A Concept for a Mars Boundary Layer Sounding Balloon: Science Case, Technical Concept and Deployment Risk Analysis

The Mars Exploration Program Analysis Group has identified measurements of the state and the variability of the Martian atmosphere as high priority investigations for the upcoming years. Balloon-borne instruments could bridge the gap in both temporal and spatial resolution in mesoscale distances between local, stationary landers and global orbiter observations. The idea to use a balloon system for such a purpose is not new in essence and has been proposed already in past decades. While those concepts considered an aerial deployment during entry and descent, the concept outlined in this study revisits a launch off the payload deck of a lander from the Martian surface. This deployment option profits today mainly from the technological advance in micro-electronics and sensor miniaturization, which enables the design of a balloon-probe significantly smaller than earlier proposed systems. This paper presents the feasibility assessment for this instrument and gives further details on the scientific and operational concept, a strawman sensor suite, its system components and the associated size and budget estimates. It is complemented by the analysis scheme proposed to assess, manage and mitigate the deployment risk involved in automatically launching such a balloon-system off a planetary surface

Penetration and performance testing of the HP³ Mole for the InSight Mars mission

During the development and the qualification of the Heat Flow Physical Properties Package (HP³) instrument (developed by the German Aerospace Center), which is part of the NASA Mars mission InSight, its self-propelling subsurface probe, the HP³ Mole was used in several penetration tests. Here, the performance of the Mole to reach the target depth, to avoid or overcome obstacles on its path, and its directional stability in the subsurface is elaborated. The different test beds and set ups are described and the results are presented. The deep penetration tests (DPT), with the purpose to reach the target depth, are the most important performance tests and therefore the results are investigated in more detail in section 2. Full functional tests (FFT), which showed the performance and degradation of the mechanism inside the Mole, are presented in section 3. Additional penetration and life cycle tests are described in section 4. The testing has demonstrated that the HP³ Mole meets all of its penetration requirements with margin

Mars Regolith Properties as Constrained from HP3 Mole Operations and Thermal Measurements

The Heat Flow and Physical Properties Package HP3 onboard the Nasa InSight mission has been on the surface of Mars for more than one Earth year. The instrument's primary goal is to measure Mars' surface heat flow through measuring the geothermal gradient and the thermal condunctivity at depths between 3 and 5m. To get to depth, the package includes a penetrator nicknamed the "Mole" equipped with sensors to precisely measure the thermal conductivity. The Mole tows a tether with printed temperature sensors; a device to measure the length of the tether towed and a tiltmeter will help to track the path of the Mole and the tether. Progress of the Mole has been stymied by difficulties of digging into the regolith. The Mole functions as a mechanical diode with an internal hammer mechanism that drives it forward. Recoil is balanced mostly by internal masses but a remaining 3 to 5N has to be absorbed by hull friction. The Mole was designed to work in cohesionless sand but at the InSight landing a cohesive duricrust of at least 7cm thickness but possibly 20cm thick was found. Upon initial penetration to 35cm depth, the Mole punched a hole about 6cm wide and 7cm deep into the duricrust, leaving more than a fourth of its length without hull friction. It is widely agreed that the lack of friction is the reason for the failure to penetrate further. The HP3 team has since used the robotic arm with its scoop to pin the Mole to the wall of the hole and helped it penetrate further to almost 40cm. The initial penetration rate of the Mole has been used to estimate a penetration resistance of 300kPa. Attempts to crush the duricrust a few cm away from the pit have been unsuccessful from which a lower bound to the compressive strength of 350kPa is estimated. Analysis of the slope of the steep walls of the hole gave a lower bound to cohesion of 10kPa. As for thermal properties, a measurement of the thermal conductivity of the regolith with the Mole thermal sensors resulted in 0.045 Wm-1K-1. The value is considerably uncertain because part of the Mole having contact to air. The HP³ radiometer has been monitoring the surface temperature next to the lander and a thermal model fitted to the data give a regolith thermal inertia of 189 ± 10 J m-2 K-1 s-1/2. With best estimates of heat capacity and density, this corresponds to a thermal conductivity of 0.045 Wm-1K-1, consistent with the above measurement using the Mole. The data can be fitted well with a homogeneous soil model, but observations of Phobos eclipses in March 2019 indicate that there possibly is a thin top layer of lower thermal conductivity. A model with a top 5 mm layer of 0.02 Wm-1K-1 above a half-space of 0.05 Wm-1K-1 matches the amplitudes of both the diurnal and eclipse temperature curves. Another set of eclipses will occur in April 2020

The InSight-HP³ mole on Mars: Lessons learned from attempts to penetrate to depth in the Martian soil

