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1.
This paper analyzes the capture of comets into Halley-type and Jupiter-family orbits from the nearparabolic flux of the Oort cloud. Two types of capture into Halley-type orbits are found. The first type is the evolution of near-parabolic orbits into short-period orbits (with heliocentric orbital periods P < 200 years) as a result of close encounters with giant planets. This process is followed by a very slow drift of cometary orbits into the inner part of the Solar System. Only those comets may pass from short-period orbits into Halley-type and Jupiter-family orbits, which move in orbits with perihelion distances q < 13 au. In the second type of capture, the perihelion distances of cometary orbits become rather small (< 1.5 au) during the first stage of dynamic evolution under the action of perturbations from the Galaxy, and then their semimajor axes decrease as a result of diffusion. The capture takes place, on average, in 500 revolutions of the comet about the Sun, whereas in the first case, the comet is captured, on average, after 12500 revolutions. The region of initial orbital perihelion distances q > 4 au is found to be at least as important a source of Halley-type comets as the region of perihelion distances q < 4 au. More than half of the Halley-type comets are captured from the nearly parabolic flux with q > 4 au. The analysis of the dynamic evolution of objects moving in short-period orbits shows that the distribution of Centaurs orbits agrees well with the observed distribution corrected for observational selection effects. Hence, the hypothesis associating the origin of Centaurs with the Edgeworth-Kuiper belt and the trans-Neptunian region exclusively should be rejected.  相似文献   

2.
This study analyzes the evolution of 2 × 105 orbits with initial parameters corresponding to the orbits of comets of the Oort cloud under the action of planetary, galactic, and stellar perturbations over 2 × 109 years. The dynamical evolution of comets of the outer (orbital semimajor axes a > 104 AU) and inner (5 × 103 < a (AU) < 104) parts of the comet cloud is analyzed separately. The estimates of the flux of “new” and long-period comets for all perihelion distances q in the planetary region are reported. The flux of comets with a > 104 AU in the interval 15 AU < q < 31 AU is several times higher than the flux of comets in the region q < 15 AU. We point out the increased concentration of the perihelia of orbits of comets from the outer cloud, which have passed several times through the planetary system, in the Saturn-Uranus region. The maxima in the distribution of the perihelia of the orbits of comets of the inner Oort cloud are located in the Uranus-Neptune region. “New” comets moving in orbits with a < 2 × 104 AU and arriving at the outside of the planetary system (q > 25 AU) subsequently have a greater number of returns to the region q < 35 AU. The perihelia of the orbits of these comets gradually drift toward the interior of the Solar System and accumulate beyond the orbit of Saturn. The distribution of the perihelia of long-period comets beyond the orbit of Saturn exhibits a peak. We discuss the problem of replenishing the outer Oort cloud by comets from the inner part and their subsequent dynamical evolution. The annual rate of passages of comets of the inner cloud, which replenish the outer cloud, in the region q < 1 AU in orbits with a > 104 AU (～ 5.0 × 10?14 yr?1) is one order of magnitude lower than the rate of passage of comets from the outer Oort cloud (～ 9.1 × 10?13 yr?1).  相似文献   

3.
An overview is given of close encounters of nearly parabolic comets (NPCs; with periods of P > 200 years and perihelion distances of q > 0.1 AU; the number of the comets is N = 1041) with planets. The minimum distances Δmin between the cometary and planetary orbits are calculated to select comets whose Δmin are less than the radius of the planet’s sphere of influence. Close encounters of these comets with planets are identified by numerical integration of the comets’ equations of motion over an interval of ±50 years from the time of passing the perihelion. Close encounters of NPCs with Jupiter in 1663–2011 are reported for seven comets. An encounter with Saturn is reported for comet 2004 F2 (in 2001).  相似文献   

