Owing to the low surface gravity of the Rosetta target comet 46P/Wirtanen, a means of anchoring the Rosetta Lander to the cometary surface will be necessary. This task can be accomplished by firing an anchor into the cometary soil immediately after touchdown to prevent a rebound of the spacecraft from the surface or subsequent ejection by other forces, and to allow for mechanical activities (drilling, etc.) at the landing site.
The rationale for anchoring is examined, based on estimates of the main forces likely to act on the spacecraft after landing. We report on the development of an anchoring device using a pyrotechnic gas generator as a power source and an instrumented anchor.
In addition to the anchoring function, which is the primary purpose of this system, the integration of acceleration and temperature sensors into the tip offers the possibility to determine some important material properties of the cometary surface layer. The accelerometer is designed to measure the deceleration history of the projectile and is thus expected to give information on how the material properties (in particular strength) change within the penetrated layer(s), while the temperature sensor will measure temperature variations at the depth at which the anchor finally comes to rest. As the mechanical properties of the material are not known, it is difficult to predict the final depth of the anchor with any great certainty, but it may well be greater than that reached by any other of the lander's instruments.
The instrumented anchor will be part of the MUPUS experiment, selected to form part of the Rosetta Lander payload. We report on results of laboratory simulations of anchor penetration performed at the Institut für Weltraumforschung, Graz, and compare these with models of projectile penetration. The value of the results expected from the penetrometry experiment in the context of an improved understanding of cometary processes is discussed.
The MUPUS physical properties experiment has been selected for the payload of the lander to be deployed to the surface of comet 46P/Wirtanen as part of the Rosetta mission. One of the constituent instruments of MUPUS is a densitometer. This will measure the bulk density of the surface layers of the comet nucleus to a depth of about 0.35 m.
The technique employed is a simple attenuation method using a 137Cs source (662 keV photons) at the tip of the MUPUS thermal probe, to be inserted into the nucleus surface. Detectors at the surface will measure the attenuated count rate. Due to the dependence of required integration time on density and depth, an algorithm for budgeting the available operation time during the penetration process is proposed.
Laboratory experiments have been performed to examine the spectrum of 137Cs emission as seen by a Cadmium Telluride (CdTe) detector through water. CdTe detector technology is being studied as an alternative to the Geiger tubes previously used in Compton backscatter densitometers on the Moon and Venus (Surkov et al., 1990).
Results show that exponential attenuation of the 662 keV radiation can be seen, together with scattered radiation at lower energies.
A Compton Backscatter Densitometer has been proposed for the RoLand probe in order to measure the bulk density of material near the surface of a comet nucleus. RoLand is to be deployed from a spacecraft in orbit around the target comet, as part of the international Rosetta mission. The basis for the use of this technique on RoLand is explained. Densitometers have been used twice before in planetary exploration, however the RoLand design aims to employ a lower energy radioisotope source so that a lightweight detection system can be used. Monte Carlo simulation of scattering and absorption in semi-infinite bulk materials has been used to investigate the design parameters, specifically the variation of backscattered count rate with density measurement can be made provided that the basic elemental composition of the material is known.
At the 1996 Summer Session of the International Space University, held in Vienna, 53 space professionals from 18 countries addressed the world's future programme of solar-terrestrial exploration and applications. Their mandate was, through an international perspective, to explore and document strategies which will increase our understanding of the Sun and its effects and help us apply solar knowledge for the benefit of mankind.
The changed global political, economic and technological environment within which space activities take place has created both obstacles and opportunities for solar exploration and applications. The economic risks of insufficient knowledge of solar phenomena are greater now than ever before, for both terrestrial technologies and space-based systems. The blurring of the line between basic and applied sciences and the movement towards interdisciplinary science missions has created a favourable climate for joint science and applications endeavours. The Ra report is a call to action. It presents an international Strategic Framework, containing programmes for the Near-, Mid- and Far-Term. It integrates solar science and applications, capitalises on global resources and talents and harmonises with the political and economic environment. We recommend the immediate establishment of a Working Group for International Solar Exploration and Applications. This would ensure the implementation of a Strategic Framework, synchronise efforts in different countries, facilitate the interaction between science and applications and help combine the output into products useful on a global scale.
Due to the Rosetta mission and the planned in situ investigation of the landing vehicle RoLand, the surface properties of comets have become a new focus of research. The estimated low gravity makes it necessary to anchor the lander immediately after touchdown to prevent a rebound from the surface and to allow movements of mechanical devices during measurements. On the other hand the anchoring device is probably that part of the lander which is probing into deeper layers than any other instrument. Therefore the anchor is equipped with an accelerometer and a temperature sensor. By measuring the deceleration history of the anchoring event the tensile strength of the penetrated layers could be derived. The temperature sensor would provide a useful addition to the grid of temperature measurements taken by other instruments of the Rosetta mission. We report here about experiments made with the preliminary design of the RoLand anchor and the interpretation of the results with respect to an improved understanding of cometary surface layers and related processes.
