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1.
David Asher Mark Bailey Apostolos Christou John McFarland Mark Muir Paul Rafferty 《Astronomy& Geophysics》2002,43(2):2.19-2.21
A possible naked-eye comet that may have been important in early cometary theory is announced by D J Asher , M E Bailey , A Christou , J McFarland , M F Muir and P P Rafferty .
Early indications sugest that Comet C/2002 (Ikeya-Zhang), discovered on 1 February 2002, may brighten to naked-eye visibility in late March 2002. It has also been suggested that it may be identical to one of the brighter comets of the 16th or 17th centuries, namely C/1532 R1 or C/1661 C1. The first of these, observed for more than 100 days towards the end of 1532, played an important role in the development of cometary theory. The second, although identified by Halley as having an orbit similar to that of the comet C/1532 R1, was not seen on its predicted return in 1788/1789 and so was presumably unrelated. Here we present long-term orbital integrations of C/2002 C1 which suggest that it orginated from the Oort cloud, and will be ejected again, within ˜0.3 Myr. There is a chance of 10–20% that it will end its life by falling into the Sun during a Halley-type phase of cometary evolution. The discovery of Ikeya-Zhang so closeto perigee by two amateur astonomers highlights the need for surveys covering both hemispheres to discover long-period and intermediate-period comets on Earth-crossing orbits. 相似文献
Early indications sugest that Comet C/2002 (Ikeya-Zhang), discovered on 1 February 2002, may brighten to naked-eye visibility in late March 2002. It has also been suggested that it may be identical to one of the brighter comets of the 16th or 17th centuries, namely C/1532 R1 or C/1661 C1. The first of these, observed for more than 100 days towards the end of 1532, played an important role in the development of cometary theory. The second, although identified by Halley as having an orbit similar to that of the comet C/1532 R1, was not seen on its predicted return in 1788/1789 and so was presumably unrelated. Here we present long-term orbital integrations of C/2002 C1 which suggest that it orginated from the Oort cloud, and will be ejected again, within ˜0.3 Myr. There is a chance of 10–20% that it will end its life by falling into the Sun during a Halley-type phase of cometary evolution. The discovery of Ikeya-Zhang so closeto perigee by two amateur astonomers highlights the need for surveys covering both hemispheres to discover long-period and intermediate-period comets on Earth-crossing orbits. 相似文献
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Observational evidence is sought that the long-term (104 yr) action of a mean motion resonance with Jupiter can produce structure in a meteoroid stream, concentrating meteoroids in a dense swarm. More specifically, predictions tabulated by Asher & Clube of enhanced meteor and fireball activity from a Taurid Complex swarm in the 7:2 resonance are compared with observational data collected in Japan over several decades. The swarm model was proposed for reasons independent of the observations analysed here, and these newly considered data are shown to be consistent with it. This allows increased confidence in the Taurid swarm theory, and more generally could mean that resonant trapping is a dynamical mechanism affecting a significant amount of meteoroidal material in the inner Solar system. 相似文献
5.
Prof. Asher P. Schick 《GeoJournal》1979,3(4):351-360
The impact of fluvial geomorphic processes on settlement in deserts is tie d to two conflicting factors: the extreme rarity of geomorphic events on the one hand, and the violence of the geomorphic forces coupled with a minimal resistance to erosion on the other.The unique character of desert floods calls for a careful geomorphic evaluation in the siting of settlements and transportation facilities. On the beneficial sid e, the geomorphic processes involved enable, under some conditions, the local exploitation of floodwaters which percolate in the alluvium of the upper tributaries, as well as the utilization of the hydraulic geometry for estimating discharge characteristics.On a larger scale, human interference often disrupts the delicate quasi-equilbrium between erosion and sedimentation in deserts. Although our poor understanding of many aspects of the mechanism of response makes it difficult to separate the effects from those of climatic changes, enough information is available to document the detrimental effect of inconsiderate management on both the regional and local levels. 相似文献
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We present observational data for two long-period and three dynamically new comets observed at heliocentric distances between 5.8 to 14.0 AU. All of the comets exhibited activity beyond the distance at which water ice sublimation can be significant. We have conducted experiments on gas-laden amorphous ice samples and show that considerable gas emission occurs when the ice is heated below the temperature of the amorphous-crystalline ice phase transition (T∼137 K). We propose that annealing of amorphous water ice is the driver of activity in comets as they first enter the inner Solar System. Experimental data show that large grains can be ejected at low velocity during annealing and that the rate of brightening of the comet should decrease as the heliocentric distance decreases. These results are consistent with both historical observations of distant comet activity and with the data presented here. If observations of the onset of activity in a dynamically new comet are ever made, the distance at which this occurs would be a sensitive indicator of the temperature at which the comet had formed or represents the maximum temperature that it has experienced. New surveys such as Pan STARRS, may be able to detect these comets while they are still inactive. 相似文献
8.
