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NASA NTRS1980

Ultraviolet albedos of Uranus and Neptune

Observations on Uranus, Neptune, and four G-type stars which were obtained with the five channel ultraviolet spectrometer aboard the Astronomical Netherlands Satellite are reported. The paper uses these data to derive geometric albedos between 1800 and 3300 A for Uranus and between 2200 and 3300 A for Neptune. It is shown that the albedos are in good agreement with ground measurements near 3300 A and with OAO 2 measurements near 2500 A. In addition, it is noted that the albedos are smaller than theoretical predictions for deep pure H2 Rayleigh-Raman scattering atmospheres, which confirms earlier predictions that additional sources of ultraviolet absorption are required in the upper atmosphere of these planets. Consideration is given to the possibility that the Uranian atmosphere contains an extra absorber, noting that the identification of this absorber is difficult from broad-band spectrophotometric measurements.

Savage, B. D.↗

NASA NTRS1980

Five-micrometer measurements of Uranus and Neptune

Five-micron brightness temperatures and brightness temperature upper limits for Uranus and Neptune have been obtained which are substantially lower than those of Jupiter and Saturn and which correspond to a geometric albedo of approximately 0.01, in agreement with results reported by Gillett and Rieke (1977). Phosphine and CH3D, which are observed at 5 microns on Jupiter and Saturn, are discussed as possible sources of opacity at 5 microns in the atmospheres of Uranus and Neptune.

Macy, W. W., Jr.↗

NASA NTRS1980

Measurements of the H2 4-0 quadrupole bands of Uranus and Neptune

It is found that the equivalent widths of the lines of the 4-0 H2 quadrupole band on Uranus and Neptune are substantially smaller than the values found by some previous observers. An analysis of the results based on a range of atmospheric models yields H2 abundances of 240 + or - 60 km-amagats for Uranus and greater than approximately 200 km amagats for Neptune.

Smith, W. H.↗

NASA NTRS1981

The Sun among stars. IV - Albedos of Uranus and Neptune and the solar color

Geometric albedos in 48 adjacent 50 A bands from 3250 to 5600 A have been derived from observations of Uranus and Neptune. The solar analog found in earlier papers (Hardorp 1978, 1980) was chosen for these reductions, so these albedos are more reliable systematically than earlier ones and allow a choice among the scattering models of Savage et al. (1980). Green methane bands are stronger on Neptune. Strong solar absorption lines are found to be partially filled in by Raman-scattering. Neglect of this effect caused Croft et al. (1972) to find a solar color that is too blue. It probably also affected the classification of G-type stars in the Michigan Spectral Catalogue as well as Garrison's (1979) interpretation of IUE observations.

Hardorp, J.↗

NASA NTRS1981

No evidence of rings around Neptune

The results of two observations of stellar occultations of Neptune to determine if the planet has a ring system are reported. The sightings were made from Mt. Stromlo, Mauna Kea, and Cerro Tololo, noting that an equatorial ring would subtend only two arcsec of view. An upper accretion limit was defined to set the region around Neptune where rings, rather than satellites, could form. The intensities of the starlight from the two selected stars were recorded by photometers on magnetic tape during the occultation period. One of the stars did not occult, but passed through the entire region where a ring system might be present. No definitive evidence for rings was found, although an optical depth for a Neptunian ring was calculated at 0.07, with a width of more than 5 km and a radius of 31,400 km.

Elliot, J. L.↗

NASA NTRS1982

Nuclear electric propulsion mission to Neptune

A nuclear electric propulsion (NEP) mission to Neptune is studied. Results of trajectory and propulsion parameter optimization are presented which lead to a selection of key electric propulsion system parameters. Spiral earth escape and spiral capture modes are assumed in the optimization studies. Flight time vs payload for Space Transportation System (STS)/Centaur injection and Triton-aided orbit capture at Neptune are also briefly investigated as methods of improving mission performance. Radiation effects of the earth spiral phase are discussed. The effects of NEP technology on science payloads and mission and system designs are evaluated. NEP is shown to be a very flexible and high performing propulsion technology.

Nock, K. T.↗

NASA NTRS1983

Absolute spectrophotometry of Neptune - 3390 to 7800 A

Comparisons are undertaken of the results of Neptune absolute spectrophotometry from 3390 to 7800 A (having 10-A resolution over 3390-6055 A and 20-A resolution over 6055-7800 A) with both filter photometry and synthetic spectra computed on the basis of a parameterization proposed by Podolak and Danielson (1977) for aerosol scattering and absorption. A CH4/O2 ratio of between 0.01 and 0.1 is derived for the convectively mixed portion of the Neptune atmosphere, which constrains the optical properties of hypothetical aerosol layers.

Bergstralh, J. T.↗

NASA NTRS1984

Consistency tests of cosmogonic theories from models of Uranus and Neptune

The planetary ratios of ice to rock (I/R) abundances expected in Uranus and Neptune are derived on the basis of several cosmogonic theories. For both Uranus and Neptune, the value of I/R lies between about 1.0 and 3.6. This value is difficult to reconcile with a scenario in which N and C are accreted primarily in the form of N2 and CO. It is consistent with some versions of both giant protoplanet theories and equilibrium accretion theories.

