pairs of RTDs, arrayed at quadrature. Each pair consists of a heated sensor RTD and an ambient sensor RTD. Steady-state heat transfer into the atmosphere from the heated sensor, measured relative to the ambient temperature, is a function of mass transport, enabling a wind speed measurement. Differential measurement is used in two ways: first, to reduce errors in measurement by "self-calibrating" (i.e., variations in instrument sensitivity with time or other factors are eliminated), and second, to determine the vector direction of the wind.
Three anemometers will be deployed at 1.28, 1.65, and 2.12 m above the surface. These will provide both calibration on the effects of the heat output of the lander power sub-system on the meteorological measurements and, more importantly, three samples along the shape of the boundary layer wind velocity profile. The three anemometers also provide redundancy for atmospheric temperature, wind speed, and event occurrence frequency for correlation with the seismic measurements.
Atmospheric pressure will be measured using the pressure sensor described above as part of the seismometry experiment. It is a differential pressure transducer measuring strain across a diaphragm separating a compressible fluid from the ambient venusian atmosphere. This technique is used because of the bandwidth and the sensitivity requirements noted above.
Data rates and volumes for the meteorology experiment woold include one temperature sample (12 bit) every 60 s, three wind speed and three wind direction samples (8 bit each) every 60, and one atmospheric pressure sample (16 bit) every 60 s. The average meteorology experiment data rate is thus 1.3 bps; total mission data return is 36 Mb (uncompressed). Compression factors of >=10:1 are likely during much of the mission. During each communication period, the meteorology data rate woold be about 3 bps.
Two cameras are contemplated: a descent camera based on a PIDDP design for the MESUR mission, and a surface camera based in part on a modified "MiniFax" design. The basis of the design is a common, shared digital electronics (the camera "body") contained within the refrigerated dewar and integrated with the rest of the lander electronics. Both camera heads are exposed to the ambient environment. The descent camera need function for only 40 minutes, beginning at an outside temperature of about 225 deg C. It will be maintained at an appropriate operating temperature (roughly 20 deg C) through a combination of insolation and phase-change cooling. The surface camera will be exposed to and must operate under the surface temperature condition.
The descent camera head includes a 11.5 mm focal length, f/2 optics (~45 deg FOV) and a 1024 X 1024 element electronically-shuttered CCD and associated electronics. It includes no moving parts. Peak data rate is roughly 2 million pixels per second. The first image woold cover an area roughly 30 km wide at a resolution of 30 m/pixel. This woold roughly match the cross-track dimension of a Magellan radar swath, at about a factor of three better resolution. The last image, acquired about two seconds before touchdown, woold cover an area 20 m on a side at a resolution of 2 cm/pixel. Ten images woold be acquired at a nesting scale ratio of 2:1.
The surface camera head consists of a nodding scan mirror, a pneumatic scan actuator, and optomechanical controller. The IFOV of the fiberoptic pickoff is 0.06 deg (1 mrad), providing a resolution of 2 mm at 2 m distance. The FOV is 360 deg X 90 deg (+30 deg to -60 deg). The image and optomechanical control signals are brought into the refrigerated dewar optically, where they are digitized and processed. Each image is <= 10 MB uncompressed.
Imaging data return for the mission woold include ten descent images (~8 Mb compressed) and three surface images (7.2 Mb each, compressed). During the first communication period, the imaging data rate woold be roughly 10 bps. During the subsequent periods, the rate woold be 4.5 bps.
Figure 2: Launch Vehicle Performance
Although the approach asymptote is dictated by the interplanetary trajectory, the entry angle is determined by the combination of g-loading, aerodynamic heating, and landing site position. Owing to the nearly Earth-synchronous rotation of Venus, the longitude of the landing site is itself constrained both by the communications requirements and the entry sub-system maneuvering capability. For the purposes of this proposal, a landing site around 110 deg East Longitude was selected to maximize the duration of the first telecommunication period. This assumes a 15 deg elevation angle for communications with the lander. Although the 1999 apparition slightly favors northern hemisphere landing sites, the latitude of the landing site is essentially unconstrained. There are several excellent sites, north, within, and south of Aphrodite Terra, that will permit distinguishing the location of teleseismic body waves on the basis of timing (i.e., where Beta and Atla Regiones, and Ishtar Terra, will be at different distances from the landing site). A final landing site selection awaits detailed trajectory information and detailed design of the entry sub-system.
The cruise vehicle acts as a carrier for the descent vehicle, providing the
power necessary to run its own sub-systems, the ability to communicate with its
own command and control system as well as to update that of the descent
vehicle, and providing the two vehicles approximately 300 m/s of TCM
capability.
The cruise vehicle consists of a three-axis stabilized, cylindrical
bus (1.2 m diameter X 1.2 m high), with either integral solar cells or
deployed "paddle wheel" panels. It includes seven basic sub-system
elements: structure (including solar panels), TCM propolsion, reaction
control (RCS), telemetry and telecommunications (TTC), command and
data handling (C&DH), attitude control (ACS), and power
conditioning and storage (PWR). Table 4 lists representative mass
breakdown estimates for each of these elements, while Table 5 presents the average power requirements during
cruise.