The technique of Very Long Baseline Interferometry (VLBI) is very important to modern radio astronomy; many widely-spaced antennae operate in co-ordination to obtain high resolution images of astronomical radio sources. Longer baselines provide higher resolution. However, ground-based VLBI is constrained by the diameter of the Earth; baselines cannot be longer than 12760 km. The next logical step is to extend interferometric baselines into space; two projects currently under development (RadioAstron and VSOP) each aim to launch an antenna into Earth orbit to perform VLBI in co-ordination with ground-based arrays. However the advantages of Space VLBI (SVLBI) cannot be fully realised without operating two antennae in orbit. This would enable VLBI observations to be made at the long wavelengths absorbed by the ionosphere (1-30 MHz) and would make full use of orbital mechanics to improve image quality and provide operational flexibility. In this paper the ideas behind Space VLBI are presented, along with the results of a computer simulation of two-satellite SVLBI aperture plane coverage. A pair of satellite orbits is suggested, followed by an analysis of a low-thrust orbital manoeuvring system which would introduce great operational flexibility.
The single scattering model for gamma backscatter density gauges has been extended to describe how the total detected count rate changes in response to localised density variations within the material. This extended model suggests there is a spatial region where density perturbations have a contradictory effect on the measured density value, an effect that has already been shown experimentally by previous workers. Here we compare their results with those predicted by application of the extended single scattering model. Since a complete description of their experimental apparatus was not available, only a crude fit could be achieved. Nevertheless all the basic features of the data could be reproduced.
The University of Kent's Unit for Space Sciences and Astrophysics has designed and is building the prototype of a dust impact analyser (DEBIE) as part of an ESA contract. The flight model (four sensor heads and electronics) will be built in Finland and will fly initially on ESA's Proba spacecraft. This configuration could be reproduced for a Lunar orbiter mission at moderate cost. Each sensor unit comprises an impact momentum sensor with a plasma stage. Measurements of the dust flux environment in Lunar orbit could be compared with those in LEO. The effect of gravitational enhancement in Lunar orbit are much reduced compared to LEO, allowing a more direct measurement of the dust flux environment at 1 AU. Unlike LEO a lunar orbit will be free from the contaminating effects of artificial space debris.
Temperature-Controlled Quartz Crystal Microbalances (TQCMs) provide a simple and reliable method to search for evaporated volatiles. A commercially- available TQCM sensor, the Mark 20, of mass 20 g has an extensive flight history and could be mounted on a Lunar South Pole lander or rover to search for small amounts of ambient volatiles or other condensible material. The experiment could operate passively or in conjunction with an active system to heat or mechanically disturb the surface material. Such thermal or mechanical action could result from the action of other experiments or systems, for instance a line heat source thermal conductivity probe, sampling drill or even the action of a rover's wheels on the soil.
ESA's Rosetta mission (currently planned for launch in 2003) involves deploying two landers from an orbiting spacecraft onto the surface of a comet nucleus. Both will perform in situ analysis of the nucleus. An experiment package for the proposed RoLand (Rosetta Lander) probe is under development at the University of Kent. It will incorporate a collection of small sensors to help characterize the physical properties of the comet nucleus. A description of the sensors, their modes of operation and the potential scientific return are given.
Experiments have been made on the low speed penetration of a regolith simulant under partial vacuum conditions with an instrumented free-fall penetrator. The aim of the tests was to evaluate the role that the presence or otherwise, of an atmosphere, plays in impact phenomena. The deceleration of the penetrator and the normal tip force are both measured simultaneously during the impact.
The dynamic behaviour of projectiles impacting planetary surfaces can be measured to derive mechanical properties of the target material. Several such dynamic penetrometers will be used on Mars, the Moon, Titan, and a comet nucleus. However, solid bodies in the Solar System exhibit a wide range of surface atmospheric and gravitational conditions and previous workers have shown that changes in ambient pressure or gravity can significantly alter penetration dynamics. This presents a challenge for the terrestrial calibration of penetrometers. We have applied a penetration model to low speed impacts in air and vacuum, with the aim of quantifying any differences in a target's measured properties. A 1.05 kg instrumented penetrometer was dropped onto two cohesionless granular materials at speeds of around 2.7 m s-1. The apparatus was located in a vacuum chamber, allowing tests to be made at low pressures. An initial upper limit for the mean deviatoric stress (a measure of material strength) was found in each case by dividing the gravitational potential energy lost (during the penetrometer's fall and penetration) by the volume penetrated. This value can be reduced using the projectile's recorded deceleration and a penetration model that includes friction and dynamic resistance. Good fits between the recorded and modelled deceleration were obtained for a range of values of dynamic drag coefficient and coefficient of friction. Initial comparison of the air and vacuum drops performed so far suggests behaviour consistent with that described by previous workers, namely that pore pressure aids penetration in loose materials but inhibits penetration in heavily compacted materials, and that these effects are larger for smaller grain sizes.