4.
The near-Earth objects and their potential threat to our planet   总被引：1，自引：0，他引：1  
The near-Earth object (NEO) population includes both asteroids (NEAs) and comet nuclei (NECs) whose orbits have perihelion distances q<1.3 AU and which can approach or cross that of the Earth. A NEA is defined as a “potentially hazardous asteroid” (PHA) for Earth when its minimum orbit intersection distance (MOID) comes inside 0.05 AU and it has an absolute magnitude H<22 mag (i.e. mean diameter > 140 m). These are big enough to cause, in the case of impact with Earth, destructive effects on a regional scale. Smaller objects can still produce major damage on a local scale, while the largest NEOs could endanger the survival of living species. Therefore, several national and international observational efforts have been started (i) to detect undiscovered NEOs and especially PHAs, (ii) to determine and continuously monitor their orbital properties and hence their impact probability, and (iii) to investigate their physical nature. Further ongoing activities concern the analysis of possible techniques to mitigate the risk of a NEO impact, when an object is confirmed to be on an Earth colliding trajectory. Depending on the timeframe available before the collision, as well as on the object’s physical properties, various methods to deflect a NEO have been proposed and are currently under study from groups of experts on behalf of international organizations and space agencies. This paper will review our current understanding of the NEO population, the scientific aspects and the ongoing space- and ground-based activities to foresee close encounters and to mitigate the effects of possible impacts.  相似文献   

5.
We analyze the dynamical evolution of Jupiter-family (JF) comets and near-Earth asteroids (NEAs) with aphelion distances Q>3.5 AU, paying special attention to the problem of mixing of both populations, such that inactive comets may be disguised as NEAs. From numerical integrations for 2×106 years we find that the half lifetime (where the lifetime is defined against hyperbolic ejection or collision with the Sun or the planets) of near-Earth JF comets (perihelion distances q<1.3 AU) is about 1.5×105 years but that they spend only a small fraction of this time (∼ a few 103 years) with q<1.3 AU. From numerical integrations for 5×106 years we find that the half lifetime of NEAs in “cometary” orbits (defined as those with aphelion distances Q>4.5 AU, i.e., that approach or cross Jupiter's orbit) is 4.2×105 years, i.e., about three times longer than that for near-Earth JF comets. We also analyze the problem of decoupling JF comets from Jupiter to produce Encke-type comets. To this end we simulate the dynamical evolution of the sample of observed JF comets with the inclusion of nongravitational forces. While decoupling occurs very seldom when a purely gravitational motion is considered, the action of nongravitational forces (as strong as or greater than those acting on Encke) can produce a few Enckes. Furthermore, a few JF comets are transferred to low-eccentricity orbits entirely within the main asteroid belt (Q<4 AU and q>2 AU). The population of NEAs in cometary orbits is found to be adequately replenished with NEAs of smaller Q's diffusing outward, from which we can set an upper limit of ∼20% for the putative component of deactivated JF comets needed to maintain such a population in steady state. From this analysis, the upper limit for the average time that a JF comet in near-Earth orbit can spend as a dormant, asteroid-looking body can be estimated to be about 40% of the time spent as an active comet. More likely, JF comets in near-Earth orbits will disintegrate once (or shortly after) they end their active phases.  相似文献   

6.
In 1946, E. Sevin postulated the global vibrations of the Sun with a period P 0 = 1/9 day and a “wavelength” L 0 = c × P 0 = 19.24 AU and predicted the tenth planet at a mean distance of 4.0 × L 0 ≈ 77.0 AU from the Sun (c is the speed of light). The global vibrations of the Sun, precisely with the period of 1/9 day, were actually detected in 1974. Recently, the largest Kuiper Bell object 2003 UB313, or Eris, with an orbital semimajor axis ≈ 3.5 × L 0 ≈ 67.5 AU was discovered. We adduce arguments for the status of Eris as our tenth planet: (i) the object is larger and farther from the Sun than Pluto and (ii) the semimajor axis of Eris agrees well with the sequence of planetary distances that follows from the resonance spectrum of the Solar system dimensions (with the scale L 0 and for all 11 orbits, including those of Pluto, Eris, and the asteroid belt). We point to a mistake of the Prague (2006) IAU Assembly, which excluded Pluto from the family of planets by introducing a new, highly controversial class of objects—“dwarf planets.”  相似文献   

