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NASA NTRS1982

Improved downlink frequency calculations for Voyager 2

Voyager 2 and her sister Voyager 1 were launched, respectively, in August and September 1977. The object of these spacecraft was to conduct exploratory investigations of the Jupiter and Saturn planetary systems and the interplanetary medium between Earth and Saturn. In April 1978 the Voyager 2 redundant receiver and the loop capacitor in the prime spacecraft receiver failed, leaving the Voyager Project with a major problem: how to communicate with the spacecraft and get the data back.

Ricardo, A. L.↗
NASA NTRS1995

Radial evolution of the solar wind from IMP 8 to Voyager 2

Voyager 2 and Interplanetary Monitoring Platform (IMP) 8 data from 1977 through 1994 are presented and compared. Radial velocity and temperature structures remain intact over the distance from 1 to 43 AU, but density structures do not. Temperature and velocity changes are correlated and nearly in phase at 1 AU, but in the outer heliosphere temperature changes lead velocity changes by tens of days. Solar cycle variations are detected by both spacecraft, with minima in flux density and dynamic pressure near solar maxima. Differences between Voyager 2 and IMP 8 observations near the solar minimum in 1986-1987 are attributed to latitudinal gradients in solar wind properties. Solar rotation variations are often present even at 40 AU. The Voyager 2 temperature profile is best fit with a R(exp -0.49 +/- 0.01) decrease, much less steep than an adiabatic profile.

Richardson, John D.↗
NASA NTRS1979

Plasma wave observations near Jupiter - Initial results from Voyager 2

The Voyager 2 flyby of Jupiter, which occurred in July 1979, provided the second opportunity to study plasma waves in the vicinity of Jupiter (first measurements made by Voyager 1 in March 1979). Because of the somewhat different trajectory and plasma conditions at Jupiter, the Voyager 2 mission provided new perspectives for analyzing many of the phenomena detected by Voyager 1, and also revealed the presence of several new types of plasma waves. A survey is presented of the initial results from the Voyager 2 plasma wave instrument, with special emphasis on the new observations and comparisons with the Voyager 1 results. The data base for the present discussion starts with the first detection of radio emissions from Jupiter about six months before closest approach and ends about two weeks after closest approach

Gurnett, D. A.↗
NASA NTRS1983

Phoebe - Voyager 2 observations

Voyager 2 images of Phoebe obtained over a period of 24 hours provide information on the size, rotation rate, surface markings, and photometric properties of this mysterious object. Phoebe is approximately equidimensional: its longest diameter, about 230 km, is only 10 percent greater than the shortest-diameter (about 210 km). A prograde-rotation period of 9.4 + or - 0.2 hours was determined from both the disk-integrated light curve and by tracking individual markings. Because of the limited resolution of the images (11 pixels across the disk) crater counts cannot be made. The geometric albedo of Phoebe is longitudinally variable from 0.046 to 0.060 (clear filter, lambda = 0.47 micron). The most prominent surface markings are brighter patches at high northern and southern latitudes that have reflectances as much as 50 percent greater than the dark, bland areas. These patches are scattered and do not appear to constitute polar caps. The Voyager color data agree with earlier ground-based spectra that show that Phoebe has a flatter spectrum than does the dark side of Iapetus; this observation is not consistent with simple contamination of Iapetus by debris from Phoebe.

Thomas, P.↗
NASA NTRS1979

Infrared observations of the Jovian system from Voyager 2

As on Voyager 1, the infrared spectroscopy and radiometry instrument on Voyager 2 returned a large number of spectral and radiometric data on Jupiter and its satellites for varying conditions of latitude, longitude, local time, and phase and emission angles. The Voyager 2 average spectrum is slightly noisier than that from Voyager 1. The paper presents selected examples of results from the Voyager 2 infrared investigation, with particular reference to latitudinal variation of the abundance ratio of ethane to acetylene, atmospheric thermal structure, Jupiter's ring, and satellites.

