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1.
We have estimated close asteroid encounters with the Earth by numerical integrations of a system with the Sun, 9 planets, and 188 near-earth-asteroids during the period 1994–4600. Asteroids approach the Earth from directions within 30? around the Sun in more than 20% of encounters with the closest distance less than 0.01 AU. Since ground-based observations cannot detect these objects, we should develop space-borne and/or lunar observatories in a short time to allow enough warning time before a catastrophic collision.  相似文献   

2.
The Sun's gradual brightening will seriously compromise the Earth'sbiosphere within 109 years. If Earth's orbit migrates outward,however, the biosphere could remain intact over the entiremain-sequence lifetime of the Sun. In this paper, we explore thefeasibility of engineering such a migration over a long timeperiod. The basic mechanism uses gravitational assists to (in effect)transfer orbital energy from Jupiter to the Earth, and therebyenlarges the orbital radius of Earth. This transfer is accomplishedby a suitable intermediate body, either a Kuiper Belt object or a mainbelt asteroid. The object first encounters Earth during an inward passon its initial highly elliptical orbit of large ( 300 AU)semimajor axis. The encounter transfers energy from the object to theEarth in standard gravity-assist fashion by passing close to theleading limb of the planet. The resulting outbound trajectory of theobject must cross the orbit of Jupiter; with proper timing, theoutbound object encounters Jupiter and picks up the energy it lost toEarth. With small corrections to the trajectory, or additionalplanetary encounters (e.g., with Saturn), the object can repeat thisprocess over many encounters. To maintain its present flux of solarenergy, the Earth must experience roughly one encounter every 6000years (for an object mass of 1022 g). We develop the details ofthis scheme and discuss its ramifications.  相似文献   

3.
Bromage  B.J.J.  Alexander  D.  Breen  A.  Clegg  J.R.  Del Zanna  G.  DeForest  C.  Dobrzycka  D.  Gopalswamy  N.  Thompson  B.  Browning  P.K. 《Solar physics》2000,193(1-2):181-193
Coronal holes on the Sun are the source of high-speed solar wind streams that produce magnetic disturbances at the Earth. A series of multi-wavelength, multi-instrument observations obtained during the 1996 Whole Sun Month campaign examined a large coronal hole in greater detail than ever before. It appeared on the Sun in August, and extended from the north pole to a large active region in the southern hemisphere. Its physical and magnetic structure and subsequent evolution are described.  相似文献   

4.
The spatial three-body problem of the Sun, Earth, Jupiter is studied over a finite time interval comparable with the age of the solar system. Some basic concepts of Nekhoroshev's theory are adopted; however, because of the finite time horizon considered, the canonical transformation scheme can be stopped with a third-order remainder. The overall effect of Jupiter is estimated from its maximal gravitational forces acting on the orbital elements of the Earth. This is done both analytically and numerically. The conservation of energy and angular momentum are rigorously taken into account. As part of a rather extensive programme which has not yet been completed, the effects of the first-order resonances and of a typical third-order rest term are estimated. Both contributions are found to be at their largest when the two osculating ellipses are coplanar. The third order force examined is zero at opposition by time reversal symmetry, but its maximal value lies close to this constellation. It can give rise to a 5% fluctuation of the semimajor axis of the Earth, not before 12 billion years, provided the eccentricity and inclination of the Earth are confined to 0e0.2 and 0i3/4, respectively. The self-consistent check of the two latter conditions is left to a future study. The results of this paper rely on a certain adiabatic approximation.  相似文献   

5.
It is pointed out that while the long-periodic variations of the elements of the terrestrial orbit around the Sun are probably sufficient to account for the frequency-spectrum of recurrent ice-ages established from the geological record of climatic changes experiences by the Earth in the course of the past half a million years, such kinematic phenomena cannot account naturally for the sudden onset of the Ice Age at the end of the Tertiary Epoch; nor for the fact that previous Ice Ages (in the Permian or pre-Cambrian times) are separated by milder (ice-free) intervals lasting 100 times longer than the periods of intermittent glaciation). Other astronomical phenomena (such as the galactic orbit of the solar system, which may cause our Earth temporarily to pass through different types of galactic climate; or temporary fluctuations in the energy output of the Sun), as well as geophysical phenomena (changes in atmospheric chemistry, and consequent fluctuations of the greenhouse effect), may have to be invoked to account for the geological facts by their combined effects.Lecture delivered on 11 October 1979 before the Serbian Academy of Sciences in Belgrade, Yugoslavia, at an International Conference in honour of Milutin Milankovich.  相似文献   

