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NASA NTRS1982

Rings of Uranus - A review of occultation results

The structure and orbits of the nine confirmed rings of Uranus are reviewed based on occultation observations from ring discovery in March 1977 through April 1981. Observations of stellar occultations by Uranus have revealed the rings to be characterized by narrow widths (up to 100 km) and sharp edges, with possible internal structure, and to have orbits fitting the coplanar precession model, and showing a decreasing eccentricity with semimajor axis. Occultation results have also allowed the determination of the J2 and J4 coefficients of the Uranian gravity potential, the coordinates of the ring plane pole, planetary equatorial radius and ellipticity and the rotation period. The sharp edges and narrow widths of the rings may be explained by small satellites orbiting near the rings, and the uniform precession of three of the rings is attributed to self-gravity. Future plans for Uranus ring observations include further occultation studies, the Voyager 2 fly-by, and Space Telescope data.

Elliot, J. L.↗

NASA NTRS1984

Exploring the Solar System with Stellar Occultations

By recording the light intensity as a function of time when a planet occults a relatively bright star, the thermal structure of the upper atmosphere of the planet can be probed. The main feature of stellar occultation observations is their high spatial resolution, typically several thousand times better than the resolution achievable with ground-based imaging. Five stellar occultations have been observed. The main results of these observations are summarized. Stellar occultations have been observed on Uranus, Mars, Pallas, Neptune and the Jovian Ring.

Elliot, J. L.↗

NASA NTRS1983

Radio scintillations observed during atmospheric occultations of Voyager: Internal gravity waves at Titan and magnetic field orientations at Jupiter and Saturn

The refractive index of planetary atmospheres at microwave frequencies is discussed. Physical models proposed for the refractive irregularities in the ionosphere and neutral atmosphere serve to characterize the atmospheric scattering structures, and are used subsequently to compute theoretical scintillation spectra for comparison with the Voyager occultation measurements. A technique for systematically analyzing and interpreting the signal fluctuations observed during planetary occultations is presented and applied to process the dual-wavelength data from the Voyager radio occultations by Jupiter, Saturn, and Titan. Results concerning the plasma irregularities in the upper ionospheres of Jupiter and Saturn are reported. The measured orientation of the irregularities is used to infer the magnetic field direction at several locations in the ionospheres of these two planets; the occultation measurements conflict with the predictions of Jovian magnetic field models, but generally confirm current models of Saturn's field. Wave parameters, including the vertical fluxes of energy and momentum, are estimated, and the source of the internal gravity waves discovered in Titan's upper atmosphere is considered.

Hinson, D. P.↗

NASA NTRS1986

Manned Mars mission sunlight and communication occultations

Calculations are presented for the 1999 opposition class mission and a procedure for obtaining singlar occultation data for any other given Mars mission is given. Occultation data for a Mars orbiter in a 24.5 hour parking orbit and a Mars base were calculated for: sunlight occultation - the time in darkness; and radio communication occultation - the communication losses between the lander and the orbiter, the lander and Earth, and orbiter and Earth.

Jack Mulqueen↗

NASA NTRS1986

Photometry of occultation candidate stars. II - Uranus 1985-1990 and Saturn 1986-1991

K magnitudes are given for seven Uranus-occulted stars, together with V and I magnitudes for 20 stars that will be occulted by the Uranian rings in 1986-1990 and 15 that will be occulted by the rings of Saturn in the course of 1986-1991. Occultation stars are noted to be up to 1 mag brighter in the K bandpass than could be predicted on the basis of V-I colors alone; this is explainable in terms of typical degrees of interstellar reddening.

French, L. M.↗

NASA NTRS1986

Occultation studies with small telescopes

The data which can be acquired by observing occultations of stars by asteroids, the Pluto-Charon system, and comets using small telescopes are reviewed. A network of small telescopes may be the only possible means of observing a few specific types of occultations. The occultations are usually sufficiently bright so that small aperture telescopes can easily obtain high signal/noise ratios. The techniques and equipment are accessible to both professional, amateur and student astronomers, which is a favorable situation since only a small percentage of the nearly-hourly occurrence of occultations of stars by minor and distant planetary bodies are ever observed. Techniques employed for measuring the dimensions of asteroids by combining chords measured by distributed small telescopes are described and examples are provided of asteroids for which complete or incomplete size/density mappings have been made.

