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The satellites of Uranus all have significant non-zero eccentricities and in the case of Miranda a significant inclination as well. The Voyager 2 encounter with Uranus in January 1986, provided more accurate estimates of the masses and sizes of the satellites. Orbital history and possible tidal heating were reexamined by using the Voyager data. The relevant orbital dynamics equations are described.
Several factors that affected the continuing expedition of Voyager 2 to Uranus are discussed. The hydrazine propellant and electrical power supplies on the spacecraft, and the radio signal strength and sun sensor and star tracker sensitivities are examined. The use of the on-board computer to steady the spacecraft and reduce image smear is described. The Voyager employs radiometric and optical data to navigate. The changes required to improve the transmission capabilities of the spacecraft and to eliminate the on-board hardware failures are analyzed.
Knowledge of the Uranian atmosphere took a quantum leap when the Voyager 2 spacecraft made a close approach in January 1986. Among the highlights of the early results are measurements of the atmospheric composition (He mole fraction = 0.15; Ch4 mole fraction = 0.02), rotation period of the deep interior (17.24 h), thermal structure and dynamics (the planet is coldest at midlatitudes; retrograde equatorial winds are inferred), and upper-atmosphere processes (a phenomenon called electroglow is only partly understood and is recognized as an important process n all the major-planet atmospheres examined by Voyager).
The Voyager spacecraft to Jupiter and beyond was designed with the intent that it would be immune to the deleterious effects of space plasma charging. Such effects include electrostatic fields disrupting science data, external electrostatic discharge (ESD) events, and also internal ESDs. This paper describes the design features incorporated into the Voyager spacecraft, the rationale for those features, the results in the Jovian environment, and present thinking about appropriate design to avoid ESD problems caused by space charging.
The present Voyagers I and II measurements of Saturn A ring azimuthal brightness variations in reflected light are noted to be in general agreement with earth-based measurements. Voyager images of the rings in light transmitted through them also indicate the presence of these brightness variations, but with greater amplitude and an about 65-deg phase discrepancy with those seen in reflection. These differences in photometric behavior are qualitatively accounted for in terms of the widespread presence of particle wakes in the A ring.
The observations of the Jovian aurora made by the Voyager UV spectrometer (UVS) during 1979 are analyzed, with special consideration given to the model fitting process. Several different estimates of the function specifying auroral arc geometry were tried, with the Broadfoot et al. (1981) estimate, found by probing with the tip of the UVS slit, giving the best agreement; this agreement is slightly better than the agreement with the Io torus footprint computed by Roederer et al. (1977). The results suggest that the UVS observations are more sensitive to the surface field geometry than are the Voyager flyby in situ observations. The results of a study of intensity maximum positions indicate that the particles exciting the spatially variable portion of the aurora are drifting west, implying either that these particles are electrons or that they are positive ions drifting east more slowly than the corotation lag of the Io torus region carries them to the west. The latter case is most consistent with the high-energy charged particle measurements.
A comparison between Voyager 2 UV spectrometer data for stellar occultations of the Uranian ring system obtained at 0.11 microns and 2.2-micron earth-based occultation data reveals the anticipated factor-of-two reduction in observed optical depths relative to those observed from earth. This is due to Voyager's proximity to the rings, which allows light diffracted out of the direct beam by ring particles to be replaced by light diffracted into the direct beam light from other particles, and further permits the placing of firm lower limits on typical particle sizes of 0.3 and 1 cm for the epsilon and delta rings, respectively. As a function of true anomaly, the epsilon ring profile is noted to remain very similar in shape and essentially constant in equivalent depth, even very near periapse.