6.
Ovenden's hypothesis suggesting former existence of a planet of 90 Earth masses which supposedly filled the Titius-Bode gap in the asteroid belt and then suddenly disappeared 16 million years ago, is critically examined by the morphological method. It is shown that an explosive removal, however improbable, could have led to the formation of the asteroids from a non-explosive core (the nuclear charge being placed outside of it), but that life on Earth would have been completely destroyed by three successive blasts-one from the direct impact of the ejecta of the planet, another from the increased radiation suddenly emitted by the Sun when hit by the ejecta, and a third one (arriving, however, first) from the radiation emitted by the nuclear explosion. The geological record of the continuity of Life on Earth for the past 109 years definitely excludes the possibility of such an explosion in the late Tertiary.The other mode of removal of the planet-in a gravitational encounter with an intruder either from interstellar space or from the unexplored outskirts of the solar system, under the condition of not having disturbed the existing regularity of planetary orbits-is not only extremely improbable, to be expected once during 100 million times the age of the solar system; but it would leave no asteroids behind, all of the previously existing primaeval asteroids having been rapidly eliminated in encounters with the hypothetical planet.Whatever the merits of Ovenden's long-range calculations of the secular perturbations of coplanar circularized planetary orbits, the hypothesis of a massive planet to have existed in the asteroidal region and then recently to have suddenly disappeared, belongs to the realm of the impossible. After such a hypothetical event, either we would not be here on Earth, or there would be no asteroids in their present place between Jupiter and Mars.Paper dedicated to Professor Hannes Alfvén on the occasion of his 70th birthday, 30 May 1978.  相似文献   

7.
The Sun-Earth-Moon system is modeled by the restricted problem of three bodies, and the curves of zero velocity are used to define the limits of stability of the Moon's orbit about the Earth. By holding the relative distances fixed, and maintaining the circular velocities of the Earth and Moon while their masses are varied by a common factor (=m E/m E=m M/m M), it is found that the possibility of the Moon leaving Earth orbit and orbiting the Sun exists for the range of values 0.005<<0.4.  相似文献   

8.
A. Hewish  S. Bravo 《Solar physics》1986,106(1):185-200
Observations on a grid of 900 radio sources have been used to map and to track large-scale structures in the solar wind at distances of 0.6–1.5 AU from the Sun. Most of the disturbances were shells of enhanced density followed by high-speed streams lasting for several days, although more stable corotating interaction regions were also observed. Ninety-six disturbances were mapped during August 1978–September 1979 and those of the erupting stream-type were usually accompanied by shocks and geomagnetic activity if they encountered the Earth. Back-projection to the Sun indicated sources that were always associated with coronal holes. Possible associations with solar flares and disappearing filaments occurred but on many occasions no flare or filament activity was evident anywhere on the disc within a suitable time interval. It is concluded that erupting streams are transients generated by coronal hole activity. Evidence is presented which further suggests that coronal mass ejections of the curved-front variety may be identified with these erupting streams.  相似文献   

9.
We study the hypothetical conditions for interstellar clouds dense enough to produce glaciations on the Earth. A simple differential formula (adequate to give lower limits to dust absorption) is used to relate mean temperatures and visual albedos today and during the glacial eras. For this, the geological and oceanographical records of the Pleistocene are used. The temperature decays are associated to an absorption of the solar light in visual magnitudes m v. As the effective albedo, integrated in all wavelengths are lower than the corresponding visual value, the adoption of a visual scale leads to an underestimation of the actual amount of dust. A minimum dust absorption m v= 0.02 mag, necessary to start a glacial era is then obtained. This should mean interstellar clouds with dust densities of 4100 mag. pc -1 and sizes of 0.3 pc or more, taking into account the time span of the glacial eras and the mean velocity of the Sun with respect to the LSR. Such clouds were never observed and are uncompatible with what is known from the interstellar medium: the glaciation clouds should be clouds with densities 50–100 times above the tolerable value for gravitational stability; on the other hand, the necessary number of clouds per cubic parsec should produce the collapse of the galactic disc as a whole. From a comparative analysis of the temperatures of the other planets it seems to be that the actual thermal temperatures in their surfaces depend less than one expects from the visual albedos. From this it is raised the suspicion that the cause of the ice ages was the Sun itself.  相似文献   

10.
We have numerically integrated the orbits of 18 fictitious fragments ejected from the asteroid 6 Hebe, an S-type object about 200km across which is located very close to theg=g 6 (orv 6) secular resonance at a semimajor axis of 2.425AU and a (proper) inclination of 15° .0. A realistic ejection velocity distribution, with most fragments escaping at relative speeds of a few hundredsm/s, has been assumed. In four cases we have found that the resonance pumps up the orbital eccentricity of the fragments to values >0.6, which result into Earth-crossing, within a time span of 1Myr; subsequent close encounters with the Earth cause strongly chaotic orbital evolution. The closest Earth and Mars encounters recorded in our integration occur at miss distances of a few thousandths ofAU, implying collision lifetimes <109 yr. Some other fragments affected by the secular resonance become Mars-crossers but not Earth-crossers over the integration time span. Two bodies are injected into the 3 : 1 mean motion resonance with Jupiter, and also display macroscopically chaotic behaviour leading to Earth-crossing. 6 Hebe is the first asteroid for which a realistic collisional/dynamical evolutionroute to generate meteorites has been fully demonstrated. It may be the parent body of one of the ordinary chondrite classes.  相似文献   

