長谷川 直

Impact ejecta with velocities exceeding the escape velocity of planetary bodies become meteorites and dust particles in interplanetary space. We present a new method that allows simultaneous measurement of the size and velocity of the largest high-velocity ejecta. High-speed camera images revealed the time required for the ejecta to reach the secondary target, and ejecta size was determined after the experiment by analyzing the craters formed upon their impact on the secondary target. We defined the size─velocity relationships of submillimeter ejecta with velocities exceeding 1 km s⁻¹, focusing on the largest detectable ejecta in our experiments. The results show that millimeter-sized meteoroids impacting the rocky surfaces of planetary bodies at 7 km s⁻¹ eject particles up to a few tens of micrometers in size toward interplanetary space at velocities exceeding the escape velocity of the body, even when it is greater than 1 km s⁻¹....

In space, asteroids have collided with planets and satellites throughout their long history, and many types of organic molecules have been synthesized. In particular, on satellites with gas atmospheres such as titan, a satellite of saturn, asteroid impacts cause hot reactions in a hot gas plume. To investigate the effects of asteroid impacts, simulation experiment has been carried out using a two-stage light-gas gun in nitrogen gas. we used a small polycarbonate bullet to impact on an iron-based target in nitrogen gas at a pressure of 100 kpa and a speed of approximately 7 km/s. As a result, many carbonaceous nanoparticles were produced. By analyzing the produced soot, using a water extraction method, a dabsylation method and a liquid chromatography method, we detected markedly large amounts of amino acids (glycine and alanine). The reaction process in the hot gas plume was considered. Many types of radicals collide with each other in nitrogen gas and the impact provides a suitable environment for the synthesis of amino acids and precursors of amino acids....

High velocity impact experiments were conducted on a conical shaped sand target, simulating a large-scale cratering formed in gravity-dominated regime, which could be affected by a surface topography such as curvature of bodies. The target material consists of dry quartz sand, prepared in conical shape with its vertex angle 120° A spherical Al projectile with its diameter of 2 mm was impacted vertically on the top part of a cone at the velocity from 1 to 4 km/s. After the impact, a top part of the conical target was excavated to form a shallow bowl-shaped crater on the top. The target resembled a trapezoid when observed from the side. The crater rim radius was able to be scaled by a conventional π-scaling relationship although it's radius was about 10 % smaller than that of the crater formed on semi-infinite flat surface. This might be caused by the geometrical effect of the target. The ejecta opening angle was measured at the time of crater formation and it was about 130°, where this is larger than that of the ejecta curtain, <90°, formed over the target of semi-infinite flat surface. This wider ejecta opening angle on conical target was able to be well reproduced by utilizing the Maxwell Z-model to a conical target....

Electrodynamic tethers are promising for removing debris but may be severed when they collide with orbital debris. Hollow cylindrical tethers (HCTs) had more severance-resistant designs than conventional solid cylindrical tethers (SCTs) because of their larger diameters for the same weight. However, high survivability could not be achieved if the cross-sectional shape of the HCT deviated from a cylindrical one. In this paper, we propose a novel device called a shape keeper (SK) that maintains the shape of the HCT. We propose three SKs made of different materials: aluminum, nonconductive polyurethane foam, and conductive polyurethane foam. The proposed SKs are evaluated in three aspects. First, the collision damage to a HCT with SKs is evaluated through hypervelocity collision experiments. The introduced SK functions as a bumper and suppresses tether damage depending on the collision point. Second, the debris removal time is investigated by considering the influence of the installed SKs on the debris removal mission. Third, the survivability of the HCT with SKs is assessed according to the two aforementioned evaluations. The results indicate that HCTs with SKs are superior to conventional SCTs in terms of survivability with the same weight....