2. 2. LUNAR ANALOGUE SOIL As TARGET MATERIAL

It is known that the lunar surface is covered by the so-called "lunar regolith" of about 10 m thick which is a mixture of fine-powder soil and rock fragments (e.g., [5]). Because the mechanical properties of the target material could be essential parameters to affect the dynamie resistant force and the penetration characteristics, it is important to prepare a target having similar mechanical properties to those of the lunar regolith.

We prepared a dry sand having a similar size distribution of particles to that of the lunar regolith. The relation of cumulative mass fraction with particle diameter in the present target material is shown in Figure 3. For comparison, the data for Apollo lunar soil samples are also indicated [6] . The specific gravity of the analogue soil was measured by means of a conventional water immersion pycnometry technique, and obtained a value of 2.60 ± 0.03. But the mechanical properties of granular media are more dependent on the bulk density or porosity than the true density. Therefore, we controlled the degree of compaction so that the bulk density and hardness of analogue soil were matched to those of lunar regolith as well as possible. The compaction was adjusted to make the hardness of the soil to be in a certain range of the values indicated by Apollo Soil Mechanics Experiments [ 7] . The resultant bulk density is in the range of 1 .7 to 1 .9 g/cm3. In order to secure the reproducibility of experiments, the hardness of the analogue soil was monitored by a static hardness test using a cone penetrometer similar to that used in the Apollo Soil Mechanics Experiments [ 7].

Fig.3.Particle size distribution by sieve analysis:(a) lunar analogue soil used in this experiment; (b) Apollo lunar soil samples (adopted from [6]). The horizontal axis indicates the particle diameter and the vertical axis indicates weight percent finer than d mm.

Fig.4.One example of stress hardness data for the aalogue soil sample measured by the conepenetrometer system. These profiles are obtained at an every distance of 20 cm apart along the line including the spinning gun barrel.Point C lies in the perpendicular from the spin axis on the target surface and is nearest to the impact point.