Search

The observed properties of non-gravitational effects in the motion of the short-period comets are described in connection with the possible mass-loss processes in their nuclei. The impulses on the nuclei have been found to be about 1 cm/sec to 3 m/sec per revolution. The contributions to the observed dynamic anomalies of gas- and dust-matter ejections from the nucleus into the comet's atmosphere are estimated. The existence of meteor showers is related to the characteristics of cometary activity. Methods of further investigations are suggested.

A study of the nongravitational deviations in the motion of P/Encke suggests that the nucleus of this comet can conceivably be approximated by a core-mantle model, composed of a porous solid core with embedded ices, surrounded by a thick envelope of free ices that gradually sublimates out.

Anomalous positional residuals from the orbit of Comet Halley, ascertained by Zadunaisky, can be explained by the existence of a spurious nucleus, deviating from the center of gravity of the comet (i.e., from its solid nucleus). It was moving behind the solid nucleus with a relative velocity of the order of a few meters per second. It can be understood as a slightly expanding cloud of micrometeoric grains of 10 −3 cm in size, of irregular shape.

Using a computerized technique of stream search, based on the statistical model of meteor streams, we have detected 72 additional streams in a sample of 19303 radio meteor orbits. The streams are found to have a tendency to cluster, partly along the ecliptic and partly in high-inclination orbits. Also noticed are specific relations among the detected streams, such as stream pairs, stream branches, and twin showers. A very probable association of a prominent stream, the a Capricornids, with the minor planet Adonis has been established, and possible associations of several streams with comets and minor planets of the Apollo and Albert types are also discussed. Identification of the detected radio streams with previously known streams is presented, and plans for future work are briefly outlined.

The effects of rotation of a cometary nucleus on the character of nongravitational parameters are discussed. It is suggested that the change of a nongravitational acceleration to deceleration (and vice versa) may be related to a precessional motion of the nucleus, or to secular variations in the lag angle coupled with severe orbital modifications.

A new model has been developed for analyzing the dynamical properties of meteor streams. It is based on the Southworth-Hawkins D-criterion of the similarity of two orbits in terms of the differences in their Keplerian elements. In contrast to the earlier stream searches, the statistical rather than the determinate concept is adopted for the definition of a stream. In particular, the proposed method emphasized the significance of the character of the D-distribution of meteors with respect to the mean orbit of a meteor stream. The mean orbital elements are computed as a weighted mean of the elements of individual meteors with D smaller than a certain limit. This process is iterated until the mean orbit converges. The calculations were performed on a high speed computer by means of a program that solved the iteration procedure and listed the meteors according to increasing D, thus giving the D-distribution to be studied. To test the method, empirical D-distributions were obtained for some major streams from the Harvard- Smithsonian Radio Meteor Project survey of more than 19 000 orbits. We have assumed the empirical D-distribution for each stream to consist of an intrinsic Maxwellian D-distribution of the stream, contaminated by a sporadic-background D-distribution. Eliminating the latter by analyzing artificial samples of meteors with randomly distributed orbits, we have developed a graphical method for determining the intrinsic dispersion, σ, in the stream's D-distribution. Model calculations suggest that as a result of planetary perturbations, σ increases secularly. There is some evidence that σ could serve to estimate the mean age of streams, but more studies on this subject are necessary.

The model for dust comets, formulated by Finson and Probstein, which had previously been tested only on Comet Arend-Roland 1957 III, has been successfully applied to three calibrated photographic plates of Comet Bennett. The size distribution, emission rate, and initial velocities of dust particles emitted from the comet's nucleus are given.

Orbital elements and nongravitational parameters are derived from observations at every apparition of the periodic comets Honda-Mrkos-Pajdusakova, Faye, Tempel 2, Biela, Brorsen, and Tempel-Swift. For all except the first comet, the observations go back a century and more, although the last three comets have failed to reappear for some considerable time. The circumstances of the splitting of P/Biela are studied, and it is shown that the motion of the primary component was scarcely affected; it is also demonstrated that, if the primary still exists, it may pass only 0.05 AU from the earth in November 1971. An up-to-date list of mass-loss rates from comets is presented. It is found that, while most of the reliable determinations indicate that the cometary nongravitational effects decrease with time, there are a few cases where the effects increase slightly. The former situation is discussed in terms of a nuclear core-mantle model, implying that these comets will eventually evolve into inert, asteroidal objects, while the nuclei of the other comets are interpreted as coreless, eventually to disappear completely (or almost completely).