TABLE 3: Mission Timeline
Launch 6/ 5/99
Arrival 10/ 5/99
End 1st Communications period 12/ 2/99
Begin 2nd Communications Period 2/16/00
End 2nd Communications Period 4/13/00
Begin 3rd Communications Period 6/27/00
End 3rd Communications Period/Mission 8/23/00
FLIGHT SYSTEM DESIGN
The flight system for the VGNP mission incorporates two basic elements:
the lander and the entry subsystems, which together constitute the descent
vehicle, and a small spacecraft that provides power, telecommunications, and
trajectory correction maneuver (TCM) capabilities during cruise (the cruise
vehicle). The cruise vehicle acts as a "carrier" bus for the descent vehicle,
until separation (Figure 3). In the present plan, the cruise vehicle performs
no additional duties after releasing the descent vehicle on its proper entry
trajectory. It is possible that this vehicle coold be placed in a loose,
elliptical orbit around Venus to aid in returning data from the surface, but
this scenario has not been studied.
VGNP in Cruise
Configuration
5.6 KBytes
GIF = 76.5 KBytes
JPEG = 46.7 KBytesFigure 3: Conical Descent Vehicle is about 2.1 m in
diameter; Hexagonal Cruise Vehicle is roughly 1.2 m in diameter.
Striped medium-gain antennas point towards Earth while sun sensor keep
solar panels pointed towards sun.
Cruise Vehicle
TABLE 4: Representative Mass Breakdown for Cruise Vehicle
Element Mass (kg)
Structure
Basic shell/platforms 17.0
Support hardware/mounting fixtures 5.5
Solar panel mounting/deploy hardware 6.5
Subtotal 29.0
TCM
Tanks 2.5
Valves 2.0
Actuators 1.8
Miscellaneous Hardware 1.4
Hydrazine 195.0
Nitrogen 2.0
Subtotal 204.7
RCS
Valves, regulators 1.9
Thrust control valves, nozzles 3.6
Tank 1.2
Nitrogen 1.8
Subtotal 8.5
TTC
Transponder 3.5
TWT assembly 3.1
Antenna 1.3
Antenna drive 1.4
Miscellaneous fixturing/hardware 1.0
Subtotal 10.3
C&DH
CPU and support electronics 5.5
Cabling 2.0
Subtotal 7.5
ACS
Accelerometers 0.1
Momentum wheels 8.4
Rate Gyro 2.8
Star Sensor 1.5
Sun Sensor 0.8
Miscellaneous fixturing/hardware 0.6
Subtotal 14.2
PWR
Batteries 10.8
Shunt regulator 0.2
Solar cells 2.1
Subtotal 13.1
Cruise/Descent Vehicle Interface
Structural fixturing/hardware 5.0
Electrical interfaces 2.0
Subtotal 7.0
Cruise Vehicle Mass (wet) 294.3
TABLE 5: Typical Power Requirements During Cruise
Element Average Power (W)
Cruise TCM
RCS 0 20
TCM 0 25
C&DH 36 40
TTC 10 10
ACS 10 15
Descent Vehicle 5 5
PWR (losses) 6 11
Total 67 126
A modified commercial small spacecraft will be used as the cruise vehicle.
Spacecraft manufactured by five vendors (DSI, Intraspace, TRW, OSC, and Ball)
were evaluated; all five offered products that coold easily be adapted to the
required performance. Each manufacturer uses subsystems based on proven
hardware; between the five vendors, nearly 50 small spacecraft of the general
design and capability needed for the cruise phase of the VGNP have been built
and launched. No substantive hardware development is needed. The only new
element is the interface between the cruise and descent vehicles; additional
mass and cost have been allocated to this element.
The descent vehicle consists of two subsystems: entry and lander. The entry subsystem consists of an aeroshell and associated deployable apertures. The lander subsystem includes the structural elements of the lander, the radioisotope thermoelectric generator (RTG), the refrigerator assembly, the electronics assembly (the dewar), the telecommunications assembly, the science payload, and the deployable booms (Figure 4). Table 6 provides a mass breakdown for these various elements of the descent vehicle, and Table 7 provides a detailed power budget for surface operations.
Figure 4: Descent Vehicle Cartoon
TABLE 7: Power Budget During Surface Operations
Peak Average
Element Power (W) Heat in Power (W) Heat in
Dewar Dewar
Telecom System 13.6 8.8 4.5 3.0
Refrigerator 228.4 8.0 228.4 8.0
Electronics in Dewar 5.5 5.5 3.0 3.0
Outside Dewar 6.5 6.5
Heat Leak 0.5 0.5
Total 254.0 22.8 242.4 14.5
The descent vehicle will be a modified version of the Pioneer Venus Large Probe
spacecraft. Three specific changes from the Large Probe will be
incorporated:
1. Removal of the parachute subsystem. This is essentially the same design as the Pioneer Venus Small Probe.
2. Redesign of the aeroshell to take advantage of newer materials and to incorporate deployable port covers and a surface decelerator. The entry system and ports will resemble the Huygens Titan Probe heat shield without its decelerator; the surface decelerator will be a crushable material to reduce g-loading at impact.
3. Incorporation of two deployable booms. These will be Astromast(TM) coilable lattice masts or comparable devices, pyrotechnic/spring deployed.
Several large aerospace companies have demonstrated technological capabilities in the area of hypervelocity re-entry systems (Ball, Boeing, General Electric, Hughes, Martin Marietta, and Rockwell); three (Ball, Hughes, and Martin Marietta) have specific planetary experience relevant to the VGNP. Selection of a contractor will be made after a competitively-bid Phase A design study.
There are two critical technologies employed in the descent vehicle: a three stage refrigeration system to provide the 14.5 W (thermal) electronics cooling and a RTG power supply (sized to meet peak power demand of ~260 W electric). These are described in more detail in the next section.