7.
At the beginning of this century, the SOHO space observatory discovered near-Sun comets with perihelion distances q ≈ 0.05 AU, which remained observable over several close encounters with the Sun. This became one of the surprises in studying the small bodies of the Solar System. Currently, there are objects that have already been observed in four (342P) and five (321P, 322P, and 323P) apparitions. In the present work, the estimates of nongravitational effects are obtained for these objects based on the pair-wise linkage of the apparitions. The calculations show that the observations of these objects are poorly represented if solely the gravitational forces are considered. The magnitude of nongravitational effects in the semimajor axis noticeably changes with time. The motion of all comets is significantly affected by the components of nongravitational forces that are perpendicular to the orbital plane.  相似文献   

8.
A substantial fraction of the Edgeworth-Kuiper belt objects are presently known to move in resonance with Neptune (the principal commensurabilities are 1/2, 3/5, 2/3, and 3/4). We have found that many of the distant (with orbital semimajor axes a > 50 AU) trans-Neptunian objects (TNOs) also execute resonant motions. Our investigation is based on symplectic integrations of the equations of motion for all multiple-opposition TNOs with a > 50 AU with allowance made for the uncertainties in their initial orbits. Librations near such commensurabilities with Neptune as 4/9, 3/7, 5/12, 2/5, 3/8, 4/27, and others have been found. The largest number of distant TNOs move near the 2/5 resonance with Neptune: 12 objects librate with a probability higher than 0.75. The multiplicity of objects moving in 2/5 resonance and the longterm stability of their librations suggest that this group of resonant objects was formed at early formation stages of the Solar system. For most of the other resonant objects, the librations are temporary. We also show the importance of asymmetric resonances in the large changes in TNO perihelion distances.  相似文献   

9.
The telescope SBG (D = 0.42 m, F = 0.76 m) at the Kourovka Astronomical Observatory of the Ural Federal University has undergone an upgrade in 2005–2006. A CCD camera (Apogee Alta U32) and a new drive system were installed, and a new system for telescope and observation control was implemented. This upgrade required verifying the astrometric quality of the telescope. The data processing approaches tested when searching for the optimum CCD image processing technique combined TYCHO2 and UCAC2 catalogues with various reduction models and methods for choosing reference stars. Lorentzian and Moffat profiles were used in the measurement of pixel coordinates. It was demonstrated that the accuracy of SBG observations of main-belt asteroids with precisely determined orbits depends on their brightness and varies from 0.06” (11.5 m ) to 0.4” (18.5 m ). Regular SBG observations of comets and asteroids (mostly near-Earth and potentially hazardous ones) have been performed since 2007. Coordinates of 8515 positions of 720 asteroids and more than 1000 positions of 40 comets were obtained. The RMS deviations of observed coordinates from their calculated values are typically smaller than 1”: the average deviations for asteroids are 0.33” (in right ascension) and 0.34” (in declination); the corresponding values for comets are 0.37” (in α) and 0.38” (in δ). The results of observations are sent to the Minor Planet Center and are used to determine orbits more accurately and solve other fundamental and applied problems.  相似文献   

10.
The evolution of orbits of asteroids found in the IRAS and WISE albedo databases was calculated numerically from 2005 to 2016. It follows from the analysis of the obtained results that a certain nongravitational effect (NGE) currently affects the motion of a considerable fraction of main-belt asteroids with diameters up to 40 km. This conclusion agrees with the available data regarding the axial rotation of asteroids. The NGE manifests itself in an increase in the semimajor axes of orbits of low-albedo asteroids relative to the semimajor axes of orbits of high-albedo bodies. The NGE-induced rate of elongation of semimajor axes of asteroids with albedos рv < 0.1 may be as high as (2–8) × 10–8 AU/year. Errors in orbital elements of asteroids (unrelated to the accuracy of observational data used to determine these orbital elements) were found in one of the MPC catalogues for 2003 in the process of estimation of the accuracy of calculations.  相似文献   

