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1.
Semi-quantitative investigation is made of hazard expected from an asteroidal impact in the Pacific. An impact ofd (diameter) = 200 m asteroid has a probability of hitting somewhere in the Pacific once in 15000 y. By carrying out a Monte Carlo simulation, such an impact, on average, is shown to create a tsunami as high as 16, 14, 15, and 21 m at Japan, Taiwan, Shanghai and Hawaii, respectively. Wooden houses, stone and brick houses, and reinforced concrete buildings are likely to be demolished by tsunamis of height 2, 7 and 20 m respectively. Thus, there is a probability of 1% or so that most of the artificial constructions on the coast lines of the Pacific be destroyed in the next century by an asteroidal impact. 相似文献
2.
S. Yabushita K. Wada T. Takai T. Inagaki D. Young E. T. Arakawa 《Astrophysics and Space Science》1986,124(2):377-388
The microorganism model of interstellar grains is investigated by spectroscopy from the infrared (IR), visible to the ultraviolet (UV) wave regions.E. coli, yeast and spores ofBacillus subtilis exhibit absorption bands at =3.1 and 9.7 m; they also exhibit several absorptions at 68 m which are in agreement with the observed IS extinction curves.To obtain the extinction curves in the visible and UV regions, dry films of microorganism are prepared on a MgF2 plate or synthesized quartz plate and their spectra measured. In the wavelength region 190400 nm, conventional spectrophotometers are adopted for the measurement. The extinction curve of the film ofE. coli is similar to the observed IS curve.For the wave-range 100<<400 nm, a vacuum UV spectrometer is adopted to avoid absorptions due to O2 in the atmosphere. The extinction spectra by this method are in agreement with the result obtained by the conventional method where comparison is possible. The extinction curves ofE. coli and yeast are such that they incrase towards the short wavelength and exhibit a peak at -190 nm, which is different from the well-known IS peak at =220 nm. It remains to be seen whether interstellar low temperatures (1040 K) can shift the peak position in the extinction curve of biochemical materials. 相似文献
3.
S. Yabushita 《Earth, Moon, and Planets》1984,31(1):43-47
Careful study by Owen of the displacements of the continents renders it likely that the Earth radius increased by almost 20% during the past 200 million years. Ramseys proposal that the Earth liquid core is a high-pressure phase of the mantle material is adopted to calculate the Earth radius under variableG. It is shown that ifG varies according to the large-numbers hypothesis of Dirac, the primordial Earth would have had radius 700 km less than the modern value. If it is assumed that the Earth began to respond and, hence, expand only 2 × 108 yr B.P., the expansion as large as 700 km is shown to be energetically plausible. 相似文献
4.
Arguments are presented to suggest that surface layers of the nuclei of periodic comets consist of crystallized rather than
amorphous water ice and thermal modelling of such nuclei is presented. The rate of sublimation of water from a rotating nucleus
is found to be greater than that from a uniformly heated nucleus. When the model is applied to P/Halley, the sublimation rate
at perihelion is found to be 8.1 × 1029 mol s−1 for a nucleus rotating with a period of 50 hours and 7.6 × 1029 for a uniformly heated nucleus on the premise that the effective radius of the nucleus is 2.5 km. The total sublimation of
water per revolution is 5.38 × 1036 molecules for P/Halley and 3.91 × 1036 molecules per P/Crommelin. The result so obtained is discussed in relation to the observational data. 相似文献
5.
Following the work of Whipple and Stefanik, radioactive heating by uranium, thorium and pottasium of a cometary nucleus is discussed. The assumed composition is that of interstellar medium. If thermal diffusivity is 10–4 cm2s–1, the central temperature of a nucleus with radius 10 km can be above 50 K, while if the thermal diffusivity is 5 × 10–3, the central temperature can be only 25 K or so. Volatile gases such as N2 and CO will flow toward the outer part of the nucleus and are lost in their first several approaches to the sun. This mechanism appears capable of explaining the depletion of N2 and CO relative to the interstellar abundance. It is argued that unfamiliar activity of comet Bowell could be explained by sublimation mainly of N2 and CO. 相似文献
6.
