Engineering topics
Publications and source records attributed to Millis, R. L..
Results of a computer search for occultations of AGK3 and SAO stars by 91 of the larger asteroids are presented. Thirty-eight events occurring in 1982 and 39 in 1983 are identified. Nominal ground tracks are plotted for the more promising occultations.
The Comet Bradfield 1979 X provided the most recent opportunity to obtain quantitative photometric data on a comet. Although the comet was not unusually luminous, its very close approach to earth made it possible to observe it from the time of discovery at a heliocentric distance of just over 0.5 AU until it was beyond 1.5 AU from the Sun. The observations conducted are discussed along with aspects of data reduction. It was found that the OH production of comets can be experimentally studied on the basis of a routine procedure. The Comet Bradfield 1979 X was one of the gassiest comets ever observed, comparable to Comet P/Encke. The production rates of all molecular species were found to decline sharply with heliocentric distance.
A photometric investigation of Comet P/Encke was undertaken using the 2.2 m telescope and the 0.6 m telescope. The two primary scientific objectives were: (1) measurement of the production rates of OH, CN, C3, and C2 as a function of heliocentric distance; and (2) determination of the comet's rotational period by searching for periodic brightness variations in the inner coma. Synoptic observation in the Canary Islands; Cambridge, Massachusetts; Flagstaff, Arizona; and Mauna Kea were planned for the study of the comet's rotation.
The occultation on Dec. 11, 1979, of AG + 0 deg 1022 by Juno was observed photoelectrically from 15 sites distributed across the occultation track. The observations are well represented by a mean elliptical limb profile having semimajor and semiminor axes of 145.2 + or 0.8 and 122.8 + or - 1.9 km, respectively. The corresponding effective diameter of Juno is 267 + or - 5 km, where the uncertainty has been conservatively increased to reflect the presence of limb irregularities clearly seen in the observations. Published radiometric and polarimetric diameters for Juno are 6% to 7% smaller than the occultation result. No secondary occultations attributable to possible satellites of Juno were recorded at any of 23 photoelectrically equipped observing sites.
Results of narrowband filter photometry are presented for comets 1977g, 1978b, 1978f, 1978j, 1979c, and 1979i. There appears to be a remarkable homogeneity among comets with regard to the relative production rates for those species which are readily measured, specifically C2, CN, C3, and OH. Most comets appear to show identical relative abundances, except for a rather well-defined variation with heliocentric distance. Molecular production rates appear to be unrelated to either the emission-to-continuum (gas-to-dust) ratio or the dynamical age of the comet. The correlation with visual magnitudes is excellent, though the slope is not what could be intuitively expected.
Results are presented for narrow-band photometry of four recent periodic comets. The observed fluxes are used to estimate carbon radical production rates, gas-to-dust ratios, and the colors of the dust grains. It is found that in most respects the periodic comets exhibit the same range of properties typical of parabolic comets. Comet P/Chernykh is one of the dustiest comets on record, and its dust is physically different from that of the other comets considered. A correlation between the gas-to-dust ratio and the color of the grains is tentatively proposed. The need for homogeneous data on a large number of comets is emphasized.
The May 29, 1978, occultation of SAO 85009 by Pallas was observed photoelectrically at seven widely spaced sites. The observations are well represented by an elliptical apparent limb profile having semimajor and semiminor axes of 279.5 + or - 2.9 and 262.7 + or - 4.5 km, respectively. Combining these results with published information on the light curve and rotational pole position, Pallas's mean diameter is found to be 538 + or - 12 km, which yields a mean density for Pallas of 2.8 + or - 0.5 g/cu cm and a visual geometric albedo of 0.103 + or - 0.005. The diameter of Pallas as determined from this occultation is significantly smaller than the values derived by radiometric, polarimetric, and double-image techniques.
Of the available earth-based techniques for determining asteroid diameters, observation of stellar occultations involving asteroids is clearly the most direct. The high degree of accuracy achievable by this method has already been demonstrated in the case of Pallas, whose mean diameter has been measured with a standard error of plus or minus 2%. In this paper the problems, results and prospects of the stellar occultation technique are reviewed. It is shown that, with the use of a network of small, portable telescopes, the method is currently applicable to a large number of asteroids. The best results can be expected for asteroids of large angular diameter and regular shape. The potential of lunar occultation observations for asteroid diameter measurements is also briefly discussed.
All available timing data for the occultations of SAO 158687 on March 10, 1977, by the cited rings of Uranus are analyzed. Least-squares fits to the data are performed using a model which postulates that rings alpha, beta, gamma, and delta are circular and coplanar. A solution obtained under the assumption that the ring plane coincides with the plane of the satellite orbits is adopted which yields radii of 44,844 km for ring alpha, 45,799 km for ring beta, 47,746 km for ring gamma, and 48,423 km for ring delta. The uncertainties in these values are discussed along with the apparent shapes and inclinations of these main rings. The mean radii estimated for the other rings are: 47,323 km for ring eta, 42,663 km for ring 4, 42,360 km for ring 5, and 41,980 km for ring 6.
