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1.
We observed 34 comets using the 24 μm camera on the Spitzer Space Telescope. Each image contains the nucleus and covers at least 106 km of each comet's orbit. Debris trails due to mm-sized or larger particles were found along the orbits of 27 comets; 4 comets had small-particle dust tails and a viewing geometry that made debris trails impossible to distinguish; and only 3 had no debris trail despite favorable observing conditions. There are now 30 Jupiter-family comets with known debris trails, of which 22 are reported in this paper for the first time. The detection rate is >80%, indicating that debris trails are a generic feature of short-period comets. By comparison to orbital calculations for particles of a range of sizes ejected over 2 yr prior to observation, we find that particles comprising 4 debris trails are typically mm-sized while the remainder of the debris trails require particles larger than this. The lower-limit masses of the debris trails are typically 1011 g, and the median mass loss rate is 2 kg/s. The mass-loss rate in trail particles is comparable to that inferred from OH production rates and larger than that inferred from visible-light scattering in comae.  相似文献   

2.
Abstract— We report on studies of the Fe, Ca, and K atom densities in the trails of meteors. The measurements of the densities were taken simultaneously and in a common volume by three ground-based lidars. We report and analyze the data obtained during two nights of Leonid showers (1996 and 1998 November 16/17) and of one night five days after the 1998 Leonids. The lidar-observed trails of Leonids differ from those of other meteor showers in both their mean altitude and in mean metal composition. The Leonid trails show a highly depressed Ca/Fe abundance ratio in comparison to CI meteoritic composition. Our observations are interpreted with the help of a numerical model that describes the ablation processes occurring during the high-speed entry of meteoroids into the Earth's atmosphere. We conclude that for the lidar-observed meteoroids, the ablation process occurs differentially for the three elements. This leads to a mixture of metals in the meteor trails, the composition of which is strongly altitude dependent and at any one altitude deviates significantly from a CI meteoritic composition. The model predicts differing altitudes and durations of trail observations for different showers, allowing us to tentatively assign the origin to the observed trails.  相似文献   

3.
Abstract— A time adjustment from the Earth's centre to the observer's location is quantified and shown to improve the residuals between the observed time of pronounced Leonid showers and the time calculated from perturbed dust trails. A correlation with the exact time of closest approach to the nominal dust trail orbit would allow a further improvement in the residuals. However, there is no evidence for such a correlation, which suggests that the dust trail cross section is substantially elongated within the orbital plane. These results indicate that approaches to dust trails within 0.0005 AU are predictable with an uncertainty of ～5 min. Application of this correction to satellites confirms the utility of two strategies to minimise the impact risk. The time adjustment could be applied to any meteor stream.  相似文献   

4.
One minute counts obtained during the meteor outburst of α-Monocerotids on November 22, 1995, are analyzed in order to examine the possibility of filamentary structure in the stream profile. None is found. It is argued that far-comet type outbursts are due to the Earth's passage through the dust trail of a long period comet, thus offering a direct means of studying such comet dust trails. Hence, the meteor stream activity curve is the first accurate cross section of dust densities through a comet dust trail.  相似文献   

5.
E Lyytinen 《Icarus》2003,162(2):443-452
Long-period comets have narrow one-revolution old dust trails that can cause meteor outbursts when encountered by Earth. To facilitate observing campaigns that will characterize and perhaps help find Earth-threatening, long-period comets from their trace of meteoric debris, we use past accounts of outbursts from 14 different showers to calculate the future dust trail positions near Earth’s orbit. We also examine known near-Earth, long-period comets and identify five potential new showers, which can be utilized to learn more about these objects. We demonstrate that it is the one-revolution trail that is responsible for meteor outbursts. A method that calculates in what year these showers are likely to return and at what hour is presented. The calculations improve on earlier approximate methods that used the Sun’s reflex motion to gauge the trail motion relative to Earth’s orbit.  相似文献   

6.
The article provides an explanation of stronger than expected by the Author Draconids 2011 activity basing on the assumption of unusually high density of 1900 trail of the comet 21P Giacobini-Zinner. Also, a revised prediction for Draconids 2014 is presented, which should also be caused by 1900 trail. For this prediction a “vertical trails” approach is used. This approach is described in the article.  相似文献   