11.
This paper analyzes the distribution of the orbits of near-Earth minor bodies from the data on more than 7500 objects. The distribution of large near-Earth objects (NEOs) with absolute magnitudes of H < 18 is generally consistent with the earlier predictions (Bottke et al., 2002; Stuart, 2003), although we have revealed a previously undetected maximum in the distribution of perihelion distances q near q = 0.5 AU. The study of the orbital distribution for the entire sample of all detected objects has found new significant features. In particular, the distribution of perihelion longitudes seriously deviates from a homogeneous pattern; its variations are roughly 40% of its mean value. These deviations cannot be stochastic, which is confirmed by the Kolmogorov-Smirnov test with a more than 0.9999 probability. These features can be explained by the dynamic behavior of the minor bodies related to secular resonances with Jupiter. For the objects with H < 18, the variations in the perihelion longitude distribution are not so apparent. By extrapolating the orbital characteristics of the NEOs with H < 18, we have obtained longitudinal, latitudinal, and radial distributions of potentially hazardous objects in a heliocentric ecliptic coordinate frame. The differences in the orbital distributions of objects of different size appear not to be a consequence of observational selection, but could indicate different sources of the NEOs.  相似文献   

12.
The existence of gaps in the perihelion distribution of the orbits of multikilometer-sized asteroids that approach the orbits of terrestrial-group planets is confirmed. This property of the orbits of large asteroids suggests the existence among them of a family of last meteorite parent bodies. Astrophysical data were considered for S-asteroids of the Main Belt and those that approach terrestrial planets. The u–x color index, which is related to the position of the absorption band at 950 nm in the asteroid spectra, was chosen for a qualitative comparison of the surface composition of these asteroids (identical or differing composition). The u – x color-index distributions were analyzed statistically according to the perihelion (q) and mean heliocentric distances (a) of the S-asteroids. It is shown that these distributions are -shaped, peaking at q 1.8 AU and a 2.2 AU. The wings of the distributions can be approximated by linear regressions. A comparison of the u – x color-index mean values for S-asteroids in the regions of the Earth and Mars and of the Main Belt prompts the conclusion that the last meteorite parent bodies in the vicinity of the orbits of the Earth and Mars come primarily from various regions of the asteroid Main Belt.  相似文献   

13.
Kan Liou  Chin-Chun Wu 《Solar physics》2016,291(12):3777-3792
Interplanetary magnetic field and solar wind plasma density observed at 1 AU during Solar Cycle 23?–?24 (SC-23/24) minimum were significantly smaller than those during its previous solar cycle (SC-22/23) minimum. Because the Earth’s orbit is embedded in the slow wind during solar minimum, changes in the geometry and/or content of the slow wind region (SWR) can have a direct influence on the solar wind parameters near the Earth. In this study, we analyze solar wind plasma and magnetic field data of hourly values acquired by Ulysses. It is found that the solar wind, when averaging over the first (1995.6?–?1995.8) and third (2006.9?–?2008.2) Ulysses’ perihelion (\({\sim}\,1.4~\mbox{AU}\)) crossings, was about the same speed, but significantly less dense (\({\sim}\,34~\%\)) and cooler (\({\sim}\,20~\%\)), and the total magnetic field was \({\sim}\,30~\%\) weaker during the third compared to the first crossing. It is also found that the SWR was \({\sim}\,50~\%\) wider in the third (\({\sim}\,68.5^{\circ}\) in heliographic latitude) than in the first (\({\sim}\,44.8^{\circ}\)) solar orbit. The observed latitudinal increase in the SWR is sufficient to explain the excessive decline in the near-Earth solar wind density during the recent solar minimum without speculating that the total solar output may have been decreasing. The observed SWR inflation is also consistent with a cooler solar wind in the SC-23/24 than in the SC-22/23 minimum. Furthermore, the ratio of the high-to-low latitude photospheric magnetic field (or equatorward magnetic pressure force), as observed by the Mountain Wilson Observatory, is smaller during the third than the first Ulysses’ perihelion orbit. These findings suggest that the smaller equatorward magnetic pressure at the Sun may have led to the latitudinally-wider SRW observed by Ulysses in SC-23/24 minimum.  相似文献   

