Akira Fujiwara

Destruction of basaltic bodies by high-velocity impact

As a simulation of collisional processes among solid bodies of various sizes in the solar system, polycarbonate projectiles of mass 0.37 g were impacted against cubic basaltic rocks of about 2 to 10 cm and larger with a velocity of 2.6 km/sec. The corresponding energy imparted per unit mass of target ranges from about 106 to 109 ergs/g. The phenomena are classified into four categories with increasing target size: (1) complete destruction, (2) remaining core, (3) transition region, and (4) crater formation. Empirical formulas for the cumulative mass of the fragments and the mass of the maximum fragment are given. The similarity of these formulas is briefly discussed. The experimental results are applied to the examination of the hypothesis that a single Martian satellite was once ruptured by impact, leaving the present two satellites. It is suggested that the radius of the parent satellite was larger than about 30 km.

Velocity distribution of fragments formed in a simulated collisional disruption

Abstract The velocity distribution of fragments resulting from catastrophic disruption was determined experimentally which is of great importance to understand the collisional evolution of planetary bodies. Basalt and alumina spheres 6 cm in diameter were shattered by nylon spheres 7 mm in diameter at velocities of 3 ∼ 4 km/sec. From high-speed photographic records taken from two orthogonal directions, velocity, initial position, and size of a few hundreds of fragments were obtained using an image processor. The three-dimensional fragment velocity determined for some prominent fragments is expressed as the − 1 6 power of fragment mass for fragment sizes larger than a few millimeters. The fraction of energy, momentum, and angular momentum partitioned into the large major fragments, core, and the fast fine ejecta from near the impact site were evaluated. About 1% of the initial kinetic energy was found to be partitioned into the major, large fragments. Cores were minor carriers of energy and momentum, while fine ejecta played a significant role as carriers of the three kinetic quantities.

Experimental study on the velocity of fragments in collisional breakup

Abstract The motion of fragments following a catastrophic destruction by either a normal or an oblique impact at 2.5–2.9 km sec−1 into cubic and spherical basalt targets was studied with a high-speed framing camera. Velocities at the antipodes of the targets vary as (E/M)0.75 (E = impact energy; M = target mass) and are lower than 200 m sec−1 at E/M ⪅ 109 ergs g−1. Excluding fine-grained particles from the impact site, 70 to 80% by mass fraction of the fragments have velocities lower than twice the antipodal velocity. Comminution and ejection energies wasted in this mass fraction were a few percent of the impact energy at E/M ⪅ 5 × 107 ergs g−1. During a catastrophic impact into asteroids some of the fragmented bodies can be reconcentrated by mutual gravitation.

On the mechanism of catastrophic destruction of minor planets by high-velocity impact

Abstract For the study of the collisional breakup of minor planets, semiquantitative interpretations of the catastrophic destruction of cubic rock targets by high-velocity impact reported by A. Fujiwara, G. Kamimoto, and A. Tsukamoto (1977, Icarus 31, 277–288) are attempted. The conditions for transition and core-type destruction are derived from consideration of the side surface spallation, or central spallation, and the back surface spallation caused by the rarefaction pulses. In the derivation, the strengths of the original P-pulse at the crater rim and the bottom are given by application of the failure criterion. As the condition for complete destruction, an empirical rule is proposed. It is found that the critical sizes for these destructions vary as E0.40 and the previously reported E M - scaling is only an approximate rule (E, impact energy; M, target mass). The possibilities of extending of these results to larger bodies of any other shape are discussed.

Velocity and spin of fragments from impact disruptions: I. An experimental approach to a general law between mass and velocity

Abstract A new velocity distribution for fragments from impact disruption of a gypsum sphere, a previous distribution for basalt and alumina targets, and basalt antipodal velocity data were compared using the center of mass system (CMS). The results show that the CMS velocity of fixed—mass fragments is within a range of an order of magnitude, despite the different materials and collisional conditions, as long as the projectile size is relatively unchanged. The CMS velocity is proportional to ∼ −( 1 3 ∼ 1 6 ) power of the mass, and this relationship is discussed in relation to the ejection model proposed for cratering by Melosh (1984, Icarus 59, 234–260, 1987, Int. J. Impact Eng. 5, 483–492).