7.
Particles of mass less than about 1 gm are a minor fraction of the total matter impinging on the Earth averaged over millennia time scales. However, these particles dominate during a single particular year and produce the most obvious evidence of incoming extra-terrestrial matter in the form of ablation trails in the atmosphere which are visible at night as meteors.Observations of meteors give astronomical information on the composition, structure, and cometary associations of the particles. The composition is deduced from optical spectra of meteors, whilst telescopic studies of the trails during formation give information on the physical structure of the particles. Any cometary associations are deduced from measurement of meteor orbits determined photographically, using television, or by radar.Meteors occur in the atmosphere at heights from about 70 to 120 km. Optical observations are restricted to night-time and usually under conditions of low moonlight. A typical television based detector can record +8M meteors with a sporadic rate of 15–20 per hour and velocities accurate to about 3%. The luminosity of the trail is strongly dependent on the velocity of the meteoroid (to about the third power).Radar observations of meteors are unrestricted by weather or time of day, and can readily detect meteors at least two orders of magnitude smaller in mass than those detectable optically. Again the observations are heavily biased toward the higher velocities as the electron line density varies approximately asV 3.5. However, the higher the velocity of the meteoroid the greater the height of the meteor trail, and the reduced probability of radar detection due to rapid diffusion of the trail. Thus radar observations tend to select meteors in the intermediate velocity range 30–40 km s–1.  相似文献   

8.
We report results from two station, short-baseline (< 100 m) high resolution measurements of faint meteors (limiting meteor magnitude +9) with the goal of measuring their optical trail widths. Meteors were observed using two 0.40 m Newtonian telescopes (field of view ˜0.4 degrees) equipped with image intensifiers. Both telescopes were vertically oriented in a fixed mount and pointed to the same field of view. One system used a gated image intensified camera allowing the transverse velocity component to be measured. The widest trail captured, out of a total of 34 common events measured by both optical systems, had a full-width to half-maximum of 1.37±0.71 m. The widest trail overall was captured by the gated system only, and was found to have a full-width of ∼ ∼10 m. The brightness variation across this trail was found to be best represented by a Lorentzian. Most trails were smaller than our resolution limit and hence we could only place upper limits on their optical width. These were generally less than 1 m after correction for instrumental effects. Four meteors were found to have heights near 65 km and very low transverse velocities. These may be indicative of a largely unreported high density asteroidal component at these faint meteor magnitudes.  相似文献   

9.
We calculate the position of dust trails from comet 8P/Tuttle, in an effort to explain unusual Ursid meteor shower outbursts that were seen when the comet was near aphelion. Comet 8P/Tuttle is a Halley-type comet in a 13.6-year orbit, passing just outside of Earth's orbit. We find that the meteoroids tend to be trapped in the 12:14 mean motion resonance with Jupiter, while the comet librates in a slightly shorter period orbit around the 13:15 resonance. It takes 6 centuries to decrease the perihelion of the meteoroid orbits enough to intersect Earth's orbit, during which time the meteoroids and comet separate in mean anomaly by 6 years, thus explaining the 6-year lag between the comet's return and Ursid outbursts. The resonances also prevent dispersion along the comet orbit and limit viewing to only one year in each return. We identified past dust trail encounters with dust trails from 1392 (Dec. 1945) and 1378 (Dec. 1986) and predicted another outburst on 2000 December 22 at around 7:29 and 8:35 UT, respectively, from dust trails dating to the 1405 and 1392 returns. This event was observed from California using video and photographic techniques. At the same time, five Global-MS-Net stations in Finland, Japan, and Belgium counted meteors using forward meteor scatter. The outburst peaked at 8:06±07 UT, December 22, at zenith hourly rate ∼90 per hour, and the Ursid rates were above half peak intensity during 4.2 h. We find that most Ursid orbits do scatter around the anticipated positions, confirming the link with comet 8P/Tuttle and the epoch of ejection. The 1405 and 1392 dust trails appear to have contributed similar amounts to the activity profile. Some orbits provide a hint of much older debris being present as well. This work is the strongest evidence yet for the relevance of mean motion resonances in Halley-type comet dust trail evolution.  相似文献   

10.
The spatial structure of meteor streams, and the activity profiles of their corresponding meteor showers, depend firstly on the distribution of meteoroid orbits soon after ejection from the parent comet nucleus, and secondly on the subsequent dynamical evolution. The latter increases in importance as more time elapses. For younger structures within streams, notably the dust trails that cause sharp meteor outbursts, it is the cometary ejection model (meteoroid production rate as a function of time through the several months of the comet’s perihelion return, and velocity distribution of the meteoroids released) that primarily determines the shape and width of the trail structure. This paper describes how a trail cross section can be calculated once an ejection model has been assumed. Such calculations, if made for a range of ejection model parameters and compared with observed parameters of storms and outbursts, can be used to constrain quantitatively the process of meteoroid ejection from the nucleus, including the mass distribution of ejected meteoroids.  相似文献   