During the Voyager 1 spacecraft flyby of Titan on November 12, 1980, an intense broadband low frequency electric field noise was observed in the inbound wake. This so-called sheath noise is believed to be generated by an ion beam-plasma interaction between the ions in the corotational Saturnian magnetospheric plasma and nearly stationary plasma created by the ionization of Titan's atmosphere. The analysis is based on the information from Voyager 1 and reasonable assumptions. The results agree quite well with the observation. The analysis shows that the instability only occurs where the density of the corotational ions is comparable to the density of ions originating from Titan's atmosphere. The growth rate is high enough to generate the observed noise, and the calculated and observed frequency ranges are in good agreement.
Voyager 2 imaging observations over a wide range of phase angles are used to determine the fundamental photometric parameters for the five largest satellites of Uranus. Over the spectral range covered by Voyager cameras (approximately 350-600 nm) the disk-averaged colors are moderately gray (no redder than the spectrum of Saturn's satellite Phoebe). Geometric albedos range from 0.19 for Umbriel to 0.40 for Ariel. Phase coefficients determined generally between phase angles of 10 and 60 deg vary from 0.021 mag/deg for Ariel to 0.028 mag/deg for Miranda. Phase integrals lie in the range of 0.5-0.65. The Bond albedos are about 0.1 for Umbriel and about 0.2 for the other satellites.
The characteristics and distribution of the small particles detected by the plasma-wave and radio-astronomy instruments during the Voyager 2 encounter with Uranus in January 1986 are examined. The nature of the observational data and the analysis techniques employed are discussed, and the results are presented in graphs. Particles of rms mass about 2.6 ng and radii of a few microns were found in an impact region of thickness 3840 km, with a maximum number density of 1600/cu m about 280 km from the equatorial plane, shortly after Voyager 2 crossed the plane of the Uranian rings. The possible origin of the particles in the rings or in small satellites outside the rings is considered.
The paper presents incontrovertible evidence for the beaming of the Jovian decametric radiation, based on simultaneous observations at about 22 MHz from one or both of the Voyager spacecraft and one or the other of two ground-based observatories. The data are used to test the hollow-cone beaming models that have been proposed to account both for the long-term statistics of the ground-based observations and for the arc structures discovered in the dynamic spectral plots of Voyager data. It is concluded that occurrences of non-Io-related Source A events are determined by the corotation with the inner Jovian magnetosphere of curved-sheet beams, and those of Io-related Source B storms by a similar type of beam that moves with the northern foot of the Io flux tube. Both types of beam can be approximated by hollow-cone beam sectors. Rough measurements of the observed portions of some of the beams have been made.
The Very Large Array (VLA) radio telescope is being instrumented at 8.4 GHz to receive telemetry from Voyager 2 during its encounter with Neptune in 1989. The procedure in which the 27 antennas have their phases adjusted in near real time so that the signals from the individual elements of the array can be added coherently is examined. Calculations of the expected signal to noise ratio, tests of the autophasing process at the VLA, and off-line simulations of that process are all presented. Various possible procedures for adjusting the phases are considered. It is shown that the signal to noise ratio at the VLA is adequate for summing the signals from the individual antennas with less than 0.1 dB of loss caused by imperfect coherence among the antennas. Tropospheric variations during the summer of 1989 could cause enough loss of coherence to make the losses higher than 0.1 dB. Experiments show that the losses caused by the troposphere can probably be kept below 0.2 dB if the time delay inherent in the phase adjustment process is no longer than approx. 5 secs. This relatively small combining loss meets the goal estabished to minimize the bit error rate in the Voyager telemetry and implies adequate autophasing of the VLA.
The Voyager Plasma Science (PLS) instrument is capable of returning integral (DC) current measurements, similar in some respects to measurements made with a Langmuir probe or a retarding potential analyzer, although there are significant differences. The integral measurements were made during a calibration sequence in the solar wind, during Cruise Science Maneuvers, and within the magnetospheres of Jupiter and Saturn by Voyager 1. After the failure of the PLS experiment following the Saturn encounter, that instrument was placed in the DC return mode returning possibly usable data from early 1981 through early 1985. The DC return measurements are difficult to interpret and are above threshold values only for relatively large fluxes; the determination of the measured current level is dependent on the operating temperature of the preamplifiers which further complicates the interpretation. Nevertheless, these measurements can be used to determine the efficiency of the suppressor grid at preventing the loss of secondary electrons off the collector plate. Some DC return measurements have been invaluable in aiding in the interpretation of some electron plasma measurements not previously understood. It is found that electron spectra can be significantly modified by the presence of second generation secondary electrons produced by either first generation secondaries or photoelectrons on the support ring of the negative high voltage modulator grid within the instrument housing.
