Akiko M. Nakamura

The Rubble-Pile Asteroid Itokawa as Observed by Hayabusa

During the interval from September through early December 2005, the Hayabusa spacecraft was in close proximity to near-Earth asteroid 25143 Itokawa, and a variety of data were taken on its shape, mass, and surface topography as well as its mineralogic and elemental abundances. The asteroids orthogonal axes are 535, 294, and 209 meters, the mass is 3.51 × 1010 kilograms, and the estimated bulk density is 1.9 ± 0.13 grams per cubic centimeter. The correspondence between the smooth areas on the surface (Muses Sea and Sagamihara) and the gravitationally low regions suggests mass movement and an effective resurfacing process by impact jolting. Itokawa is considered to be a rubble-pile body because of its low bulk density, high porosity, boulder-rich appearance, and shape. The existence of very large boulders and pillars suggests an early collisional breakup of a preexisting parent asteroid followed by a re-agglomeration into a rubble-pile object.

Regolith migration and sorting on asteroid Itokawa.

High-resolution images of the surface of asteroid Itokawa from the Hayabusa mission reveal it to be covered with unconsolidated millimeter-sized and larger gravels. Locations and morphologic characteristics of this gravel indicate that Itokawa has experienced considerable vibrations, which have triggered global-scale granular processes in its dry, vacuum, microgravity environment. These processes likely include granular convection, landslide-like granular migrations, and particle sorting, resulting in the segregation of the fine gravels into areas of potential lows. Granular processes become major resurfacing processes because of Itokawas small size, implying that they can occur on other small asteroids should those have regolith.

Detailed images of asteroid 25143 Itokawa from Hayabusa.

Rendezvous of the Japanese spacecraft Hayabusa with the near-Earth asteroid 25143 Itokawa took place during the interval September through November 2005. The onboard camera imaged the solid surface of this tiny asteroid (535 meters by 294 meters by 209 meters) with a spatial resolution of 70 centimeters per pixel, revealing diverse surface morphologies. Unlike previously explored asteroids, the surface of Itokawa reveals both rough and smooth terrains. Craters generally show unclear morphologies. Numerous boulders on Itokawas surface suggest a rubble-pile structure.

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.

Size-frequency statistics of boulders on global surface of asteroid 25143 Itokawa

The surface of asteroid 25143 Itokawa is covered with numerous boulders although gravity is very small compared with that of other asteroids previously observed from spacecraft. Here we report the size-frequency statistics of boulders on the entire surface of Itokawa based on high-resolution images (1 pixel ≈0.4 m) obtained by the Hayabusa spacecraft. There are 373 boulders larger than 5 m in mean horizontal dimension on the entire surface—0.393 km2—and the number density is nearly 103/km2. The cumulative boulder size distribution on the entire surface has a power-index of −3.1 ± 0.1. For the east and west sides and the head and body portions of Itokawa, the power-index of the size distributions and the number densities of boulders of these areas are thought to be similar from the statistical point of view. A global mapping of boulders shows that there is no apparent correlation in the locations of boulders and craters. The ratio of the total volume of the boulders to the total excavated volume of the craters on Itokawa is ≈25% when only craters larger than 50 m in mean diameter are considered, and this ratio is extremely larger than that on Eros and the Moon, respectively. The origin of boulders on the surface of Itokawa was examined quantitatively by calculating the number of boulders and the size of the largest boulder using a model based on impact cratering experiments. The result indicated that the boulders on the surface of Itokawa cannot solely be the product of craters. Our results suggest that the boulders originated from the disruption of the larger parent body of Itokawa, as has been described in previous papers (Fujiwata et al., Science, 312, 1330–1334, 2006; Saito et al., Science, 312, 1341–1344, 2006).

Catastrophic disruption experiments: recent results

Abstract This paper presents a review of the progress in the field of catastrophic disruption experiments over the past 4 years, since the publication of the review paper by Fujiwara et al. (Asteroids II, pp. 240–265, University of Arizona Press, Tucson, 1989). We describe the development of new techniques to produce shattering impacts relevant to the study of the collisional evolution of the asteroids, and summarise the results from numerous experiments which have been performed to date, using a variety of materials for both the impactor and the targets. Some of these, such as ice-on-ice, loose aggregates and pressurised targets, are quite new and have provided novel and exciting results. Some of the gaps existing previously in the data on fragment ejection-angle distributions, as well as translational and rotational velocity fields (including fine fragments) have been filled, and these new results will be surveyed.

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).

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.

OBSERVATIONAL EVIDENCE FOR AN IMPACT ON THE MAIN-BELT ASTEROID (596) SCHEILA

An unexpected outburst was observed around (596) Scheila in 2010 December. We observed (596) Scheila soon after the impact using ground-based telescopes. We succeeded in the detection of a faint linear tail after 2011 February, which provides a clue to determine the dust ejection date. It is found that the dust particles ranging from 0.1-1 ?m to 100 ?m were ejected into the interplanetary space impulsively on December 3.5 ?1.0 day. The ejecta mass was estimated to be (1.5-4.9)?108?kg, suggesting that an equivalent mass of a 500-800?m diameter crater was excavated by the event. We also found that the shape of the light curve changed after the impact event probably because fresh material was excavated around the impact site. We conclude that a decameter-sized asteroid collided with (596) Scheila only eight days before the discovery.

Interpretation of (596) Scheila's Triple Dust Tails

Strange-looking dust cloud around asteroid (596) Scheila was discovered on 2010 December 11.44-11.47. Unlike normal cometary tails, it consisted of three tails and faded within two months. We constructed a model to reproduce the morphology of the dust cloud based on the laboratory measurement of high-velocity impacts and the dust dynamics. As a result, we succeeded in reproducing the peculiar dust cloud by an impact-driven ejecta plume consisting of an impact cone and downrange plume. Assuming an impact angle of 45 Degree-Sign , our model suggests that a decameter-sized asteroid collided with (596) Scheila from the direction of ({alpha}{sub im}, {delta}{sub im}) = (60 Degree-Sign , -40 Degree-Sign ) in J2000 coordinates on 2010 December 3. The maximum ejection velocity of the dust particles exceeded 100 m s{sup -1}. Our results suggest that the surface of (596) Scheila consists of materials with low tensile strength.