Geophysical Research Letters, 20, 2715-2717, 1993
(Received: November 8, 1993; accepted: November 11, 1993)
Copyright 1993 by the American Geophysical Union.
Paper number 93GL03380
In October, 1992 the Pioneer Venus Orbiter entered the atmosphere of Venus, ending nearly 14 years of observations at Venus. Prior to the entry into the atmosphere and subsequent loss of the spacecraft careful management of spacecraft resources had allowed the acquisition of much low altitude data over the nightside of the planet. The long duration of the Pioneer Venus mission has enabled us to study the ionosphere and atmosphere of Venus under different levels of solar activity.
The Pioneer Venus Orbiter (PVO) was inserted into orbit around Venus on December 4, 1978. The spacecraft was instrumented to carry out both in situ and remote observations of the atmosphere and ionosphere of Venus, and also to monitor the solar wind [Colin, 1980]. On insertion the PVO orbital period was nominally 24 hours, with apoapsis near 12 Venus radii (planetocentric, 1 R = 6052 km) and periapsis around 140 km altitude. The orbital inclination was 105.6°, and the latitude of periapsis was near 17° N. The Pioneer Venus instruments are described in IEEE Trans. Geosci. Remote Sens., GE-18 , while a comprehensive collection of papers on the early mission results is given in J. Geophys. Res., 85(A13) .
At the beginning of the Pioneer Venus mission the spacecraft periapsis was kept in the 140 km altitude range through apoapsis maneuvers. These maneuvers were no longer carried out after three seasons of nightside periapsis passes of the Venus ionosphere and the periapsis altitude was allowed to rise through solar gravitational perturbations. The orbiter periapsis altitude for the entire Pioneer Venus mission is shown as a function of time in Figure 1. The highest periapsis altitude was around 2309 km, and occurred on July 4, 1986 (orbit 2768). After this the periapsis altitude began to descend, and final atmospheric entry of the orbiter was expected to occur in late 1992, around orbit 5000, nearly 14 years after orbit insertion.
The lifetime of the Pioneer Venus Orbiter spanned more than a solar cycle, as shown in Figure 2. On comparison with Figure 1 it can be seen that solar activity was high early in the Pioneer Venus mission when periapsis altitude was maintained at low altitudes. Solar activity was minimum when periapsis altitude was maximum. As the spacecraft descended to lower altitudes solar activity again increased. However, solar activity was at an intermediate level when the orbiter periapsis altitude finally decreased to very low altitudes similar to those encountered early in the mission. One of the main goals of the entry phase (i.e., the last sequence of low periapsis altitude orbits prior to final entry) was to compare ionospheric and atmospheric conditions for different levels of solar activity.
After nearly 14 years of operations, some of the spacecraft systems had degraded. The solar cells were no longer as efficient as they used to be, providing a spin averaged current of 4 A, compared to the 13 A obtained early in the mission. In addition, the battery capacity was somewhat less than at the beginning of the mission, although occasional deep discharges during apoapsis eclipses did partially recondition the batteries. As a consequence, careful power management was required. By the end of the mission only a few short sequences of data could be acquired per orbit. The main science goal of the entry phase operations was the acquisition of periapsis data. However, at this time the spacecraft was in Earth occultation and eclipse. To conserve battery power the spacecraft transmitter was turned off during eclipse and the instrument data were stored in the spacecraft Data Storage Unit (DSU). On exit from eclipse and occultation the transmitter was turned on and the stored data telemetered to the earth. Short spans of data were also acquired in "real time" at higher altitudes for orbit determination and solar wind "snapshots".
Fig. 1. Pioneer Venus Orbiter periapsis altitude as a function of time. Early in the mission the periapsis altitude was maintained near 140 km. Later the periapsis altitude rose and fell due to solar gravitational perturbations. The orbiter entered the atmosphere in late 1992.
Fig. 2. Solar activity over the duration of the Pioneer Venus mission. The 10.7 cm flux and sunspot numbers are shown. Early in the Pioneer Venus mission solar activity was high. Solar activity was decreasing at the end of the mission.
The solar gravitational perturbation was such that the rate of change of periapsis altitude was greatest near midnight. At this local time periapsis altitude decreased by about 2 km per day. This height is comparable to the neutral atmosphere scale height [see, e.g., Kasprzak et al., 1993]. Very close monitoring of the spacecraft was therefore required in determining when the periapsis raising maneuver should occur. The main constraint was the amount of expected spacecraft heating due to drag. A maneuver was scheduled whenever the predicted drag for the next periapsis pass exceeded 0.7 m/s. The magnitude of the periapsis raising maneuver was chosen so that the next maneuver would occur roughly five orbits later. Prior to the entry period it was estimated that of the original 70 lbs (32 kg) of hydrazine propellant roughly 5 lbs (2.3 kg) remained, but with a high degree of uncertainty (J. W. Dyer, personal communication, 1992). The estimated 5 lbs of propellant would have been sufficient to raise the spacecraft periapsis altitude enough for final atmospheric entry to occur on the dayside.
While it was hoped that sufficient propellant remained, it was by no means certain that the spacecraft would survive through to the dayside ionosphere. The spacecraft had exceeded its nominal lifetime by many years, and the failure of any of several subsystems could mark the end of the mission. It appears, however, that there was insufficient propellant to complete the sequence of periapsis raising maneuvers. The spacecraft velocity increment fell short of that expected during the apoapsis maneuver prior to orbit 5050. Furthermore, the subsequent attitude trim maneuver did not reorient the spacecraft by the expected amount (M. A. Smith, personal communication, 1992).
Since the spacecraft was in Earth occultation during the periapsis passes, it was not possible to track the spacecraft in real time through periapsis. In addition, any data acquired would not be telemetered to the Earth if the spacecraft was lost. No telemetry was received from the spacecraft after periapsis on orbit 5056 (October 8, 1992), and the last periapsis data were acquired on orbit 5055 (October 7,1992).
