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At least 1,279 records · Page 71
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NASA NTRS1986

First results from the Giotto magnetometer experiment at comet Halley

The Giotto magnetometer experiment at comet Halley has for the first time provided magnetic field measurements in all the important spatial regions characterizing the front-side interaction between the solar-wind magnetoplasma and a cometary atmosphere. Upstream waves of cometary origin have been observed at distances greater than two million km from the comet, both inbound and outbound. A cometary bow shock has been identified at 1.15 million inbound on the dawn side and a thick quasi-parallel cometary bow shock outbound. A turbulent magnetosheath has been observed further inside. A magnetic pile-up region has been identified inside 135,000 km, inbound, and 263,000 km, outbound, with fields up to 57 and 65 nT, respectively. A cavity region with essentially zero magnetic field has been discovered, with a width of 8500 km along the trajectory around closest approach.

Neubauer, F. M.↗
NASA NTRS1986

Dust jets in comet Halley observed by Giotto and from the ground

A hypothesis for the short-lived, intermittent discrete sources of dust on the nucleus surface of comet Halley has been confirmed by closeup imaging by a camera aboard the Giotto probe. A previous conclusion for the nonexistence of jet emission from the dark side of the nucleus, reached on the basis of analysis of ground-based data, is also confirmed. Attention is given to such issues as possible spin vector variations for comet Halley over the 76-year period, the survival of dust sources, and the positions of the prime meridian at the two returns to the sun.

Sekanina, Z.↗
NASA NTRS1986

Space observations of comet Halley

Observations of comet Halley by the Solar Maximum Mission, the International Ultraviolet Explorer, the Pioneer Venus Orbiter, the Dynamics Explorer 1, and by sounding rockets are reviewed. Visual images taken through the wide-band blue filter showed an extended coma elongated approximately in the antisolar direction. Preperihelion observations show definite evidence for both a plasma and a dust tail. The ultraviolet observations indicate that an extensive hydrogen cloud extends to well over 10 billion km from the nucleus. The derived water production rate for the comet showed very large changes around the time of perihelion. The rocket observations showed strong H I, O I, and C I emissions, several CO fourth positive bands, and the C II emissions at 1335 A extending into the tail.

Brandt, J. C.↗
NASA NTRS1986

Interplanetary trajectory design for the Mariner Mark II Comet Rendezvous/Asteroid Flyby mission

The primary purpose of the first mission utilizing a Mariner Mark II spacecraft will be to rendezvous with the short-period comet Wild 2 and conduct a proximal examination during the comet's quiescent and active phases. The purpose of the present study is to elucidate, using an accurate dynamic model, some of the characteristics of the interplanetary trajectory that are relevant to the mission design. Optimal transfer trajectories are generated, constrained by the Shuttle/Centaur launch vehicle capability and the total spacecraft mass and performance. Asteroid flyby opportunities are examined, and the effects of selected asteroid encounters on mission performance are discussed.

Myers, M. R.↗
NASA NTRS1986

Early photometry of comet p/Halley - Development of the coma

The observation of comets over a large range of heliocentric distances can provide important information, taking into account, for instance, the appearance of true cometary characteristics. Photometric evidence is provided regarding coma formation at approximately 5.9 AU, giving attention to aspects of water-ice sublimation. New photometry of p/Halley was obtained when the comet was at 5.9 and 5.1 AU, during 1984 October and 1985 January, respectively. The new observations are compared with data obtained at larger heliocentric distances. It is argued that sustained mass loss from the nucleus began at about 5.9 AU, at a rate which is consistent with production by sublimation from a predominantly water-ice nucleus.

Meech, K. J.↗
NASA NTRS1986

Decametric radio emission from comets - An attempt at detection

In a brief discussion of radio emission of comets, it is pointed out that an occurrence of nonthermal radio emissions would be expected only at decametric or longer wavelengths. An attempt is reported to detect emission at decameter wavelengths during the International Halley Watch trial run on Comet P/Crommelin in March 1984. No plasma effects in the tail were observed either at visible or at radio wavelengths. Assuming free-free emission from an optically thin gas, the observations make it possible to place an upper limit in the range from 560 to 8900 electrons per cu cm on the electron density in the tail. The Teepee Tee array employed in the investigations has a frequency coverage of 15-125 MHz, and a sensitivity of approximately 1 Jy, (the highest currently available from the ground at low frequencies).

