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NASA NTRS1984

Evolution of the ultraviolet coma of comet Austin (1982g.)

Ultraviolet spectra of comet Austin (1982) obtained in July 1982 at heliocentric distances ranging from 1.10 to 0.81 AU preperihelion with the IUE reveal that the comet is very similar in chemical composition and appearance to comet Bradfield (1979X). In addition, the derived H2O production rate is found to vary with heliocentric distance as r to the -3.6, similar to the r to the -3.7 behavior found for comet Bradfield. It is pointed out, however, that the limited sample rate precludes the observation of the short-term variations which could be produced by a rotating nonuniform cometary nucleus.

Feldman, P. D.↗
NASA NTRS1984

Hydrogen production rate from comet Austin 1982g

Meaningful measurements with respect to the cometary Balmer-alpha (H-alpha) emission are difficult and require the use of special equipment. The first ground-based observations of H-alpha emission from a cometary hydrogen corona were conducted on comet Kohoutek 1973 XII with a large-aperture Fabry-Perot spectrometer installed at the McMath solar telescope at Kitt Peak National Observatory. The present investigation is concerned with the second ground-based observations of cometary H-alpha emission carried out during the apparition of comet Austin 1982g. A 150 mm dual-etalon Fabry-Perot spectrometer was employed in the experiment. Use was made of an observatory which is designed for the high spectral resolution study of faint extended sources such as interstellar and geocoronal emission lines. The investigation demonstrates that hydrogen production rates from comets as faint as about 7th magnitude can be routinely measured from the ground at minimal cost.

Shih, P.↗
NASA NTRS1984

A radio source occultation experiment with comet Austin 1982g, with unusual results

A radio source occultation by comet Austin 1982g was observed on September 15-16, 1982. A change in the apparent position of 1242 + 41 by 1.3 arcsec occurred when the source was 220,000 km away from the cometary ion tail. If this change was due to refraction by the cometary plasma, it indicates an electron density of the plasma of about 10,000/cu cm. When the radio source was on the other side of the plasma tail, at a distance of 230,000 km, the position angle of the electric vector of the radio source changed gradually over about 140 deg within two hours. This observation cannot be explained in terms of ionospheric Faraday rotation, and results from either an intrinsic change in the radio source or Faraday rotation in the cometary plasma due to a change in the direction and/or strength of the magnetic field. In the latter case, the cometary coma must have an electron density and a magnetic field strength orders of magnitude larger than current theories predict.

De Pater, I.↗
NASA NTRS1986

Activities of the Department of Astronomy and McDonald Observatory of the University of Texas at Austin

McDonald Observatory of the University of Texas at Austin reports on its activities during the period 1 Jan. 1986 to 30 June 1986. Extensive observations of Halley's Comet were obtained. The comet exhibited large variability; moreover, its variability was much more rapid than can be accounted for by water vaporization as the sole controller of activity. Jupiter satellite Io's atmosphere was found to be distended by more than the equilibrium scale height but less than for unimpeded streaming into space. The atmosphere is at least temporarily bound to IO. Uranus' (3-0) H2 quadrupole line shapes require a modification of Baines and Bergstralh's standard model which incorporates at high altitude absorbing haze in addition to the lower haze layer. A fraction of normal H2 equal to 0.25 + or 0.10 is derived, in good agreement with the standard model. This result is unchanged when the preliminary temperature structure derived by the Voyager Radio Occultation Experiment is used instead of Appleby's model c. Out of the six Pluto-Charon mutual events observed this year, data were obtained on four. Preliminary analysis is yielding improved estimates for the diameters, masses, densities, and albedos of these objects.

Smith, H. J.↗
NASA NTRS1989

Low elevation angle KU-band satellite measurements at Austin, Texas

At low elevation angles, the propagation of satellite signals is affected by precipitation as well as by inhomogeneties of the refractive index. Whereas precipitation causes fades for relatively small percentages of time, the refractive index variability causes scintillations which can be observed for most of the time. An experiment is now under way in Austin, Texas, in which the right hand circularly polarized 12 GHz beacon of INTELSAT-V/F10 is observed at a 5.8 deg elevation angle, along with the radiometric sky temperature, rainfall rate, humidity, pressure, temperature, and wind speed and direction. The objective of these measurements is to accumulate a database over a period of 2 years and to analyze the probabilities and dynamical behavior of the signal variations in relation to the meteorological parameters. The hardware and software used for the data acquisition and analysis is described and the results from the first year of measurements are presented.

