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At least 1,189 records · Page 66
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NASA NTRS1979

The ion drive program - Comet rendezvous issues for SEPS developers

Preliminary steps have been taken in a joint high-priority project between NASA and the European Space Agency, whereby a Solar Electric Propulsion System (SEPS), using ion drive of a 25-30 kilowatt power level, will be utilized for the first time, as part of the Space Transportation System, in powering a probe to be deployed toward Halley's comet in 1985 and a separate spacecraft which will rendezvous with the Temple 2 comet in 1988 and study it for one year. Unlike ballistically-launched vehicles, an unprecedented long-term interaction between the SEPS, the primary source of power and attitude control, and the spacecraft, responsible for data handling (at a rate of 10-120 kilobits per second), and command and telecommunications to earth (requiring capabilities at both X- and S-band frequencies, for dual-frequency navigational tracking), is required, as mission phases alternate between powered flight and science data-taking. Different design sensitivities are presented graphically.

Atkins, K. L.↗
NASA NTRS1979

Flyby- and rendezvous-type comet missions from the standpoint of large-particle dust experiments

The measurement by cometary flyby and rendezvous missions of the dust particles emitted from a cometary nucleus and believed to be the major source of the interplanetary dust is considered. The impact rate per unit area of particles in a given size range on a spacecraft dust sensor is calculated in terms of the distance between comet and probe and the particle emission rate and velocity. It is concluded that large-particle (greater than 100-micron) dust analysis experiments cannot be conducted on flyby missions to short-period comets since the required miss distance is orders of magnitude smaller than the targeting error, while for a rendezvous mission much greater miss distances are possible due to the longer period of cometary contact. The problem of designing dust sensors to operate at the subkilometer per second intercept velocities of a rendezvous mission is also noted.

Sekanina, Z.↗
NASA NTRS1980

Spectrophotometry of Comet West

Postperihelion observations of Comet West (1975n = 1976 VI) have been made with a Fourier transform spectrometer at heliocentric distances from 0.57 to 1.68 AU. Measurements were made of the emission bands of C2, CN, C3, CH, and NH2, as well as the emission lines of Na D and forbidden (O I), and the flux in the continuum in nine different bandpasses. Several ratios of the band strengths of CN have been used to determine the two free parameters in the fluorescence equilibrium model of CN of Danks and Arpigny (1973). From the values of the parameters it is inferred that the vibrational transition probability for the ground electronic state is between 0.025 and 0.075 per sec and that the ratio of oscillator strengths between the (0-0) bands of the violet and red systems is between 25 and 30. When corrected for field-of-view effects, NH2 shows no systematic variation in abundance relative to C2 while CH shows a small increase. The cometary continuum is found to be slightly redder than the solar continuum, consistent with results for other bright, dusty comets. The equivalent width of the Delta u = 0 sequence of C2 shows a marked decrease at r(H) = 1.2 AU.

Ahearn, M. F.↗
NASA NTRS1980

Navigation accuracy analysis for the Halley flyby phase of a dual comet mission using ion drive

A dual comet (Halley Flyby/Tempel 2 Rendezvous) mission, making use of the solar electric propulsion system, is under consideration for a 1985 launch. This paper presents navigation accuracy analysis results for the Halley flyby phase of this mission. Orbit determination and guidance accuracies are presented for the baseline navigation strategy, along with the results of a number of sensitivity studies involving parameters such as data frequencies, data accuracies, ion drive thrust vector errors, comet ephemeris uncertainties, time lags associated with data processing and command sequence generation, probe release time, and navigation coast arc duration.

Wood, L. J.↗
NASA NTRS1980

Brightness profiles of CO/+/ in the ionosphere of Comet West /1976 VI/

The observed sunward and tailward brightness profiles of the (2-0) band of the (A2Pi-X2Sigma) system of CO(+) (4260 A) in Comet West (1976 VI) are established over a range of heliocentric distance from 0.440 to 0.842 AU. From these data the variation with heliocentric distance of the CO(+) production rate is investigated along with the size of the contact surface (the diameter of the onset of the tail). A photoionization model is proposed for the production of CO(+) in Comet West in which CO photoionization is considered to be the principle source of CO(+); the model is consistent with the observational data and the MHD theories.