14.
Physical lifetimes and end-states of short-period comets are analysed in connection with the problem of the maintainance of the zodiacal dust cloud. In particular, the problem of the comet-asteroid relationship is addressed. Recent studies of the physical properties of Apollo-Amor asteroids and short-period comets (e.g., Hartmann et al., 1987) show significant differences between them, suggesting that they are distinct classes of objects. A few percent of the active SP comets might become asteroidal-like bodies in comet-type orbits due to the buildup of dust mantles. The remainder probably disintegrate as they consume their volatile content so their debris can only be observed as fireballs when they meet the Earth. Unobservable faint SP comets — i.e., comets so small (m 1014 g) that quickly disintegrate before being detected, might be a complementary source of dust material. They might be completely sublimated even at rather large heliocentric distances (r - 3 AU). Yet the released dust grains can reach the vicinity of the Sun by Poynting-Robertson drag. The mass associated with unobservable SP comets with perihelion distances q 3 AU might be comparable to that computed for the sample of observed SP co-mets with q 1.5 AU. It is concluded that SP comets (from the large to the unobservable small ones) may supply an average of several tons/sec of meteoric matter to the zodiacal dust cloud.  相似文献   

15.
We report the results of position, photometric, and polarimetric observations of two near-Earth asteroids made with the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences. 1.2-hour measurements of the photometric variations of the asteroid 2009 DL46 made onMarch 8, 2016 (approximately 20m at a distance of about 0.23 AU from the Earth) showed a 0.m2-amplitude flash with a duration of about 20 minutes. During this time the polarization degree increased from the average level of 2–3% to 14%. The angle of the polarization plane and the phase angle were equal to 113° ± 1° and 43°, respectively. Our result indicates that the surface of the rotating asteroid (the rotation period of about 2.5 hours) must be non-uniformly rough. Observations of another asteroid—1994 UG—whose brightness was of about 17m and which was located at a geocentric distance of 0.077 AU, were carried out during the night of March 6/7, 2016 in two modes: photometric and spectropolarimetric. According to the results of photometric observations in Johnson’s B-, V-, and R-band filters, over one hour the brightness of the asteroid remained unchanged within the measurement errors (about 0.m02). Spectropolarimetric observations in the 420–800 nm wavelength interval showed the polarization degree to decrease from 8% in the blue part of the spectrum to 2% in the red part with the phase angle equal to 44?, which is typical for S-type near-Earth asteroids.  相似文献   

16.
A simple method for numerical integration of the equations of motion of small bodies of the Solar System is proposed, which is especially efficient in studying the orbits with small perihelion distances. The evolution of orbits of 121 numbered asteroids with perihelion distances q < 1.2 AU is investigated over the time interval of years 2000–2100 with allowance made for the gravitational influence of nine planets and three largest asteroids. The circumstances of close encounters of asteroids with the Earth and other terrestrial planets are presented.  相似文献   

17.
We used more than 250 000 high-precision American and Russian radar observations of the inner planets and spacecraft obtained in the period 1961–2003 to test the relativistic parameters and to estimate the solar oblateness. Our analysis of the observations was based on the EPM ephemerides of the Institute of Applied Astronomy, Russian Academy of Sciences, constructed by the simultaneous numerical integration of the equations of motion for the nine major planets, the Sun, and the Moon in the post-Newtonian approximation. The gravitational noise introduced by asteroids into the orbits of the inner planets was reduced significantly by including 301 large asteroids and the perturbations from the massive ring of small asteroids in the simultaneous integration of the equations of motion. Since the post-Newtonian parameters and the solar oblateness produce various secular and periodic effects in the orbital elements of all planets, these were estimated from the simultaneous solution: the post-Newtonian parameters are β = 1.0000 ± 0.0001 and γ = 0.9999 ± 0.0002, the gravitational quadrupole moment of the Sun is J2 = (1.9 ± 0.3) × 10?7, and the variation of the gravitational constant is ?/G = (?2 ± 5) × 10?14 yr?1. The results obtained show a remarkable correspondence of the planetary motions and the propagation of light to General Relativity and narrow significantly the range of possible values for alternative theories of gravitation.  相似文献   

