Search

At least 16 fragments were detected in images of comet C/1999 S4 (LINEAR) taken on 5 August 2000 with the Hubble Space Telescope (HST) and on 6 August with the Very Large Telescope (VLT). Photometric analysis of the fragments indicates that the largest ones have effective spherical diameters of about 100 meters, which implies that the total mass in the observed fragments was about 2 × 10 9 kilograms. The comet's dust tail, which was the most prominent optical feature in August, was produced during a major fragmentation event, whose activity peaked on UT 22.8 ± 0.2 July 2000. The mass of small particles (diameters less than about 230 micrometers) in the tail was about 4 × 10 8 kilograms, which is comparable to the mass contained in a large fragment and to the total mass lost from water sublimation after 21 July 2000 (about 3 × 10 8 kilograms). HST spectroscopic observations during 5 and 6 July 2000 demonstrate that the nucleus contained little carbon monoxide ice (ratio of carbon monoxide to water is less than or equal to 0.4%), which suggests that this volatile species did not play a role in the fragmentation of C/1999 S4 (LINEAR).

The progress in the understanding of the Tunguska object is reviewed in the light of evidence presented in numerous recent investigations, which appeared following the publication of my 1983 paper on the object's proposed asteroidal nature. The issues addressed extensively in the present review involve: (i) the fundamental characteristics of the event, such as the object's energy, altitude and velocity at the time of its terminal explosion and the dynamic pressure involved; (ii) the problem of atmospheric fragmentation of very massive impactors and the implications for their ablation and deceleration; (iii) new analysis of the orientation of the Tunguska object's heliocentric orbit based on the best data on the apparent radiant of the fireball's atmospheric trajectory; and (iv) comparison with the findings of other recent investigations, including compositional studies. Also employed in the arguments are the results now available on the impacts of comet Shoemaker-Levy 9's fragments into Jupiter and the findings of a recent comparative study of two huge fireballs (one cometary, one stony, both involving impactors several meters across) observed with the cameras of the European Network of fireball monitoring. It is concluded that hypotheses based on presumed cometary origin of the Tunguska object encounter unsurmountable difficulties and that the interpretation of the event as a fall of a small stony or carbonaceous asteroid is not only plausible, but virtually certain.

Comet P/Faye 1991 XXI (1991 n) was observed with the planetary camera (PC) of the Hubble Space Telescope (HST) between October 29 and November 21, 1991, when its geocentric distance was in the range 0.62–0.67 AU. The resulting high resolution—a single PC pixel projected to a distance of ∼20 km at the comet—made it possible to clearly discriminate the nucleus and to study the dust coma within ∼100 km from the nucleus. The spherical aberration which affected the early operation of the HST has severely complicated the analysis and image restoration using the Richardson–Lucy method has failed to give satisfactory results. The coma is dominated by the point spread function (PSF) of the nucleus up to a radial distance of ∼100 km and cannot be recovered. Beyond, it is affected by the spherical aberration up to approximately 750 km and was analyzed by comparison with a grid of models convolved with appropriate PSFs. Beyond 750 km, it remains unaffected and can be studied directly. From the outer to the innermost regions, the coma presents an elongated shape which may be explained either by an active source on the nucleus or, more likely, by a projection effect of the dust tail. If this second interpretation is correct, then the temporal evolution of the comet is very slow and smooth, suggesting an extended source of dust on the nucleus. The dust production rate, corrected for the projection effect of the tail, amounts to 125 kg sec −1 .

STARDUST, a Discovery-class mission, will return intact samples of cometary dust and volatiles from comet P/Wild 2, as well as samples of the interstellar dust moving through the solar system. Dust capture utilizes aerogel, a microporous silica that is capable of intact capture of hypervelocity particles. A navigation camera, an in situ dust analyzer, and a dust flux monitor complete the payload. The Wild 2 flyby takes place in January 2004, with Earth return in January 2006.

Fragmentation dominates the ablation process of cometary type impactors during their atmospheric flight. Models that account for it imply that a kilometer-sized nuclear fragment of comet Shoemaker-Levy 9 has disintegrated almost completely by the time of its terminal explosion, at a pressure level of 14 and 5×1015grams apiece.

