1. INTRODUCTION

Several planetary missions to use a hard landing probe, called a "penetrator" have been proposed (e.g., LUNAR-A, Deep Space 2, Mars-96, Rosetta). Utilization of penetrators for planetary explora-tion has many advantages over soft landing probes. The most obvious advantage of the penetrator will be its cost-effective capability of deploying scientific instruments on planetary surface. The penetrator will also make it possible to deliver scientific instruments into the planetary subsurface for in-situ chemical analysis and/or heat flow measurements, otherwise those measurements would require drilling holes from the surface.

The Institute of Space and Astronautical Science (ISAS) plans to undertake a lunar mission named as LUNAR-A, which will be launched by M-V Iaunch vehicle of ISAS in 2002 fiscal year. The main objective of the LUNAR-A mission is to explore the lunar interior using seismometry and heat flow measurements. The scientific objectives would be almost impossible without penetrators under current severe mass and budget constraint of the LUNAR-A mission.

This paper will present some results of experiments made during the course of development of LUNAR-A penetrator. The LUNAR-A penetrator requires that it (the top of the penetrator) should rest in the lunar regolith at a depth deeper than 1 m and at attitude angle (angle between the penetrator body axis and the vertical) Iower than 60 degrees. The first requirement comes from need of assuring temperature stability of the instruments and the second one comes from enabling measurement of heat flow and tele-communication between the penetrator and the mother orbiting s pacecraft.

The depth and attitude of a penetrator after penetration are influenced by impact velocity, mass, cross-sectional area (base area), and nose-shape of the penetrator, mechanical properties of a target material, incidence angle, impact attack-angle, and others. Many experimental studies have been made to clarify the effects of those parameters on impact dynamics of penetrator penetration into geologic materials (e.g., [1] [2] [3] [4]). However, almost all of these studies are limited to the case that the impact is normal to the target surface. Although normal incidence of the penetrator on the target simplifies the problem of penetrator dynamics, such an ideal condition is rarely met in actual planetary explorations; there always exists a possibility that the penetrator will hit on planetary surface at oblique incidence and with a finite attack-angle. Oblique impact with a finite attack-angle is inevitable for real missions due to slight (even if it may be small) error of separating the penetrator from the spacecraft, error of the attitude control of the penetrator, unexpected topography of planetary surface and others. Both the oblique incidence and a finite attack-angle will affect the penetration dynamics significantly because rotational torque will be applied to the penetrator at impact and during the course of penetration.

To date, as far as we know, the effects of oblique incidence and attack-angle on the penetration dynamics have not been investigated experimentally, possibly because it is difficult to simulate the impact condition with oblique incidence and attack-angle in a controlled fashion. In the present study, we report on the experimental setup to study these effects on penetration of a scale-model of LUNAR-A penetrator and results from the experiments.