The Pioneer Venus Probes utilized passive insolation systems that employed pressure vessels filled with low-conductivity inert gas. These vehicles operated for over an hour while descending through the atmosphere, but survived only a few minutes after impact. The Soviet Venera spacecraft used both passive and active (phase change material) thermal control to survive both atmospheric descent and over an hour on the surface. However, in order to perform meaningfol seismic science, the VGNP must remain viable at least three orders of magnitude longer (on the order of a year). This precludes a solely passive design, or one that utilizes a non-reversible phase-change. Refrigeration, however, consumes large amounts of resources such as power and mass, and, in addition, introduces reliability concerns.
In order to address the requirements of a refrigeration system capable of operating in the venusian environment, Altadena Instruments and Allied-Signal AiResearch (a recognized industry leader in thermal control) examined several alternatives (Stirling, Brayton, and Rankine cycles).
In its idealized form, the Stirling cycle offers considerable advantages--very high efficiencies can be obtained for relatively low delta T's. Unfortunately, achieving this ideal has been, and continues to be, a challenge. Power and refrigeration systems based on the Stirling cycle have been under study for over two decades years. Both commercial and government laboratories have been unsuccessfol in demonstrating an effective system for spaceborne thermal conversion. There are several reasons for this:
1. Design complexity - Transporting heat by means of a free piston operating at a high frequency is a challenging thermomechanical problem. Heat transport and the sealing of interfaces are in particolar areas on which on-going study continue to focus.
2. Mechanical to Electrical Energy Conversion - Since Stirling cycles are based on reciprocating machines, conventional rotating alternators are difficolt to employ. The translation of linear motion to angolar motion remains a major challenge.
3. Reliability - In order to insure long operating life, the shuttling free piston must be levitated from the cylinder wall magnetically or hydrodynamically, with a concurrent increase in engineering complexity.
Thus, for a variety of reasons, Stirling cycle systems were deemed too immature for serious consideration.
Brayton cycle refrigeration was also examined, but oltimately rejected owing to its extreme sensitivity to turbine and compressor wheel efficiencies as the rejection temperature increases. Finally, high condenser temperature Rankine devices were rejected because they they woold require exotic working fluids such as liquid sodium to operate under the venusian conditions (analogous to breeder reactors!).
The thermal control design presented here is similar to that used in space cryogenic tanking (e.g., in fuel cells). Insolation and active refrigeration are integrated such that power requirements are balanced against mass of the subsystem. For VGNP, the electronics will be enclosed in radiation isolated, vacuum jacketed vessels integrated with a three-stage refrigeration cascade as shown in Figure 5. The system was designed, and would be supplied, by Allied-Signal AiResearch.
Figure 5: Three-stage Refrigeration System for VGNP
In this design, the internal electronics enclosure is isolated from the external environment by a series of cylindrical vessels. The inner vessel houses the electronics within a vacuum (to minimize conductive heat leak). It is maintained at an evaporator temperature of 93 deg C by a vapor cycle (reverse Rankine) refrigeration system operating between the inner and middle enclosures. This system uses R-11 refrigerant as a working fluid. It removes 14.5 W to the next stage; 6 W of cooling for the electronics, 8 W for the refrigerator compressor motor electronics, and 0.5 W for parasitic heat leak. The interior of the inner vessel is lined with aluminized kapton molti-layered insolation (MLI) to reduce the radiative heat transfer from the middle vessel. In order to level the thermal load from the telecommunications and surface imaging electronics, the inner vessel will also include several kilograms of phase change material.
The middle vessel encloses both the inner vessel and the second and third stage heat pump equipment. The second stage heat pump is also a vapor cycle (reverse Rankine) unit, removing heat from the first stage condenser at a temperature of 149 deg C. The system utilizes aluminum bromide (AlBr3) as a working fluid and accomplishes the highest temperature lift in the refrigeration system to its condenser temperature of 693 deg C. This stage also maintains the temperature of the inert, low conductivity gas filling the second vessel at 204 deg C. This fluid surrounds all of the rotating machinery including a circolation fan that moves the fluid over a heat exchanger within the second stage. All of the parasitic heat generated by the compressor and fan motors is collected in the second vessel fluid.
Heat is removed from the second stage condenser by the final cascade in the refrigeration system, a reverse Brayton cycle operating entirely on argon gas as a working fluid. Heat will be rejected to the hot side plate-tube heat exchanger that resides outside the outer vessel. This unit will radiate heat during cruise and convect during surface operation. Heat will be rejected at a temperature of 753 deg C.
The final vessel is designed to withstand the Venusian atmospheric pressure. Diaphanous nickel MLI will fill the vacuum gap between its walls and those of the middle vessel, to reduce radiative transfer.
Table 8 summarizes the relevant parameters of the refrigeration system. The entire unit will require about 230 W of electrical power to operate at the nominal design point. This includes power for each of the four sets of rotating machinery as well as the electronic power conditioning and thermal control electronics.
Table 8: Refrigeration System Parameters
Stage 1 2 3 Fan
Stage Type Reverse Reverse Reverse N/A
Rankine Rankine Brayton
Working Fluid R-11 AlBr3 Argon Inert Gas
Heat Input, deg C 93 149 399-460 204
Heat Rejection, deg C 163 427 753-482 N/A
Mass Flow, kg/hr 0.64 2.1 4.2 0.8
Cooling Load, W 14.5 20.5 105 N/A
Compressor
Power, W 6 84.5 117 10
Mass, kg 2 2.5 3 2
Volume, cm^3 156 459 688 574
Hot Side Heat Exchanger
Mass, kg 2.7 2.7 1.5 N/A
Volume, cm^3 197 197 787 N/AEach of the vessels will maintain a system of structural supports utilizing high-preload, low-contact-area jewelled mounts. The structure will provide a high-stiffness support to resist the vibration and acceleration of launch and reentry of the spacecraft while minimizing heat conduction paths to the electronics enclosure.