The Hubble Space Telescope made systematic observations of the split comet P/Shoemaker-Levy 9 (SL9) (P designates a periodic comet) starting in July 1993 and continuing through mid-July 1994 when the fragments plunged into Jupiter's atmosphere. Deconvolutions of Wide Field Planetary Camera images indicate that the diameters of some fragments may have been as large as approximately 2 to 4 kilometers, assuming a geometric albedo of 4 percent, but significantly smaller values (that is,1 kilometer) cannot be ruled out. Most of the fragments (or nuclei) were embedded in circularly symmetric inner comae from July 1993 until late June 1994, implying that there was continuous, but weak, cometary activity. At least a few nuclei fragmented into separate, condensed objects well after the breakup of the SL9 parent body, which argues against the hypothesis that the SL9 fragments were swarms of debris with no dominant, central bodies. Spectroscopic observations taken on 14 July 1994 showed an outburst in magnesium ion emission that was followed closely by a threefold increase in continuum emission, which may have been caused by the electrostatic charging and subsequent explosion of dust as the comet passed from interplanetary space into the jovian magnetosphere. No OH emission was detected, but the derived upper limit on the H2O production rate of approximately 10(27) molecules per second does not necessarily imply that the object was water-poor.

The Hubble Space Telescope observed the fragmented comet P/Shoemaker-Levy 9 (1993e) (P indicates that it is a periodic comet) on 1 July 1993. Approximately 20 individual nuclei and their comae were observed in images taken with the Planetary Camera. After subtraction of the comae light, the 11 brightest nuclei have magnitudes between approximately 23.7 and 24.8. Assuming that the geometric albedo is 0.04, these magnitudes imply that the nuclear diameters are in the range approximately 2.5 to 4.3 kilometers. If the density of each nucleus is 1 gram per cubic centimeter, the total energy deposited by the impact of these 11 nuclei into Jupiter's atmosphere next July will be approximately 4 x 10(30) ergs ( approximately 10(8) megatons of TNT). This latter number should be regarded as an upper limit because the nuclear magnitudes probably contain a small residual coma contribution. The Faint Object Spectrograph was used to search for fluorescence from OH, which is usually an excellent indicator of cometary activity. No OH emission was detected, and this can be translated into an upper limit on the water production rate of approximately 2 x 10(27) molecules per second.

The aim of the presented work was to assess the causes of injury to great nerves during varicose vein surgery and comment on the consequences.This was a retrospective study of 2344 patients operated on for primary varicose veins between the years 1980 and 2005.In three patients out of 2344 the peroneal nerve was injured. The three patients underwent neurosurgery. In the first patient transplantation of the sacral nerve was performed. In the second patient the nerve was released from ligatures, and in the third patient the nerve was first released from the cicatrice and the transposition of the tendon of the posterior tibial muscle followed. All three patients went through intensive rehabilitation. The first patient still wears peroneal splint, the limb is atrophic. In the second patient the function has been well restored and he is not disabled anymore. However, the restitution of the lower limb function is not sufficient for him to work as a teacher of physical education. The third patient still suffers from serious paresis of the peroneal nerve.Even a frequent and relatively simple intervention such as varicose vein surgery may be accompanied by serious complications affecting patients for the rest of their lives. Serious motor nerve injuries are encountered only in operations in the popliteal fossa and the fibula head. Complications are more frequent when operations are performed by young general surgeons than when they are performed by an experienced surgeon or an expert in vascular surgery. The affected patients should be referred for neurosurgery; however, results are unpredictable. In spite of an intensive rehabilitation and possible plastic surgery the patients are permanently affected.

The incidence of vascular replacement infection is reported between 0.5-6.0%. The infection of aortal valve replacement is usually managed by its removal and axilofemoral reconstruction. Replacement removal and reconstruction in situ is another alternative. The aim of our work is to give the comparison of the results obtained with the extra-anatomic reconstruction and alternative techniques.Between January 1987 and August 2008 1324 abdominal aortal replacements were performed in our department. In 14 patients (1.05%) the infection of the replacement occurred; one of the patients was operated on in another hospital. In eight patients axilobifemoral/axilopoplietal by-pass was applied (Group A). In two patients the whole replacement was removed together with in situ reconstruction (ISR) using a new bifurcation replacement. In two patients a partial resection of the replacement and the resected part substitution or femoro-femoral cross-over by-pass was performed. Three patients were treated with a total or partial replacement removal without the following reconstruction.In Group A no haemorrhage from the aortal stub occurred. The mean value of surgical management due to the replacement infection was 3.5 (1-6). For Group B the number was 5 (1-16). In Group B there were three reinfections, in Group A none. In both Group A and B one patient died; one thigh amputation was necessary in Group A, two in Group B.The partial or total removal of the replacement followed with in situ reconstruction is considered a safe and good method solving the infection of the aortal replacement in some patients. The results are comparable to those obtained with extra-anatomic reconstruction that remains the method of choice in patients with virulent infections.