Millis, Robert L.↗

NASA NTRS1988

Io hot spots - Infrared photometry of satellite occultations

Io's active hot spots, which are presently mapped on the basis of IR photometry of this moon's occultation by other Gallilean satellites, are obtained with greatest spatial resolution near the sub-earth point. A model is developed for the occultation lightcurves, and its fitting to the data defines the apparent path of the occulting satellite relative to Io; the mean error in apparent relative position of occulting satellites is of the order of 178 km. A heretofore unknown, 20-km diameter hot spot is noted on Io's leading hemisphere.

Goguen, J. D.↗

NASA NTRS1989

Triaxial ellipsoid dimensions and rotational pole of 2 Pallas from two stellar occultations

The ability to derive unique triaxial ellipsoid dimensions and a rotational pole for an asteroid is demonstrated on the basis of three stellar occultations. Two such occultations for Pallas are used to examine the locus of solutions falling along a curve in seven-dimensional parameter space. While the first occultation slightly overestimated the true size of the asteroid's outline, more plausible solutions are allowed within its errors. The occultation model is combined with speckle interferometric results. Pallas is more flattened, with a greater b/c ratio, and smaller c, than found in previous estimates.

Drummond, Jack D.↗

NASA NTRS1989

Precise measurement of asteroid sizes and shapes from occultations

The observational techniques (including photoelectric observations, television recordings, and visual timings) used to measure asteroid dimensions from occultations of stars by asteroids are discussed together with the methods of analysis appropriate to occultation data. Results are presented on the determinations of asteroid diameter, density, and internal structure measures learned from occultations of 36 asteroids. Prospects for continued effective applications of the occultation technique to asteroid studies are discussed.

Millis, R. L.↗

NASA NTRS1991

Pluto-Charon stellar occultation candidates - 1990-1995

A search to identify stars that might be occulted by Pluto or Charon during the period 1990-1995 and part of 1996 is studied. This search was made with an unfiltered CCD camera operated in the strip scanning mode, and it reaches an R magnitude of approximately 17.5 - about 1.5 mag fainter than previous searches. Circumstances for each of the 162 potential occultations are given, including an approximate R magnitude of the star, which allows estimation of the signal-to-noise ratio (S/N) for observation of each occultation. The faintest stars in the list would yield an S/N of about 20 for a 1 s integration when observed with a CCD detector on an 8 m telescope under a dark sky. The astrometric precision (+/- 0.2 arcsec, with larger systematic errors possible for individual cases) is insufficient to serve as a final prediction for these potential occultations, but is sufficient to identify stars deserving of further, more accurate, astrometric observations. Statistically, about 32 of these events to be observable somewhere on earth are expected. The number of events actually observed will be substantially smaller because of clouds and the sparse distribution of large telescopes. Finder charts for each of the 91 stars involved are presented.

Dunham, E. W.↗

NASA NTRS1991

Occultation studies of the solar system

Because of their high spatial resolution, stellar occultations have proven extremely effective for learning about planetary upper atmospheres, asteroids, and planetary rings. The occultation program at MIT involves the following: (1) identifying the scientific questions that can be answered by occultation events; (2) predicting the zone of visibility for the useful events; (3) maintaining and improving a high speed CCD camera for observing occultations; (4) obtaining the observations; and (5) reducing the data and interpreting the results.

Elliot, James L.↗

NASA NTRS1991

Probing Titan's atmosphere with a stellar occultation

The 3 July, 1989 occultation of 28 Sgr by Titan is discussed. The star was readily detectable throughout the occultation, reaching a minimum normalized flux of about 0.05. The occultation probed Titan's atmosphere in a region not studied by the Voyager spacecraft. The region is important for the aerobraking of Titan entry probes, and direct information about its properties is important for the Cassini mission. Occultation data (normalized stellar flux vs universal time) is shown in chart form for NASA supported stations, along with data from a collaborating group at the Wise observatory in Israel. Strong scintillation data of the star is noticeable in the data records, and provides information on waves/turbulence in Titan's high atmosphere.