The Leonid meteor storms of 1833 and 1966 总被引:2,自引:0,他引:2
D. J. Asher 《Monthly notices of the Royal Astronomical Society》1999,307(4):919-924
The greatest Leonid meteor storms since the late eighteenth century are generally regarded as being those of 1833 and 1966. They were evidently due to dense meteoroid concentrations within the Leonid stream. At those times, the orbit of Comet 55P/Tempel–Tuttle was significantly nearer that of the Earth than at most perihelion returns, but still some tens of Earth radii away. Significantly reducing this miss distance can be critical for producing a storm. Evaluation of differential gravitational perturbations, comparing meteoroids with the comet, shows that, in 1833 and 1966 respectively, the Earth passed through meteoroid trails generated at the 1800 and 1899 returns. 相似文献
9.
O. Vaduvescu M. Birlan A. Tudorica A. Sonka F.N. Pozo A.D. Barr D.J. Asher J. Licandro J.L. Ortiz E. Unda-Sanzana M. Popescu A. Nedelcu D. Dumitru R. Toma I. Comsa C. Vancea D. Vidican C. Opriseanu T. Badescu M. Badea M. Constantinescu 《Planetary and Space Science》2011,59(13):1632-1646
We report on the follow-up and recovery of 100 program NEAs, PHAs and VIs using the ESO/MPG 2.2 m, Swope 1 m and INT 2.5 m telescopes equipped with large field cameras. The 127 fields observed during 11 nights covered 29 square degrees. Using these data, we present the incidental survey work which includes 558 known MBAs and 628 unknown moving objects mostly consistent with MBAs from which 58 objects became official discoveries. We planned the runs using six criteria and four servers which focus mostly on faint and poorly observed objects in need of confirmation, follow-up and recovery. We followed 62 faint NEAs within one month after discovery and we recovered 10 faint NEAs having big uncertainties at their second or later opposition. Using the INT we eliminated four PHA candidates and VIs. We observed in total 1286 moving objects and we reported more than 10,000 positions. All data were reduced by the members of our network in a team effort, and reported promptly to the MPC. The positions of the program NEAs were published in 27 MPC and MPEC references and used to improve their orbits. The O−C residuals for known MBAs and program NEAs are smallest for the ESO/MPG and Swope and about four times larger for the INT whose field is more distorted. For the astrometric reduction, the UCAC-2 catalog is recommended instead of USNO-B1. The incidental survey allowed us to study statistics of the MBA and NEA populations observable today with 1–2 m facilities. We calculate preliminary orbits for all unknown objects, classifying them as official discoveries, later identifications and unknown outstanding objects. The orbital elements a, e, i calculated by FIND_ORB software for the official discoveries and later identified objects are very similar with the published elements which take into account longer observational arcs; thus preliminary orbits were used in statistics for the whole unknown dataset. We present a basic model which can be used to distinguish between MBAs and potential NEAs in any sky survey. Based on three evaluation methods, most of our unknown objects are consistent with MBAs, while up to 16 unknown objects could represent NEO candidates and four represent our best NEO candidates. We assessed the observability of the unknown MBA and NEA populations using 1 and 2 m surveys. Employing a 1 m facility, one can observe today fewer unknown objects than known MBAs and very few new NEOs. Using a 2 m facility, a slightly larger number of unknown than known asteroids could be detected in the main belt. Between 0.1 and 0.8 new NEO candidates per square degree could be discovered using a 2 m telescope. 相似文献
10.
The past 12 years have seen significant steps forward in the science and practice of coastal flood analysis. This paper aims to recount and critically assess these advances, while helping identify next steps for the field. This paper then focuses on a key problem, connecting the probabilistic characterization of flood hazards to their physical mechanisms. Our investigation into the effects of natural structure on the probabilities of storm surges shows that several different types of spatial-, temporal-, and process-related organizations affect key assumptions made in many of the methods used to estimate these probabilities. Following a brief introduction to general historical methods, we analyze the two joint probability methods used in most tropical cyclone hazard and risk studies today: the surface response function and Bayesian quadrature. A major difference between these two methods is that the response function creates continuous surfaces, which can be interpolated or extrapolated on a fine scale if necessary, and the Bayesian quadrature optimizes a set of probability masses, which cannot be directly interpolated or extrapolated. Several examples are given here showing significant impacts related to natural structure that should not be neglected in hazard and risk assessment for tropical cyclones including: (1) differences between omnidirectional sampling and directional-dependent sampling of storms in near coastal areas; (2) the impact of surge probability discontinuities on the treatment of epistemic uncertainty; (3) the ability to reduce aleatory uncertainty when sampling over larger spatial domains; and (4) the need to quantify trade-offs between aleatory and epistemic uncertainties in long-term stochastic sampling. 相似文献