Major progress has recently been achieved in the understanding of the coma morphology of dust comets. The conceptual models for the various types of feature observed in the coma (jets, spirals, halos, fans, etc.), their computer simulation, and the hydrodynamic models for jet formation are critically reviewed, and evidence for anisotropic, strongly collimated flows of ejecta emanating from discrete active regions (vents) on the rotating cometary nuclei is presented. The techniques employed to generate synthetic comet images that simulate the observed features are described and their relevance to the primary objectives of coma-morphology studies is exemplified. Modelling of temporal variations in the water emission from discrete active regions suggests that production curves asymmetric with respect to perihelion should be commonplace. Critical comparisons are offered with the activity profiles of Encke’s comet and with light curves of disappearing comets and comets that undergo outbursts. Discussed next is the potential evolutionary significance of nuclear shape, the fractional area of the surface that is active, the vents’ erosion rate, and available information on their activation, aging, dormancy, migration, rejuvenation, and extinction. Recent developments in the understanding of the processes that cause the nongravitational perturbations of cometary motions are reviewed and the observed discontinuities are identified with birth of new sources and/or deactivation of old vents.

[1] The Cometary and Interstellar Dust Analyzer (CIDA) instrument analyzes the composition of individual grains in the cometary coma. As each particle impacts a silver plate, the high-impact energy due to the relative velocity of the spacecraft as it flies through the coma causes the elements and molecular compounds in the particle to become ionized. Using a fast time-of-flight mass spectrometer, a complete set of ions are detected for each impact, from a mass range of 1 (atomic hydrogen) up to a few thousand atomic mass units, encompassing all elements in the periodic table and many molecules, such as organic compounds. This experimental technique has already been applied with excellent success at Halley's comet, and the CIDA derivative instrument is flying on the Stardust mission, which will encounter comet Wild 2 in January of 2004. The data returned will give clues to the elemental and chemical composition of the dust component of this comet.

The authors present their experience in late infection of aortoilliacofemoral reconstruction by means of the Dacron vascular prosthesis. They found such complication in 8 of 620 (1.5%) reconstructions in that region in period 1988-1998. There is essential position of scintigraphy (Le 99mTc-HMPAO) and CT in diagnostic of location and extent of an affection. Conservative approach and local surgery were not permanently successful. The alternative approach is radical removing of the whole graft. The alternative of aortofemoral reconstruction is axillobifemoral bypass using ePTFE graft in case of an infection of the bifurcation graft.

The authors give an account of their experience with establishment of an arteriovenous fistula for dialysis using interposition with a DIASTAT-GORE prosthesis between the axillary artery and the contralateral axillary or inner jugular vein and its localization on the anterior thoracic wall. The method is apt in patients with a primarily unsuitable or secondarily surgically exhausted autologous circulation of the upper extremities.

The authors use, when establishing secondary arteriovenous anastomoses for haemodialysis, the blood vessels of the cubital fossa. If it is possible from the anatomical and functional aspect, they connect the superficial venous vessels to the arterial ones by means of a perforating vein. If the blood vessels of the forearm are of poor quality, they use this procedure as the primary approach. After two years the group of 38 established fistulas comprises 87% which can be completely used and none of them had to be modified.

In this information we wanted to call your attention to the newer method of treatment of chronic renal insufficiency--CAPD. The process of execution, the advantages and disadvantages by using this method as well as our experience during last two years are described. The number of cured patients is 22 persons with very good results. The longest time of treatment has been 22 month, the complications by 4 persons only, that is 18 per cent. For the patient the method is relatively unpretentious, safe, effective and from the economic point of view advantageous. It is possible to recommend this method for larger use.

