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Whipple, F. L.

Publications and source records attributed to Whipple, F. L..

At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1979

Scientific need for a cometary mission

Known facts about comets are reviewed including their organic and inorganic content. Photographs are used to show the differences in the physical appearances of the three types of comets. Space missions will provide the opportunity to determine the sequence of events that led to their formation and that of the solar system; how volatiles arrived on earth; and the basis for the existence of life on earth; and the source of the outer planetary system.

Whipple, F. L.↗
NASA NTRS1978

On the nature and origin of comets and their contribution to planets

The rotation of cometary nuclei and certain properties of cometary ices are discussed. Observations show that comets were formed at extremely low temperatures and probably contain amorphous ices that give off exothermal energy on mild heating. While a large comet has a slow rotation period, many smaller typical comets appear to be rotating rapidly, indicating that encounters among them were frequent during formation. An estimate for the product of the relaxation time for encounters and the mean space density near the end of comet formation is presented, and a time scale of approximately 1,000,000 yr for comet accumulation is suggested. Some probable and possible contribution of comets to the solar system are summarized.

Whipple, F. L.↗
NASA NTRS1978

Rotation period of comet Donati

Consideration is given to the rotation period of comet Donati (1858 VI) whose haloes were approximate parabolic envelopes having foci near the apparent nucleus and vertices toward the sun forward from the tail axis. The regularity and sharpness of the halves suggest that they represent the repetitive ejection of material from an active area which is exposed to solar radiation as the cometary nucleus rotates. Bobrovnikov's results (1954) are used to evaluate the linear expansion velocity of such haloes. This calculation is applied to the comet and a linear correction is used to assess the results.

Whipple, F. L.↗
NASA NTRS1978

Cometary brightness variation and nucleus structure

The Bobrovnikoff and Beyer photometric data for more than 100 comets analyzed for intrinsic brightness variations, before and after perihelion, according to some inverse power law of solar distance. The Oort and Schmidt classification of comet age is extended and applied with Marsden's new determinations of inverse semimajor axis. A physical interpretation of the data involves the quick loss of a frosting of supervolatile materials from new comets, and for all comets, the development of an insulating crust after perihelion. The orientation of the axes of rotating comets is shown to be an important factor in cometary brightness variations. A speculation is made concerning the axis of rotation of Comet Kohoutek.

Whipple, F. L.↗
NASA NTRS1977

The reality of comet groups and pairs

Although the common genetic origin of the Kreutz family of sun-grazing comets has generally been accepted, there remains uncertainty with regard to genetic identity among other groups of comets whose orbital elements are nearly alike. Porter (1952) has listed a number of such groups, and Opik (1971) has made a statistical study of the orbits of 472 comets with aphelion distances beyond Saturn. Opik lists 97 groups that show similarities among their three angular elements, calculates an overall probability of some 10 to the -39th power that these similarities could have occurred by chance, and thus concludes that 60% or more of such comets fall into genetic groups containing from two to seven members. This paper explores the statistical reality of Opik's groups utilizing the Monte Carlo method of statistics as well as ordinary probability theory. The conclusion is reached that except for a few pairs, the similarity among orbital elements within the groups is no greater than random expectation.

Whipple, F. L.↗
NASA NTRS1977

National Geodetic Satellite Program, Part II: Smithsonian Astrophysical Observatory

A sequence of advances in the determination of geodetic parameters presented by the Smithsonian Astrophysical Observatory are described. A Baker-Nunn photographic system was used in addition to a ruby-laser ranging system to obtain data for refinement of geodetic parameters. A summary of the data employed to: (1) derive coordinates for the locations of various tracking stations; and (2) determine the gravitational potential of the earth, is presented.

Arnold, D. A.↗
NASA NTRS1977

Comets: Data, problems, and objectives

A highly abridged review of new relevant results from the observations of Comet Kohoutek is followed by an outline summary of our basic knowledge concerning comets, both subjects being confined to data related to the nature and origin of comets rather than the phenomena (for example, plasma phenomena are omitted). The discussion then centers on two likely places of cometary origin in the developing solar system, the proto-Uranus-Neptune region versus the much more distant fragmented interstellar cloud region, now frequented by comets of the Opik-Oort cloud. The Comet Kohoutek results add new insights, particularly with regard to the parent molecules and the nature of meteoric solids in comets, to restrict the range of the physical circumstances of comet formation. A few fundamental and outstanding questions are asked, and a plea made for unmanned missions to comets and asteroids in order to provide definitive answers as to the nature and origin of comets, asteroids, and the solar system generally.

Whipple, F. L.↗
NASA NTRS1977

The constitution of cometary nuclei

The nongravitational term in the expression for the total force acting on a comet is calculated, and an upper limit is obtained for the product of the radial nongravitational term times the radius times the square root of the albedo. This condition is satisfied for ten periodic comets with q no greater than 1.5 AU, and the activity of these comets is consistent with control by H2O ice. Some of the comets must be spotty to account for their low albedo values. The effect of cosmic rays on comets, leading to frosting of their surface, is discussed.