Secure anchoring of the Rosetta Lander to the cometary surface over its lifetime is of vital importance for the success of the mission. This issue has become even more critical since we know from recent observations that the P/Wirtanen nucleus is extremely small (nominally 800 m radius) and starts emitting gas and dust already at a rather large distance from the Sun (3.3 - 3.5 AU). In this paper we estimate the forces which may act on the Lander after touchdown on the surface and show that anchoring is an absolute necessity. It is found that the main forces counteracting gravity are the gas drag from sublimating ice, followed by centrifugal force. Electrostatic repulsion and solar radiation pressure are negligible in comparison. The forces arising from seismic events and Lander operations (drilling, deployment, etc.) are also briefly considered. We demonstrate that the tension achievable in the anchoring cable should be sufficient to hold the Lander to the surface, even in the worst possible case for P/Wirtanen.
At the ISSI Workshop 'Origin and Composition of Cometary Material' a short questionnaire was devised by the 'Critical Measurements for the Future' Working Group and distributed to the attendees. The aim was to find out what they thought were the 'critical questions' and the key measurements needed to find answers. Results from the 15 respondents are collated and summarised.
On touchdown at the nucleus of comet Wirtanen in 2012, the Rosetta Lander will fire an anchoring harpoon into the surface material. This is necessary not only to avoid rebound of the Lander and supply a reaction force against operations such as drilling, but also to prevent its subsequent ejection by gas drag from sublimating volatiles. Here we present the results of recent laboratory work on the anchoring system, including test shots performed into a variety of target materials. Two sensors of the MUPUS experiment are mounted within the anchor projectile- a temperature sensor (ANC-T) for long-term sub-surface temperature measurements, and a piezo-electric shock accelerometer (ANC-M). The acceleration history measured by ANC-M during the anchor's penetration will yield information on the strength of the cometary material, down to a depth of one metre or more. This should enable the detection and location of layers having distinctly different strength, whose existence is predicted by models of cometary material.
The MUPUS ANC-M piezo-electric accelerometer sensor, mounted in the Rosetta Lander's anchoring harpoon, will record the deceleration history of the anchor as it penetrates the surface material of comet Wirtanen. Initial processing of the data involves use of the sensor's transfer function, double integration and remapping to obtain acceleration vs. depth. Information on the granularity, strength and possible layering of the material may then be derived from such penetrometry data. Detailed evaluation of the data is, however, a complex inversion problem which demands extensive modelling. Here we outline two of the modelling approaches we have used so far. For the second we present model deceleration profiles generated to simulate penetration in a layered target.
Successful ways of using space science and technology as a vehicle for science education are explored. We report on activities centred on two educational forums - "extra-curricular", in the form of special lectures delivered to over 150 pupils from different schools in one session, and "curricular" activities where the scientist entered a school for several days to become an integral part of the pupils' class. Space science and technology were applied to the school context within the framework of the UK's National Curriculum for ages 13-17. These activities were part of the Pupil Researcher Initiative, run by Sheffield Hallam University on behalf of the Particle Physics and Astronomy Research Council (PPARC), and the ESSTeL lectures sponsored by PPARC's Public Understanding of Science and Technology initiative. We give an account of the preparation of both initiatives on the part of the teacher and give an insight into the practicalities of organising the events. We comment on the advantages of using space as an educational tool and its ability to motivate the target audience.
The MUPUS instruments, part of the tentative payload for the Rosetta lander RoLand, are aimed to study the energy balance of the nucleus/coma interface and the evolution of key thermal and mechanical parameters. The surface temperature will be measured with an infrared thermal mapper. The temperature profile and the thermal conductivity in the first tens of centimeters of the nucleus will be measured with a penetrator equipped with temperature sensors that can be actively heated to measure the conductivity with the line-heat-source method. The tip of the penetrator will carry a penetrometer to measure the hardness of the top nucleus layers and a Compton backscatter densitometer will be used to measure the density. The RoLand anchor will be equipped with an additional temperature sensor and penetrometer. The longevity of RoLand offers the opportunity to study the energy balance and the metamorphosis of thermal and mechanical parameters over a significant portion of one orbital revolution of the Rosetta target comet. The acquired data will, in addition, help to understand the onset of activity, gas and dust emission, which will be mainly monitored by the orbiter. The thermal sensors will provide a ground truth for IR data from the orbiter.