Combi, M. R.↗
NASA NTRS1979

The elemental and isotopic abundances of H, C, N, O in comets

The elemental abundances of light elements in comets is discussed by considering gas and dust separately. It is noted that the icy conglomerate model of the nucleus (Whipple, 1957) has been confirmed, and that the dust-to-gas mass ratio has been estimated within a factor of two for two recent comets. It is found that the total amount of oxygen available is also nominally in solar abundance; this implies that the oxygen of the solar nebula has probably been quantitatively bound in compounds that have been completely condensed and accreted in cometary nuclei. In addition, the amount of hydrogen is clearly depleted by a factor of 2,000, suggesting that the only hydrogen present is that linked in molecular compounds of H, C, N and O.

Delsemme, A. H.↗
NASA NTRS1980

Radio observation of comet Meier /1978f/ in 18-cm OH lines

Observations of 18-cm OH spectral lines in comet Meier (1978f) with the 1000-ft. Arecibo telescope show spatial resolution of the OH coma by the 2.9 arcmin beam (=3.7 x 10 to the 5th km). The data agree with predictions of the solar Fraunhofer spectrum-pumping theory of comet OH excitation. On the assumption that the OH parent molecule (e.g., H2O) has a Haser-model scale length of about 1.0 x 10 to the 5th km at heliocentric distance 1 AU, an OH scale length of less than 10 to the 6th km, and probably near 1.0 x 10 to the 5th km is derived. Assuming a recently calculated value of the OH lifetime and the solar radiative pumping model, the results indicate an OH production rate of 10 to the 29th Kayser at heliocentric distance 2 AU.

Giguere, P. T.↗
NASA NTRS1980

Comets and cometary missions

The paper deals with the planning phase of a combined NASA/ESA dual comet mission to the two periodic comets, P/Halley and P/Temple-2. The project is planned for a FY 81 start. If approved, the launch would occur in July or August 1985, using Shuttle IUS twin stage. The solar powered ion propulsion engine will provide continuous thrust virtually throughout the three-year flight until the P/Temple-2 rendezvous in July 1988. En route to the Temple-2 rendezvous, an ESA Halley probe will be deployed to encounter Halley about 73 days before perihelion, when the heliocentric distance of Halley is about 1.5 AU.

Mendis, D. A.↗
NASA NTRS1981

Water production models for Comet Bradfield (1979 l)

The IUE observations of Comet Bradfield (1979 l) made 10 January 1980 to 3 March 1980 permit a detailed study of water production for this comet. Brightness measurements are presented for all three water dissociation products, H, O, and OH, and comparisons are made with model predictions. The heliocentric variation of the water production rate was derived.

Weaver, H. A.↗
NASA NTRS1981

Photometric and CCD direct image observation of comet Encke

Attempted detection of periodic variations in brightness of the comet Encke is described. Viewing problems due to the position, faintness, and rate of motion of the comet are discussed. The failure of attempts to perform photoelectric photometry and CCD imaging is described. Photometric observations of the prototype Earth crosser, (1862) Apollo, are described and a photoelectric light curve of observations made during a four-hour period is presented.

Mccrosky, R. E.↗
NASA NTRS1980

Production of carbon, sulfur, and CS in Comet West

Results from ultraviolet spectrographic observations of Comet West (1975n) in the 1620-3960 A wavelength range are presented. Objective grating imagery of the comet reveals for the first time emission from sulfur atoms and CS molecules as well as previously observed lines or bands in carbon atoms and CO(+), CO2(+) and OH molecules. Some evidence exists for emission in the C2 Mulliken bands and in the CO Cameron bands. On the basis of the observations, production rates of C (ground state), C(1D), S, and CS are inferred to be 3.8 x 10 to the 29th per sec, 4.4 x 10 to the 29th per sec, 1.7 x 10 to the 28th per sec, and greater than approximately 3.1 x 10 to the 27th per sec, respectively.

