The statistics of Earth-approaching asteroids are first summarized, and an enhanced frequency of objects smaller than 100 meters is noted. Superposed on these random hazards may be a periodic one of new comets due to galactic tides of the Oort Cloud with a period of 26-36 Myr (Rampino, 1998). New asteroids and comets are being found evermore frequently because new telescope-and-detector systems are coming on line. These are intended primarily for the discovery of dangerous objects, but a beginning has been made with the study of statistics of main-belt asteroids. In addition to trans-Neptunian objects, cis-Neptunian “Centaurs” are recognized, which may be a link in the evolution of short-period comets and thereby contribute to the flux of Earth approachers. With the new equipment coming on line, we are beginning to see that the global hazard will be mostly quantified within a few decades. We do see a shortage in astrometric follow up fainter than about the 20th magnitude.

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We conducted 10 shots of high-velocity oblique impact experiments (1.95-3.52 km/s) using nylon projectiles and spherical mortar targets. Large craters were formed, but these targets were not disrupted by the impacts. We then calculated the efficiencies of momentum transfer from the projectile to the post-impact target for each experiment. The efficiencies of angular momentum transfer from the translational motion of the projectiles to the rotation of the post-impact targets were also derived. A representative efficiency of angular momentum transfer was calculated to be 0.17 for random successive collisions. The efficiency was applied to an equation expressing the precession angle of asteroids. It is shown that 1989ML, target of Japan-US asteroid-sample-return-mission (MUSES-C) would be tumbling.

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The new profiles of the space missions aimed at asteroids and comets, moving from fly-bys to rendezvous and orbiting, call for new spaceflight dynamics tools capable of propagating orbits in an accurate way around these small irregular objects. Moreover, interesting celestial mechanics and planetary science problems, requiring the same sophisticated tools, have been raised by the first images of asteroids (Ida/Dactyl, Gaspra and Mathilde) taken by the Galileo and NEAR probes, and by the discovery that several near-Earth asteroids are probably binary. We have now developed two independent codes which can integrate numerically the orbits of test particles around irregularly shaped primary bodies. One is based on a representation of the central body in terms of “mascons” (discrete spherical masses), while the other one models the central body as a polyhedron with a variable number of triangular faces. To check the reliability and performances of these two codes we have performed a series of tests and compared their results. First we have used the two algorithms to calculate the gravitational potential around non-spherical bodies, and have checked that the results are similar to each other and to those of other, more common, approaches; the polyhedron model appears to be somewhat more accurate in representing the potential very close to the body's surface. Then we have run a series of orbit propagation tests, integrating several different trajectories of a test particle around a sample ellipsoid. Again the two codes give results in fair agreement with each other. By comparing these numerical results to those predicted by classical perturbation formulae, we have noted that when the orbit of the test particle gets close to the surface of the primary, the analytical approximations break down and the corresponding predictions do not match the results of the numerical integrations. This is confirmed by the fact that the agreement gets better and better for orbits farther away from the primary. Finally, we have found that in terms of CPU time requirements, the performances of the two codes are quite similar, and that the optimal choice probably depends on the specific problem under study.

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This paper deals with the effect of a third body on the apsidal motion of two bodies. The specific case involves a third body-planet Jupiter and the apsidal line motion of a minor planet that orbits the Sun and has its apsidal line go through the major axis of an ellipse. The third body (Jupiter) which satisfies the Langrangian solution will affect the apsidal line motion and therefore affects the ascending and descending motions of the minor planet. In this case no analytical solutions can be obtained, and therefore specific assumptions are made along with numerical solutions. For convenience, we adopt the Lagrangian solution in the three-body problem and obtain quasi-analytical results, which are used to evaluate the effect of the planet on the dΩ/dt (Ω ascending node) of each minor planet. This method is beneficial for improving our knowledge of the orbital elements of the asteroids, and perhaps even much smaller effects such as the effects of the planets on the interplanetary dust complex. Information on the latter may be provided by using this method to investigate Jupiter's effect on the inclination of the symmetry surface of the zodiacal dust cloud.

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We investigate the formation of spatial structure in dense, self-gravitating particle systems such as Saturn's B-ring through local N-body simulations to clarify the intrinsic physics based on individual particle motion. In such a system, Salo (1995) showed that the formation of spatial structure such as wake-like structure and particle grouping (clump) arises spontaneously due to gravitational instability and the radial velocity dispersion increases as the formation of the wake structure. However, intrinsic physics of the phenomena has not been clarified. We performed local N-body simulations including mutual gravitational forces between ring particles as well as direct (inelastic) collisions with identical (up to N ≈ 40000) particles. In the wake structure particles no longer move randomly but coherently. We found that particle motion was similar to Keplerian motion even in the wake structure and that the coherent motion was produced since the particles in a clump had similar eccentricity and longitude of perihelion. This coherent motion causes the increase and oscillation in the radial velocity dispersion. The mean velocity dispersion is rather larger in a more dissipative case with a smaller restitution coefficient and/or a larger surface density since the coherence is stronger in the more dissipative case. Our simulations showed that the wavelength of the wake structure was approximately given by the longest wavelength λ_cr = 4π²GΣ/κ² in the linear theory of axisymmetric gravitational instability in a thin disk, where G, Σ, and κ are the gravitational constant, surface density, and a epicyclic frequency.

