A review of penetrometers for subsurface access on comets and asteroids

Abstract

Abstract-The characterization of comet and asteroid interiors will eventually require in situ exploration with drills, penetrators/penetrometers, hypervelocity impactors, excavators or other devices. Because they offer desirable scientific capabilities and relative mechanical simplicity, penetrators and penetrometers, which use only axial force to push beneath the surface, are a good choice for near-term missions. Penetrometers are instruments, generally deployed from a larger vehicle, that measure subsurface mechanical properties and may also contain additional scientific instruments. There are three basic types: "fast" penetrometers are released from above and plunge into the surface. Static and dynamic (collectively referred to as "slow") penetrometers use, respectively, a constant slow penetration speed and periodic hammering impulses. The low gravity environment of asteroids and comets presents a key challenge to instrument deployment and also greatly affects the mechanical properties of surface materials, and in turn penetrometer performance. The Rosetta mission, currently en route to comet 67P/Churyumov-Gerasimenko, will be the next mission to try both fast and slow, dynamic penetrometry, when it arrives in 2014. We present some new concepts of static penetrometers for small body exploration that are adapted to the low gravity environment. The low gravity environment also presents challenges for the testing of penetrometers on Earth and a number of previous solutions are described and new methods suggested. In the next generation of missions to study comets and asteroids, penetrometers could provide important data on their mechanical, seismic, thermal, electromagnetic, and chemical characteristics, as well as sample collection.

Sediment identification using free fall penetrometer acceleration-time histories

“…Acceleration-time histories obtained from the impact of projectiles in geological materials have been used to evaluate the nature of the targeted material in areas like the study of planetary bodies (Garry et al 1999;Ko ¨mle et al 2001;Glaser et al 2008) and study of freshwater sediment in paleo-limnological research (Spooner et al 2004).…”

An On-Orbit CubeSat Centrifuge for Asteroid Science and Exploration

“…The resisting force will be recorded at 100 Hz as the actuator extends from above the regolith to within d max ~1cm (size of the largest particles) from the bottom. The design is inspired by the slow mode of penetration of comet penetrometers [25].…”

Ultrasonically Assisted Hammer-Action Penetrators in Planetary Regolith

“…Parameters. Hammering probes for use in low-gravity environments utilize springs to deliver the required impact, requiring careful considerations of the reacted forces as these can cause the penetrator to jump out of the hole (Glaser, Ball, and Zacny 2008). These probes incorporate suppressors into the spring systems to reduce the reacted force below the skin friction, however this would require significant development and optimization to be used for these tests.…”

“…This is particularly valuable for small planetary bodies, due to the low surface gravity and thus lower available weight. These moles utilize an internal mass to hammer the probe through the granular surface, and have been used in several missions such as Beagle-2, the Philae lander, and the 2018 InSight mission (Glaser, Ball, and Zacny 2008;Hansen-Goos and Grott 2014). A balance between progression rate and structural damage to the internal components of the probe is essential, as impulses can reach potentially destructive levels, as demonstrated with the PLUTO probe aboard Beagle-2 exhibiting hammer strikes as high as 8,000 m/s 2 (Pinna et al 2001;Glaser, Ball, and Zacny 2008).…”