A software package enabling engineers to conduct experiments to determine the actual performance of long constraint-length convolutional codes over the Voyager 1 communication link directly from the Jet Propulsion Laboratory (JPL) has been developed. Using this software, engineers are able to enter test data from the Laboratory in Pasadena, California. The software encodes the data and then sends the encoded data to a personal computer (PC) at the Goldstone Deep Space Complex (GDSC) over telephone lines. The encoded data are sent to the transmitter by the PC at GDSC. The received data, after being echoed back by Voyager 1, are first sent to the PC at GDSC, and then are sent back to the PC at the Laboratory over telephone lines for decoding and further analysis. All of these operations are fully integrated and are completely automatic. Engineers can control the entire software system from the Laboratory. The software encoder and the hardware decoder interface were developed for other applications, and have been modified appropriately for integration into the system so that their existence is transparent to the users. This software provides: (1) data entry facilities, (2) communication protocol for telephone links, (3) data displaying facilities, (4) integration with the software encoder and the hardware decoder, and (5) control functions.
Observations of a Jovian decametric non-Io-A noise storm made from Voyager 1, the University of Florida Radio Observatory, the University of Texas Radio Astronomy Observatory, and the Jupiter station at Goddard Space Flight Center at frequencies of 26.3, 22.2, 20.0, and 18.0 MHz were found to be correlated. The activity observed at the ground stations occurred 68 min after the corresponding activity at Voyager 1. After correction is made for propagation time differences, this delay is reduced to 34 min. It is demonstrated that at each frequency the envelope of the individual-event beams occurring during the storm (some or all of which are associated with dynamic spectral arcs) is a quasi-constant structure that corotates with the inner Jovian magnetosphere, and that the width of this envelope beam is frequency dependent. The width increases as frequency is decreased, mainly because of the change in position of the trailing-edge beam boundary. Evidence for a relatively slow temporal change in beam geometry is also presented.
The January 24, 1986 radio occultation of Voyager 2 by the Uranian rings yielded fine-resolution, 50- and 200-m optical depth profiles at a 3.6 cm wavelength for the nine pre-Voyager rings; these are found to be opaque, reaching optical depths as great as 6-8 in rings gamma and epsilon, which are the only two rings possessing extremely sharp inner and outer edges. Rings 6, 5, 4, and delta share a morphology characterized by a sharp outer-edge transition and a quasi-exponential inner edge one. In ring eta, this behavior is reversed, and ring beta exhibits both diffuse edges and the smallest of the observed opacities.
Incontrovertible evidence is presented for the beaming of the Jovian decametric radiation, based on simultaneous observations at about 22 MHz from one or both of the Voyager spacecraft and one or the other of two ground-based observatories. The data are used to test the hollow-cone beaming models that have been proposed to account both for the long term statistics of the ground-based observations and for the arc structures discovered in the dynamic spectral plots of Voyager data. It is concluded that occurrences of non-Io-related Source A events are determined by the corotation with the inner Jovian magnetosphere of curved-sheet beams, and those of Io-related Source B storms by a similar type of beam that moves with the northern foot of the Io flux tube. Both types of beam can be approximated by hollow-cone beam sectors.
The life expectancy of the Voyager spacecraft as they enter interstellar space, and what they will find along the way, are examined. The range of options available to mission planners as to which instruments to keep alive and which to shut off for the interstellar voyage is addressed. The way that the spacecraft will be operated in interstellar space is described.