Vogel, Wolfhard J.↗
NASA NTRS1989

Overview of computational control research at UT Austin

An overview of current research activities at UT Austin is presented to discuss certain technical issues in the following areas: (1) Computer-Aided Nonlinear Control Design: In this project, the describing function method is employed for the nonlinear control analysis and design of a flexible spacecraft equipped with pulse modulated reaction jets. INCA program has been enhanced to allow the numerical calculation of describing functions as well as the nonlinear limit cycle analysis capability in the frequency domain; (2) Robust Linear Quadratic Gaussian (LQG) Compensator Synthesis: Robust control design techniques and software tools are developed for flexible space structures with parameter uncertainty. In particular, an interactive, robust multivariable control design capability is being developed for INCA program; and (3) LQR-Based Autonomous Control System for the Space Station: In this project, real time implementation of LQR-based autonomous control system is investigated for the space station with time-varying inertias and with significant multibody dynamic interactions.

Bong, Wie↗
NASA NTRS1990

Optical and near-IR imaging observations of comet Austin 1989c1

Near-nucleus imaging observations of comet Austin (1989c1) were carried out by the Japanese CCD imaging team. Six telescopes were used to monitor the time variation of the near-nucleus images in C2, CN, H2O, and Na continuum in the optical region, and in J, H, and K bands in the near-IR region. A featureless, round shape of the comet was revealed in all images. Although some of the jet features are recognized by using an image enhancement technique, the azimuthal difference of the intensity distribution is about 10 percent. The images in the H2O band show complex ion structures near the nucleus.

Watanabe, J.↗
NASA NTRS1990

Fabry-Perot observations of comet Austin

Preliminary results of a program to observe Comet Austin (1990c1) from 16 April to 4 May and from 11 May to 27 May 1990 using the West Auxiliary of the McMath Solar Telescope on Kitt Peak, Arizona were presetned. The observations were made with a 15 cm duel-etalon Fabry-Perot scanning and imaging spectrometer with two modes of operation: a high resolution mode with a velocity resolution of 1.2 km/s and a medium resolution mode with a velocity resolution 10 km/s. Scanning data was obtained with an RCA C31034A photomultiplier tube and imaging data was obtained with a Photometrics LN2 cooled CCD camera with a 516 by 516 Ford chip. The results include: (1) information on the coma outflow velocity from high resolution spectral profiles of (OI)6300 and NH2 emissions, (2) gaseous water production rates from medium resolution observation of (OI)6300, (3) spectra of H2O(+) emissions in order to study the ionized component of the coma, (4) spatial distribution of H2O(+) emission features from sequences of velocity resolved images (data cubes), and (5) spatial distribution of (OI)6300 and NH2 emissions from medium resolution images. The field of view on the sky was 10.5 arcminutes in diameter. In the imaging mode the CCD was binned 4 by 4 resulting in 7.6 sec power pixel and a subarray readout for a field of view of 10.5 min.

Schultz, David↗
NASA NTRS1990

The near ultraviolet spectra of comets P/Brorsen-Metcalf and Austin

Results are reported on spectrophotometric observations of comets P/Brorsen-Metcalf and Austin from 3000 to 3600 A at a spectral resolution of about 1.8 A. The strongest features are the OH(A-X) 0-0 and 1-1 bands, and the NH(A-X) 0-0 bands. For the first time, the OH(A-X) 0-1 band was clearly found. The existence of the CN(B-X) 2-1 and 3-2 bands were verified and measured. A feature at 3258 A that was first seen in uncalibrated spectra was detected, and was identified as the NH singlet (c-a) 0-0 transition. The CO2(+) features at 3378, 3504, and 3512 A were also firmly identified. This ion was reported as being present in the tail of Comet Bester (1984 I) by Swings and Page (1950). The identification of a weak feature at 3547 A was proposed as the fundamental transition of H2CO, which would make this the first optical cometary detection of this molecule which is very abundant in giant molecular clouds.

Cochran, William D.↗
NASA NTRS1990

Rocket observations of the ultraviolet spectrum of comet Austin (1989c1)

Long slit ultraviolet spectra of Comet Austin (1989c1) were obtained from a sounding rocket experiment launched from White Sands Missile Range, New Mexico, on 21 April 1990 at 1015 UT. The instrument, known as the Faint Object Telescope, consisted of f/15.5 Dall-Kirkham telescope, a Rowland Circle spectrograph and a microchannel plate intensifier coupled to a Reticon diode array. An onboard television camera transmitted images to the ground during flight, which permitted real-time maneuvers in order to center the comet in the entrance slit. The comet was held near the center of the slit for approximately 270 seconds by sending six pointing corrections, all but one of which was less than 30 arcseconds. The comet's parameters at the time of observation, along with the characteristics of the instrument, are given.

Sahnow, D. J.↗
NASA NTRS1990

IUE observations of comet Austin (1989c1)

Observations of comet Austin (1989c1) were made using the International Ultraviolet Explorer (IUE) satellite on December 22, 1989 (preperihelion) and on nine dates between 7 and 24 May, 1990 (postperihelion). A summary of observation dates and parameters is given. The ultraviolet spectrum is characteristic of that of non-dusty comets. Some preliminary results are presented for OH and CS production rates and dust-reflected solar continuum emission based upon the long wavelength, low resolution mode spectra; photometry using the IUE Fine Error Sensor; a detection of the CO fourth positive system in the short wavelength, low dispersion spectra; and an upper limit on S2.