The MUPUS instruments for RoLand are aimed to study the energy balance of the nucleus/coma interface and the evolution of key thermal and mechanical parameters. The surface temperature will be measured with an infrared thermal mapper. The temperature profile and the thermal conductivity in the first tens of cm of the nucleus will be measured with a penetrator equipped with sensors that can be actively heated to measure the conductivity with the line-heat-source method. The tip of the penetrator will carry a penetrometer to measure the hardness of the top layers and a Compton backscatter densitometer will be used to measure the density. The RoLand anchor will be equipped with an additional temperature sensor and penetrometer. The longevity of RoLand allows to study the energy balance and the material properties over a significant portion of the orbital period of the target comet. The data will help to understand the onset of activity, and of gas and dust emission. The thermal sensors will provide ground truth for IR data from the orbiter.
One of the goals of the ESA interplanetary mission Rosetta (which will investigate the short period comet Wirtanen in the years 2011-2013) is to measure the main physical properties of the surface material on the cometary nucleus. This will help us to understand many characteristic cometary phenomena, even including plasma processes. In this contribution we outline the means by which the strength of the nucleus surface layers will be determined, thus obtaining some sort of stratigraphy down to a depth of one metre or more. In particular we present model results for the 'deceleration history' of the Rosetta Lander's anchoring harpoon in the cometary soil and discuss how sliding friction between the projectile and the soil affects the deceleration profile and the final penetration depth.
The longevity of the Rosetta lander RoLand offers the opportunity to study the energy balance and the metamorphosis of thermal and mechanical parameters over a significant portion of one orbital revolution of the Rosetta target comet. While most RoLand experiments focus on composition and chemistry, the MUPUS instruments, part of the tentative payload for the Rosetta lander RoLand, are aimed to study the energy balance of the nucleus/coma interface and the evolution of key thermal and mechanical parameters. Unlike planetary evolution, cometary evolution is influenced by the energy input at the surface. The near surface layers are accessible with some effort and may thus be directly studied. A penetrator equipped with temperature sensors and heaters (MUPUS-PEN) aims to measure the vertical temperature distribution (PEN-TP) and the thermal conductivity (PEN-THC) in the first tens of centimeters of the nucleus as they evolve with time. A combined evaluation of the PEN-TP and PEN-THC data will allow to understand vertical surface heat flow into or from the comet nucleus and the energy balance of the comet. The surface temperature will be measured with an infrared thermal mapper (MUPUS TM). Both thermal sensors will provide a ground truth for IR data from the orbiter. The PEN-M sensor will measure mechanical properties like hardness and grain size during penetration. A Compton backscatter densitometer (CBD) will be used to measure the density. The RoLand anchor(s) will be equipped with an additional temperature sensor and penetrometer. The results will help to understand the onset of activity, gas and dust emission, which will be measured by the orbiter. Understanding the dominating processes and their time scales allows to determine the present state of the surface material ("Is the matter found close to the surface pristine?") as well as extrapolation both into the past and the future.
The Rosetta mission to comet P/Wirtanen, due for launch in 2003 and arrival in 2011, aims to deploy two landers to the surface of the nucleus to perform experiments in situ. In order to characterise the physical properties of the near-surface layers, a measurement of bulk density is required. This, combined with measurements of thermal, mechanical and structural properties as well as composition, will help us understand the nature of cometary material and the degree to which the surface layers have evolved during the history of the solar system. For the bulk density measurement the method of gamma densitometry has been chosen. The densitometer is part of the MUPUS physical properties experiment package for the RoLand lander. This poster outlines the concept of the instrument and recent develoments in its design as a result of the payload accommodation process.
The strength of the surface material to be encountered by a spacecraft landing on one of the Solar System's solid bodies is an important design parameter. A number of surface missions are currently either planned or en route for the Moon, Mars, Titan, asteroids and comets. For such unfamiliar targets as comets the surface strength is of particular concern. Here we report on development of the Rosetta Lander's anchoring device. The first- and probably deepest- in situ science measurements on the Wirtanen nucleus will be made by a piezoelectric shock accelerometer (MUPUS ANC-M) mounted inside the Rosetta Lander's harpoon anchor, to be shot into the surface at about 60 m/s on touchdown.
