Engineering topics

Millis, Robert L.

Publications and source records attributed to Millis, Robert L..

15 records

NASA NTRS1992

Comets - Groundbased observations of spacecraft mission candidates

Ground-based narrowband photometry results are presented for nine candidate comets for flyby and/or rendezvous missions. The comets include Churyumov-Gerasimenko, d'Arrest, Encke, Grigg-Skjellerup, Honda-Mrkos-Pajdusakova, Kopff, Tempel 1, Tempel 2, and Wild 2. On the basis of measured OH production rates and a model of the sublimation of water from the surface, limits are derived on the size of each cometary nucleus. A detailed analysis of the characteristics of these nine viable mission candidates can furnish the bases for the prioritization of targets of prospective missions.

Osip, David J.↗

NASA NTRS1992

15 years of comet photometry: A comparative analysis of 80 comets

In 1976 we began a program of narrowband photometry of comets that has encompassed well over 400 nights of observations. To date, the program has provided detailed information on 80 comets, 11 of which have been observed on multiple apparitions. In this paper we present the observed range of compositions (molecular production rate ratios) and dustiness (gas production compared with AF-rho) for a well sampled group of comets. Based on these results we present preliminary analysis of taxonomic groupings as well as the abundance ratios we associate with a 'typical' comet.

Osip, David J.↗

NASA NTRS1991

Comet Levy (1990c) - Groundbased photometric results

Emission bands of OH, NH, CN, C3, and C2 have been isolated, together with continuum points at 3650 and 4845 A, by narrowband filter photometry of Comet Levy; all species are noted to exhibit strong asymmetry near perihelion, in association with production rates for most species that are about twice as large before perihelion as after it in the 1-3 AU distance range. The asymmetry of the OH production rate was almost double that of the other species, and exhibited maximum production over a month before other species.

Schleicher, David G.↗

NASA NTRS1991

Occultation studies of the solar system

Occultations of stars by planets, satellites, planetary ring systems, and asteroids offer opportunities to study the occulting bodies in ways not otherwise possible from the surface of the Earth. For example, one can detect even an extremely tenuous atmosphere and can measure the temperature and density profiles of the atmosphere in regions not ordinarily sampled by spacecraft. Occultations also permit direct measurement of the size and shape of solar system objects too small to be directly resolved by groundbased telescopes. The accuracy of such determinations is typically 1-2 percent and, moreover, is independent of the distance to the object. In this investigation, upcoming occultations through wide-ranging computer searches are identified, and accurate predictions for the more important events are provided. Selected occultations are observed with specially designed portable photometric equipment.

Millis, Robert L.↗

NASA NTRS1991

The 15 years of comet photometry: A comparative analysis of 80 comets

In 1976, a program of narrowband photometry of comets was initiated that has encompassed well over 400 nights of observations. To date, the program has provided detailed information on 80 comets, 11 of which were observed during multiple apparitions. The filters (initially isolating CN, C2, and continuum and later including C3, OH, and NH) as well as the detectors used for the observations were changed over time, and the parameters adopted in the reduction and modeling of the data have likewise evolved. Accordingly, we have re-reduced the entire database and have derived production rates using current values for scalelengths and fluorescence efficiencies. Having completed this task, the results for different comets can now be meaningfully compared. The general characteristics that are discussed include ranges in composition (molecular production rate ratios) and dustiness (gas production compared with Af(rho)). Additionally an analysis of trends on how the production rates vary with heliocentric distance and on pre- and post-perihelion asymmetries in the production rates of individual comets. Possible taxonomic groupings are also described.

Osip, David J.↗

NASA NTRS1990

Standard stars for photometry of comets

Magnitudes are presented in 10 bandpasses for 63 stars that can be used as standards in photometric observations of comets. The bandpasses are, in addition to those for three (UV, blue, and red) continuum points, those which measure C2, C3, CN, CO(+), H2O(+), OH, and NH emissions. After correction for extinction, the derived magnitudes were transformed to a zero point such that all filters showed magnitude 5.88 for the star HD 3379. The internal agreement among the present observations and the agreement obtained between the observations and independently published measures indicate that the derived magnitudes are generally accurate to + or - 0.01 mag.

Osborn, Wayne H.↗

NASA NTRS1990

Periodic variations in the activity of Comet P/Halley during the 1985/1986 apparition

A search for periodic variation in the production of gas and dust by Comet Halley has been performed using narrowband photometric measurements from four sites - Lowell Observatory, Mauna Kea Observatory, Perth Observatory, and Cerro Tololo Inter-American Observatory. The method of phase dispersion minimization was applied to observations made during 164 observing nights between September 1985 and June 1986. A clear-cut variation, with a period near 7.4 days, was present throughout the postperihelion window. Less conclusive evidence of a similar period has been found in the pre-perihelion data. No indication of a shorter period or of strong sporadic activity exists in the data. The observations require that Halley's nucleus returns to essentially the same orientation with respect to the sun approximately every 7.4 days except for longer-timescale seasonal evolution. This fact precludes certain proposed models of nuclear motion.

