SMART-1: Impact Prediction Based on Laboratory Impact Studies
The target was a tray of sand. The sand was similar in some respects to that defined as lunar regolith stimulant. The geological classification of the sand was: Lower Greensand, Leighton Buzzard, Beds, UK. It is free from silt, clay and organic matter. The grain shape was rounded to sub-rounded. The size of the grains was 90 – 150 microns, with a minimum of 85% inside the stated size range. This compares to the JSC-A Lunar Soil Simulant. JSC-1 is a glass rich basaltic ash considered to well approximate the lunar mare soil. JSC-1 simulant has a mean particle size of 98 – 117 microns, comparable to that here.
The sand tray was placed for use on a tilting table which had a capture cell behind it to catch any ricochet fragments from the projectile. 3 shots were done, at glancing angles of 10°, 5° and 2°. All angles are accurate to 0.1°. The data presented here are for the shot at 2°. The impact speed was 2.18 kms-1. The projectile left a crater than is shown in Figure 2. The internal crater dimensions (i.e. measured in the original undisturbed plane of the target surface) are:
- Length (L) = 36.8 mm
Width (W) = 24.0 mm
Circularity (C = L/W) = 1.53
Depth (D) = 4.0 mm
Volume (V) = 2.1 cm3
Ricochet angle = 2.9°
The crater is very shallow and flat bottomed. Usually crater shape is given by depth/diameter. Here given the asymmetric crater we average L and W and give a shape parameter of 2D / (L + W) = 0.13.
The projectile mass (M) was 12 mg. Taking the ratio V/M we obtain V/M = 0.175 m3 kg-1. If we ignore all issues of scaling and simply assert that this ratio is a constant at all sizes, then we can take the SMART-1 mass as 290 kg and obtain a crater volume of 52 m3. This is within the range previously predicted by the SMART-1 campaign (10 – 80 m3). Part of this volume of target material has gone into the crater rim walls (raised above the original surface level). As a crude approximation we suggest that 50% of the excavated material is in the walls, leaving 26 m3 of material ejected away from the crater. The crater will be very shallow, perhaps 2 m deep (as measured from the original surface level), with rim walls only 1 m high. The crater length is predicted to be 6.9 m and the width 4.5 m.
The ricochet projectile makes a single, not quite circular entrance hole in our ejecta capture system. This suggests it rebounded as a single object but had undergone some deformation. The penetration depth into the capture cell is compatible with impact by an object travelling at 1.6 – 1.8 kms-1. So there has been a slight (10 – 20%) loss of speed during the impact. These predictions are for an impact of 2°. The crater elongation will increase if a lower angle of 1° occurs as expected.
References
Burchell M.J., Cole M.J., McDonnell J.A.M., and Zarnecki J.C. 1999. Hypervelocity Impact Studies Using the 2 MV Van de Graaff Accelerator and Two-Stage Light Gas Gun of the University of Kent at Canterbury. Meas. Sci. Technol. 10; 41 – 50.
Contact Details
M.J. Burchell, M.J. Cole
Centre for Astrophysics and Planetary Science
School of Physical Sciences
Univ. of Kent
Canterbury
Kent CT2 7NH
United Kingdom
Phone +44 (0)1227 823248
Email: M.J.Burchellkent.ac.uk
http://astro.kent.ac.uk/
http://www.kent.ac.uk/physical-sciences/main/staff/mjb.htm