ESA Science & Technology - SMART-1: Impact Prediction Based on Laboratory Impact Studies

Figure 1: Two stage light gas gun at the University of Kent

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°

Figure 2: Impact crater from incidence at 2°, of a 2.03 mm diameter aluminium sphere at 2.18 kms-1. Impact direction is from the bottom. Impact direction from bottom of the image and is marked with an arrow. The scale bar shown is 10 mm.

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