EXPERIMENT TO SIMULATE IMPACT INTO LOW-DENSITY ASTEROIDS

进行了实验和理论研究，以了解风化层是否存在于小行星表面。风化层是由陨石高速撞击所喷出的碎片堆积而成。其速度低于逃逸速度。我们确定的速度的碎片喷射的影响，实验和理论。特别是由于最近的太空任务报告显示存在密度非常低的小行星，我们考虑到了这一点。靶体由通过加热烧结的玻璃珠制成。通过改变烧结周期，可获得约4%至80%的孔隙率。用直径为2 ~ 7 mm的尼龙弹以3 ~ ★km/s的速度撞击靶板。高速摄影机记录了撞击现象。大部分破片沿靶面法线沿着射出，测量了破片在靶面上的落点位置和靶面上的弹坑形状。根据前一次测量，获得了速度小于约20 m/s时的碎片速度-尺寸分布。结果表明，碎片速度随孔隙率的增加而减小，陨石坑直径在孔隙率约为40%时出现最大值，据此建立了速度-尺寸分布的标度律，并利用该标度律用数值方法研究了小行星风化层厚度的发展。结果发现，即使在10 - 1公里大小的小行星上，强度较弱的物质也存在风化层。这些结果与最近的Near-Shoemaker航天器获得的事实相一致，即433 Eros表面有风化层。

Experimental and theoretical studies were performed to understand whether regolith exists or not on the asteroid surface. The regolith is formed by accumulation of the fragments wliich are ejected by high-velocity impacts of meteoritic bodies but. whose velocity is lower than the escape velocity. We determined velocity of the fragments ejected by the impact both experimentally and theoretically. Especially since recent reports from space missions shows the existence of asteroids of very low density, we take this point into account. Target bodies were made of glass beads sintered by heating. Porosity of about 4 to 80 per cent was made by changing the sintering period. Nylon projectiles of 2 mm to 7 mm in diameter were impacted to the targets at velocity 3-★km/s. Impact phenomena were recorded by high-speed cameras. Most of fragments were ejected along the target surface normal, and the location of their landing site on the floor and crater shape on the targets were measured. From the former measurement fragment velocity-size distribution at the velocity less than about 20 m/s was obtained. It was shown quantitatively that fragment velocity decreases with increasing of porosity.Also found was that the crater diameter has a maximum at porosity of about 40 per cent. From these results a scaling law of velocity-size distribution was constructed and using this time development of asteroid regolith thickness was pursued by numerical method. As the result it was found that, regolith exists even on the asteroids of 10 - 1 km in size for the weak strength material. These results are compatible with the fact obtained by recent Near-Shoemaker spacecraft that 433 Eros has regolith on its surface.