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At least 289 records · Page 16
All sourcesNASA NTRSDOE OSTI
NASA NTRS1972

Reality of comet nucleus.

The prime problem of a comet mission must be to settle whether the cometary nucleus has an actual tangible material existence, or whether it arises from some optical effect present only at times within comets. The absence of any large particles in a comet seems to be demonstrated by certain meteor showers. A feature that would seem to indicate that a comet consists primarily of a swarm of particles is that the coma in general contracts as the comet approaches the sun, roughly in proportion within the distance, and then expands again as it recedes.

Lyttleton, R. A.↗
NASA NTRS1973

The comet-meteor stream complex.

The genetic relationship between short-period comets and meteor streams is investigated. It is shown that mechanisms exist for the radial and the longitudinal focussing of particles in meteor streams with characteristic time scales of agglomeration significantly smaller than those of any of the known dispersive processes. Consequently, it is claimed that meteor streams may not merely form a sink for short-period comets but may also form a source. A likely origin for the volatiles observed in such comets is suggested. It is finally stressed that this reciprocity in the genetic relationship between short-period comets and meteor streams should form an important consideration in any attempt at accounting for the observed population of short-period comets.

Mendis, D. A.↗
NASA NTRS1974

A survey of possible missions to the periodic comets in the interval 1974 - 2010

Catalogs are developed to survey the mission possibilities for the short period comets. In the first the physical and pertinent orbital characteristics are given for 65 short period comets. The second catalog is one containing the predicted perihelia for each of the 65 comets between 1974 and 2010. Geometry is included to indicate feasibility of Earth-based observation and sighting within 100 days of perihelion. The comets are divided on the basis of size and activity into three groups from the data in the first catalog: primary, secondary and low interest. The perihelia are separated into two groups: satisfactory and not satisfactory on the basis of earth-comet distance.

Bender, D. F.↗
NASA NTRS1974

Observations of comet Kohoutek (1973f)

The coma region Comet Kohoutek (1973f) was observed with a 40-mm proximity focused image intensifier tube and filters to isolate principal emissions. The low brightness of the comet limited the results, but it was possible to compare the intensity profiles of C2 and continuum in the coma. It appears that Comet Kohoutek was a typical comet in that it did not exhibit unusual coma structure similar to that of Comet Bennet. Photographs taken with the 42 cm Schmidt showed typical type 1, type, 2 and antitail morphology, with increased type 2 activity after perihelion passage. A technique for eliminating the type 2 (dust) component of the tail is given by means of photographic subtraction of blue and red sensitive layers of a color photograph.

Larson, S.↗
NASA NTRS1974

Infrared observations of Comet Kohoutek /1973f/

Comet Kohoutek has been observed at wavelengths between 1.25 and 12.5 microns before and after perihelion passage extending to comet-sun distances of 1 AU. The luminosity and the variation of brightness with comet-sun distance in the infrared are extraordinarily similar to those of Comet Ikeya-Seki (1965f). Apart from an apparent 'silicate' emission feature near 10 microns, the spectrum of the comet between 3.5 and 12.5 microns is close to that expected from emission by grey particles. Hotter particles and scattered sunlight produce the bulk of the 1.25- to 3.5-micron emission.

Gatley, I.↗
NASA NTRS1975

Do comets play a role in galactic chemistry and gamma-ray bursts

This paper explores the plausibility of the assumption that enough material from interstellar space is locked up in comets to reduce significantly the apparent growth rate of 'heavy' elements (mass greater than He) and, therefore, the present abundance of heavy elements in the interstellar medium and in the disk stars. A related suggestion concerns the influx of comets on neutron stars as a source of gamma-ray bursts. Although no interstellar comets have been observed, reasonable upper limits to the observed numbers and masses do not rule out the first suggestion. The gamma-ray-burst suggestion appears unlikely. A surprisingly large total mass of comets could be gravitationally bound to the sun and, of course, to other stars, but remain undetected. The nature of the 1908 Tunguska explosion is discussed briefly. It was probably not an encounter with an active comet.

Whipple, F. L.↗
NASA NTRS1975

Comets and planets - Their interrelated origin

This report gives a concise summary of the conclusions of cosmogonic studies linking the origin of comets with the origin of the Jovian planets at the dawn of the solar system. Cometary nuclei are shown to be original planetesimals ejected from and returning to the solar system. Formation processes for comets are described in detail, emphasizing the preplanetary rings of the Jovian planets, condensation and accretion from diffuse matter, and the origin of Oort's sphere. Physical and mechanical properties of comets are discussed, including the composition of cometary gases, the size and composition of cometary nuclei, the placement of comets in Oort's sphere, and the density of nuclei in that region. Unresolved difficulties in models of planet and comet formation are considered.