Gergely, T. E.↗
NASA NTRS1986

Approach strategy for the rendezvous with comet Tempel 2

The methods used to locate the comet nucleus and safety navigate through the debris surrounding the comet are discussed. The approach to the nucleus is in two parts: a high-speed (1.9 km/s) leg which ends with the spacecraft 200,000 km sunward of the nucleus, and a low-speed (25 m/s) leg which traverses the 200,000 km to the nucleus. The nucleus will be recovered by the narrow-angle camera 30 days prior to the completion of the high-speed inbound leg. The debris density is low enough to permit the safe passage of the spacecraft through the surrounding cometary debris.

Kakuda, R. Y.↗
NASA NTRS1986

Imaging studies of comets

Extensive photography was conducted on comet Giacobini-Zinner in support of the flyby of the International Cometary Explorer (ICE) through the plasma tail on 9/11/85, and a dense series of plates was secured of Halley's Comet during the 7-month interval approximately centered on perihelion, i.e., 1985 November - 1986 May. The hundreds of high quality plates of Halley contain copious numbers of dramatic plasma events, including DE's, helical waves, and turning tail rays. Initial study of the plate material lends support to the magnetic sector boundary model of DE's developed by Niedner and Brandt.

Brandt, J. C.↗
NASA NTRS1986

Spatially resolved quantitative spectroscopy of comets

The pristine nucleus of a comet, in particular, its chemical composition is studied. More immediate goals are to quantitatively compare the mass-loss-rates of various periodic comets, and to map out the distribution of coma region ions and neutral radicals with the hope of indirectly determining their parent molecules.

Spinrad, H.↗
NASA NTRS1986

Astrometric observations of comets and asteroids and subsequent orbital investigations

The 155-cm reflector was used for observations of comets and minor planets on 32 nights during April-October. The distribution was fairly uniform, ranging from 45 in June and September to 28 in July: on September 1 a total of 17 observations was made. A table is included which lists the 230 measurements published during the semester. These include six additional measurements going back to 1984 and the republication of a previously misidentified 1976 observation as a new discovery. Thirty-two of the observations refer to comets, 83 to numbered minor planets, and the remainder to unnumbered minor planets. Among the older-numbered minor planets observed were the Earth-approaching objects (3103) 1982 BB, (3199) 1982 RA (Nefertiti), (3361) 1982 HR and (3362) 1984 QA (Khufu), some of which were of interest for radar experiments. Unnumbered Earth-approaching objects observed were 1963 RH, 1983 RD, 1985 TB and 1986 DA, as well as the new discoveries 1986 JK, 1986 LA and 1986 PA.

Mccrosky, R. E.↗
NASA NTRS1986

Comet P/Halley 1910, 1986: An objective-prism study

V. M. Slipher of the Lowell Obs. collected a large amount of spectroscopic data during the 1910 apparition of Halley's comet. Three of his post perihelion objective-prism plates were selected, digitized, and subjected to modern digital data reduction procedures. Some of the important steps in the analysis where: (1) Density to intensity conversion for which was used 1910 slit spectra of Fe-arc lamp on similar plates (Sigma) and derived an average characteristic curve; (2) Flux calibration using the fact that during the period June 2 to 7 1910 P/Halley was very close (angular distance) to the bright star Alpha Sex (A0III, V-4.49), and the spectra of both star and comet were recorded on the same plates. The flux distribution of Alpha Sex was assumed to be similar to that of the standard star 58 Aql and derived a sensitivity curve for the system; (3) Atmospheric extinction using the standard curve for the Lowell Obs.; (4) Solar continuum subtraction using the standard solar spectrum binned to the spectral resolution. An example of a flux-calibrated spectrum of the coma (integrated over 87,000km) before the subtraction of solar continuum is presented.

Carsenty, U.↗
NASA NTRS1987

To catch a comet 2: Technical update on CAN-DO

Since the presentation of, To Catch a Comet, was given at the last G.A.S. Symposium, many events have impacted the CAN-DO Comet Halley program. This paper summarizes the changes to the payload and its mission, including improvements in camera control and CAN-DO's participation in the Halley Armada.

Obrien, Thomas J.↗
NASA NTRS1987

Astrometric observations of comets and asteroids and subsequent orbital investigations

The 155-cm reflector of the Smithsonian Astrophysical Observatory was used for observations of comets and minor planets on 35 nights during October l986- April l987. The distrubution was fairly uniform November-February, but poor weather permitted only 21 observations during March. Table I lists the 423 measurements published (in the MPCs) since the last report, two of them actually made at the very beginning of May l987. Forty-six of the observations refer to comets, 193 to numbered minor planets (numbered, that is, by the end of the semester; only 11 of them refer to minor planets already numbered at the time of the last report), and the remainder to unnumbered minor planets.