Periapsis altitude as a function of local time is shown in Figure 3. The figure shows that the spacecraft very nearly survived through to the dayside ionosphere, only three more periapsis raising maneuvers were required. However, survival of the spacecraft into the dayside ionosphere was the most optimistic of outcomes for the entry phase. That the spacecraft was able to take data for as long as it did is a major achievement, and the entry phase operations were highly successful, with significant new data having been acquired prior to the loss of the orbiter.
As noted earlier, one of the main goals of the entry phase was to investigate the Venus atmosphere and ionosphere under different solar activity conditions than those prevalent during the early Pioneer Venus mission. Variations in solar activity are expected to have a strong effect on the nightside ionosphere. As discussed by Knudsen et al. , there are two sources of ionization for the nightside ionosphere of Venus at solar maximum. The first is ionization due to electron precipitation [Gringauz et al., 1979]. The second is ion transport from dayside to nightside. The ion transport is expected to be reduced during solar minimum due to the lower altitude of the ionopause at the terminator as a consequence of reduced ionospheric plasma pressure. At solar maximum intervals of high solar wind dynamic pressure are expected to also restrict the day to night transport of plasma. It is thought that "disappearing ionospheres" [Cravens et al., 1982] are a consequence of reduced transport at solar maximum.
Fig. 3. Periapsis altitude as a function of local time. The final periapsis data were acquired on orbit 5055 (October 7, 1992). The actual periapsis altitude () is plotted up to orbit 5055, the pre-entry phase prediction (+) is plotted for later orbits. Prior to the entry phase it was hoped that some 9 periapsis raising maneuvers would be sufficient to allow the spacecraft to survive through to the dayside ionosphere. The figure shows that most of the low altitude observations were acquired in the post-midnight local time sector.
Many of the papers in this section on the Pioneer Venus entry phase specifically address the variation with solar cycle of the ionosphere and atmosphere. The nightside ionosphere is indeed found to be significantly reduced from the solar maximum condition, being more reminiscent of "disappearing ionospheres". This suggests that day to night ion transport was reduced during the entry phase, although results from modeling of the ionosphere indicates that some day to night transport was still occurring during the entry phase. Since the entry phase occurred during solar intermediate conditions, rather than solar minimum, some day to night transport might still be expected. The neutral atmosphere, on the other hand, shows very little variability from solar maximum to solar intermediate conditions, especially on the nightside. The nightside neutral atmosphere appears to be insulated from solar cycle dependent changes in the dayside thermosphere.
A variety of wave phenomena were detected during the entry phase. Neutral density waves, with wavelengths of several 100 km, were observed as were plasma density fluctuations with wavelengths of the order 1 km. Higher frequency plasma waves were also measured.
Several papers present modeling results and theoretical analysis pertaining to the entry observations. The collection of papers is rounded out with papers on higher altitude observations of the Venus magnetotail and magnetosheath.
Acknowledgments. There are many individuals and institutions who should be acknowledged after 14 years of a highly successful mission. First, I wish to thank the Hughes Aircraft Company for manufacturing such an excellent spacecraft, and the NASA Ames Research Center for supporting the Pioneer Venus project. I especially would like to thank the following individuals: R. M. Goody, D. M. Hunten, and N. W. Spencer for suggesting a Venus entry probe mission, which grew to be Pioneer Venus; C. F. Hall, the original Pioneer Venus Project Manager; S. Dorfman who managed the Pioneer Venus program for Hughes; H. C. Brinton, Program Scientist at NASA Headquarters; R. O. Fimmel (Project Manager), R. Craig (previous Project Science Chief), L. E. Lasher (current Project Science Chief), and L. Colin (Project Scientist) for their excellent management of the project; J. W. Dyer, D. W. Lozier, M. A. Smith, R. W. Jackson, M. N. Wirth, and J. R. Phillips of NASA/Ames Research Center for their Herculean efforts in managing the spacecraft during the entry operations; and N. Mottinger and R. E. Ryan of the Jet Propulsion Laboratory for navigation support and coordination with the Deep Space Network. Lastly, I wish to remember the deceased members of the Pioneer Venus community, H. Masursky, F. L. Scarf, and J. H. Wolfe, who all contributed greatly to the success of the Pioneer Venus mission.
Colin, L., The Pioneer Venus program, J. Geophys. Res., 85, 7575-7598, 1980.
Cravens, T. E., L. H. Brace, H. A. Taylor, Jr., C. T. Russell, W.
C. Knudsen, K. L. Miller,
A. Barnes, J. D. Mihalov, F. L. Scarf, S. J. Quenon, and A. F. Nagy, Disappearing
ionospheres on the nightside of Venus, Icarus, 51, 271-282, 1982.
Gringauz, K. I., M. I. Verigin, T. K. Breus, and T. Gombosi,
The interaction of electrons in the
optical umbra of Venus with the planetary atmosphere - the origin of the nighttime
ionosphere, J. Geophys. Res., 84, 2123-2127, 1979.
IEEE Trans. Geosci. remote Sens., GE-18(1), 1980.
J. Geophys. Res., 85(A13), 1980.
Kasprzak, W. T., H. B. Niemann, A. E. Hedin, S. W. Bougher,
and D. M. Hunten, Neutral
composition measurements by the Pioneer Venus neutral mass spectrometer during
orbiter re-entry, Geophys. Res. Lett., this issue, 1993.
Knudsen, W. C., A. J. Kliore, and R. C. Whitten, Solar cycle
changes in the ionization sources
of the nightside Venus ionosphere, J. Geophys. Res., 92, 13,391-13,398, 1987.
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