Outgassing and dust emission from discrete regions on the nuclei of comets are investigated with regard to the processes of activation, dormancy, reactivation, and extinction. With regard to P/Halley, P/Encke, P/Tempel 2, P/Machholz, P/Takamizawa, Bowell and some other new comets, the evolution of one active region of the nucleus surface appears to be independent of the evolution of another active region. The traditional concept of deactivation as a slow and monolithic process needs to be replaced with a more dynamic concept of intermittent periods of dormancy and reactivation of individual vents, varying in size. Life spans of discrete sources of activity are estimated to be at a few hundred revolutions about the sun for comets with perihelia at heliocentric distances of less than 2 AU, if the bulk density is very low.

Motion of 16 striae through the dust tail of Comet West 1976 VI are observed over a time interval of more than three days. Initiation times for most of the 16 striae are found to coincide with the times of discrete violent bursts of dust, determined from the motions of streamers. It is also found that fragments are subjected to repulsive accelerations between 0.6 and 2.7 the solar attraction, indicating submicron-size absorbing particles, while repulsive accelerations imparted to parent articles are slightly lower than the average acceleration of fragments. The sizes of the fragments are estimated to be between one tenth and a few tenths of a micron. No effect of the Lorentz force on striae motion is detected. It is concluded that the theoretical assumption that striae are products of fragmentation of friable dust particles ejected from the nucleus agrees with observations.

The density and the mass spectrum of the dust near comet Halley have been measured by the Giotto space probe's dust impact detection system. The dust spectrum obtained at 291,000 km from the comet nucleus show depletion in small and intermediate masses; at about 600 km from the nucleus, however, the dust activity rises and the spectrum is dominated by larger masses. Most of the mass striking Giotto is noted to reside in the few large particles penetrating the dust shield. Momentum balances and energy considerations applied to an observed deceleration suggest that a large mass of the spacecraft was detached by an impact.

The new model, which has recently been proposed for the motions of fragments of the split comets, is critically examined through a comparison with the traditional approach. It is concluded that the new model, based on the premise that the rate of separation of any two fragments of a split comet is determined primarily by the differential nongravitational forces in their motions, is preferable in many a respect to the traditional hypothesis, built on the assumption that significant impulses are exerted on the fragments at the time of splitting. The new model appears to have interesting implications for the physics of cometary splitting, suggesting that the breakup mechanism may essentially be non-violent or of a low degree of violence, and for estimates of cometary masses.

properties of the surface layer of the nucleus; (b) bulk, especially rotational, characteristics Qfthe nucleus. This review is concerned with a recent breakthrough in the investigation of polar-axis orientations and spin rates, and with the implications for the surface struc­ ture of cometary nuclei. The rotation data for twelve comets are summa­ rized in Table 1. The progress achieved in the past few years has demonstrated that seem­ ingly complex relations among observed physical and dynamical proper­ ties of comets can consistently be interpreted in terms of Whipple's (1950, 1951) model of a rotating icy-conglomerate nucleus. The nucleus, prob­ ably a few kilometers in diameter for an average comet, is believed to lose mass more or less continuously, at higher rates near the sun, slowly far­ ther away. The lost mass consists of volatiles and nonvolatiles. The vola­ tile fraction is the ices or frozen gases that sublimate from the surface (or from beneath the surface) of the nucleus in response to the heating by sun­ light. The outgassing is accompanied by a momentum transferred to the nucleus in the direction opposite that of the escaping gas. The nonvolatile fraction is the dust, embedded in, or otherwise physically connected with, the ices in the nucleus and released and accelerated away from the nucleus due to a force exerted by momentum transfer from the expanding gas.

The present investigation has the objective to provide a summary of the existing evidence on the disappearance of comets and to draw conclusions regarding the physical processes involved in the disappearance. Information concerning the classification of evidence and the causes of apparent disappearance of comets is presented in a table. Attention is given to the dissipating comets, the headless sungrazing comet 1887 I, and the physical behavior of the dissipating comets and the related phenomena. It is found that all comets confined to the planetary region of the solar system decay on astronomically short time scales. However, only some of them appear to perish catastrophically. Some of the observed phenomena could be successfully interpreted. But little insight has been obtained into the character of the processes which the dissipating comets experience.