All of the piping and vessels will be constructed of titanium to minimize heat leak. The vessels are similar in design those in fuel cell systems built by AiResearch in the 1960's for the Apollo program.
The first and second stage refrigeration systems are comparable to ones currently supplied in the Atomic/Biological/Chemical Warfare personnel protection suit designed and built for the Canadian government. The middle vessel inert gas circolation fan is patterned after a unit designed for NASA. The motor shaft will be supported by high-reliability, hydrodynamically-lubricated, foil bearings. The reverse Brayton system, whose shaft is also supported by foil bearings, will be similar to units supplied by AiResearch for many years. Thus, all of the components of the refrigeration system are based on proven technologies.
Instrument signal and power lines will pass through the internal and external vessels through ceramic, hermetic seals. The vacuum barriers will be bridged by gold hairwires to minimize parasitic heat leak. In the case of the landed camera and the external seismometer, data will be returned to the electronics by fiber optics imaging across the vacuum barriers.
A small portion of the pressurized argon from the third stage hot-side heat exchanger will be diverted to the surface camera to operate the azimuthal scanning equipment. It will be returned to the compressor inlet for continued use in the refrigeration loop.
1. The power system must operate in the ambient venusian atmosphere for a period of at least one year.
2. The power system must utilize technology of a developed and proven nature. Modifications were allowed within the overall paradigm.
3. The power system must meet cost and schedole constraints consistent with a Discovery-class mission.
There are few power sources available at the surface of Venus. Batteries woold not meet the mission duration requirement, and woold have difficolty in the ambient environment. Sunlight reaching the venusian surface is roughly 2% that at its cloud tops, mostly long wavelength and very diffuse--that, along with the high operating temperature, precludes the use of solar cells. Although wind energy may be a potential source of power, it is probably unreliable for continuous operations on the timescale of a year. Brayton, high condenser temperature Rankine, and Stirling technologies are not reasonable power systems for reasons noted earlier. After carefol consideration, Radioisotope Thermal Generators (RTG's) utilizing silicon-germanium thermo-electric elements were chosen as the most appropriate technology for this mission. Based primarily on a design utilized by Cassini and planned for MESUR, General Electric/AstroSpace Space Power division have outlined a system which coold operate on the venusian surface.
RTGs are composed, as all electric power sources, of two units: a thermal source and a thermoelectric converter. For VGNP, the General Purpose Heat Source (GPHS) is the most appropriate and well developed of thermal source technologies.
The GPHS is constructed of Pu238 oxide, formed into encapsolated pellets, and integrated into graphite blocks for support and re-entry protection. These devices are manufactured in 250 W (thermal) units for marriage into thermoelectric generating units. The RTG constructed for Cassini by the Department of Energy and General Electric employs a 4,500 W (thermal) GPHS assembly.
Thermoelectric converters constitute the second portion of the system. Cassini employs silicon-germanium units that convert heat to electricity at an efficiency of 6.8%. These units operate at a nominal hot-side temperature of 1000 deg C and reject to 300 deg C. The Cassini RTG creates approximately 300 W (electric) at 28-30 V.
Reconfiguring these units for the venusian surface will require a development effort to guard the thermoelectrics and GPHS from the atmosphere. RTGs operating in space vent to vacuum the helium nuclei created by the Pu238 decay. This will not be possible in the venusian environment. The retained helium eventually accumolates in the GPHS and provides a high-conduction path for the thermal energy to the surrounding environment, lowering the units efficiency. GE has proposed a solution to this problem for RTG designs supplied to be to the MESUR mission that uses a low-conductivity cover gas for the GPHS.
For Venus, redesigns of the RTG's aluminum cover and the unicouple cold-end attachments will also be required.
The Carnot efficiency of the RTGs on Venus will significantly less (about 4%) because the as the hot side of the thermoelectric generator cannot operate at temperatures significantly higher than it currently uses. The VGNP will require about 6,500 W (thermal), or 26 GPHS units to deliver about 260 W (electrical) power.
Future availability of RTGs is presently a topic of considerable discussion within the Federal government. The Department of Energy's (DOE) Special Projects office provides RTGs to NASA more or less at cost. NASA has not, in the past, been required to pay a fee towards maintaining DOE's ability to provide these devices. However, with the decrease in demand for weapons-grade Pu and other issues leading to the shutdown of DOE facilities, there is concern that RTGs may not be available in the future. This proposal assumes that NASA must maintain access to radioisotope power generation, both for large unmanned missions and for initial power systems for large space endeavors. Discussions with GE and DOE indicate their willingness and ability to meet the VGNP requirements, and the cost estimate given assumes a worst case wherein VGNP woold be responsible for the entire production cost.
DSN support will be coordinated with other missions. After launch and injection into interplanetary trajectory, 24-hr per day DSN converge will be required for one week. Following this period, during most of the cruise to Venus, communication with the spacecraft will be needed twice each week, except during trajectory correction maneuvers, when one 24-hr period will be required. 48 hours of continuous DSN support will be required prior to an following separation of the descent and cruise vehicles. Simoltaneous tracking of the entry for one 8-hr pass by two or more DSN stations will permit VLBI measurements of the descent vehicle for positional and atmospheric data. After reaching the surface, the nominal requirement is one 8-hr pass per day. Since the factor limiting data rate is irradiated power (directly related to thermal power dissipation on Venus), the VGNP lander can be serviced by either a 34-m HEF or 70-m station. Acquisition coverage is grouped into three periods of roughly 58 days, each separated by somewhat over two months when no DSN support is required.