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Recent advances in a series of studies based on the molecular dynamics (MD) computer simulation that was performed to investigate the whole of H₂ formation process on the surface of dust grains throughout within a single model are reviewed. Amorphous water ice slabs were generated at 10 K and 70 K as a model surface of dust grains, and then the first and second incident H atoms were thrown onto the surface. The following fundamental processes of H2 formation via two H atoms' recombination, H + H -> H₂, were studied in detail; 1) the sticking of H atom onto the surface, 2) the diffusion of H atom on the surface, 3) the reaction of two H atoms on the surface, 4) the ejection of H₂ from the ice surface. The sticking probability, the mobility, the reaction probability, and the ejection lifetime were selfconsistantly obtained for the above processes. The product energy distribution of H₂ molecules formed on icy mantles of dust grains was also studied, and it was found that H₂ molecules can be highly vibrationally excited by formation pumping mechanism.

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The deflection of interstellar dust grains in the magnetic field near the heliopause depends on their surface electric charge. We study the electric charging of the grains with emphasis on the secondary electron emission because of its importance in the hot plasma environment near the heliopause. We correct previous models of the secondary electron emission that overestimate the electric charge of dust near the heliopause. Our model calculations of the grain charge, when combined with results from in situ measurements of interstellar dust in the heliosphere, place an upper limit on the magnetic field strength. We find that the detection of interstellar dust with mass of 10^-18 kg indicates the component of the magnetic field perpendicular to the interstellar dust flow to be less than 0.4 nT.

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This paper reviews major results of present studies and recent developments for future missions in the Japanese space program regarding in-situ measurement and collection of micrometeoroids and orbital debris in the near Earth space. Japan's contribution in this area began with the post flight impact analysis of the Space Flyer Unit (SFU) satellite which was returned to Earth in 1996 after 10-month exposure in space. Despite a decade later than similar efforts first conducted in the USA and Europe, it resulted in a record of over 700 hypervelocity impact signatures, which now forms the nation's first database of real space impacts being open to public in the Internet. Together with laboratory impact tests, both morphological and elemental analyses of the impact craters yielded new insights of the meteoroid to debris ratio as well as flux variation compared with the previous spacecraft. The next step was a passive aerogel exposure in the STS-85 shuttle mission in 1997. No hypervelocity impact was found there but its experience has been incorporated for designing a microparticle collector to be on-board the Japan Experiment Module-Exposed Facility of the International Space Station. All of such “passive” collection of micro-impact features, however, still leave the significant uncertainty in the quest of their origins. Therefore an aerogel-based “hybrid” dust collector and detector (HD-CAD) is currently under the development. It measures time of impact and deduces impactors' orbital and physical parameters by detecting impact flash while still capturing them intact. The system is suitable for both (1) sample return missions in LEO as well as to parent bodies of meteoroids, i.e., comets and asteroids, and (2) one-way mission to where the thermal and plasma environment is such that impact induced plasma detectors may suffer from significant noise, e.g., a Mercury orbiter and a solar probe. Together with unambiguous dust samples from a comet by STARDUST and an asteroid by MUSES-C as references, the HD-CAD in the LEO will be able to deduce the accretion rates of the cometary and asteroidal dust grains on the Earth.

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POLDER-sensor onboard the satellite ADEOS has observed the reflected light from Earth surface-atmosphere system at multi-viewing angles. These directional measurements include significant information of scattering particles. The POLDER possesses high radiometric sensitivity and multi-channels in the visible and near infrared wavelengths, and measures polarization as well as radiance. This work intends to show that its multi-spectral and multi-directional measurements are useful to retrieve aerosol characteristics. The basic algorithm for aerosol retrieval is based on light scattering simulations of polarization field, where the heterogeneous aerosol model according to Maxwell-Garnett mixing rule is considered. Monthly global distribution maps showing the Ångström exponent and the optical thickness of aerosols were obtained. From these aerosol maps, it can be seen that the aerosol distribution derived from POLDER radiance and polarization data coincides with that from the radiance data obtained independently from OCTS, and atmospheric aerosols exhibit certain intrinsic feature on a global and on a seasonal scale.

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For the purpose of simulating the surface alteration process called “space weathering”, experiments of pulse laser irradiation, proton implantation, and laser irradiation to proton implanted samples were performed and reflectance spectra of altered materials were measured. To simulate the impact heating by micrometeorite bombardments, we made a new apparatus using a pulse laser whose pulse duration is 6-8 nanoseconds, comparable with a timescale of micrometeorite impacts. We find that the degree of space weathering, i.e., change of reflectance spectrum should depend on mineral composition. Laser irradiation onto olivine produces the largest reduction of albedo and the highest reddening of reflectance spectrum. In general, variation of olivine spectra is much larger than that of pyroxenes. Depths of absorption bands do not change in the scaled spectra. The olivine spectrum after the laser irradiation can match spectra of some olivine asteroids within a subtype of S-type asteroids. Comparison of Vesta spectrum with altered pyroxene spectra suggests that Vesta surface would be relatively older than olivine asteroids. We also investigate the influence of solar wind proton and pyroxene FeO content. The proton implantation causes small changes in olivine and enstatite spectra. Implanted protons do not influence spectral change by the laser irradiation: the laser irradiation and the proton implantation do not produce multiplicative but additive changes on the reflectance spectra. FeO content of pyroxenes does not relate to the degree of reflectance change.

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