Hanel, R.↗
NASA NTRS1979

Radio science with Voyager at Jupiter - Initial Voyager 2 results and a Voyager 1 measure of the Io torus

About 22 hours after its closest approach to Jupiter, Voyager 2 passed behind the planet as viewed from the earth. Although the spacecraft was geometrically occulted for nearly two hours, the radio links between it and the earth were maintained almost continuously because of the refraction of the signals in Jupiter's south polar atmosphere. A figure shows the plane-of-the-sky geometry of this grazing occultation and preliminary data on the intensity of the spacecraft radio signals as received by the tracking station at Goldstone, California. The intensity data indicate a classic atmospheric occultation profile and the effects of turbulence and ionospheric focusing and defocusing. Analysis of the dispersive ionospheric refraction data yields preliminary profiles for the topside ionosphere at 66.7 deg S (entry in the evening) and 50.1 deg S (exit in the morning) that are reversed with respect to corresponding Voyager 1 profiles in terms of plasma concentration at a fixed altitude. Preliminary reduction of the preencounter occultation of Voyager 1 by the Io torus gives an average plasma density of about 1000 e/cu cm.

Eshleman, V. R.↗
NASA NTRS1979

Planetary radio astronomy observations from Voyager 2 near Jupiter

The Voyager 2 Planetary Radio Astronomy experiment to Jupiter has confirmed and extended to higher zenomagnetic latitudes results from the identical experiment carried by Voyager 1. The kilometric emissions discovered by Voyager 1 often extended to 1 megahertz or higher on Voyager 2 and often consisted of negatively, or less frequently, positively drifting narrowband bursts. On the basis of tentative identification of plasma wave emissions similar to those detected by Voyager 1, the plasma torus associated with Io appeared somewhat denser to Voyager 2 than it did to Voyager 1. The paper reports on quasi-periodic sinusoidal or impulsive bursts in the broadcast band range of wavelengths (800 to 1800 kHz). A Faraday effect appears at decametric frequencies, which probably results from propagation of the radiation near its sources on Jupiter. Finally, the occurrence of decametric emission in homologous arc families is discussed.

Warwick, J. W.↗
NASA NTRS1989

The Voyager 2 mission to Neptune

Voyager 2 and its twin, Voyager 1, were launched in 1977. Both spacecraft investigated Jupiter's and Saturn's systems. Voyager 2 continued on to fly past Utranus in 1986 and Neptune in 1989, while Voyager 1 headed out of the solar system. The mission at Neptune presented many engineering and scientific challenges. Neptune is about 30 Astronomical Units (AU) from the sun and earth, resulting in extremely low lights levels (nearly 1000 times lower than at earth) and in communication distances of nearly 4.5 billion kilometers. To compensate for the long communication distances, several new techniques were developed. As at Uranus, an onboard backup computer compressed the imaging data. In addition, the data return was further improved by electronically arraying and expanding several receiving antennas. As a result, the data rates from Neptune were about the same as they were from Saturn, even though the distance was three times greater. Several changes were made in the onboard software to optimize Voyager's operations at the very low light levels at Neptune. Finally, to obtain the maximum information from the Neptune encounter, a trajectory was selected which passed within just 5000 kilometers of Neptune's atmosphere, but which also posed several possible environmental hazards.

Haynes, Norman R.↗
NASA NTRS1995

Forward and reverse shocks in the outer heliosphere: Observations from Voyager 2

Observations from Voyager 2 as it moved from 10 to 14 deg S heliographic latitude in the period from 1992 through 1994 were used to gather statistics on the relative number of forward and reverse shocks. These results can be used to compare with observations from the Ulysses spacecraft which moved from 6 deg S to 70 deg S heliographic latitude during that time period. The Ulysses observations are in agreement with a 3-D, MHD model of the evolution of a steady tilted-dipole solar wind flow configuration prevalent in 1993. The model predicts and the Ulysses observations confirm a preponderance of reverse shocks at Ulysses latitudes poleward of streamer-belt latitudes. A preliminary scan of the Voyager data supports the complementary prediction of the model that forward fronts should dominate at large heliocentric distances near the heliographic equatorial plane during the same time period.