11.
We discuss the main mechanisms affecting the dynamical evolution of Near-Earth Asteroids (NEAs) by analyzing the results of three numerical integrations over 1 Myr of the NEA (4179) Toutatis. In the first integration the only perturbing planet is the Earth. So the evolution is dominated by close encounters and looks like a random walk in semimajor axis and a correlated random walk in eccentricity, keeping almost constant the perihelion distance and the Tisserand invariant. In the second integration Jupiter and Saturn are present instead of the Earth, and the 3/1 (mean motion) and v 6 (secular) resonances substantially change the eccentricity but not the semimajor axis. The third, most realistic, integration including all the three planets together shows a complex interplay of effects, with close encounters switching the orbit between different resonant states and no approximate conservation of the Tisserand invariant. This shows that simplified 3-body or 4-body models cannot be used to predict the typical evolution patterns and time scales of NEAs, and in particular that resonances provide some fast-track dynamical routes from low-eccentricity to very eccentric, planet-crossing orbits.On leave from the Department of Mathematics, University of Pisa, Via Buonarroti 2, 56127 Pisa, Italy, thanks to the G. Colombo fellowships of the European Space Agency.  相似文献   

12.
The Hat Creek two-element interferometer has been used to study the quiet Sun at 22 GHz. A statistical analysis of output of the interferometer clearly shows the existence of time variations on the quiet Sun with time scales 180 s. The observations suggest that the fine structure on the quiet Sun might consist of two components - one which varies with the time scales 3 min and the other being relatively stable. The average visibility amplitude indicates that the fine structure on the Sun has a typical angular size of 6. The observation that the variance and the mean of the visibility amplitudes depend in the same way on the projected baseline suggests that the transient sources have angular size similar to the average size of the fine structure on the quiet Sun. Power spectra of the output of the interferometer show no significant periodicity.On leave of absence from Tata Institute of Fundamental Research, Bombay, India.  相似文献   

13.
Photographs of the Sun in predominantly L radiation (centered at 1215.67 Å) with 3 spatial resolution were taken from an Aerobee rocket shortly after fourth contact by the Moon on the eclipse day of 1972 July 10. This preliminary reporting of the results describes the instrument and shows two of the photographs taken. The supergranulation is manifest and active regions and filaments are well resolved over the entire disk. Densitometer traces across the disk are presented giving the flux incident on the Earth from active regions, cell boundaries, and filaments.  相似文献   

14.
It is known that the observed secular accelerations of the Sun and Moon are not consistent with the tidal interactions of the Earth with the Sun and Moon. Following Dicke, the hypothesis of variable constant of gravity is adopted and expressions for the accelerations are derived. It is shown that if the theoretical ratio of the acceleration is equated the observed one, a unique value for —/G can be calculated. Adopting the accelerations obtained by Fotheringham, Newton, Muller and Stephenson, and Stephenson, it is found that — /G ranges from 1.4 × 10–11 to 3.3 × 10–11 yr–1. This estimate is consistent with the one based upon the comparison of the lunar accelerations measured with respect to atomic and ephemis times.  相似文献   

15.
Some properties of solar radio bursts observed at the Earth are mainly due to propagation effects in the corona. A radio echo of short-time narrow-band bursts is observed by a decameter radioheliograph on the basis of UTR-2 antenna. Propagation effects are manifested in the marked regular change of the burst intensity-time profile at 25 MHz during a half-rotation of the Sun. A displacement of limb diffuse bursts deep into the solar atmosphere of 1.5 - 2R has been also found during the burst lifetime.  相似文献   

16.
The aim of this paper has been to discuss a possibility of the formation of an inertial system of coordinates, precise within 0″.001, with the aid of the measures of mutual angular distances of quasars by the VLBI technique used in radio-astronomy. Such a system depends on neither the rotation, nor revolution of the Earth around the Sun — a fact which permits a separation of the astronomical and geophysical aspects of the problem. The proposed system should permit us to resolve some general astrometric problems — such as of the precession and nutation of the Earth as well as of the motion of the terrestrial axis of rotation within the Earth, or fluctuations in the angular veoocity of terrestrial rotation.  相似文献   