Budzien, S. A.↗
NASA NTRS1990

OH radio observations of comets P/Brorsen-Metcalf (1989o), Okazaki-Levy-Rudenko (1989r), Aarseth-Brewington (1989a1), and Austin (1989c1) at the Nancay radio telescope

The 1667 MHz and 1665 MHz transitions of the OH radical were recently monitored in several comets with the Nancay radio telescope: P/Brorsen-Metcalf (1989o) (August 4 to October 31, 1989), Okazaki-Levy-Rudenko (1989r) (October 3 to December 2, 1989), Aarseth-Brewington (1989a1) (December 8 to 30, 1989), and Austin (1989c1) (February 15 to June 14, 1990). Present gas expansion measurements obtained from the analysis of the line shapes are presented and the long term variations of the water production rate, as measured from the OH radio lines. On October 13, 1989, the occultation of a background source by comet Okazaki-Levy-Rudenko (1989r) was fortuitously observed. A preliminary report of this observation is given. Further analysis of the data is continuing.

Bockelee-Morvan, D.↗
NASA NTRS1990

Silicates and variability in comets Okazaki-Levy-Rudenko (1989r) and Austin (1989c1)

Comet Okazaki-Levy-Rudenko (1989r) was studied over a wide range of wavelengths from the UV to the submillimeter. Using visual magnitude estimates from the IAU Circulars and reported submillimeter fluxes in conjunction with the thermal infrared spectroscopy and filter photometry, the nature of the grains and the behavior of the comet can be assessed and compared to other comets studied under this program. In addition, preliminary results from a collaborative infrared study of Comet Austin are presented which address the nature of the dust in that comet. Progress on a model for cometary properties and brightness behavior is discussed.

Russell, Ray W.↗
NASA NTRS1990

A tail-wagging event in comet Austin

On the morning of 21 May 1990, a sequence of images of Comet Austin was obtained in the light of H2O+. The filter was centered at 602.5 nm with FWHM of 5.0 nm. At least two waves were followed out through the main ion tail structure. During the course of the observations, 2.5 hours, two new tail rays were seen to form and undergo similar wave motion. Several condensations, possibly representing disconnected material, were seen to develop and move out along the tail ray with a velocity of about 60 km/sec.

Klinglesmith, Daniel A.↗
NASA NTRS1991

The spectrum of Comet Austin from 910 to 1180 A

A spectrum of Comet Austin (1988 c1) has been obtained from 910 to 1180 A. Three bright emission lines were detected, including a forbidden oxygen line (1128 A), which are attributable to radiative pumping of neutral oxygen by solar Lyman-beta. The relative strengths of the observed features should prove to be a useful diagnostic of the physical conditions and radiation fields in cometary comae. In addition, the absence of strong spectral features from highly volatile species such as He, Ar, or N2 can be used to place constraints on the thermal environment under which the comet was formed and has been processed.

Green, James C.↗
NASA NTRS1991

Time-lapse CCD imagery of plasma-tail motions in Comet Austin

The appearance of the bright comet Austin 1989c1 in April-May of 1990 allowed us to test a new imaging instrument at the Joint Observatory for Cometary Research (JOCR). It is a 300mm lens/charge coupled device (CCD) system with interference filters appropriate for cometary emissions. The 13 frames were made into a time-lapse movie showing the evolution of the plasma tail. We were able to follow at least two large-scale waves out through the main tail structure. During the sequence, we saw two new tail rays form and undergo similar wave motion.

Klinglesmith, Daniel A., III↗
NASA NTRS1992

Near-ultraviolet spectroscopy of Comet Austin (1989c1)

Comet Austin (1989c1) was observed post-perihelion at a heliocentric distance near 1.25 AU. The wavelength range was from the atmospheric cutoff at 3000 to 4000 A. The coma spectra were calibrated into flux units and the contaminating sky spectrum and solar scattered light continuum were subtracted, leaving an ultraviolet spectrum of about 1.5-A resolution and excellent signal-to-noise ratio. The spectrum is dominated by emissions from OH, NH, CH, C3, and CN, some of the weaker emissions of which are seen here for the first time. More bands of CO(2+) were found than in any previous investigation and several intensity anomalies were noted; H2CO, OH(+), NCN, N(2+), and CN(+) may be present. Several emission features well above the noise level remain unidentified. The relative intensities of the OH and CN bands agree with the predictions of resonance fluorescence when one considers the potential effects of contamination by other molecules. The effects of the ozone absorption spectrum are not fully removed by the data-reduction process, although this does not affect these results.

Valk, Jacobus H.↗