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The two-dimensional axisymmetric hydrocode model of free particles is applied to the calculation of response of a comet nucleus to a meteorite impact. The nucleus is assumed to be spherical with a radius of 1 km. It is composed of a porous granular mixture of water ice and of mineral. Initial temperature is 50 K. Porosity is v = 0.6 and the mean density is p = 400 kg me3. Impactor radius is equal to 1 m and its mean density is equal to that of the nucleus. Impact velocity is 10 km se'. A normal impact is considered. Particular.forms for the equations of state (EOS) for the real medium (water ice and rock) as well as for an artificial medium of very low-density (40 kg mJ) filling up the voids are used. The cohesion is included in the constitutive model of the constituents. Numerical modelling provides the time dependent fields of pressure, density, temperature, and particle velocity in the vicinity of an impact point. The evolution of the field of temperature correlated with the function for kinetics of amorphous to crystalline phase transition permits discussion of the impact-induced crystallization of presumably initially amorphous ice. 0 1999 COSPAR. Published by Elsevier Science Ltd. MODEL OF COMET NUCLEUSThere exists a large diversity in details concerning modeling of comet nuclei. However, the dominating abundance of water ice as well as the high porosity of nuclei are well established. Other features are more or less controversial. Among them is the state of the ice: amorphous or crystalline, see e.g. Rickman (199 1) and Kauchi et al. (1994). The last authors concluded: "Icy grains which formed the Uranian and Neptunian satellites and comets were initially amorphous, if they were formed from the icy grains preserved from the molecular cloud stage". The form of ice depends on temperature of formation of a comet nucleus, on contents of radioactive nuclei including short lived Al% (PriahGk et al. 1987), on history of a comet from formation epoch till the recent time, and on actual properties of comet's orbit, in particular on perihelion distances. Intrinsic radioactivity, if sufficiently intense, can heat a presumably amorphous ice and transform it to the crystalline form. Energy of solar radiation or accumulated energy of many impacts can transform ice from amorphous to crystalline form in a layer spreading downward from the surface level to a certain depth. Study of impact induced phase transition from amorphous to crystalline phase of water ice is one of the goals of this work. This phase transition driven by solar flux of energy incident on comet surface was recently studied by Podolak and Prialnik ( 1996).We suppose that the grains of water ice and of mineral as well as the voids are randomly mixed. They form the nucleus that is statistically or "macroscopically" uniform, Figure 1. However, our two-dimensional (2D) geometry requires axial symmetry: the z-axis follows the direction of impactor movement and is perpendicular to the target; it is directed downward into the nucleus, with z = 0 at the ...

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The article is devoted to the preliminary concept of the Future Planetary Defense System (FPDS) emphasizing astroballistics. This paper is intended to support international efforts to improve the planetary security of Earth. The work covers three areas of knowledge: astronautics, astrodynamics, and astroballistics. The most important part of the presented article is dynamic, contact combat modeling against small, deformable celestial bodies. For these purposes, the original, proprietary hydrocode of the free particle method (HEFPM-G) with gravity was used. The main aim of combat is to redirect potentially hazardous objects (PHOs) to orbits safe for Earth or destroy them. This concept’s first task is to find, prepare, and use dynamic three-dimensional models of the motion of celestial bodies and spacecraft or human-crewed spaceships in the solar system’s relativistic frame. The second task is to prepare the FPDS’ architecture and computer simulation space missions’ initial concepts in the internal part of the solar system. The third and main task covers simulating, using hydrocodes, and selected methods of fighting 100 m diameter rock material asteroids.