Rotation of fragments in catastrophic impact

Abstract The rotation of fragments produced by catastrophic impacts into basalt targets was investigated using framing camera records taken by Fujiwara and Tsukamoto (1980, Icarus 44 , 142–153). Most of the cores have low rotation rates, of the order 1 rev · sec −1 or less. Many spall fragments have high rotation rates in the strong shear field produced in the target material by impact.

Velocity of finer fragments from impact

Abstract We describe the method and the result of a new experiment to obtain velocity distribution of fine ejecta fragments, from a few to a hundred microns in size, produced from basalt targets by impacts of nylon projectiles at a velocity of 3.7 km s −1 . The size distribution of holes perforated by the ejecta fragments on thin films and foils placed around the targets was investigated, and the size-velocity relation was determined with the aid of an empirical formula for threshold penetration (McDonnell and Sullivan, Hypervelocity Impacts in Space , Unit for Space Sciences, University of Kent, 1992). The velocity of the fastest fragments, at a given size, is from the extrapolation of the size-velocity relation for 1–100 mm fragments (Nakamura and Fujiwara, Icarus 92 , 132–146, 1991; Nakamura et al , Icarus 100 , 127–135, 1992). The laboratory results are also compared with those obtained from the study of secondary craters around large lunar craters (Vickery, Icarus 67 , 224–236, 1986, Geophys. Res. Lett . 14 , 726–729, 1987). All these data provide a smooth size-velocity relationship in the normalized fragment size range of four orders of magnitude.

Complete fragmentation of the parent bodies of Themis, Eos, and Koronis families

Abstract The fragmentation of the parent asteroids of the Themis, Eos, and Koronis families is investigated by considering mutual gravitational effects among the fragmented bodies. The masses of the parent asteroids and the kinetic and gravitational energies of the fragmented bodies are estimated. Comparison of these results and data from the laboratory impact experiments leads to the conclusion that the parent asteroids of the three families were completely fragmented at E p / M of 10 8 erg/g or more ( E p , impact energy; M , parent mass). However, since most of the fragments had low relative velocities many reaccumulated through mutual gravitation. The larger members in these families should have the rubble pile structures and hydrostatic equilibrium figures.

Impact fracture patterns on phobos ellipsoids

Abstract Phobos-ellipsoid models made of clay were fragmented by the impact of high-velocity projectiles to examine the idea proposed by P. Thomas, J. Veverka, and T. Duxbury ((1978) Nature 273 , 282–284) that the grooves on Phobos are the manifestation of fractures produced by the Stickney-forming impact. The fracture lines on the models consist of two sets. One is concentric around the impact site and along E lines, which are defined as the intersecting lines of the ellipsoid surface and a set of spherical surfaces with the center of the spheres at the impact site. The other runs radially from the impact site and along P lines, which are defined as the lines crossing E lines perpendicularly on the ellipsoid surface . Some patterns of the grooves originating radially from the crater Stickney on Phobos are very similar to the P lines. The gridded topography, hummocky groove sections, and smooth topography on Phobos could have been formed by the fracture or associated surface disturbances due to the wave induced by the Stickney-forming impact, because they are distributed along the E lines surrounding the converging point of the P lines. All the models except one showed that the density of the fractures east of the impact site is greater than that of those to the west. Fracture patterns similar to one of the most prominent groove sets, which converge and diminish into the region of about (270°, 0°) were not produced by the impact on the ellipsoid of uniform constituent. These grooves would have been produced by the opening of preexisting cracks by the Stickney-forming impact. Other grooves also seem to be affected by such latent cracks.

Energy partition into translational and rotational motion of fragments in catastrophic disruption by impact: An experiment and asteroid cases

Abstract Translational kinetic energy E t and rotational kinetic energy E r of the fragments produced in the laboratory catastrophic disruption of a basalt sphere by the impact of a high-speed projectile were determined. It was found that the maximum value of E t / E t is of the orders of 10 −2 , which is not so different in the order of magnitude from the value estimated for family asteroids in spite of the great difference of the scale. However, the laboratory value is significantly higher than the value for the family.