18.
Sedna is the first inner Oort cloud object to be discovered. Its dynamical origin remains unclear, and a possible mechanism is considered here. We investigate the parameter space of a hypothetical solar companion which could adiabatically detach the perihelion of a Neptune-dominated TNO with a Sedna-like semimajor axis. Demanding that the TNO’s maximum value of osculating perihelion exceed Sedna’s observed value of 76 AU, we find that the companion’s mass and orbital parameters (m c , a c , q c , Q c , i c ) are restricted to $$m_c>rapprox 5\hskip.25em\hbox{M}_{\rm J}\left(\frac{Q_c}{7850\hbox{ AU}} \frac{q_c}{7850\hbox{ AU}}\right)^{3/2}$$ during the epoch of strongest perturbations. The ecliptic inclination of the companion should be in the range $45{\deg}\lessapprox i_c\lessapprox 135{\deg}$ if the TNO is to retain a small inclination while its perihelion is increased. We also consider the circumstances where the minimum value of osculating perihelion would pass the object to the dynamical dominance of Saturn and Jupiter, if allowed. It has previously been argued that an overpopulated band of outer Oort cloud comets with an anomalous distribution of orbital elements could be produced by a solar companion with present parameter values $$m_c\approx 5\hskip.25em\hbox{M}_{\rm J}\left(\frac{9000\hbox{ AU}}{a_c}\right)^{1/2}.$$ If the same hypothetical object is responsible for both observations, then it is likely recorded in the IRAS and possibly the 2MASS databases.  相似文献   

19.
We provide our estimates of the intensity of the gamma-ray emission with an energy near 0.1 TeV generated in intergalactic space in the interactions of cosmic rays with background emissions. We assume that the cosmic-ray sources are pointlike and that these are active galactic nuclei. The following possible types of sources are considered: remote and powerful ones, at redshifts up to z = 1.1, with a monoenergetic particle spectrum, E = 1021 eV; the same objects, but with a power-law particle spectrum; and nearby sources at redshifts 0 < z ≤ 0.0092, i.e., at distances no larger than 50 Mpc also with a power-law particle spectrum. The contribution of cosmic rays to the extragalactic diffuse gammaray background at an energy of 0.1 TeVhas been found to depend on the type of sources or, more specifically, the contribution ranges from f ? 10?4 to f ≈ 0.1, depending on the source model. We conclude that the data on the extragalactic background gamma-ray emission can be used to determine the characteristics of extragalactic cosmic-ray sources, i.e., their distances and the pattern of the particle energy spectrum.  相似文献   

20.
We analyze our earlier data on the numerical integration of the equations of motion for 274 short-period comets (with the period P<200 yr) on a time interval of 6000 yr. As many as 54 comets had no close approaches to planets, 13 comets passed through the Saturnian sphere of action, and one comet passed through the Uranian sphere of action. The orbital elements of these 68 comets changed by no more than ±3 percent in a space of 6000 yr. As many as 206 comets passed close to Jupiter. We confirm Everhart’s conclusion that Jupiter can capture long-period comets with q = 4–6 AU and i < 9° into short-period orbits. We show that nearly parabolic comets cross the solar system mainly in the zone of terrestrial planets. No relationship of nearly parabolic comets and terrestrial planets was found for the epoch of the latest apparition of comets. Guliev’s conjecture about two trans-Plutonian planets is based on the illusory excess of cometary nodes at large heliocentric distances. The existence of cometary nodes at the solar system periphery turns out to be a solely geometrical effect.  相似文献   

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