Opik, E. J.↗
NASA NTRS1976

Predicted favorable visibility conditions for anomalous tails of comets

Visibility conditions are used to list the comets that have displayed a sunward dust tail during the time of earth's passage through the orbital plane of the comet. A computer program describing the conditions for this type of antitail observability was applied to the Catalogue of Cometary Orbits (Marsden, 1975), starting with the comets of 1737. It is shown that only about 20-30% of the nearly parabolic comets that should have displayed an antitail at the node were actually observed to do so. There appears to be a general absence of antitails among the short period comets.

Sekanina, Z.↗
NASA NTRS1976

A Comparison of the Composition of New and Evolved Comets

The intensity ratio of the continuum to the molecular emissions was estimated in the spectra of eight-five comets. Four conclusions are drawn: (1) There is no readily apparent difference in continuum to emission intensity ratio between new and more evolved comets; (2) an intrinsic distribution of this characteristic does occur; (3) periodic comets with weak continua are derived from new comets with the same property; and (4) no weakening of the continuum in general occurs following perihelion passage. The infrared evidence for comet Encke suggests that the faintness of its continuum may be caused by a size distribution containing only particles larger than about 10 um.

Donn, B.↗
NASA NTRS1977

Opportunities for ballistic missions to Halley's comet

Alternative strategies for ballistic missions to Halley's comet in 1985-86 are described. A large scientific return would be acquired from a ballistic Halley intercept in spite of the high flyby speeds that are associated with this mission mode. The possibility of retargeting the cometary spacecraft to additional comets after the Halley intercept also exists. Two cometary spacecraft of identical design would be used to carry out four separate cometary encounters over a 3 year period. One spacecraft would intercept Halley's comet before its perihelion passage in December 1985 and then go on to comet Borrelly with an encounter in January 1988. The other spacecraft would be targeted for a postperihelion Halley intercept in March 1986 before proceeding toward an encounter with comet Tempel 2 in September 1988.

Farquhar, R. W.↗
NASA NTRS1977

On the origin of the magnetic field in type-1 comet tails - A reply to Ershkovich

Ershkovich (1976) employed the observed geometry of the type 1 tail of comet Arend-Roland 1957 III to show that any magnetic field present there cannot be of 'internal' origin but must have been 'captured' from the magnetized solar wind. Such an 'external' origin for the magnetic field in type 1 comet tails is not disputed, but several other points raised by Ershkovich are discussed. These concern the generation of substantial magnetic fields within the coma during periods when the comet's heliocentric distance is typically not greater than 1 AU, Ershkovich's use of the pressure balance across the tail 'tangential discontinuity' to estimate that the 'captured' magnetic field is of the order of the solar wind-field, his interpretation of certain observations of motions in the tail of comet Arend-Roland, and his assertion that the distant comet tail will be transformed into a turbulent wake due to the Kelvin-Helmholtz instability. It is argued that the 'captured' magnetic field should be substantially amplified relative to the solar-wind field, that the pressure-balance equation is of limited use, and that the real configuration of a type 1 tail must be treated in detail before any definite conclusions can be reached about the Kelvin-Helmholtz instability.

Mendis, D. A.↗
NASA NTRS1977

The ionospheres and plasma tails of comets

The paper reviews the current state of knowledge about cometary plasma (type I) tails and ionospheres. Observational statistics for type I tails are examined along with spectroscopic observations of plasma tails, identified ion species in such tails, and the morphology of cometary plasma tails and ionospheres. Evidence for a strong interaction between comets and the solar wind is evaluated on the basis of observations of plasma-tail orientations, large accelerations of tail structures, and correlations between disturbances in type I tails and solar-wind or geomagnetic disturbances. The use of comets as solar-wind probes is discussed, the nature of comet-solar-wind interactions is investigated, and ionization sources for cometary gases are considered. Hydrodynamic models of comet-solar-wind interaction are summarized, and the structure and ion chemistry of cometary ionospheres are studied. Observations suggesting that significant magnetic fields are associated with comets are briefly reviewed and interpreted.