Smith, A. M.↗
NASA NTRS1981

A CCD tracker for closed-loop instrument pointing at Halley's comet

An optical tracker based on CCD technology has been developed for the closed-loop pointing of the narrow-angle imaging system of the proposed Halley's comet intercept mission at the comet nucleus. The target tracker, needed to assure the accurate pointing of science instruments as the nucleus rapidly passes the spacecraft, is a component of the pointing control system, which also includes a two-axis articulated instrument platform and high-resolution optical encoders to measure platform orientation relative to spacecraft structure. The device was designed on the basis of a CCD imaging array to calculate target centroid position based on the optical intensity of all pixels containing the image, and a processor to perform spacecraft attitude control, pointing control and target tracker control and data processing functions. The tracker has been configured to operate in both target acquisition and tracking modes. Preliminary simulation results and past experiments on similar trackers indicate that excellent tracking performance can be achieved, and a breadboard version of the tracker is currently being fabricated.

Stanton, R. H.↗
NASA NTRS1981

Negative ions in comets

Negative ion sources in comets are identified and cometary plasma effects caused by negative ions are examined. The primary negative ion sources are shown to be: (1) for the inner coma - photodissociation of HCN, electron attachment of OH, and collision with alkalis; (2) in the vicinity of the nucleus - interplanetary dust collisions with the nucleus; and (3) for both the contaminated solar wind region and sporadic discharges in the nonhomogeneous inner coma plasma - dissociative electron attachment and charge inversion during keV positive ion scattering by cometary dust. Negative ion abundance for Halley's Comet has been estimated to be 10 to the -6th - 10 to the -10th of electron densities.

Wekhof, A.↗
NASA NTRS1981

Spectrophotometry of faint comets: The asteroid approach

Observing programs at optical (0.35-0.8 micron) and near-infrared (1.1-2.4 micron) wavelengths, directed at the acquisition of reflection spectra of faint and distant comets, are described. The ultimate goal is to obtain spectrophotometric measurements of comets for which a significant part of the light is expected to be reflected by the solid surface of the nucleus.

Degewij, J.↗
NASA NTRS1981

Photographic observations of comets at Lowell Observatory

Observations of Halley's comet at Lowell Observatory cover a period from November 10, 1909 to May 17, 1911: 334 direct photographs were taken, 118 objective prism spectra, and 32 slit spectrograms of the nucleus, many of them including 5 arc minutes of the surrounding coma. Just one morning's observations are illustrated as an example of utilizing every possible resource available at the Observatory at the time. Observational improvements developed since then are described and some suggestions for the coming return of Halley's comet are made.

Giclas, H. L.↗
NASA NTRS1981

The interactive astronomical data analysis facility - image enhancement techniques to Comet Halley

PDP 11/40 computer is at the heart of a general purpose interactive data analysis facility designed to permit easy access to data in both visual imagery and graphic representations. The major components consist of: the 11/40 CPU and 256 K bytes of 16-bit memory; two TU10 tape drives; 20 million bytes of disk storage; three user terminals; and the COMTAL image processing display system. The application of image enhancement techniques to two sequences of photographs of Comet Halley taken in Egypt in 1910 provides evidence for eruptions from the comet's nucleus.

Kinglesmith, D. A., III↗
NASA NTRS1981

Plans for Comet Halley

The configuration, trajectories, and payloads of a spinning Earth-orbiting satellite designed by ESA for a post-perihelion flythrough (Giotto mission) of Comet Halley are described. Experiments approved for use on two identical three-axis stabilized Venera spacecraft destined to encounter the comet after releasing probes to Venus are listed as well as the instruments to be carried on Japan's Planet-A flyby mission. The main features launch windows are given for each spacecraft.

Rahe, J.↗
NASA NTRS1981

Comets: Gases, ices, grains and plasma

The program and abstracts of the 97 papers delivered at the colloquium are presented. Cometary nuclei, comet dust, the coma, ion tails, several comet missions, and cometary origin and evolution were discussed.

Wilkening, L. L.↗