Lazarus, A. J.↗
NASA NTRS1982

Infrared observations of the Saturnian system from Voyager 2

During the passage of Voyager 2 through the Saturn system, infrared spectral and radiometric data were obtained for Saturn, Titan, Enceladus, Tethys, Iapetus, and the rings. Combined Voyager 1 and Voyager 2 observations of temperatures in the upper troposphere of Saturn indicate a seasonal asymmetry between the northern and southern hemispheres, with superposed small-scale meridional gradients. Comparison of high spatial resolution data from the two hemispheres poleward of 60 deg latitude suggests an approximate symmetry in the small-scale structure, consistent with the extension of a symmetric system of zonal jets into the polar regions. Longitudinal variations of 1 to 2 K are observed. Disk-averaged infrared spectra of Titan show little change over the 9-month interval between Voyager encounters. By combining Voyager 2 temperature measurements with ground-based geometric albedo determinations, phase integrals of 0.91 plus or minus 0.13 and 0.89 plus or minus 0.09 were derived for Tethys and Enceladus, respectively. The subsolar point temperature of dark material on Iapetus must exceed 110 K. Temperatures (and infrared optical depths) for the A and C rings and for the Cassini division are 69 plus or minus 1 K (0.40 plus or minus 0.05), 85 plus or minus 1 K (0.10 plus or minus 0.03), and 85 plus or minus 2 K (0.07 plus or minus 0.04), respectively.

Hanel, R.↗
NASA NTRS1980

Airglow from Jupiter's nightside and crescent - Ultraviolet spectrometer observations from Voyager 2

The Voyager Ultraviolet Spectrometer has made extensive observations of airglow from Jupiter's dark-side equatorial latitudes. The brightness of H Lyman alpha, the only emission detected, varies between 700 and 1000 rayleighs (R) as a function of longitude. The dark side of Jupiter is illuminated by sky background Lyman alpha arising from resonance scattering of the solar Lyman alpha line by the neutral hydrogen of the interstellar medium. Calculations show that resonance scattering of this sky background by hydrogen in Jupiter's thermosphere will produce about 300 R of Lyman alpha emission. The additional Lyman alpha observed is probably excited by electrons and protons precipitating at equatorial latitudes. Based on the 500-R upper limit set here on the dark-side H2 Lyman and Werner bands, and the Lyman alpha measurements, the exciting particles are thought to have a soft energy spectrum and deposit about 0.04 erg/sq cm per sec in the atmosphere. There is evidence for an asymmetrical precipitation pattern associated with the longitudinal variation in Lyman alpha emission, and a suggestion of a strong day-night difference in precipitation as well.

Mcconnell, J. C.↗
NASA NTRS1983

The Voyager 2 scan platform anomaly

The two Voyager spacecraft were launched in the summer of 1977, flew past Jupiter in 1979, and reached Saturn in 1980 and 1981. The Voyager Attitude and Articulation Control Subsystem (AACS) controls the positioning of a scan platform on which scientific instruments are mounted. On August 26, 1981, it was discovered that the azimuth axis of the scan platform of Voyager 2 had not completed the commanded slew. The profile of the failed slew command resembled a similar scan failure on Voyager 1 on February 23, 1979. In that case, contamination of the output gear mesh in the azimuth actuator was identified as the most probable cause of the failure. The problem was solved by cycling the azimuth actuator until the contaminant broke down and smooth operation was restored. A similar treatment applied to the Voyager 2 problem failed. Details of the Voyager 2 anomaly investigation are discussed.

Marchetto, C. A.↗
NASA NTRS1984

The far-ultraviolet energy distribution of Sirius B from Voyager 2

Observations of Sirius obtained with the Voyager 2 ultraviolet spectrometer clearly reveal the presence of flux from the white dwarf Sirius B at wavelengths between 950 and 1100 A. These observations are in good agreement with all previous ultraviolet observations of Sirius B, and in particular with the IUE observations of Boehm-Vitense, Dettmann, and Kapranidis. A joint analysis of the Voyager 2 and IUE observations yields a temperature range of 26,000-28,000 K. A reexamination of current ultraviolet, visible, and X-ray observations produces good general agreement, but no single, mutually consistent, temperature for Sirius B. The Voyager 2 observations can be used to place a firm upper limit of 28,000 K on the temperature of Sirius B.