Schleicher, David G.↗

NASA NTRS1990

Photometric narrowband CCD imaging of comets P/Brorsen-Metcalf and Austin (1989c1)

Simultaneous wide-field CCD images and conventional aperture photometry were obtained of Comets P/Brorsen/Metcalf (1989o) and Austin (1989c1). These data allow direct testing of absolute calibration of the CCD images and the ability to generate full-come, continuum-subtracted emission band images. Preliminary photometric calibration for a portion of the Brorsen-Metcalf observations yields reduced emission band and continuum fluxes which agree to within about 10 percent with those from the photoelectric observations. Conventional photometry of Comet Austin shows unusual variations among the production rates of the different species as a function of heliocentric distance.

Schleicher, David G.↗

NASA NTRS1989

The nucleus of Comet P/Tempel 2

Simultaneous optical photometry and IR radiometry of Comet P/Tempel 2 are presented. Periodic variations of brightness are present and in phase at all wavelengths. Because the optical and thermal rotational light curves are in phase, it is concluded that the variations are caused by the changing apparent cross section of an elongated nucleus rotating with a period near 8.9 h. The variation of flux with aperture makes it possible to separate the contributions of the nucleus and the coma. The contribution by the coma is about 25 percent at maximum light in the optical and undetectable at the level of 10 percent at all times in the thermal IR.

A'Hearn, Michael F.↗

NASA NTRS1989

Revised scale lengths for cometary NH

New values for the Haser model scale lengths of NH are derived from extensive narrow-band photometry of comet P/Halley. The observations require a parent scale length which is 5-10 times larger than previously published values. It is shown that the newly determined scale lengths also give better agreement with recent observations of two other comets: P/Borrelly and Liller. When combined with fluorescence efficiency calculations by others, the present results indicate that NH production in typical comets, relative to that of other molecular species, is about 20 times greater than previously believed.

Schleicher, David G.↗

NASA NTRS1988

Occultation studies of the solar system

Occultations of stars by planets, satellites, planetary ring systems, asteroids, and comets provide valuable opportunities to probe the Solar System in ways otherwise impossible from the surface of the earth. For example, one can precisely measure the size and shape of objects which are much too small to be resolved directly, accurately map the structure and transparency of ring systems, and detect the faintest trace of an atmosphere. In this investigation, researchers identify upcoming occultations through wide-ranging computer searches, provide accurate predictions for the more important events, and observe selected occultations with our specially designed portable photometric equipment. During the past year, researchers produced accurate predictions for an occultation of AG+40 degrees 0783 by 324 Bamberga on 8 December 1987 and coordinated efforts to observe this event. The occultation was successfully observed at 13 sites including two manned by Lowell Observatory astronomers.

Millis, Robert L.↗

NASA NTRS1988

An investigation of the nucleus and coma of Comet P/Arend-Rigaux

Narrow-band photometry of Comet P/Arend-Rigaux in the visible and near-ultraviolet are presented for six nights during the 1884/1985 apparition, including three nights of simultaneous observations in the thermal infrared made with the NASA IRTF. The results indicate a period of rotation of 13.47 hr, dimensions for the nucleus of 13 x 8 x 8 km (under the assumption of prolate spheroid shape), and a steady decrease in the comet's gas production from a month before perihelion to more that two months after perihelion. Results for the nucleus and the coma suggest that the nucleus is totally covered with a nonvolatile mantle.

Millis, Robert L.↗

NASA NTRS1987

Occultation studies of the Solar System

The planetary occultation program began at Lowell Observatory in 1973 with a worldwide campaign to observe mutual occultations and eclipses of the Galilean Satellites. Then the temperature profile of the Martian atmosphere was measured from data taken during the occultation of epsilon Geminorum, the Rings of Uranus were discovered as they occulted SAO 158687, and the dimensions of Pallas were measured when that minor planet occulted SAO 85009. In 1979 the present grant was initiated, providing funds for portable photometric instrumentation used to observe occultations by asteroids as well as by Uranus and Neptune. Software for predicting occultations of catalog stars by asteroids, planets, and comets was written in 1983. Lowell currently provides most of the available predictions for asteroid occultations. Realizing in 1983 that the lack of a high-quality astrometric telescope dedicated to occultation work was limiting progress, an 18-inch, F/8 lens was acquired and adapted to an existing mounting at Lowell. Although acquisition of the lens and implementation of the new telescope has been accomplished primarily with non-grant funds, the instrument makes a major contribution to occultation research.

Millis, Robert L.↗

NASA NTRS1987

Observations of the 22 April 1982 stellar occultation by Uranus and the rings

The 1.5 m telescope of Teide Observatory at Teneriffe was used to observe an occultation of the stellar object KME 14 by the Uranus ring system. The telescope was equipped with a two-channel IR flux collector, with one passband centered at 4490 A and the other near 8800 A. Data processing procedures that were used are detailed. The ring profiles were analyzed with a square-well model, which accounted for diffraction and finite source size while treating each ring as sharp-edged and uniformly translucent. An emersion lightcurve was obtained and inverted to yield a temperature profile of the upper atmosphere of Uranus, which displayed a temperature maximum (at 8 microbar) near the equator.

Millis, Robert L.↗

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.↗