The NASA InSight lander mission to Mars payload includes the Heat Flow and Physical Properties Package HP3 to measure the surface heat flow. The package was designed to use a small penetrator - nicknamed the mole - to implement a vertical string of temperature sensors in the soil to a depth of 5 m. The mole itself is equipped with sensors to measure a thermal conductivity-depth profile as it proceeds to depth. The heat flow is calculated from the product of the temperature gradient and the thermal conductivity. To avoid the perturbation caused by annual surface temperature variations, the measurements need to be taken at a depth between 3 m and 5 m. The mole is designed to penetrate cohesionless soil similar in rheology to quartz sand which is expected to provide a good analogue material for Martian sand. The sand would provide friction to the buried mole hull to balance the remaining recoil of the mole hammer mechanism that drives the mole forward. Unfortunately, the mole did not penetrate more than 40 cm, roughly a mole length. The failure to penetrate deeper is largely due to a cohesive duricrust of a few tens of centimeter thickness that failed to provide the required friction. Although a suppressor mass and spring as part of the mole hammer mechanism absorb much of the recoil, the available mass did not allow designing a system that fully eliminated the recoil. The mole penetrated to 40 cm depth benefiting from friction provided by springs in the support structure from which it was deployed and from friction and direct support provided by the InSight Instrument Deployment Arm. In addition, the Martian soil provided unexpected levels of penetration resistance that would have motivated designing a more powerful mole. The low weight of the mole support structure was not sufficient to guide the mole penetrating vertically. Roughly doubling the overall mass of the instrument package would have allowed to design a more robust system with little or no recoil, more energy of the mole hammer mechanism and a more massive support structure. In addition, to cope with duricrust a mechanism to support the mole to a depth of about two mole lengths should be considered

The InSight HP3 Penetrator (Mole) on Mars: Soil Properties Derived from the Penetration Attempts and Related Activities

The NASA InSight Lander on Mars includes the Heat Flow and Physical Properties Package HP3 to measure the surface heat flow of the planet. The package uses temperature sensors that would have been brought to the target depth of 3–5 m by a small penetrator, nicknamed the mole. The mole requiring friction on its hull to balance remaining recoil from its hammer mechanism did not penetrate to the targeted depth. Instead, by precessing about a point midway along its hull, it carved a 7 cm deep and 5–6 cm wide pit and reached a depth of initially 31 cm. The root cause of the failure – as was determined through an extensive, almost two years long campaign – was a lack of friction in an unexpectedly thick cohesive duricrust. During the campaign – described in detail in this paper – the mole penetrated further aided by friction applied using the scoop at the end of the robotic Instrument Deployment Arm and by direct support by the latter. The mole tip finally reached a depth of about 37 cm, bringing the mole back-end 1–2 cm below the surface. It reversed its downward motion twice during attempts to provide friction through pressure on the regolith instead of directly with the scoop to the mole hull. The penetration record of the mole was used to infer mechanical soil parameters such as the penetration resistance of the duricrust of 0.3–0.7 MPa and a penetration resistance of a deeper layer (> 30 cm depth) of 4.9±0.4 MPa. Using the mole’s thermal sensors, thermal conductivity and diffusivity were measured. Applying cone penetration theory, the resistance of the duricrust was used to estimate a cohesion of the latter of 2–15 kPa depending on the internal friction angle of the duricrust. Pushing the scoop with its blade into the surface and chopping off a piece of duricrust provided another estimate of the cohesion of 5.8 kPa. The hammerings of the mole were recorded by the seismometer SEIS and the signals were used to derive P-wave and S-wave velocities representative of the topmost tens of cm of the regolith. Together with the density provided by a thermal conductivity and diffusivity measurement using the mole’s thermal sensors, the elastic moduli were calculated from the seismic velocities. Using empirical correlations from terrestrial soil studies between the shear modulus and cohesion, the previous cohesion estimates were found to be consistent with the elastic moduli. The combined data were used to derive a model of the regolith that has an about 20 cm thick duricrust underneath a 1 cm thick unconsolidated layer of sand mixed with dust and above another 10 cm of unconsolidated sand. Underneath the latter, a layer more resistant to penetration and possibly containing debris from a small impact crater is inferred. The thermal conductivity increases from 14 mW/m K to 34 mW/m K through the 1 cm sand/dust layer, keeps the latter value in the duricrust and the sand layer underneath and then increases to 64 mW/m K in the sand/gravel layer below

Optimization of the InSight HP³-Mole

The German Aerospace Center (DLR) is providing the HP³-payload for the InSight-Mission of NASA. InSight is the 12th part of the Discovery program and is supposed to investigate the seismic activities, geodesy and heat transport of the interior of the mars [RD 13]. HP³ is the abbreviation for Heat Flow & Physical Properties Package. It consists of a heat probe that will measure the heat flow of the mars in depths of up to five meters. To get the probe there it is integrated in the HP³-Mole. The HP³-Mole is a non-rotating drill that acts like a self-hammering nail. An internal hammering mechanism, which is a three mass system, is pushing the Mole with every stroke deeper in the Martian soil. These three masses are linked to each other via two springs. In this thesis, several methods of improving the advancing speed of the Mole are investigated. The changes that provide a high enhancement will be integrated in the Prototype-Model, resp. the Engineering-Model of the Mole

A mole deployed mass spectrometer for sub-surface volatile detection and characterisation at airless bodies