S. Yabushita 《Earth, Moon, and Planets》1996,74(3):183-189
Heat transport at very high temperatures is governed by a nonlinear diffusion equation, and in order to estimate the efficiency of nuclear explosions near the surface of a near earth object (NEO) in the orbit deflecting, it is required to solve the non-linear diffusion equation. Here, the solution is obtained by similarity considerations. It is shown that the earlier (approximate) value of the efficiency obtained by Simonenko et al. of the fraction of absorbed energy to the total incident energy is an overestimate by 60%. Other differences are in the propagation velocity of the thermal front and in the time required for hydrodynamical motion to start.A brief comparison is also made with the explosion where neutrons are the major products. 相似文献
7.
S. Yabushita 《Earth, Moon, and Planets》1992,58(1):57-63
The data set of Grieve which provides diameters and ages of craters is analyzed to obtain periodicity of the formation rate and decay constant of craters. It is confirmed that large craters (D 10 km) do not exhibit any periodicity while small ones appear to satisfy the Broadbent criterion for quantum (periodicity) hypothesis at P 29.5 myr. The result is consistent with a recent study of Bailey who showed that large craters are largely due to asteroids. Allowing for the decay of craters, an excess of 4 small craters within the nearest past is detected. In this sense, one may argue that the solar system is now in a moderate comet shower. 相似文献
8.
S. Yabushita 《Celestial Mechanics and Dynamical Astronomy》1992,54(1-3):161-178
Claim for periodicity in the crater formation rate is reinvestigated using a criterion proposed by Broadbent, and data sets of Rampino and Stothers and of Grieve are shown to satisfy the periodicity criterion (P 30 Myr).On the other hand, currently observed impactors are mainly asteroids, while long and short periodic comets whose fluxes may vary by external disturbances occupy only a small fraction. Using a Monte Carlo simulation, constraints are obtained for the dispersion Q(Myr) from an exact periodicity and for the periodic components (F
tp) in the signals for their periodicity to be detected. It is found that for = 5, 6 and 7 Myr, F
tp, would have to be 40% or greater, 60% or greater and 80% or greater, respectively. These constraints are used to discuss whether the giant molecular cloud perturbations can give rise to the periodicity in the impact events. The amplitude of the solar Z-motion need to be some 100pc for = 6 Myr, which requires the periodic component (SP and LP comets, if the former originate from the latter) to be 60%, while for = 7 Myr, the periodic component need to be 80%. The GMC perturbation model consistent with the periodicity appears to be the one where the amplitude is 100pc and the periodic component - 60% of the impactors. If SP comets mainly originate from a source such as the hypothetical Kuiper belt, the GMC perturbation would not be consistent with the periodicity. 相似文献
9.
Shin Yabushita 《Monthly notices of the Royal Astronomical Society》1998,299(1):244-248
The efficiency of absorption of X-rays generated by a nuclear explosion at the surface of an asteroid, estimated earlier, is used to calculate the explosion yield needed to deflect the orbit of an asteroid. Following the work of Ahrens &38; Harris, it is shown that a recoil velocity of 1 cm s−1 is required to deflect an asteroid from a collision course with the Earth, and the necessary yield of explosion energy is estimated. If it is assumed that the scaling law between the energy and the diameter of the resulting crater, obtained from experiments carried out on the Earth, remains valid on the asteroid surface, where gravity is much weaker, an explosion energy of 8 and 800 megaton (Mton) equivalent of TNT would be required for asteroids of diameter 1 and 10 km respectively. If, on the other hand, the crater diameter is proportional to a certain power of the gravity g , the power being determined from a dimension analysis, 130 kton and 12 Mton would be required to endow asteroids of diameters 1 and 10 km with the required velocity, respectively. The result indicates that in order to estimate the required explosion energy, a better understanding of cratering under gravity much weaker than on the Earth would be required. 相似文献
10.
S. Yabushita 《Earth, Moon, and Planets》1991,52(1):87-92
Maximum possible acceleration due to out-gassing from cometary nuclei is calculated for H2O and CO(N2) molecules. It is found that the maximum excess velocity at great distance is 0.18 km s–1 so that excess velocities less than this value are compatible with the non-gravitational acceleration due to non-symmetric out-gassing. On the other hand, Comet 1975q and comet 1955V have excess velocities 0.81 and 0.80 km s–1 respectively. These comets may be regarded as the candidates for possible interstellar comets. 相似文献