EXPERIMENT TO SIMULATE IMPACT INTO LOW-DENSITY ASTEROIDS

进行了实验和理论研究，以了解小行星表面是否存在风化层。风化层是由陨星体高速撞击抛射出的碎片堆积而成的。它的速度小于逃逸速度。我们从实验和理论两方面确定了撞击产生的碎片的速度。特别是最近来自太空任务的报告显示存在密度非常低的小行星，我们考虑到这一点。靶体由玻璃微珠加热烧结而成。通过改变烧结时间，孔隙率约为4%至80%。直径2mm ~ 7mm的尼龙弹丸以3 ~★km/s的速度撞击目标。撞击现象由高速摄像机记录下来。大部分碎片沿目标表面法向抛射，并测量了其在地面上的着陆点位置和目标上的弹坑形状。由前一种测量得到了在速度小于20 m/s时碎片的速度-尺寸分布。定量结果表明，破片速度随孔隙度的增大而减小。还发现陨石坑的直径在孔隙度约为40%时最大。从这些结果中建立了速度-尺寸分布的标度规律，并利用这个时间用数值方法研究了小行星风化层厚度的发展。结果表明，对于弱强度物质，即使在10 ~ 1 km大小的小行星上也存在风化层。这些结果与最近的近鞋匠号宇宙飞船获得的事实相一致，即爱神星433的表面有风化层。

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.