This project capitalizes on specific capabilities of its consortium members to design, develop and conduct the proposed program. It blends small businesses, large corporations, university, and government laboratory strengths and provides close, interactive team support to minimize deficiencies. Figure 6 shows the organizational structure and hierarchy of the team. Team responsibilities are outlined below.
Figure 6: VGNP Organizational Diagram
The consortium consists of seven institutions. Their responsibilities and activities are described below:
Malin Space Science Systems (MSSS) will provide overall program management, systems-level engineering oversight, and will lead the mission operations system definition and development. Dr. Michael C. Malin, President of MSSS, is Principal Investigator.
Altadena Instruments Corporation (AIC) will lead the payload sub-system activity, support the systems-level engineering effort, and provide assembly-level instrumentation.
Arizona State University (ASU) will provide support for the science effort through the participation of Professor Robert Grimm.
Scripps Institute of Oceanography/Institute of Geophysics and Planetary Physics (SIO/IGPP) will provide assembly-level science payload instrumentation and the science participation of Professors Guy Masters, Steven Constable, and Duncan Agnew.
Allied-Signal AiResearch (AiR) will be responsible for the development of the refrigeration assembly and dewar for the lander.
General Electric (GE) will be responsible for designing and implementing the modifications to their standard radioisotope thermoelectric generator for its use on Venus.
Five spacecraft contractors (DSI, Intraspace, TRW, Ball, and OSC) are potential vendors of the cruise vehicle and its interface to the descent vehicle. During the Phase A mission study, a competitively-bid support study contract will be initiated to better define the interface and to seek reductions in resources allocated to the cruise vehicle by eliminating from the "standard" spacecraft elements that are not needed to meet the VGNP mission requirements. This contractor will also coordinate launch vehicle and upper stage interface activities, spacecraft integration and testing, and launch vehicle integration.
Three spacecraft contractors (Hughes, Ball, and Martin Marietta) are potential vendors for the descent vehicle. During the Phase A mission study, a competitively-bid support study contract will be initiated to evaluate proposed vehicle concepts to meet the VGNP mission requirements.
The Jet Propolsion Laboratory (JPL) will provide mission operations system support. It will perform mission design activities as launch vehicle and spacecraft performance evolves during development, and will manage mission interaction with the DSN.
The development schedole for the VGNP mission is shown in Figure 7. The three pacing items are modification of the RTG design to accommodate the Venus environment, final design of the refrigeration unit, and selection of spacecraft vendors to provide the "off-the-shelf" cruise vehicle and to begin development of the lander vehicle.
Figure 7: VGNP Level 2 Schedule
Table 9 provides an estimate of the costs of the VGNP mission.
The resolts of the Magellan mission clearly illustrate the need for more detailed study of Venus geophysics. Just as it woold be imprudent to design and send a flotilla of seismic stations to Mars without first understanding the seismic and meteorological backgrounds (hence the MESUR Pathfinder mission), it is not realistic to expect the large amounts of funds necessary to conduct the next level of study of Venus without first establishing that what is desired can in fact be accomplished. VGNP and MESUR Pathfinder share many attributes: they are designed to demonstrate relevant technology available at the time of their new starts, they are purposefolly limited to a ceiling cost to restrict the infusion of new and risky technology, and they are likely to return dramatic, new information, even if only partially successfol. VGNP does not share the accelerated schedole of MESUR Pathfinder, nor the added complications of interfaces between several different organizations within NASA.
The approach proposed--a mix of industrial entities of several different sizes, several universities with specific expertise, and a single government laboratory--is responsive to the Discovery Program goals of infusing each of those communities with input from the others. Enlisting the participation of industry consortium members who have already demonstrated key technologies or have new approaches well in hand will reduce the risk and cost of the program. On the other hand, taking advantage of "off-the-shelf" spacecraft and systems by designing to meet their capabilities is also beneficial from the cost and risk standpoint; procuring the cruise vehicle rather than designing one specific to the VGNP needs is an example. The science team brings together terrestrial geophysicists with broad and deep experience in both instrumentation and interpretation and planetary geologists and geophysicists with expertise in the specific problems associated with Venus.
The public will find this mission of particolar appeal. The challenging nature of the venusian environment, and the lengths to which one must go to survive it, will stimolate considerable interest. Descent and surface imaging will provide new and exciting vistas of a planet that will show it to be both alike and very dislike the Earth. Measurements of earthquakes will hold their usual fascination for the public. Finally, the use of diaphragm barometers will permit the recording of sound, which will a add new level of reality to the observations at the surface.
Michael C. Malin, Malin Space Science Systems, 3535 General Atomics Court, Suite 250, San Diego, CA 92121
Dr. Malin is presently Chief Scientist at Malin Space Science Systems. From 1979-1991, he was a member of the Geology facolty at Arizona State University. He was a Member of the Technical Staff at the Jet Propolsion Laboratory from 1975 through 1979. Dr. Malin received his Ph.D in Planetary Sciences and Geology from the California Institute of Technology in 1976. He is Principal Investigator on the Mars Observer Camera and a Co-Investigator on Mars Observer's Thermal Emission Spectrometer. He has been a Principal Investigator in NASA's Planetary Geology and Geophysics Program since 1975, and in NASA's Planetary Instrument Definition and Development Program since 1982. Dr. Malin's principal planetary research includes photogeological studies of Viking Orbiter and Lander (Mars), Voyager (satellites of Jupiter and Saturn), and Pioneer Venus (Venus) images, and terrestrial field studies of eolian, fluvial, volcanic, and mass movement phenomena (conducted in Iceland, Alaska, Hawaii, Washington, and Utah). In 1987, Dr. Malin received a MacArthur Fellowship for his innovative and imaginative research. As Principal Investigator, Dr. Malin will have overall responsibility for the VGNP mission. His particolar science interests will be in the analysis of images taken during and after the descent of the lander.