11.
2001年狮子座流星雨,是几百年来难得有机会在中国观测到的一场流星暴。在我们收集到的照相观测资料中,发现了一些特殊的、还未引起广泛注意的流星现象。"蛇行"的流星以及螺旋形并带有辐条结构的流星余迹,早在我国古代记录中就可以找到印证。现代的照相技术,不仅直观地显示了文字难以描述的颜色、形态和变化,而且提供了可永久性进行测量的记录。其中包括罕见的对流星余迹的双站观测资料。用常规的"高空风作用"难以解释这些观测现象。它们可能反映了流星带电体与地球磁场的相互作用。  相似文献   

12.
In this paper, we use radar observations from a 50 MHz radar stationed near Salinas, Puerto Rico, to study the variability of specular as well as non-specular meteor trails in the E-region ionosphere. The observations were made from 18:00 to 08:00 h AST over various days in 1998 and 1999 during the Coqui II Campaign [Urbina et al., 2000, Geophys. Rev. Lett. 27, 2853–2856]. The radar system had two sub-arrays, both produced beams pointed to the north in the magnetic meridian plane, perpendicular to the magnetic field, at an elevation angle of approximately 41 degrees. The Coqui II radar is sensitive to at least two types of echoes from meteor trails: (1) Specular reflections from trails oriented perpendicular to the radar beam, and (2) scattering, or, non-specular reflections, from trails deposited with arbitrary orientations. We examine and compare the diurnal and seasonal variability of echoes from specular and non-specular returns observed with the Coqui II radar. We also compare these results with meteor head echo observations made with the Arecibo 430 MHz radar. We use common region observations of these three types of meteor echoes to show that the diurnal and seasonal variability of specular trails, non-specular trails, and head echoes are not equivalent. The implications of these results on global meteor mass flux estimates obtained from specular meteor observations remains to be examined.  相似文献   

13.
The asteroid 3200 Phaethon is suggested as a candidate for direct impact research. The object is considered to be an extinct comet and the parent of the Geminid meteor shower. One could say that this provides a possible argument for a space mission. Based on such a mission, this paper proposes to investigate the nature of the extinct comet and the additional interesting possibility of artificially generated meteor showers.
Dust trail theory can calculate the distribution of a bundle of trails and be used to show in which years artificial meteors would be expected. Results indicate that meteor showers will be seen on Earth about 200 yr after the event, on 2022 April 12.  相似文献   

14.
Jupiter and Saturn produce important gravitational impulses on meteoroids released by comet 109P/Swift-Tuttle. The meteoroids from this comet once released follow retrograde orbits that during their periodic approaches to these planets (within 1.6 and 0.9 A.U., respectively) are impulsed gaining orbital energy. This perturbation effect is translated into a net inward shift in the node of the perturbed meteoroids. Such geometry with Jupiter occurred in 2004 over a meteoroid trail ejected by this comet during the 1862 A.D. return of the comet to perihelion. In order to study the predicted outburst produced by one-revolution meteoroids, the Spanish Photographic Meteor Network (SPMN) performed an extensive campaign. As a part of this observational effort here are presented 10 accurate meteoroid orbits. We discuss their origin by comparing them with the theoretical orbital elements of the dust trails intercepting the Earth during the 2004 Perseid return.  相似文献   

15.
Most of the participants in the ACM 96 conference would have made use of facilities in a building named for Georges-Louis Leclerc, the Compte de Buffon (1707–1788). Buffon made many major contributions to the natural sciences, and may be considered to be one of the founders of planetary science. He proposed a theory for the origin of the planets which involved a massive comet having an oblique impact upon the Sun, the ejected material condensing so as to form a regular system of planets. Amongst his mathematical contributions is what is known as Buffon's Needle, whereby experimental evaluations of π may be made by randomly dropping a needle onto a set of parallel lines of separation greater than the needle length, and accumulating the fraction of times that the needle cuts one of the lines. Near-Earth asteroid (NEA) trails imaged onto a CCD chip provide a two-dimensional analogue of this, and where the pixel size is very large (this having some advantages for NEA searching) an analysis based on Buffon's Needle provides probabilities of the NEA trail lying within one, two or three pixels, such probabilities affecting the chances of detection. It is therefore appropriate that Buffon and his contributions to studies of comets and asteroids be remembered in these conference proceedings.  相似文献   