Whipple, F. L.↗
NASA NTRS1976

The study of the physics of cometary nuclei

A semiannual progress report describing the work completed during the period 1 September 1975 to 29 February 1976 on the physics of cometary nuclei was given. The following items were discussed: (1) a paper entitled ""A speculation about comets and the earth'', (2) a chapter entitled"" The physics of comets'' for ""Reviews of Astronomy and Astrophysics'', (3) continuing work on split comets, and (4) results dealing with a new application of nongravitational solar-radial forces as a measure of comet nucleus dimensions and activity.

Whipple, F. L.↗
NASA NTRS1976

A speculation about comets and the earth

Assuming that some 100 earth masses of excess icy conglomerate material accumulated in the Uranus-Neptune region, it is suggested that dispersal by perturbations could have transferred a large fraction of that material into Jupiter-crossing orbits, with a time constant for dissipation of the order of 10 to 100 million years. The possible formation of a 'cometary nebula' within Jupiter's orbit is considered along with the accumulation of a significant amount of cometary material on earth. Calculations are presented which show that the cometary flux within Jupiter may have attained a million times its present value for a period of 10 to 100 million years, that the current flux on earth adds only about one ten-millionth to one hundred-millionth of an earth mass in 4.6 billion years, and that the accretion rate scales only with the local density of the cometary material being collisionally destroyed. The possibility is discussed that the solar wind was unable to cope with the large cometary flux within Jupiter, resulting in a 'choke up' of cometary debris in the inner solar system and providing the cometary nebula with a density many orders of magnitude higher than the present value.

Whipple, F. L.↗
NASA NTRS1976

Physical processes in comets

The paper discusses physical processes in comets which involve solar and nuclear radial forces that affect the motions of gases and icy grains, gas-phase chemistry very close to the nuclei of large comets near the sun, sublimation of icy grains, dissociation of parent molecules into radicals and of radicals into atoms, and ionization by sunlight and collisions. The composition and dimensions of nuclei are examined along with variations in intrinsic brightness, the nature of volatiles, gas production rates in the coma, characteristics of icy grains in the coma, and the structure of streamers, ion tails, and dust tails. The structure of the coma is described in detail on the basis of spectroscopic observations of several comets. The origin of comets is briefly reviewed together with the relation of comets to earth, the interplanetary complex, and the interstellar medium. Desirable future observations are noted, especially by space missions to comets.

Whipple, F. L.↗
NASA NTRS1976

The status of cometary science

The problem of determining the chemical composition and physical structure of comet nuclei are discussed in light of recent analytical improvements. The development of radio and infrared techniques, improved sensing equipment, and the ability to conduct observations in space using satellites or skylabs are highlighted. The studies of two major comets, Kohoutek and West 1975n, are presented as examples of the utilization of these improved analytic tools.

Whipple, F. L.↗
NASA NTRS1975

The study of the physics of cometary nuclei

Research in the area of split and hyperbolic comets, orbital calculations, and interstellar comets is discussed. Other topics discussed include the role of comets in galactic chemistry, gamma ray bursts, and predicted favorable visibility conditions for anomalous tails of comets.

Whipple, F. L.↗
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 NTRS1974

Comets: Data, problems and objectives

Relevant results from the observations of Comet Kohoutek are presented with an outline summary of basic knowledge concerning comets, both subjects being confined to data related to the nature and origin of comets rather than the phenomena (for example, plasma phenomena are omitted). Places of cometary origin in the developing solar system are discussed, along with the proto-Uranus-Neptune region versus the much more distant fragmented interstellar cloud region, now frequented by comets of the Opik-Oort cloud. The Comet Kohoutek results add new insights, particularly with regard to the parent molecules and the nature of meteoric solids in comets, to restrict the range of the physical circumstances of comet formation.

Whipple, F. L.↗
NASA NTRS1973

Radial pressure in the solar nebula as affecting the motions of planetesimals

Growing planetesimals and a range of drag laws depending on the Reynolds number and on the ratio of particle size to mean free path are considered. Particles spiral in the direction of positive gradient, thus being concentrated toward toroidal concentrations of gas. The effect increases with decreasing rates of particle growth, i.e., with increasing time scales of planet formation by accretion. In the outer regions, where evidence suggests that comets were formed and Uranus and Neptune were so accumulated, the effect of the pressure gradient is to clear the forming comets from those regions. The large mass of Neptune may have developed because of this effect, perhaps Neptune's solar distance was reduced from Bode's law, and perhaps no comet belt exists beyond Neptune. In the asteroid belt, on a slow time scale, the effect may have spiraled planetesimals toward Mars and Jupiter, thus contributing to the lack of planet formation in this region.

Whipple, F. L.↗
NASA NTRS1972

Cometary Nuclei: Models

Arguments for the existence of some kind of icy-conglomerate cometary nucleus are put forward, and the role of clathrates in the condensation of comets from the solar nebula is discussed. The division of cosmic materials into three types is described, and it is suggested that whereas Jupiter, and to a large extent Saturn, condensed directly from gas and the terrestrial planets and asteroids collected from planetesimals of earthy material, the comets were formed as snow balls in the vicinity of Uranus and Neptune, with these two planets themselves representing accumulations of comets. The evidence for the existence of a comet belt beyond Neptune is considered, and it is concluded that at 50 AU from the sun the mass of such a belt cannot be more than that of the Earth. Finally, some of the problems associated with the mission to periodic Comet d'Arrest are discussed, in particular those concerned with the detectability of the nucleus and the hazards of a close encounter.

Whipple, F. L.↗