Mccrosky, Richard E.↗
NASA NTRS1987

Temperatures and minimum thickness of the inactive surface layer of comet Halley

The effects of a nonvolatile mantle on the thermal state of a comet nucleus are investigated. The original computer model was modified so that temperatures can be computed through a thin dust mantle to the center of a 5 km spherical nucleus in the orbit of P/Halley. No attempt is made to simulate the formation of the mantle. Results are obtained for various specified values of initial mantle thickness and thermal conductivity to determine their effects on temperature profiles through the mantle. The minimum thickness of mantle that can withstand ejection by sublimating gasses is also calculated as a function of mantle thermal conductivity. Calculations were performed for ten or more orbits until temperatures in the mantle reached a near steady state. Results indicate that mantles as thin as 4 cm and 14 cm, for thermal conductivities of 600 and 6000 ergs/cm/s K, respectively, will remain intact. Surface temperatures as high as 511 K at perihelion and 400 K at the position of spacecraft encounter were computed at 0 degree latitude for an upright, rotating nucleus. Ice interface temperatures were raised by different amounts during each orbit, depending on mantle thickness and thermal conductivity, until steady state was reached. These results imply that relatively small nonvolatile masses emplaced randomly in comet nuclei could produce an irregular, permanently mantled surface and could also account for the apparently random location of active areas.

Fanale, F. P.↗
NASA NTRS1986

Plasma structures in Comet Halley

Large-scale plasma phenomena based on results of spacecraft probing of comets Halley and Giacobini-Zinner and on International Halley Watch are discussed. The data show that: the basic mechanism for the comets' interaction with the solar wind (Alfven, 1957) is correct; that the turn-on and turn-off of plasma activity follows the description of Mendis and Flammer (1984); and that disconnection events are associated with sector boundary crossings and an essential element in the morphology of developed plasma tails (Niedner and Brandt, 1978).

Brandt, John C.↗
NASA NTRS1986

Observations of heavy energetic ions far upstream from Comet Halley

On March 25, 1986, when the ICE spacecraft came within 28 million km of the nucleus of comet Halley, and for several days around this time, bursts of heavy ions were observed by the ICE energetic ion experiment. The bursts were observed only during periods when the solar wind velocity was considerably higher than its nominal value. The characteristics of these ions, in particular their anisotropies, were examined. Using the well known formulae for transformation of distributions from the solar wind frame of reference to the spacecraft frame, the angular distributions expected from either protons, or heavy ions from the water group, were studied, showing that the measurements are consistent with heavy ions, and not with protons. Other sources of heavy ions are considered, and the most likely source of these ions is comet Halley.

Sanderson, T. R.↗
NASA NTRS1986

Angular and energy distribution of low energy cometary ions measured in the outer coma of Comet Halley

During the early phase of the Giotto encounter with comet Halley, at distances from the nucleus greater than 350,000 km, the neutral mass spectrometer was operated in a mode allowing the measurement of low energy ions. Data reveal two important features of the outer coma: the presence of a sharp discontinuity in the plasma flow at 550,000 km from the nucleus which results in a significant decrease of the plasma flow accompanied by an increase in temperature; and the detection of newly born ions identified as O(+) and CO(+), at distances from the comet greater than 800,000 km.

Berthelier, J. J.↗
NASA NTRS1986

The upstream region, foreshock and bow shock wave at Halley's Comet from plasma electron measurements

Halley plasma electron parameters from 2.7 million km from the comet nucleus to the bow shock wave at 1.1 million km and beyond are surveyed. The features of the electron foreshock lying outside the shock to a distance of 230,000 km are described. It is a region of intense solar wind-comet plasma interaction in which energetic electrons are prominent. Several spikes of electrons whose energies extend to 2.5 keV appear in front of the shock. These energetic electrons may be accelerated in the same way electrons are accelerated at the Earth's bow shock to energies of 1 to 10 keV. The direction of the electron bulk flow direction changes abruptly between 1920 and 1922 UT, and the flow speed begins a sharp decline at the same time. It is suggested that the spacecraft entered the bow shock wave between 1920 and 1922 UT. Electron density variations at Halley are very much smaller than those at Giacobini-Zinner.

Anderson, K. A.↗