Orbital observational studies and investigations carried out on returned lunar samples have inferred the presence of a significant reservoir of volatile compounds, in particular water. It seems very likely that these volatile compounds will be concentrated at the poles and other locations, such as at depth, where low temperatures exist to provide cryogenic traps. However, the full inventory of these species, their concentration and their sources are unknown To address some of these issues a volatile analysis instrument is required that is specifically designed for identification and quantification of volatiles. The Lunar Volatile Resources Analysis Package is part of the provisional payload for the ESA European Lander and aims to measure the chemical abundance, chemical and isotopic composition of volatiles from regolith samples and the lunar exosphere. It is important to understand, and limit where possible the contamination of the lunar surface by external volatiles in order that measurements are scientifically meaningful. Any controlled soft landing sequence will contaminate the locality of the lander with rocket propellant by-products. Therefore, to allow meaningful scientific return careful considerations must be given to the sampling campaign, i.e. sampling from below or outside of the contamination zone. To this end, it is considered that the use of a sub-surface penetrating mole offers the possibility of accessing material from within the contamination layer and then at increasing depth into pristine sub-surface material. We report on the design and characterisation of an instrumented sub-surface penetrating mole device with an integrated miniaturised ion trap mass spectrometer capable of performing in-situ volatile detection and characterisation at the moon and any other airless body. The design uses the heritage gained from the Ptolemy Mass Spectrometer and the Beagle 2 sub-surface penetrating mole device. We have aimed for a very highly integrated package with technology which is capable of flight readiness within this decade. Major drivers used in instrument design are science capability, and technology elements which include resources (mass, power, volume), maturity (heritage, TRL), ruggedness, contamination, radiation and telemetry and commanding issues

Mechanical analysis of a Solar Array hinge based on a 180 folded flexible printed circuit board

Flexible Printed Circuit Boards (Flex PCB) are commonly used for the electrical routing on spacecrafts (S/C). The implementation of Flex PCB as structural material and harness for the Gossamer Solar Array (GoSolAr) is currently studied by the German Aerospace Center (DLR). The Flex PCB as well as the other parts of the array are folded and stowed into a small volume for transport into space and deployed once the spacecraft starts operating. The folding areas, also referred to as hinge, are subject of this paper. The hinges and their design have major impact on the required deployment force and geometry. The aim is to get a better understanding of the relevant parameters, which are the deployment force, the angles between photovoltaic generators and the deployed distance. Deployment experiments were carried out on four different design options, which were then compared to each other with respect to their deployment behaviour. Important aspects for this evaluation were the controllability during the deployment and the force required for tensioning the array. Main differences of the evaluated designs were the amount of elastic deformation energy stored in the different configurations (stowed and deployed). The experiments revealed that a design, which has its minimum of elastic deformation energy in stowed configuration is beneficial to be used for GoSolAr. Furthermore, a numerical model of the hinge based on the FEM is introduced. The experiments were used to correlate the model. This model can be further used for design studies and optimization. The design of Flex PCB as hinges adds a mechanical function to its otherwise only electrical purpose and therefore opens a whole new field of applications, especially for thin film and lightweight structures

HP3 Instrument Support System Structure development for the NASA/JPL Mars Mission InSight

On May 05, 2018 NASA JPL launched its mission to Mars called “InSight”. Main objective of this mission is to gain more knowledge about the evolution of terrestrial planets and to more precisely determine properties of core, mantle and crust of Mars. One of a number of different scientific instruments onboard the lander there will be HP3 (Heat Flow and Physical Properties Package), which was developed by the German Aerospace Center (DLR). It will be operated on Martian ground to measure the heat flow through the Martian outer crust. It uses a hammering mechanism which will pull a tether approx. 5 m into the soil. The hammering device is equipped with foil heaters on the outer hull and the tether is equipped with temperature elements. Both is needed for the determination of the thermal conductivity of the surrounding regolith and the measurement of the temperature gradients in the ground. There is the need of a separate system to be able to perform those activities on the surface. This system is called the “HP3 Support System”. Its main task is to ensure a stable, nearly perpendicular position of the hammering mechanism relative to the soil on the Martian surface before initial penetration. It furthermore houses the instruments for length measurement and serves as electrical connection to the lander. The paper will give an overview of the development and the qualification of the structure of the Support System. It will focus on the mechanical design, the analysis of the structural dynamics but in particular on the testing which includes standard environmental testing but also numerous development tests that are very mission specific. The mechanical design of the Support System is mainly driven by a unique set of requirements derived from the working environment on Mars, the deployment from the lander deck and the mechanically separated operation on the surface. The instrument design will be explained to show, which design elements were implemented to ensure proper functionality. Various development tests had to be performed during the Support System structure development. Besides the standard qualification tests, special tests were developed to show compliance of the instrument design to the requirements. Such tests are: Separation Tests from the lander deck in cold environment under various tilt angles, Tether Deployment Tests, under various temperatures, foldings and routings, Feet Sliding Resistance Tests on sand with different slopes. The paper will give an overview on all tests necessary for the support system qualification and will describe test setups and the results