17.
Corotating solar wind streams emanating from stable coronal structures provide an unique opportunity to compare the response of planetary ionospheres to the energy conveyed in the streams. For recurrent solar conditions the signal propagating outward along spiral paths in interplanetary space can at times exhibit rather similar content at quite different downstream locations in the ecliptic plane. Using solar wind measurements from plasma detectors on ISEE-3, Pioneer Venus Orbiter (PVO) and Helios-A, as well as in-situ ion composition measurements from Bennett Ion Mass Spectrometers on the Atmosphere Explorer-E and PVO spacecraft, corotating stream interactions are examined at Earth and Venus. During May–July 1979 a sequence of distinct, recurrent coronal regions developed at the Sun. Analysis of these regions and the associated solar wind characteristics indicates a corrresponding sequence of corotating streams, identifiable over wide distances. The time series of solar wind velocity variations observed at Earth, Venus, and the Helios-A positions during June–July attests to intervals of corotating stream propagation. The characteristics of the stream which passed Earth on July 3, are observed at Helios-A and at Venus (PVO) about 8 days later, consistent with the spiral path propagation delay times between the locations in the ecliptic plane. On July 3, Earth and Venus have a wide azimuthal separation of about 142 . Although the planetary environments are distinctly different, pronounced and somewhat analagous ionospheric responses to the stream passage are observed at both Earth and Venus. The response to the intercepted stream is consistent with independent investigations which have shown that the variability of the solar wind momentum flux is an important factor in the solar wind-ionosphere interaction at both planets.  相似文献   

18.
Satellite observations of the heliospheric current sheet indicate that the plasma flow velocity is low at the center of the current sheet and high on the two sides of current sheet. In this paper, we investigate the growth rates and eigenmodes of the sausage, kind, and tearing instabilities in the heliospheric current sheet with the observed sheared flow. These instabilities may lead to the formation of the plasmoids and kink waves in the solar wind. The results show that both the sausage and kink modes can be excited in the heliospheric current sheet with a growth time 0.05–5 day. Therefore, these modes can grow during the transit of the solar wind from the Sun to the Earth. The sausage mode grows faster than the kink mode for < 1.5, while the streaming kink instability has a higher growth rate for > 1.5. Here is the ratio between the plasma and magnetic pressures away from the current layer. If a finite resistivity is considered, the streaming sausage mode evolves into the streaming tearing mode with the formation of magnetic islands. We suggest that some of the magnetic clouds and plasmoids observed in the solar wind may be associated with the streaming sausage instability. Furthermore, it is found that a large-scale kink wave may develop in the region with a radial distance greater than 0.5–1.5 AU.Also at Department of Earth and Space Science, University of Science and Technology of China, Hefei Anhui 230029, China.  相似文献   

19.
We present the use of the analytical solutions of the planets and of the Moon's motion in the determination of the quantities which relate the barycentric and the geocentric coordinate systems and of the expressions of precession-nutation. The computation of the precession and nutation quantities are built with the analytical theories of the motion of the Moon, the Sun and the planets of the Bureau des longitudes. We take into account the influence of the Moon, the Sun and all the planets on the potential of the Earth limited to C j,0 for j from 2 to 5, C 2,2, S 2,2, C 3, S 3, , for from 1 to 3 and C 4,1, S 4,1. We determine the 3 Euler angles , , and 2 calculating the components of the torque of the external forces with respect to the geocenrer in the case of the rigid Earth. The equations are solved by iterations and so are taken into account the nutations-on-the-nutations effects. We have determined the analytical variations of the angles and w fixing the equator with respect to the ecliptic J2000. We find, in w, a secular term of –26.5026 mas per century. The analytical solution of the precession-nutation has been compared to a numerical integration over the time span 1900–2050. The differences do not exceed 16 µas for and 12 µas for .  相似文献   

20.
Except for protons, the chemical composition of solar cosmic rays is very similar to the abundance of the elements at the photosphere of the Sun. If we consider the relative abundance ratio of protons to -particles (P/) at constant rigidity, this ratio is highly variable from one solar cosmic ray event to another. This ratio observed at the Earth, however, decreases monotonically with time from the onset of solar flares and, furthermore, is dependent on the heliocentric distance of the parent flares from the central meridian of the solar disk. P/'s which have been measured before the onset of SC geomagnetic storms change from 1.5 to 50 or more, being a function of the westward position of the source from the east limb of the Sun. These variations with respect to time and heliocentric distance suggest that the propagation of solar cosmic rays is strongly modulated in the interplanetary space. The major part of the -particles seem to propagate as if they are trapped within the magnetic clouds which produce SC geomagnetic and cosmic ray storms at the earth.The chemical composition and rigidity spectra of solar cosmic rays suggest that solar cosmic rays are mainly accelerated by the Fermi mechanism in solar flares. The observed variation of P/'s is produced mainly through the difference between the propagation characteristics of protons and -particles.NAS-NRC Associate with NASA.  相似文献   

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