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At least 199 records · Page 11
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NASA NTRS1973

Approximations to the mean surface of Mars and Mars atmosphere using Mariner 9 occultations

When the Mariner 9 spacecraft passed behind Mars in its orbiting phase, the radio beam was cut off by some physical feature. These occultations occurred during the months of November and December 1971 and again in April and May 1972. The fairly good global spread of these data was used to estimate the shape of the Martian globe, as definable by mathematical shapes (ellipsoidal and spherical harmonic). The refraction profiles allowed the estimation of pressure near the occultation point; these, corrected back to an arbitrary fixed value of pressure, chosen to be 6.1 mb, were used as radii to an isobaric surface. Preliminary results showed a figure displacement from the mass center with a best fit southeast by 2.85 km. There was also an equatorial displacement in the direction of about 100 deg W. Atmospheric approximations for the mass center to 6.1-mb pressure distances were fit to an ellipsoid.

Cain, D. L.↗
NASA NTRS1972

The atmosphere of Mars from Mariner 9 radio occultation measurements.

Mariner-9 S-band radio occultation measurements conducted during November and December 1971 are described in terms of new information that was obtained about the shape and atmosphere of Mars. The arrival of the spacecraft coincided with a severely obscuring global dust storm, and the effect of the dust in the atmosphere was reflected in the reduced temperature gradients that were measured in the daytime near-equatorial atmosphere, indicating (1) heating of the atmosphere by solar radiation absorbed by dust and (2) simultaneous cooling of the surface. In general, surface atmospheric pressures measured in the equatorial regions are in very good agreement with previous radio-occultation and earth-based measurements. A global disparity in pressures strongly suggests that the physical shape of Mars is more oblate than the shape of its gravitational equipotential surface.

Kliore, A. J.↗
NASA NTRS1972

The shape of Mars from the Mariner 9 occultations.

The extinction time of the radio signal, as the Mariner 9 spacecraft was occultated by Mars, together with an accurate ephemeris of the spacecraft were used to determine radii from the mass center to the occulting feature. Similarly estimations were made of the radius to a point where the pressure reached a certain fixed value. Several simple models were proposed to fit both sets of radii data.

Cain, D. L.↗
NASA NTRS1973

An atmosphere on Ganymede from its occultation of SAO 186800 on 7 June 1972.

The observational data obtained during the occultation are of sufficient quality to determine the occultation radius and to support the inference that Ganymede does possess at least a modest atmosphere. Assuming a circular cross section, the diameter of Ganymede was found to be 5271 km. Effects of the atmosphere on the accuracy of the value obtained for the Ganymede diameter are discussed.

Carlson, R. W.↗
NASA NTRS1974

Stellar occultations

It is important that future occultations by Titan be predicted well in advance, and imperative that they be observed adequately. Stellar occultations can yield reliable temperature/number density profiles of Titan's upper atmosphere near the density level. The temperature profiles can be used to discriminate between atmospheric compositions having high mean molecular weights, and those having low mean molecular weights. Detection of spikes at several wavelengths with high time resolution gives useful limits on the helium content of the atmosphere.

Veverka, J.↗
NASA NTRS1974

An occultation satellite system for determining pressure levels in the atmosphere

A two-satellite microwave occultation system is described that will fix, as an absolute function of altitude, the pressure-temperature profile generated by a passive infrared sounder. The 300 mb pressure level is determined to within 24 m rms, assuming the temperture errors produced by the infrared sensor are not greater than 2 K rms. Error caused by water vapor in the radio path is corrected by climatological adjustments. A ground test of the proposed system is described. A microwave signal propagating between two mountain tops was found to be subject to periods of intense fading. Computer analysis of the raypath between the transmitting and receiving stations indicates that multipath and defocusing were responsible for this fading. It is unlikely that an operational pressure-reference-level system will be subject to the deep fades observed in the ground test, because the phenomena are associated with lower altitudes than the closest approach altitude of an occultation-system raypath.

Morrison, A. R.↗
NASA NTRS1974

Preliminary results on the atmospheres of Io and Jupiter from the Pioneer 10 S-band occultation experiment

The preliminary analysis of data from the Pioneer 10 S-band radio occultation experiment has revealed the presence of an ionosphere on the Jovian satellite Io having an electron density peak of about 60,000 electrons per cubic centimeter at an altitude of approximately 60 to 140 kilometers. This suggests the presence of an atmosphere having a surface number density of about 10 billion to one trillion per cubic centimeter, corresponding to an atmospheric surface pressure of between .1 and 10 nanobar, at or below the detection threshold of the Beta Scorpii stellar occultation. A measurement of the atmosphere of Jupiter was obtained down to the level of about 80 millibars, indicating a large temperature increase at about the 20 millibar level, which cannot be explained by the absorption of solar radiation by methane alone and can possibly be due to absorption by particulate matter.