Mendis, D. A.↗
NASA NTRS1977

Lyman-alpha observations of Comet West /1975n/

The rate of hydrogen production of Comet West is studied through rocket observation of solar Lyman-alpha radiation resonantly scattered by the escaping hydrogen atoms. Two sets of Lyman-alpha exposure sequences are used to obtain computer-smoothed brightness contour (isophote) maps covering a density range of 100:1. A simple radial outflow model is applied to the contour maps to determine the rate of hydrogen production (3.2 by 10 to the 30th power atoms/sec.) Discrepancies between the observed shape of the outer isophotes and predicted models may be explained by optical depth effects, or by the presence of small pieces of the comet's nucleus distributed along the orbit. Hydrogen, carbon, and oxygen production for Comet West and Comet Kohoutek are compared; differences may be accounted for by variations in the composition or evolution of the two comets.

Opal, C. B.↗
NASA NTRS1977

A comparison of the composition of new and evolved comets

The continuum to molecular emission intensity ratio in cometary spectra is examined as an indicator of initial composition of the comets. This ratio presumably measures the dust to gas ratio in comets, or the component of gas yielding visible emissions. The spectral relationships and compositions of new comets and more evolved ones are compared. No significant difference in the continuum to emission ratio was noted between new and more evolved comets, which is in contradiction to Oort's conclusion. This suggests that no systematic change occurs in the continuum to emission ratio as comets evolve from new to periodic. Perihelion passage does not seem to produce a general weakening of the continuum, although it occurs in individual cases.

Donn, B.↗
NASA NTRS1978

The exploration of comets

The structure and origin of comets are reviewed with reference to future cometary research involving fly by and rendezvous missions. Ion-driven spacecraft, to be launched from a Shuttle in the mid-1980s, will theoretically be able to make close observations of Halley's Comet, and fly on to either Tempel 2 or Borelly's Comet during the course of the same mission. A rendezvous, whereby the spacecraft follows a comet in its path around the sun, is considered preferable to a fly by, which will allow only a few hours of observation. Suggested equipment for comet research include IR sensors, X- and Gamma-ray spectrometers, and possibly SEM to analyze the composition of cometary dust particles.

Neugebauer, M.↗
NASA NTRS1977

Navigation capability for an ion drive rendezvous with Halley's Comet

An analysis has been conducted in connection with plans for a study of Halley's Comet during its 1986 apparition. The use of low-thrust vehicles, utilizing an ion drive system, is being considered for a comet rendezvous mission. A preliminary trajectory for the Halley rendezvous mission calls for launch on June 20, 1982, followed by rendezvous on December 21, 1985. The navigation analysis described focuses on the terminal approach to Halley, the 60-day period preceding rendezvous. Navigation analysis assumptions are examined, taking into account navigation error sources, radio tracking, onboard optical data, earth-based comet observations, and orbit determination and guidance strategies. The preliminary mission design considers a rendezvous at approximately 56,000 km from the comet nucleus (6,000 km outside the dust envelope). Navigation performance is measured in terms of comet-relative position and velocity errors at encounter. Variations to the baseline navigation study provide illustrations concerning the close link between delivery accuracy and stochastic thrust errors.

Thornton, C. L.↗
NASA NTRS1977

A mission design for the Halley comet rendezvous using Ion Drive

The Ion Drive propulsion system, a derivative of the old Solar Electric Propulsion (SEP) technology is considered adequate to perform all mission objectives of a proposed Halley's comet rendezvous (scheduled for launch in 1982) except one: control of thermal energy from the concentrating solar arrays. This problem can be solved, however, by adding a separable tail probe to the baseline system. The system consists of an Ion Propulsion Module (IPM) and a Mission Module (MM). Scientific objectives include a determination of the structure of the comet nucleus, an evaluation of nucleus evolution, an assay of the comet's atmosphere and ionosphere, and a study of the interaction between the comet and the interplanetary medium. Attention is given to the navigation parameters necessary for heliocentric transfer and post-rendezvous circumnavigation of the comet.

Boain, R. J.↗
NASA NTRS1979

Report of the Comet Science Working Group

General scientific questions and measurement objectives that can be addressed on a first comet mission relate to: (1) the chemical nature and the physical structure of comet nuclei as well as the changes that occur as functions of time and orbital position; (2) the chemical and physical nature of the atmospheres and ionospheres of comets, the processes which occur in them, and the development of these atmospheres and ionospheres as functions of time and orbital position; and (3) the nature of comet tails, the processes by which they are formed, and the interaction of comets with the solar wind. Capabilities of the various instruments required are discussed.

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