In this review attention is focused on those areas of the dynamical investigation of cometary dust in which significant progress has been achieved since the previous review papers (Sekanina 1976a, 1977). Much of the progress stems from work based on the model of dust comets formulated by Finson and Probstein (1968a). Their introduction of a new technique for dust-tail studies has made it possible for the first time to gain insight into such properties of cometary dust as the size distribution function of particles shortly after emission from the comet nucleus, the distribution of particle ejection velocities, and the production of dust versus time. Since the Finson-Probstein approach is of a combined dynamical/photometric type, information on particle sizes and masses is provided indirectly, through parametric functions determined from the observed distribution of light intensity in the tail. The basic limitation is that particle radius a cannot be separated from particle density ρ, as their product is related to a directly observed quantity β, the acceleration exerted on the particle by solar radiation pressure. Expressed in units of solar attraction, β for a spherical particle is given by where c0 = 0.585 × 10−4 g/cm2 and Qpr is the integrated scattering efficiency of the particle for radiation pressure, which varies significantly with a for particles whose dimensions are smaller than the effective wavelength of solar radiation.

The last three years have witnessed a growing interest in the physical properties of the small bodies in the solar system. Perhaps the most significant impetus to research on small bodies has been the imminent arrival of Comet Halley in the inner solar system. This famous comet, which was recovered in autumn 1982, has been the object of intense study during the past year as it has approached the sun and developed a tail. Much of the international, ground-based astronomical research on Halley has been coordinated through the International Halley Watch program. Spacecraft from several nations have been successfully launched (or soon will be, we hope) and are on their way to intercept the comet and make close-up observations and in situ measurements. The commencement of spacecraft study of small bodies marks a new era in comet/asteroid science and, in coordination with ground-based and Earth-orbital observations, will result in unprecedented new knowledge about the origin of the solar system and about solar system processes. Although Halley is receiving the most attention, interest is also high in Comet Giacobini-Zinner, the vicinity of which will be probed by a diverted American spacecraft in September 1985. Upcoming spacecraft studies of comets through 1986 are described at the end of the comet section of this report. Asteroid exploration by spacecraft is also anticipated to begin in the near future. The trajectory of the NASA Gailieo Mission to Jupiter has been changed to permit close-encounter observations of the large main-belt asteroid 29 Amphitrite in December 1986; these observations will be conducted on a “best effort” basis only a few months after launch of Galileo. Interest is also high in Europe, the Soviet Union, and the United States in possible spacecraft missions to additional comets and asteroids during the 1990’s. If these efforts are pursued, there will be a concomitant ground-based effort. The last three years have also witnessed extremely productive efforts to observe small bodies from Earth orbit. For example, the Infrared Astronomical Satellite discovered a comet in 1983, which made the closest approach to the Earth of any comet in centuries. IRAS made important discoveries and measurements of other comets and also assembled an immense database on infrared brightnesses of thousands of numbered and unnumbered asteroids. The planned launch of the Hubble Space Telescope next year highlights the continuing potential for applying extremely powerful instrumental techniques to the study of comets and asteroids from above the Earth’s atmosphere.

The Finson-Probstein (1968a) method of analysis of the distribution of light intensity in dust tails has so far been applied to only a handful of comets. Yet, the results already suggest a striking diversity in the properties of the particle-size related distribution function f(β), one of three parametric functions determined. Here β is the acceleration exerted on the particle by solar radiation pressure, measured in units of solar attraction. For a spherical particle β is a function of its radius a, density ρ, and the integrated efficiency factor for radiation pressure Qpr (i.e., the ratio of the particle's effective cross-section for radiation pressure to its geometric cross-section): where c0 = 0.585 × 10−4 g/cm2.

The well-known associations of meteor streams with periodic comets and the probable cometary origin of the zodiacal dust cloud point to the importance, in the cometarg debris, of particles with masses exceeding roughly 10-6 gram. It is shown that these large particles dominate in the sunward-oriented anomalous tails of comets. Their study is essential for meaningful estimates of the mass of meteor streams and of the injection rate of the cometary debris that contributes to the zodiacal cloud. Favorable conditions for the detection of anomalous tails can be recognized in advance, as demonstrated by the successful predictions for comets Kohoutek (1973 XII) and Bradfield (1975p). To answer the question as to whether short-period comets can support the zodiacal cloud, a study of anomalous tails at future returns of these comets is considered indispensable.