Kliore, A.↗
NASA NTRS1974

The Regulus occultation light curve and the real atmosphere of Venus

An inversion of the light curve observed during the July 7, 1959, occultation of Regulus by Venus leads to the conclusion that the light curve cannot be reconciled with models of the Venus atmosphere based on spacecraft observations. The event occurred in daylight and, under the subsequently difficult observation conditions, it seems likely that the Regulus occultation light curve is marred by a systematic errors in spite of the competence of the observers involved.

Veverka, J.↗
NASA NTRS1974

Winds and the occultation experiment

A spacecraft orbiting about another planet, such as Mars or Venus, may be used to obtain data about the pressure, density, and temperature fields over the planet from multiple occultations if the orbit precesses or retrogresses. Under certain conditions successive occultations will provide mean dynamic information such as wind speeds over the time and spacing intervals. It is shown that data concerning winds may be found by comparing refractivity information rather than pressure or temperature.

Gross, S. H.↗
NASA NTRS1974

The occultation of beta Scorpii by Jupiter. II - The hydrogen-helium abundance in the Jovian atmosphere

The helium abundance in the Jovian atmosphere has been determined by a new method from our observations of the occultation of beta Scorpii by Jupiter. The occultation light curves, consisting of simultaneous records at three wavelengths with a time resolution of 0.01 sec, are dominated by sharp, well-defined intensity peaks or spikes. Because of the wavelength-dependence of the refractivity of the Jovian atmosphere, the arrival time of a given spike at the Earth is also wavelength dependent. A new method for determining the composition of a hydrogen-helium atmosphere from such data is presented, and the principal sources of error are discussed and evaluated.

Elliot, J. L.↗
NASA NTRS1974

A low resolution map of Europa from four occultations by Io

Analysis of three occultations of JII (Europa) by JI (Io) has resulted in a preliminary reflectivity map of JII for the hemisphere centered on longitude 324 deg, a measurement of 1483 plus or minus 20 km for the radius of JII, estimates of the event impact parameters, determination of the midevent times, and a visual geometric albedo of 0.74, for JII. A fourth occultation light curve was used after derivation of the results to confirm their validity.

Vermilion, J. R.↗
NASA NTRS1975

Mission analysis for satellite measurements of stratospheric constituents by solar occultation

Orbital characteristics and launch vehicle requirements for a solar occultation experiment measuring atmospheric constituents, such as aerosols or ozone, during the Nimbus-G and Applications Explorer Missions are analyzed. The experiment to be flown is basically a sun photometer which measures the spectral attenuation of solar radiation by the earth's atmosphere during spacecraft sunrise and sunset, yielding two aerosol and/or ozone stratospheric profiles per orbit. The tangent latitudes and longitudes as well as frequency of these measurements are analyzed for various spacecraft orbits to define maximum geographical coverage capability. Results indicate that a 50 deg inclined orbit for Applications Explorer provides latitude coverage from approximately 70 deg north to 70 deg south every 2-1/2 weeks. A high-moon, sun-synchronous orbit with an inclination of 99 deg for Nimbus-G will provide for coverage of occultation measurements at high latitudes near the polar regions (i.e., 64 to 80 deg north and south). The solar pointing requirements of the experiment in terms of yaw and pitch angles are also defined.

Harrison, E. F.↗
NASA NTRS1975

The atmosphere of Io from Pioneer 10 radio occultation measurements

The occultation of the Pioneer 10 spacecraft by Io (JI) provided an opportunity to obtain two S-band radio occultation measurements of its atmosphere. The day-side entry measurements revealed an ionosphere having a peak density of about 60,000 el/cu cm at an altitude of about 100 km. The topside scale height indicates a plasma temperature of about 406 K if it is composed of Na(+) and 495 K if N2(+) is principal ion. A thinner and less dense ionosphere was observed on the exit (night side), having a peak density of 9,000 el/cu cm at an altitude of 50 km. The topside plasma temperature is 160 K for N2(-) and 131 K for Na(+). If the ionosphere is produced by photoionization in a manner analogous to the ionospheres of the terrestrial planets, the density of neutral particles at the surface of Io is less than 10 to 1 trillion per cu cm, corresponding to a surface pressure of less than 10 to 1 nanobars.

