Using the inertia of spacecraft during landing to penetrate regoliths of the Solar System

Abstract

The high inertia, i.e. high mass and low speed, of a landing spacecraft has the potential to drive a penetrometer into the subsurface without the need for a dedicated deployment mechanism, e.g., during Huygens landing on Titan. Such a method could complement focused subsurface exploration missions, particularly in the low gravity environments of comets and asteroids, as it is conducive to conducting surveys and to the deployment of sensor networks. We make full-scale laboratory simulations of a landing spacecraft with a penetrometer attached to its base plate. The tip design is based on that used in terrestrial Cone Penetration Testing (CPT) with a large enough shaft diameter to house instruments for analysing pristine subsurface material. Penetrometer measurements are made in a variety of regolith analogue materials and target compaction states. For comparison a copy of the ACC-E penetrometer used on the Huygens mission is used. A test rig at the Open * Corresponding author University is used and is operated over a range of speeds from 0.9 to 3 m sand under two gravitational accelerations. The penetrometer was found to be sensitive to the target's compaction state with a high degree of repeatability. The penetrometer measurements also produced unique pressure profile shapes for each material. Measurements in limestone powder produced an exponential increase in pressure with depth possibly due to increasing compaction with depth. Measurements in sand produced an almost linear increase in pressure with depth. Iron powder produced significantly higher pressures than sand presumably due to the rough surface of the grains increasing the grain-grain friction. Impacts into foamglas produced with both ACC-E and the large penetrometer produced an initial increase in pressure followed by a leveling off as expected in a consolidated material. Measurements in sand suggest that the pressure on the tip is not significantly dependent on speed over the range tested, which suggests bearing strength equations could be applied to impact penetrometry in sand-like regoliths.In terms of performance we find the inertia of a landing spacecraft, with a mass of 100 kg, is adequate to penetrate regoliths expected on the surface of Solar System bodies. Limestone powder, an analogue for a dusty surface, offered very little resistance allowing full penetration of the target container. Both iron powder, representing a stronger coarse grained regolith, and foamglas, representing a consolidated comet crust, could be penetrated to similar depths of around two to three tip diameters, probably more if impacting with a slightly higher speed.

Detection of structure in asteroid analogue materials and Titan’s regolith by a landing spacecraft

“…A conventional spacecraft with a mass of around 100 kg landing at a few metres per second, with a penetrometer fitted to its base, has enough momentum to deliver sensors into the sub-surface of an asteroid or other planetary regolith by virtue of its high mass (Lorenz et al, 1994;Paton et al, 2015). Despite the low speed, the high mass of the spacecraft presents a momentum comparable to, if not exceeding, the values obtained by low mass, high speed penetrators.…”

“…Its dimensions are based on the Cone Penetration Testing (CPT) design used in civil engineering applications. X-PEN has previously been used to investigate measurements in analogue planetary regoliths by Paton et al (2015). The penetrometer is 40 cm in length, 3.8 cm in diameter and the conical tip has a 60…”

“…The signal from a penetrometer can be processed to obtain regolith properties such as internal friction, cohesion and particle size, e.g. Paton et al (2012aPaton et al ( , 2015.…”

“…To test full-scale landing impact tests of spacecraft penetrators for subsurface exploration (Paton et al, 2012b(Paton et al, , 2015 and support interpretation of the Huygens penetrometer measurements on Titan (Zarnecki et al, 2005) a test rig was built at the Open University in the UK. The design was constrained by experimental requirements and the restricted floor space of the laboratory.…”

“…Chitu et al (2015), would be required to maintain contact with the surface. It may also be possible to use a large rigid penetrometer to aid the landing as explored by Paton et al (2015) to absorb some of the elastic energy of the spacecraft during landing under certain circumstances.…”