Hubbard, W. B.↗

NASA NTRS1992

Long-term variations in abundance and distribution of sulfuric acid vapor in the Venus atmosphere inferred from Pioneer Venus and Magellan radio occultation studies

Radio occultation experiments have been used to study various properties of planetary atmospheres, including pressure and temperature profiles, and the abundance profiles of absorbing constituents in those planetary atmospheres. However, the reduction of amplitude data from such experiments to determine abundance profiles requires the application of the inverse Abel transform (IAT) and numerical differentiation of experimental data. These two operations preferentially amplify measurement errors above the true signal underlying the data. A new technique for processing radio occultation data has been developed that greatly reduces the errors in the derived absorptivity and abundance profiles. This technique has been applied to datasets acquired from Pioneer Venus Orbiter radio occultation studies and more recently to experiments conducted with the Magellan spacecraft. While primarily designed for radar studies of the Venus surface, the high radiated power (EIRP) from the Magellan spacecraft makes it an ideal transmitter for measuring the refractivity and absorptivity of the Venus atmosphere by such experiments. The longevity of the Pioneer Venus Orbiter has made it possible to study long-term changes in the abundance and distribution of sulfuric acid vapor, H2SO4(g), in the Venus atmosphere between 1979 and 1992. The abundance of H2SO4(g) can be inferred from vertical profiles of 13-cm absorptivity profiles retrieved from radio occultation experiments. Data from 1979 and 1986-87 suggest that the abundance of H2SO4(g) at latitudes northward of 70 deg decreased over this time period. This change may be due to a period of active volcanism in the late 1970s followed by a relative quiescent period, or some other dynamic process in the Venus atmosphere. While the cause is not certain, such changes must be incorporated into dynamic models of the Venus atmosphere. Potentially, the Magellan spacecraft will extend the results of Pioneer Venus Orbiter and allow the continued monitoring of the abundance of distribution of H2SO4(g) in the Venus atmosphere, as well as other interesting atmospheric properties. Without such measurements it will be difficult to address other issues such as the short-term spatial variability of the abundance of H2SO4(g) at similar latitudes in Venus atmosphere, and the identities of particles responsible for large-scale variations observed in NIR images.

Jenkins, J. M.↗

NASA NTRS1993

An analysis of the Voyager 2 Ultraviolet Spectrometer occultation data at Uranus - Inferring heat sources and model atmospheres

Heat source information is derived here from the Voyager 2 Ultraviolet Spectrometer occultation data of Uranus. Analytic functions for the local heat dependence on altitude are used to obtain a temperature profile by solving the heat equation. The stellar entrance and exit occultation and a solar occultation are used to infer the thermal and density structure of the atmosphere. The least squares fit solution to the solar occultation data gives one source located at 1.8 x 10 exp -5 microbar with a strength of 0.056 +/- 0.01 erg/sq cm/s. Latitudinal temperature gradients are obtained.

Stevens, Michael H.↗

NASA NTRS1992

Reduction and analysis of seasons 15 and 16 (1991 - 1992) Pioneer Venus radio occultation data and correlative studies with observations of the near-infrared emission of Venus