16.
We discuss in this letter a new cosmological model including both a non-linear derivative interaction and a Gauss–Bonnet invariant. The first interaction term appears in the theory with the Galilean shift symmetry, whereas the second term is motivated from string theory and plays a crucial role in Chern–Simons gravitational theories as well as in the renormalization of quantum field theories in curved spacetime. It is shown that the presence of both the cubic interaction and the Gauss–Bonnet invariant term supply amazingly prosperous cosmological phenomenologies, which are explored in some detail.  相似文献   

17.
Recent observations of the longest known head-tail galaxy IC 711, at long wavelengths have shown it to extend farther than previously known. At δ 74 cm, Vallée and Roger (1987) found it to stretch 17′ (930 kpc) behind the optical nucleus of this elliptical galaxy. At δ92 cm, Vallée and Strom (1988) found it to stretch 18.3′ (1 Mpc) behind the optical nucleus. What new physics can be learned from it? To that end, a theoretical simulation in 3 dimensions of the longest head-tail galaxy has been performed, building the radio trail cell by cell inside a volume of space composed of 8 million cells, with each cell being assigned a proper radio emissivity. Matching ram pressure theory and recent observations yields a primary fast decrease of the radio luminosity with time or distance from the optical nucleus of the galaxy, and a secondary periodic variation of the radio luminosity every 500 kpc. Near the end of the tail, a sudden and unpredicted flare-up of the observed width of the tail requires a sudden change in the jet's physical conditions about 2 Gyr earlier, or that of the surroundings. These simulations provide some additional constraints on the ram pressure theory. Along the tail ridge, particle aging and particle acceleration must combine to give a luminosity function that mimics an exponential decrease with time. Across the tail ridge, no edge brightening is necessary, suggesting that particle aging and particle acceleration must combine via central turbulences and central instabilities (not by edge effects). The disappearance of the radio trail at two points along the radio ridge is unexpected from the ram pressure theory. Thus particle aging and particle acceleration must combine once more to mimic a sinusoidal modulation with time of the radio luminosity, perhaps near the nucleus inside the optical galaxy.  相似文献   

18.
Using a sample of 454 Mira light curves from the ASAS survey we study the shape of the light variations in this kind of variable stars. Opposite to earlier studies, we choose a general approach to identify any deviation from a sinusoidal light change. We find that about 30% of the studied light curves show a significant deviation from the sinusoidal reference shape. Among these stars two characteristic light curve shapes of comparable frequency could be identified. Some hint for a connection between atmospheric chemistry and light curve shape was found, but beside that no or only very weak relations between light curve shape and other stellar parameters seem to exist (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)  相似文献   

19.
Shiba  Y.  Shimoda  C.  Maruyama  T.  Okumura  S.  Tomita  M.  Murasawa  A.  Ohtsuka  K.  Tomioka  H.  Hidaka  E. 《Earth, Moon, and Planets》1997,77(1):47-54
Several Leonid fireballs were successfully photographed by the Japanese Fireball Network and by other observers in Japan on 16 November, 1996. A totals of seven of these were simultaneously observed from two or more stations, from which the orbital and physical data were deduced. The radiant of these fireballs were very small, only 0.1°, similar to that of the 1991 Perseids. The 1996 Leonids showed a lower magnitude distribution index similar to those obtained in the 1961 and 1965 Leonids. All of these showers occurred before perihelion passage of the parent comet. We conclude that we have already encountered the elongated front part of the dust trail of the Leonid parent comet, where the trail is probably composed of larger dust particles. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

20.
Comparison of five X-ray observations of the intermediate polar FO Aqr reveals that the morphology of the X-ray light curve changes considerably with time. In particular, power spectra of the 1988 Ginga   and 1993 ASCA   data reveal strong sideband and orbital variations, whereas the 1990 Ginga   observation does not. This suggests that the amount of stream-fed accretion varies with epoch, and the system was accreting predominantly from a disc in 1990.   In contrast to other intermediate polars, the X-ray spin-pulse profiles show significant variations between observations, ranging from relatively sinusoidal to sawtooth-shaped at medium energies. During the 1988 and 1990 observations a notch is visible at spin phase zero, due to the presence of an interpulse at phase 0.85, which is absent during the other observations. At lower energies a narrow pulse of emission is seen at spin phase 0.2.   We interpret the pulse profile from the 1990 Ginga   observation using a model for accretion from a disc on to a dipolar magnetic field, the axis of which is offset from the white dwarf centre by ∼ 0.15 white dwarf radii. In order to account for the later occurrence of the hardness-ratio maximum in 1988 and 1993, we suggest that the accretion-rate profile changes so that accretion is favoured along the field lines which trail the magnetic pole. This also accounts for the disappearance of the interpulse and notch in 1993.  相似文献   

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