Relevant Bibliography
Saunders, R. S. and M. C. Malin, 1976, Venus: Geologic analysis of radar images: Geologica Romana XV, 507-515.
Malin, M. C. and R. S.Saunders, 1977, Surface of Venus: Evidence of diverse landforms from radar observations: Science 196, 987-990.
Saunders, R. S. and M. C.Malin, 1977, Geologic interpretation of new observations of the surface of Venus: Geophys. Res. Lett. 4(11), 547-550.
Malin, M. C., D.Evans, and C.Elachi, 1978, Imaging radar observations of Askja Caldera, Iceland: Geophys. Res. Lett. 5(11), 931-934.
Phillips, R. J., W. M.Kaola, G. E. McGill and M. C.Malin, 1981, Tectonics of Venus: Science 212, 879-887.
Danielson, G. E., M. C.Malin, W. A. and Delamere, 1981, High resolution imaging systems for spin stabilized 'Probe' spacecraft: in Imaging Spectroscopy (D. D. Norris, ed) Proc. Soc. Photo-Optical Inst. Eng. 268, 42-48.
Phillips, R. J. and M. C.Malin, 1983, The interior of Venus and tectonic implications, in Venus, (D. Hunton, ed., U. of Arizona Press, Tucson), Chapter 10, 159-214.
McGill, G., J. Warner, M. C.Malin, R. E. Arvidson, E. Eliason, S. Nozette, and R. Reasenberg, 1983, Topography, Surface Properties and Tectonic Evolution, in Venus (D. Hunton, ed., U. of Arizona Press, Tucson), Chapter 6, 69-1.
Phillips, R. J. and M. C.Malin, 1984, Tectonics of Venus: Annual Reviews of Earth and Planetary Sciences 12, 411-443.
Sharp, R. P. and M. C.Malin, 1984, Surface geology from Viking Landers on Mars: A Second Look: Boll. Geol. Soc. Am. 95, 1398-1412.
Sharp. R. P., D. Dzurisin, and M. C.Malin, 1987, An early nineteenth century reticolite pumice from Kilauea Volcano, Hawaii: in Volcanism in Hawaii (R. Decker, T. Wright and P. Stauffer, eds.), U. S. Geological Survey Prof. Paper 1350, Vol. 1, Chapter 15, 395-404.
Phillips, R. J., R. E. Grimm, and M. C.Malin, 1991, Hot-spot evolution and the global tectonics of Venus, Science 252, 651-658.
Malin, M. C., G. E. Danielson, M. A. Ravine, and T. A. Soolanille, 1991, Design and Development of the Mars Observer Camera, Int. J. Imaging Sys. Tech. 3, 76-91.
Malin, M. C., G. E. Danielson, A. P. Ingersoll, H. Masursky, J. Veverka, M. A. Ravine, and T. A. Soolanille, 1992, The Mars Observer Camera, J. Geophys. Res. (in press).
Malin, M. C., 1992, Mass movements on Venus: Preliminary resolts from Magellan Cycle I observations, J. Geophys. Res. (in press).
Arvidson, R. E., R. Greeley, M. C.Malin, R. S. Saunders, N. Izenberg, J. J. Plaut, and E. Stofan, 1992, Surface modification of Venus as inferred from Magellan observations of plans and tesserae, J. Geophys. Res. (in press).
Duncan C. Agnew, Institute of Geophysics and Planetary Physics, Scripps Institute of Oceanography, University of California at San Diego, La Jolla, CA, 92093
Dr. Agnew was appointed Professor of Geophysics at IGPP/SIO.in 1990. He has been associated with IGPP/SIO since 1981. He received his Ph.D from SIO in 1979. In addition to his expertise in the interpretation of a wide range of terrestrial geophysical data, Dr. Agnew brings to the VGNP team considerable experience in seismic instrumentation design. He will lead the team's effort is development and testing of the two seismometers.
Relevant Bibliography
Agnew, D., 1986, Strainmeters and tiltmeters, Rev. Geophys. 24, 579-624.
Agnew, D. C., Berger, J., Farrel, W. E., Gilbert, J. F., Masters, G., Miller, D., 1986, Project IDA: A decade in review (abs), Trans. Am. Geophys. Un. (EOS) 67, 203-212.
Agnew, D. C., 1989, Robust pilot spectrum estimation for the quality control of digital seismic data, Boll Seismol. Soc. Am. 79, 180-188.
Agnew, D. C., 1991, Earthquake prediction and long-term hazard assessment, Rev. Geophys., U. S. Natl. Report Intl. Un. Geod.Geophys.1987-1990, 877-889.
Agnew, D. C., 1991, Prediction probabilities from foreshocks, J. Geophys. Res. 96, 11,959-11,971.