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.

Computer enhanced high resolution groundbased images taken during Halley’s most active phases in 1985–86 exhibit all of the coma structures seen in 1910 photographs. Improved detector technology compensated for the poor observing circumstances of the current apparition yielding a wealth of complex coma structure. This structure resulted mostly from discrete dust jet activity producing spirals, arcs, and jagged-edged envelopes. Several episodes of the “multiple nucleus” and antisunward jet phenomenon were also observed. The variation in dust jet activity during the Vega and Giotto flybys is easily seen on the groundbased images. Our images of jets in the 388 nm CN (0,0) band show that they usually do not correlate very well with visible dust jets. As in 1910, the observed expansion velocities and jet curvature indicate a rotation period near 2.2 days. Because the short period dominates the shape of the jets, detailed jet modeling will be required to detect longer term rotation components such as the proposed 7.4 day period.

Analysis of the data from Giotto’s Dust Impact Detection System experiment (DIDSY) is presented. These data represent measurement of the size of dust grains incident on the Giotto dust shield along its trajectory through the coma of comet P/Halley on 1986 March 13/14. First detection occurred at some 287000 km distance from the nucleus on the inbound leg; the majority of the DIDSY subsystems remained operational after closest approach (604 km) yielding the last detection at about 202000 km from the nucleus.

Analysis of the data from Giotto's Dust Impact Detection System experiment (DIDSY) is presented. These data represent measurement of the size of dust grains incident on the Giotto dust shield along its trajectory through the coma of comet P/Halley on 1986 March 13/14. First detection occurred at some 287000 km distance from the nucleus on the inbound leg; the majority of the DIDSY subsystems remained operational after closest approach (604 km) yielding the last detection at about 202000 km from the nucleus. In order to improve the data coverage (and especially for the smallest grains, to approximately 10(-19) kg particle mass), data from the PIA instrument has been combined with DIDSY data. Flux profiles are presented for the various mass channels showing, to a first approximation, a 1/R2 flux dependence, where R is the distance of the detection point from the cometary nucleus, although significant differences are noted. Deviations from this dependence are observed, particularly close to the nucleus. From the flux profiles, mass and geometrical area distributions for the dust grains are derived for the trajectory through the coma. Groundbased CCD imaging of the dust continuum in the inner coma at the time of encounter is also used to derive the area of grains intercepted by Giotto. The results are consistent with the area functions derived by Giotto data and the low albedo of the grains deduced from infrared emission. For the close encounter period (-5 min to +5 min), the cumulative mass distribution function has been investigated, initially in 20 second periods; there is strong evidence from the data for a steepening of the index of the mass distribution for masses greater than 10(-13) kg during passage through dust jets which is not within the error limits of statistical uncertainty. The fluences for dust grains along the entire trajectory is calculated; it is found that extrapolation of the spectrum determined at intermediate masses (cumulative mass index alpha = 0.85) is not able to account for the spacecraft deceleration as observed by the Giotto Radio Science Experiment and by ESOC tracking operations. Data at large masses (10(-8) kg) recently analysed from the DIDSY data set show clear evidence of a decrease in the mass distribution index at these masses within the coma, and it is shown that such a value of the mass index can provide sufficient mass for consistency with the observed deceleration. The total particulate mass output from the nucleus of comet P/Halley at the time of encounter would be dependent on the maximum mass emitted if this change in slope observed in the coma were also applicable to the emission from the nucleus; this matter is discussed in the text. The flux time profiles have been converted through a simple approach to modeling of the particle trajectories to yield an indication of nucleus surface activity. There is indication of an enhancement in flux at t approximately -29 s corresponding to crossing of the dawn terminator, but the flux detected prior to crossing of the dawn terminator is shown to be higher than predicted by simple modelling. Further enhancements corresponding to jet activity are detected around +190 s and +270 s.