Kliore, A. J.↗
NASA NTRS1975

Observations of turbulence in the atmosphere of Venus using Mariner 10 radio occultation measurements

A technique is developed for estimating the outer scale of turbulence in a planetary atmosphere using dual-frequency radio occultation measurements. This technique is based on the frequency dependence of the temporal frequency spectra for the log-amplitude fluctuations and is particularly useful when probing the upper atmosphere where the transverse velocity of the line-of-sight path is decelerating very rapidly. It is applied to the region of strong turbulence located in the vicinity of 60 km on the dayside of the atmosphere of Venus using the Mariner 10 S/X radio occultation measurements. It is found that, contrary to earlier findings from Mariner 5, the outer scale of turbulence is at least 5 km. It appears that the outer scale of turbulence is as large as the vertical extent of the region of strong turbulence. Estimates of the structure constants for refractive index and temperature fluctuations indicate that the turbulence is stronger than that measured near the earth's tropopause.

Woo, R.↗
NASA NTRS1975

Turbulence effects in radio occultation studies of atmospheric scale heights

Atmospheric scale heights derived from radio occultation measurements of a turbulent planetary atmosphere would be accurate if the average angle of refraction through the turbulent atmosphere were equal to the refraction angle for the corresponding quiescent atmosphere. These two angles are not equal (although their difference may not be large enough to cause significant error). If necessary, it should be possible to correct for the systematic error introduced by this inequality by using measurements of signal spectra to determine characteristics of the turbulence. Sensitive dual-frequency measurements could help define the effects of turbulence in future radio occultation experiments. It does not appear that the angular offset due to turbulence has been important in past experiments, although it may become significant and require corrective analysis when improved equipment is used to probe deep into turbulent atmospheres with greater measurement precision.

Eshleman, V. R.↗
NASA NTRS1976

Temperature of the atmosphere of Jupiter from Pioneer 10/11 radio occultations

Radio-occultation data from the Pioneer 10 and 11 flybys are analyzed using an integral inversion technique combined with a model for Jupiter's shape based on gravity data in order to account for the oblateness of the planet's atmosphere. The inversion technique defines the center of refraction in terms of the radius of curvature and the normal to the equipotential surface at the closest approach point of the ray. An Abelian inversion of the data obtained during the Pioneer 10 entry, the Pioneer 10 exit, and the Pioneer 11 exit is performed by fixing the center of refraction at some average value for each occultation event. The results for all three measurements are found to be consistent and to show a temperature inversion between the 10-mb and the 100-mb levels, with temperatures of 130 to 170 K at 10 mb and 80 to 100 K at 100 mb. Major sources of error are discussed, and it is concluded that the range of reasonable temperature uncertainties at these levels does not exceed 40 K. It is noted that the temperature inversion was also inferred from Pioneer 10 IR radiometric data.

Kliore, A. J.↗
NASA NTRS1976

The occultation of Kappa Geminorum by Eros

The paper discusses predictions and observations of the occultation of Kappa Gem by (433) Eros on January 24, 1975. Several positive and negative observations made in western New England are described. Local circumstances for the occultation are reconstructed, and the size and shape of Eros are determined analytically as well as graphically. The calculations yield two extremes for the cross section: a circle 23 km in diameter or a somewhat irregular figure 20 km by 6 or 7 km. Arguments based on the expected albedo of the asteroid suggest that the circle should be warped into an ellipse 21 by 13 km or that the irregular figure might be one component of a dumbbell-like profile.

Oleary, B.↗
NASA NTRS1976

The occultation of beta Scorpii by Jupiter. VII - The angular diameters of beta Scorpii A1 and A2

A new technique for measuring angular diameters of stars, which makes use of stellar images formed by planetary atmospheres during occultations, is described. This method has been applied to light curves of the 1971 May 13 occultation of beta Sco by Jupiter, yielding the angular diameters of the two early B stars comprising the spectroscopic binary beta Sco A. An angular diameter of about .000422 arcsec for beta Sco A1 and about .000264 arcsec is found for beta Sco A2. These angular diameters are in good agreement with that obtained with the intensity interferometer for beta Sco, a star of nearly the same magnitude and spectral type as beta Sco A1. If the distance to beta Sco A were precisely known, beta Sco A1 and A2 would become fundamental standards of mass, luminosity, and radius for early B stars. Present constraints on the distance and methods by which it could be accurately determined are discussed.

Elliot, J. L.↗