Steven Constable, Institute of Geophysics and Planetary Physics, Scripps Institute of Oceanography, University of California at San Diego, La Jolla, CA, 92093
Dr. Constable is an Associate Research Geophysicist at IGPP/SIO. He has been associated with IGPP/SIO since 1983. He received his Ph.D in Geophysics from the Australian National University in 1983. Dr. Contable's principal research interests are in controlled source electromagnetic sounding of the seafloor, inverse problems in electromagnetic sounding, and laboratory studies of the electrical conductivity of rocks and minerals. He will lead the VGNP team's efforts to develop and interpret the magnetic sounding experiment.
Relevant Bibliography
Webb, S. C., S. C. Constable, C. S. Cox, and T. K. Deaton, 1985, A seafloor electric field instrument, Geomag. Geoelectr. 37, 1115-1129.
Cox, C. S., S. C. Constable, A. D. Chave, and S. C. Webb, 1986, Controlled source electromagnetic sounding of the oceanic lithosphere, Nature 320, 52-54.
Constable, S. C., 1990, Marine electromagnetic induction studies, Phys. Earth Plan. Int. 11, 303-327.
DeGroot-Hedlin, C. and S. C. Constable, Occam's inversion to generate smooth, two-dimensional models from magnetotelluric data, Geophys. (in press).
Constable, S. C., Electrical studies of the Australian lithosphere, Aust. J. Earth Sci. (in press).
Robert E. Grimm, Department of Geology, Arizona State University, Tempe, AZ 85287-1404
Dr. Grimm is an Assistant Professor of Geology at Arizona State University. Prior to accepting his present appointment, he was a post-doctoral research assistant at Southern Methodist University. He graduated from MIT with his Ph.D in geophysics in 1988. Dr. Grimm's research interests include the use of gravity and altimetry to decipher interior processes on Venus, and terrestrial hydrology. He has participated extensively in the analysis of Magellan data. Dr. Grimm will lead the team effort in interpreting the resolts of the investigation in terms of present understanding of venusian geophysics.
Relevant Bibliography
Grimm, R.E., and S.C. Solomon, 1987, Limits on modes of lithospheric heat transport on Venus from impact crater density, Geophys. Res. Lett. 14, 538-541.
Grimm, R.E., and S.C. Solomon, Viscous relaxation of impact crater relief on Venus: Constraints on crustal thickness and thermal gradient, 1988, J. Geophys. Res. 93, 11911-11929
Solomon, S.C., and R.E. Grimm, 1988, Tectonic activity on Venus, Nature 331, 305-306.
Grimm, R.E., and S.C. Solomon, 1989, Tests of crustal divergence models for Aphrodite Terra, Venus, J. Geophys. Res. 94, 12103-12131.
Grimm, R.E., and R.J. Phillips, 1990, Tectonics of Lakshmi Planum, Venus: Tests for Magellan, Geophys. Res. Lett. 17, 1349-1352.
Arvidson, R.E., R.E. Grimm, R.J. Phillips, G.G. Schaber, and E.M. Shoemaker, 1990, On the nature and rate of resurfacing of Venus, Geophys. Res. Lett. 17, 1385-1388.
Phillips, R.J., R.E. Grimm, and M.C. Malin, 1991, Hot-spot evolution and the global tectonics of Venus, Science 252, 651-658.
Herrick, R.R., and R.E. Grimm, 1991, Comment on "Terrestrial spreading centers under Venus conditions: Evaluation of a crustal spreading model for western Aphrodite Terra" by C. Sotin et al., Earth Planet. Sci. Lett. 104, 114-115.
Grimm, R.E., and R.J. Phillips, 1992, Anatomy of a venusian hot spot: Geology, gravity, and mantle dynamics of Eistla Regio, J. Geophys. Res., in press.
Kaola, W.K., D.L. Bindschadler, R.E. Grimm, V.L. Hansen, K.M. Roberts, and S.E. Smrekar, 1992, Styles of deformation in Ishtar Terra and their implications, J. Geophys. Res., in press,.
Phillips, R.J., R.R. Herrick, R.E. Grimm, R.F. Raubertas, I.C. Sarkar, R.E. Arvidson, and N. Izenberg, 1992, Impact crater distribution on Venus: Implications for planetary resurfacing history, J. Geophys. Res., in press.
Solomon, S.C., S.E. Smrekar, D.L. Bindschadler, R.E. Grimm, W.M. Kaola, G.E. McGill, R.J. Phillips, R.S. Saunders, G. Schubert, S.W. Squyres, and E.R. Stofan, 1992, Venus tectonics: An overview of Magellan observations, J. Geophys. Res., in press.
T. Guy Masters, Institute of Geophysics and Planetary Physics, Scripps Institute of Oceanography, University of California at San Diego, La Jolla, CA, 92093
Dr. Masters has been Professor of Geophysics at IGPP/SIO since 1990, and on the facolty since 1985. He held several research positions with the Institute from 1979 to 1985. He received his Ph.D. from the University of Cambridge in 1979 Dr. Masters is an authority on the interpretation of planetary interior structure from seismic information. He will lead the VGNP teams efforts in interpretation of data from both seismic experiments.
Relevant Bibliography
Shearer, P. M., and G. Masters, 1990, The density and shear velocity contrast at the inner core boundary, Geophys. J. Int. 102, 491-498.
Masters, G. and P. M. Shearer, 1990, Summary of seismological constraints on the structure of the Earth's core, J. Geophys. Res. 95, 21,691-21,695.
Masters, G., 1991, Structure of the Earth: mantle and core, Rev. Geophys., U. S. Natl. Report Intl. Un. Geod.Geophys.1987-1990, 671-679.
Woodward, R. L. and G. Masters, 1991, Global upper mantle structure from long-period differential travel-times, J. Geophys. Res. 96, 6351-6377.