Photoelectric observations of the December 23, 1977, occultation of BD -15 deg 3969 by the rings of Uranus are analyzed. The observations were made with a 1.5-m IR flux collector and clearly show the occultations of the star by the epsilon, delta, gamma, and alpha rings prior to the star's disappearance behind the planet. Detection of the beta-ring occultation is uncertain, but several features were observed which may be due to occultations of the star by objects other than the five major rings. It is noted that no other occultations with both a duration and depth comparable to the epsilon-ring event were observed between the orbit of Miranda and the limb of Uranus. The results suggest that the epsilon ring does not consist of two fragments but rather is a complete ring which may be either circular and inclined slightly with respect to the four inner rings or coplanar and elliptical.
The paper describes how observation of Uranus at far-red wavelengths during the planet's occultation of a star 3.5 magnitudes fainter led to the discovery of five rings surrounding the planet. Decreases in brightness occurring before the scheduled occultation were detected at the Perth Observatory near Bickley, Western Australia, and these decreases remained inexplicable until a study of the records showed a symmetric pattern before and after occultation by the planet. The Perth records support the planetary ring interpretation even though the planet's shadow could not be observed at Perth.
UBV photometry of J V (Amalthea) near greatest western elongation shows this satellite to be about one magnitude fainter than previously believed. It is observed to be very red, having a B-V color index near + 1.5 mag. Unlike the Galilean satellites, J V appears to be a low-albedo object.
High-time-resolution photoelectric measurements of the occultation of Epsilon Gem by Mars were obtained with the 36-in. telescope at Goddard Space Flight Center. Temperature profiles as a function of number density have been determined by inverting the immersion and emersion light curves on the assumption of a pure CO2 atmosphere. For number densities between 10 to the 13th and 10 to the 15th power per cu cm, the temperature varies in a wavelike fashion about a mean value near 150 K. These temperature profiles agree well with those derived from airborne observations by Elliot et al. (1977), indicating that the atmospheric features producing the excursions in the temperature profiles extend for at least 100 km along the Martian limb.
Postperihelion narrow-band filter photometry has been used to derive gas and dust production rates for Comet West. For heliocentric distances less than 1.25 AU, C2 and CN vary as the -2.8 power of distance, while C3 and OH (based on published measurements) vary as -2.1 power. C3 production closely parallels dust production. It is concluded that OH and C3 parents are produced at the nucleus, while CN and C2 parents are produced from the icy-grain halo. It is also found that Comet West is one of the dustiest and most productive of the recent comets, being exceeded only by Comet Bennett.
A description is presented of partial occultations of SAO 158687 by a series of rings around the planet Uranus. A 61-cm telescope at Perth Observatory in Western Australia was used in the investigations. Measurements were made using a conventional single-channel photometer equipped with an RCA C31034B photomultiplier. Uranus was centered in a 28-arc sec diameter aperture during the observations. An occultation of SAO 158687 by Uranus itself did not occur at Perth. However, five brief partial occultations of SAO 158687 were detected with the chart recorder. The midtimes, durations, and depths of these events are listed in a table.
Measurements of the brightness and B-V color index of the leading and trailing hemispheres of Iapetus during the past five apparitions of Saturn are presented. The dependence of brightness on solar phase angle is determined for the two faces of the satellite. After correction for solar phase angle and distance effects, the brighter trailing hemisphere of Iapetus was found to be constant in brightness and color index throughout the interval covered by the observations. The brightness of the dark leading hemisphere, however, decreased by about 0.2 mag over the same period, and its B-V color index reddened by 0.05 mag. These changes are interpreted as evidence for the existence of a bright southern pole cap on Iapetus.
Experience gained in obtaining atmospheric parameters, oblatenesses, and diameters of Jupiter and Mars from recent stellar occultations by these planets is used to predict what can be learned from the March 1977 occultation of the star SAO158687 by Uranus. The spectra of this star and Uranus are compared to indicate the relative instrument intensities of the two objects, the four passbands where the relative intensities are most nearly equal are listed, and expected photon fluxes from the star are computed on the assumption that it has UBVRI colors appropriate for a K5 main-sequence object. It is shown that low photon noise errors can be achieved by choosing appropriate passbands for observation, and the rms error expected for the Uranus temperature profiles obtained from the occultation light curves is calculated. It is suggested that observers of this occultation should record their data digitally for optimum time resolution.
UBV observations of asteroid 433 Eros were conducted on 17 nights during the winter of 1974/75. The peak-to-peak amplitude of the lightcurve varied from about 0.3 mag to nearly 1.4 mag. The absolute V mag at maximum light, extrapolated to zero phase, is 10.85. Phase coefficients of 0.0233 mag/degree, 0.0009 mag/degree, and 0.0004 mag/degree were derived for V, B-V and U-B, respectively. The zero-phase color of Eros (B-V = 0.88, U-B = 0.50) is representative of an S (silicaceous) compositional type asteroid. The color does not vary with rotation. The photometric behavior of Eros can be modeled by a cylinder with rounded ends having an axial ratio of about 2.3:1. The asteroid is rotating about a short axis with the north pole at 15 deg ecliptic longitude and 9 deg ecliptic latitude.