Podolak, M.↗

NASA NTRS1984

Uranus and Neptune

A scientific framework within which to plan the Voyager encounters with Uranus and Neptune was sought. Specific objectives were: (1) to assess the current state of knowledge of Uranus and Neptune, their magnetospheres, and their respective systems of satellites and rings (if any), (2) to identify important scientific issues that can be addressed effectively by Voyager, and (3) to provide an opportunity for Voyager investigators to interact with other scientists knowledgeable in the field of physical studies of the Uranian and Neptunian systems.

Jay T Bergstralh↗

NASA NTRS1984

Variability of Neptune

Earth based observers of Neptune found that the planet varies in brightness at various wavelengths in ways that suggest that changes occur in the planet's atmosphere on several different time-scales. Global inhomogeneities in high altitude haze distribution that are stable for several days permit measurements of the planet's rotation period (about 18 hours), but this stability sometimes breaks down, obscuring the diurnal lightcurve. In addition, there is an apparent long term variability of the brightness of Neptune in anticorrelation with the cycle of solar activity. This slow variability of low amplitude may be punctuated by outbursts of high altitude condensation of particles in the atmosphere, whose decay time is several months.

Dale P Cruikshank↗

NASA NTRS1984

Absolute spectrophotometry of Titan, Uranus, and Neptune 3500-10,500 A

The present absolute measurements of Titan, Uranus and Neptune geometric albedo spectra in the 3500-10,500 A range have a resolution of about 7 A, together with high SNR, in virtue of the exceptional effeciency of the spectrograph and Reticon detector employed. The high precision and spectral resolution of the data, which are in excellent agreement with the Uranus albedo measurements of Lockwood et al. (1983), make possible quantitative measurements of the effects of Raman scattering by H2 in the Uranus and Neptune atmospheres.

Neff, J. S.↗

NASA NTRS1985

The moons of Uranus, Neptune and Pluto

Voyager 2, launched in August 1977, will fly by Uranus in January, 1986, passing within 29,000 km of that planet's innermost moon, Miranda. It will subsequently encounter Neptune in August 1989, flying within 10,000 km of its inner satellite, Triton; images made of this moon by a high resolution camera are expected to reveal surface features as small as a few hundred meters in diameter. The composition of the Uranian moons will br inferred from their near-IR reflectance spectra and mean densities. While the spacecraft will not fly by Pluto, it is expected that the lessons learned from the Voyager encounters with Neptune and Uranus will expand current understanding of Pluto and its moon, Charon.

Brown, R. H.↗

NASA NTRS1985

Predicted occultations by Uranus, Neptune, and Pluto 1985-1990

Predictions are presented for 54 occultations by Uranus' ring system, 24 occultations by Uranus, 22 occultations by Neptune, and ten possible occultations by Pluto during the period 1985 through 1990. Notable occultations by Uranus occur 24 May 1985, 16 April 1987, 27 May 1990, 21 June 1990, and 7 August 1990. The best Neptune occultations are on 7 June 1985 and 25 May 1990. Of the Pluto events, there are six which are nominally observable somewhere on the earth, although uncertainties in Pluto's position combined with uncertainties in the star positions preclude prediction of a ground track until just before the events.

Mink, D. J.↗

NASA NTRS1985

Planning for VLA/DSN arrayed support to the Voyager at Neptune

Preplanning for the use of the National Radio Astronomy Observatory's Very Large Array (VLA) in support of Voyager at Neptune has been underway since early 1982. When arrayed with the Deep Space Network (DSN) antennas at Goldstone, CA, the VLA more than doubles the potential data return over the American longitude for the 1989 Voyager encounter. The background, rationale and current status of planning for VLA-DSN Arrayed Support to the Voyager at Neptune are discussed.

Layland, J. W.↗

NASA NTRS1985

A VLA experiment: Planning for Voyager at Neptune

A very large array (VLA) engineering experiment was conducted on the night of July 22, 1983 to explore one aspect of the potential for the VLA to support Voyager at its Neptune encounter in August of 1989. Specifically, the experiment tested the abiliy of the VLA to self-calibrate on a natural radio source whose effective signal strength is the same as Voyager's will be at its Neptune encounter. The experiment was successful and supported the belief that the VLA would be able to be self-calibrated with Voyager's signal.

Layland, J. W.↗

NASA NTRS1985

Variability of Neptune

Cruikshank, D. P.↗

NASA NTRS1985

The 1983 June 15 occultation by Neptune. I - Limits on a possible ring system

Observations on 15 June 1983 of an occultation of a star by Neptune from Mauna Kea, Mount Stromlo, Siding Spring, and the Kuiper Airborne Observatory show no evidence for equatorial rings between 25,300 and 200,000 km (R/N/ = 25,269 km). Within most of this region, the upper limit on the optical depth along the line of sight, for rings broader than 6 km , is 0.04, which corresponds to a normal optical depth of 0.016. These results rule out a Neptunian ring system similar to that of Saturn or Uranus, but not a system of low optical depth similar to the Jovian rings. The data show no features that appear likely to have been caused by material in the equatorial plane of Neptune near the Roche limit.

Elliot, J. L.↗

NASA NTRS1985

Shepherding model for Neptune's arc ring

A model to explain the confinement of the recently discovered incomplete arc ring around Neptune is developed. The ring may be azimuthally confined near a triangular (Trojan) point of an undiscovered satellite of Neptune. Radial diffusion of the ring particles can be prevented by shepherding torques of another moon. Two satellites with diameters of 100-200 km would be sufficient to confine the ring; such moons would be too small to have been photographed from earth.

Lissauer, J. J.↗