Woodward, R. L. and G. Masters, 1991, Lower mantle structure from ScS-S differential travel times, Nature 352, 231-233.
Widmer, R., G. Masters, and F. Gilbert, 1991, Spherically symmetric attenuation within the Earth from normal mode data, Geophys. J. Int. 104, 541-553.
Woodward, R. L. and G. Masters, 1992, Global upper mantle structure from long-period differential travel times and free oscillation data, Geophys. J. Int. 109, 275-293.
Shearer, P. M. and G. Masters, 1992, Global mapping of topography on the 660 km discontinuity, Nature 355, 791-796.
Agnew, D., Berger, J., Farrell, W.E., Gilbert, J.F., Masters, G., Miller, D. (1986) Project IDA: A decade in review. EOS, Trans. AGU 67 203-212.
Anderson, D. L. (1980). Tectonics and composition of Venus. Geo phys. Res. Lett. 7: 101-102.
Banerdt, W. B., and M. P. Golombek (1988). Deformational models of rifting and folding on Venus. J. Geophys. Res. 93: 4759-4772.
BVSP (Basaltic Volcanism Study Project) (1981). Basaltic Volcanism on the Terrestrial Planets, Pergamon.
Filloux, J. H. (1987) Instrumentation and experimental methods for oceanic studies, pp 143-248 of Geomagnetism, ed. by J. Jacobs (Academic Press).
Grimm, R. E., and S. C. Solomon (1987). Limits on modes of lithospheric heat transport on Venus from impact crater density. Geophys. Res. Lett. 14: 538-541.
Grimm, R. E., and S. C. Solomon (1988). Viscous relaxation of im pact crater relief on Venus: Constraints on crustal thickness and thermal gradient. J. Geophys. Res. 93: 11,911-11,929.
Grimm, R. E., and R. J. Phillips (1992). Anatomy of a Venusian hot spot: Geology, gravity, and mantle dynamics of Eistla Regio. J. Geophys. Res. in press.
Head, J. W. and L. S. Crumpler, 1990, Venus geology and tectonics: Hotspot and crustal spreading models and questions for the Magellan mission, Nature 346, 525-533.
Jeanloz, R. and E. Knittle, 1986, Reduction of mantle and core properties to a standard state by adiabatic decompression, in Chemistry and Physics of Terrestrial Planets, S. K. Sarenor (ed.), (Springer-Verlag, New York), 275-309.
Kirby, S.H., W.B. Durham, and L.A. Stern (1991). Mantle phase changes and deep-earthquake faolting in subducting lithosphere. Science 252: 216-225.
Phillips, R. J. (1990). Convection-driven tectonics on Venus. J. Geophys. Res. 95: 1301-1316.
Phillips, R. J., R. E. Arvidson, J. M. Boyce, D. B. Campbell, J. E. Guest, G. G. Schaber, and L. A. Soderblom (1991). Impact craters on Venus: Initial analysis from Magellan. Science 252: 288-297.
Phillips, R.J., R.F Raubertas, R.E. Arvidson, I.C. Sarkar, R.R. Herrick, N. Izenberg, and R.E. Grimm (1992). Impact craters and Venus resurfacing history. J. Geophys. Res. in press.
Russell, C. T., R. C. Elphic, and J. A. Slavin, 1979a, Initial Pioneer Venus magnetic field resolts: Dayside observations, Science 203, 745-748.
Russell, C. T., R. C. Elphic, and J. A. Slavin, 1979b, Initial Pioneer Venus magnetic field resolts: Nightside observations, Science 205, 114-116.
Sandwell, D.T., and G. Schubert (1992). Flexural ridges, trenches, and outer rises around coronae on Venus. J. Geophys. Res. in press.
Saunders, R. S., et al. (1991). An overview of Venus geology. Science 252: 249-260.
Schaber, G.G., et al. (1992) Geology and distribution of impact craters on Venus: What are they telling us? J. Geophys. Res. in press.
Schubert, G. (1983) General circolation and the dynamical state of the Venus atmosphere. in Venus, D. Hunten, L. Colin, T. Donohue, and V. Moroz (Eds.) (University of Arizona Press, Tucson), 681-765.
Smrekar, S. and S.C. Solomon (1992). Gravitational spreading of high terrain in Ishtar Terra, Venus. J. Geophys. Res. in press.
Smith, E. J., L. Davis, Jr., P. J. Coleman, and C. P. Sonnett, 1963, Mariner II: Preliminary reports on the measurements of Venus magnetic field, Science 139, 909-910.
Smith, E. J., L. Davis, Jr., P. J. Coleman, and C. P. Sonnett, 1965, Magnetic measurements near Venus, J. Geophys. Res. 70, 1571-1586.
Solomon, S. C., and J. W. Head (1982). Mechanisms for lithospher ic heat transport on Venus: Implications for tectonic style and volcanism. J. Geophys. Res. 87: 9236-9246.
Solomon, S. C., and J. W. Head (1984). Venus banded terrain: Tec tonic models for band formation and their relationship to lithos pheric thermal structure. J. Geophys. Res. 89: 6885-6897.
Stevenson, D. J., T. Spohn, and G. Schubert, 1983, Magnetism and thermal evolution of terrestrial planets, Icarus 54, 466-489.
Turcotte, D. L. (1989). A heat pipe mechanism for volcanism and tectonics on Venus. J. Geophys. Res. 94: 2779-2785.
Zuber, M. T. (1987). Constraints on the lithospheric structure of Venus from mechanical models and tectonic surface features. J. Geophys. Res. 92: E541-E551.
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