Holberg, J. B.↗
NASA NTRS1979

Magnetic field studies at Jupiter by Voyager 2: Preliminary results

The Voyager 2 magnetic field experiment is described and compared to the Voyager 1 experiment and data. The magnetosphere, the bow shock, the magnetopause, and the extended magnetic tail of Jupiter are discussed. Two crossings of the near equatorial current sheet were observed in the magnetosphere and its tail every 10 hour rotation period of the planet. A definitive mapping of the geometry and character of these enhanced plasma and depressed magnetic field regions is discussed. The interaction of the satellite Ganymede with the Jovian magnetosphere, which leads to disturbances as the Jovian magnetosphere corotates with the planet past the satellite is analyzed.

Ness, N. F.↗
NASA NTRS1979

Magnetic field studies at Jupiter by Voyager 2 - Preliminary results

The Voyager 2 magnetic field experiment, for which the instrumentation is identical to that on Voyager 1, operated flawlessly throughout the second Jupiter encounter. The paper presents a brief overview of the results obtained to date on the Jovian magnetosphere, the bow shock, the magnetopause, and the extended magnetic tail. The results and the magnetic field geometry confirm the earlier conclusion from Voyager 1 that Jupiter has an enormous magnetic tail, approximately 300-400 Jupiter radii in diameter, trailing behind the planet with respect to the supersonic flow of the solar wind. Additional observations of the distortion of the inner magnetosphere by a concentrated plasma show a spatial merging of the equatorial magnetodisk current with the current sheet in the magnetic tail. Disturbances near Ganymede are discussed.

Ness, N. F.↗
NASA NTRS1989

Plasma observations near Neptune - Initial results from Voyager 2

The plasma science experiment on Voyager 2 made observations of the plasma environment in Neptune's magnetosphere and in the surrounding solar wind. Because of the large tilt of the magnetic dipole and fortuitous timing, Voyager entered Neptune's magnetosphere through the cusp region, the first cusp observations at an outer planet. Thus the transition from the magnetosheath to the magnetosphere observed by Voyager 2 was not sharp but rather appeared as a gradual decrease in plasma density and temperature. The maximum plasma density observed in the magnetosphere is inferred to be 1.4 per cubic centimeter (the exact value depends on the composition), the smallest observed by Voyager in any magnetosphere. The plasma has at least two components; light ions (mass, 1 to 5) and heavy ions (mass, 10 to 40), but more precise species identification is not yet available. Most of the plasma is concentrated in a plasma sheet or plasma torus and near closest approach to the planet. A likely source of the heavy ions is Triton's atmosphere or ionosphere, whereas the light ions probably escape from Neptune. The large tilt of Neptune's magnetic dipole produces a dynamic magnetosphere that changes configuration every 16 hours as the planet rotates.

Belcher, J. W.↗
NASA NTRS1979

Voyager 2 encounter with the Jovian system

The Voyager 2 encounter with Jupiter is reviewed, with particular reference to the trajectory and the major sequence modifications made because of the Jupiter measurements obtained with Voyager 1. The specific Voyager 2 encounter was chosen to complement that of Voyager 1. Attention is given to results on the atmosphere, satellites and ring system, and magnetosphere.

Stone, E. C.↗
NASA NTRS1982

Magnetic field studies by Voyager 2 - Preliminary results at Saturn

Results of Voyager 2 studies of the magnetosphere and planetary magnetic field of Saturn are presented. Magnetometer studies have confirmed the results obtained by Voyager 1, indicating the magnetic field to be that of a centered dipole of moment 0.21 gauss Saturn radii-cubed, tilted approximately 0.8 deg from the rotation axis and a maximum measured field intensity of 1187 nT at latitude 17.3 deg N just before periapsis. Voyager 2 observed multiple bow shock and magnetopause crossings during its inbound and outbound trajectories, which were complementary to those of Voyager 1, including magnetopause crossing at 18.5 Saturn radii on the inbound trajectory, and at 48.4-50.9 Saturn radii outbound indicative of magnetospheric expansion due to changing solar wind conditions. Throughout the outbound passage, the magnetospheric field was observed to be relatively steady and smooth, with no evidence for any azimuthal asymmetry or magnetic anomaly. Results thus are incapable of accounting for the observed periodic modulation of the Saturnian kilometric radio emissions.

Ness, N. F.↗