Radio occultation experiments, and radio astronomical observations have suggested that significant variations (both spatial and temporal) in the abundances of sulfur-bearing gases are occurring below the Venus cloud layers. In addition, recent Near Infra-Red images of the nightside of Venus revealed large-scale features which sustain their shape over multiple rotations (the rotation periods of the features are 6.0 +/- 0.5 days). Presumably, the contrast variations in the NIR images are caused by variations in the abundance of large particles in the cloud deck. If these particles are composed of liquid sulfuric acid, one would expect a strong anticorrelation between regions with a high abundance of sulfuric acid vapor, and regions where there are large particles. One technique for monitoring the abundance and distribution of sulfuric acid vapor (H2SO4) at and below the main Venus cloud layer (altitudes below 50 km) is to measure the 13-cm wavelength opacity using Pioneer Venus Orbiter Radio Occultation Studies (PV-ORO). We are working to characterize variations in the abundance and distribution of subcloud H2SO4(g) in the Venus atmosphere by using a number of 13-cm radio occultation measurements conducted with the Pioneer Venus Orbiter near the inferior conjunction of 1991. When retrieved, the vertical profiles of the abundance of H2SO4(g) will be compared and correlated with NIR images of the night side of Venus made during the same period of time. Hopefully, the combination of these two different types of data will make it possible to constrain or identify the composition of the large particles causing the features observed in the NIR images. Considered on their own, however, the parameters retrieved from the radio occultation experiments are valuable science products.

Jenkins, Jon M.↗

NASA NTRS1995

Triton stellar occultation candidates: 1995-1999

We have completed a search for candidates for stellar occultations by Triton over the years 1995-1999. CCd strip scan images provided star positions in the relevant sky area to a depth of about 17.5 R magnitude. Over this time period, we find that Triton passes within 1.0 arcsec of 75 stars. Appulses with geocentric minimum separations of less than 0.35 arcsec will result in stellar occultations, but further astrometry and photometry is necessary to refine individual predictions for identification of actual occultations. Finder charts are included to aid in further studies and prediction refinement. The two most promising potential occultations, Tr176 and Tr180, occur in 1997.

Mcdonald, S. W.↗

NASA NTRS1991

Pluto-Charon Stellar Occultation Candidates: 1990-1995

We have carried out a search to identify stars that might be occulted by Pluto or Charon during the period 1990-1995 and part of 1996. This search was made with an unfiltered CCD camera operated in the strip scanning mode, and it reaches an R magnitude of approximately 17.5-about 1.5 mag fainter than previous searches. Circumstances for each of the 162 potential occultations are given, including an approximate R magnitude of the star, which allows estimation of the signal-to-noise ratio (S/N) for observation of each occultation. The faintest stars in our list would yield an S/N of about 20 for a 1 S integration when observed with a CCD detector on an 8 m telescope under a dark sky. Our astrometric precision (+/- 0.2 arcsec, with larger systematic errors possible for individual cases) is insufficient to serve as a final prediction for these potential occultations, but is sufficient to identify stars deserving of further, more accurate, astrometric observations. Statistically, we expect about 32 of these events to be observable somewhere on Earth. The number of events actually observed will be substantially smaller because of clouds and the sparse distribution of large telescopes. Finder charts for each of the 91 stars involved are presented.

Dunham, E. W.↗

NASA NTRS1994

Occultation Predictions Using CCD Strip-Scanning Astrometry

We are developing the method of CCD strip-scanning astrometry for the purpose of deriving reliable advance predictions for occultations involving small objects in the outer solar system. We are using a camera system based on a Ford/Loral 2Kx2K CCD with the Crossley telescope at Lick Observatory for this work. The columns of die CCD are aligned East-West, the telescope drive is stopped, and the CCD is clocked at the same rate that the stars drift across it. In this way we obtain arbitrary length strip images 20 arcmin wide with 0.58" pixels. Since planets move mainly in RA, it is possible to obtain images of the planet and star to be occulted on the same strip well before the occultation occurs. The strip-to-strip precision (i.e. reproducibility) of positions is limited by atmospheric image motion to about 0.1" rms per strip. However, for objects that are nearby in R.A., the image motion is highly correlated and their relative positions are good to 0.02" rms per strip. We will show that the effects of atmospheric image motion on a given strip can be removed if a sufficient number of strips of a given area have been obtained. Thus, it is possible to reach an rms precision of 0.02" per strip, corresponding to about 0.3 of Pluto or Triton's angular radius. The ultimate accuracy of a prediction based on strip-scanning astrometry is currently limited by the accuracy of the positions of the stars in the astrometric network used and by systematic errors most likely due to the optical system. We will show the results of . the prediction of some recent occultations as examples of the current capabilities and limitations of this technique.

Dunham, Edward W.↗