A general model is proposed which accounts for the dynamical evolution of the observed jets, envelopes and tail bands of periodic Comet Swift-Tuttle, the parent comet of the Perseid meteor stream, in terms of dust ejection from discrete active regions on the rotating nucleus. High-resolution drawings and measurements of cometary jets made upon the comet's first appearance in 1862 are used to infer a nuclear rotation period of 2.77 days, obliquity of 80 deg, and spin axis orientation with respect to the solar direction of 60 deg. The observed jets are attributed to eight discrete active regions covering not more than 1% of the cometary surface and producing bursts of duration of about 0.1 day. Calculations show that waning dust jets develop into envelopes and that old envelopes in turn become the observed tail bands. No evidence of truly violent explosions is found, and effects of active region outgassing on cometary orbital motion are negligible. Potential applications of the model to periodic Comet Halley and other comets are noted.

The nuclear properties of the periodic comet Comas Sola are studied based on a precession model applied previously to the periodic comets Encke, Kopff, and Giacobini-Zinner. The results imply that, for a few revolutions about the sun, Comas Sola was precessing more rapidly than any comet studied to date. An explanation is offered for this behavior in terms of a perturbation in the comet's obliquity shortly after the 1952 passage through perihelion. The equatorial radius of the nucleus is close to 1 km and its rotation period is 1.5-2.3 days, according to the model results. The calculated shape of the nucleus is compared with those of other comets studied using this technique.

A hypothetical continuous ejection of dust from discrete emission sources on the sunlit hemisphere of a rotating comet nucleus is tested against 34 digitally processed plates of Comet Halley over the May 5-June 6, 1910 period, as well as the reported projected lengths of six antisunward jets observed between May 14 and June 6, 1910. A map of the comet's nucleus is presented of whose 13 discrete dust sources only two can be modeled as point-like regions. Attention is given to curious cases of 'chain' activation in which the sources that intersect one another on the surface emit dust sequentially, in successive rotations.

The continuous ejection of dust from discrete emission sources on the rotating nucleus of the Comet Halley is modelled in order to explain the evolution of spiral jets which unwind from the nucleus condensation into envelopes or halos in the comet head. The model is applied to digitally processed images of three features of the comet taken from Mount Wilson plates during the 1910 fly-by. The model permits a determination of the motion and spin vector for each emission source, its cometocentric coordinates, and a function relating particle ejection velocity to the solar radiation pressure exerted on the ejecta. It is found that the obliquity of the comet orbit's plane to its equatorial plane is 45 deg, the axis of rotation period of 17.3 days. The derived function of particle ejection velocity to the solar radiation pressure implied no contribution from grains larger than 10 microns in radius. High dust loading of gas flows from the June 1910 emission sources is indicated. It is estimated that because of the favorable approach geometry of the Gioto spacecraft during its 1986 flyby, the likelihood of encountering dense jets of dust is small.

Directed outgassing from isolated sources on P/Encke's rotating nucleus is investigated based on observations made between 1868 and 1918. A model of a collimated particle flow which identifies a sunward fan-like coma with the emission cone described by the ejecta from a discrete active region is shown to account for physical observations of the comet's perihelion. The results support nucleus precession, with a time-variable rate averaging about 1 degree per revolution. It is suggested that wall collapse and other processes limit the lifespans of individual vents to only a few hundred years or so. Differences between new and old vents are considered.

A computer search for earth-approaching comets among those listed in Marsden's (1983) updated orbit catalog has identified 36 cases at which minimum separation distance was less than 2500 earth radii. A strong representation of short period comets in the sample is noted, and the constant rate of the close approaching comets in the last 300 years is interpreted to suggest the lack of long-period comets intrinsically fainter than an absolute magnitude of about 11. A comet-earth collision rate derived from the statistics of these close encounters implies an average period of 33-64 million years between any two events. This rate is comparable with the frequency of geologically recent global catastrophes which appear to be associated with extraterrestrial object impacts, such as the Cretaceous-Tertiary extinction 65 million years ago and the late Eocene event 34 million years ago.

The existing data base on certain characteristics of the periodic Comet Kopff, a candidate for a NASA rendezvous mission in the 1990s, is summarized. The comet was discovered in 1906. The data are used to calculate limits to the nuclear mass (0.7-1.5 x 10 to the 16th g), equatorial radius (1.26-1.64 km), and a geometric albedo of 0.08. An equatorial rotation velocity of 8.1-10.7 hr is projected. The estimates are highly dependent on the present photometric data, which in turn depend on the insolation rate, which if different than assumed could mean that all parameters would be revised upward significantly.