To verify the applicability of the dynamic penetrometry technique for determination of the strength distribution within a material, test shots were performed using an apparatus built at the Max-Planck-Institute for Extraterrestrial Physics in Garching/Munich. Sand, various glass foams and porous concrete were used as target materials. Their strengths approximately cover the range that might be encountered on cometary nuclei. The 'deceleration history' of a test anchor was recorded by a miniature piezoelectric accelerometer. Several data processing and modelling steps are used to extract strength profile information. We report in detail on the procedures used, and on the derivation of vertical strength profiles using a model based on that of Forrestal et al. (1981). Although developed for interpretation of data from the Rosetta Lander, the methods presented are of general value for other missions involving surface penetration, including the DS2 Mars Microprobes, Lunar-A and possibly the comet mission ST4 / Champollion.
From a scientific standpoint, early indications of the strength of the surface material and any distinct layers measured by ANC-M should prove valuable to subsequent depth-sensitive investigations on the Rosetta Lander, including the MUPUS thermal probe, seismic sounding experiments and composition analyses of material extracted by the sampling drill. Interpretation of the ANC-M data will help constrain models of the formation and evolution of the material found at the landing site and will help document the mechanical and structural context of nearby sampled material.
Outgassing and surface erosion are expected to increase the density of the surface layer of Wirtanen's nucleus and lead to the formation of a dust mantle. This process is important for the observed behaviour of the comet and its evolution. The density influences thermal, mechanical and seismic properties of the nucleus material. The MUPUS physical properties experiment on the Rosetta Lander includes a density sensor for measurements of the bulk density of the nucleus surface material. The density sensor uses a gamma ray attenuation technique to obtain a depth profile of bulk density as the MUPUS thermal probe is hammered into the comet and, following full penetration, its variation with time. We report on the instrument's design, the results of laboratory tests and calculations of the instrument's performance.
The MUPUS experiment package is now - like its parent spacecraft, the Rosetta Lander - close to delivery of the flight model. Numerous tests and calibration campaigns for most of the MUPUS sensors have been performed during recent months. For the first time we may now predict the accuracy and quality of the data to be collected by the MUPUS experiment in 2012, after touchdown of the Rosetta Lander on the target comet. We present selected test results from the MUPUS sensors: temperature sensors and accelerometers in the lander's anchors, the MUPUS TM infrared temperature sensors, the depth sensor on MUPUS PEN, the densitometer and the temperature and thermal conductivity sensors. We discuss how a combined evaluation of this data will allow us to assess the physical behaviour of the comet, its near-surface structure, its energy balance, and possible changes of physical properties as the comet approaches the Sun. We also discuss important synergies with other investigations on Rosetta.
The proposed ESA cornerstone mission BepiColombo to Mercury will be a good opportunity to study the physical properties of the near surface material. According to the current mission profile it is foreseen to land a surface science package to Mercury's high latitude regions. We propose to investigate the surface strength properties by the means of accelerometers mounted in the penetrating part of the lander. As the current design of the mission is open to either a hard landing, releasing a penetrator at touch down - or a controlled, soft descent and subsequent release of a self propelled subsurface probe, we show that both options allow the study of surface properties by the means of analysing the deceleration/force acting on the device. Thus we can derive properties like material strength profiles and texture information along the penetration path. The penetration depth can vary from several tens of centimetres up to some meters depending on the penetrator mechanics and geometry and of course on the target material properties. The best scientific return is expected when these measurements are performed in the framework of a 'physical properties' package consisting of an assembly of small and low mass sensors, measuring e.g. density and thermal properties, in addition.
The BepiColombo mission plans to carry a penetrator, with an aftbody to stay at the surface and a forebody to penetrate to a depth of several metres. One of the payload elements studied is HP3, a package to measure surface heat flow and thermal and mechanical properties of the regolith. Heat flow is a quantity of fundamental interest, constraining both the chemical composition and the interior dynamics of a planet. It is believed that the variation of heat flow over the Hermean surface is much less than that on Earth, making a single measurement more valuable. Models predict a surface heat flow of a few 10 mW/m2. Heat flow determination requires measurements of the temperature gradient and thermal conductivity at a depth sufficient to avoid daily and seasonal temperature perturbations. The depth at which these are less than a few mK is about 2m. For the temperature gradient and thermal conductivity measurements, two options are being studied. The first option has a strip of temperature sensors integrated into the forebody, while in the second option the umbilical is equipped with temperature sensors. In both cases the sensors can be heated to measure the thermal conductivity using the line heat source method. The non-thermal physical properties foreseen to be measured are the bulk density (by a gamma ray densitometer) and the mechanical and structural properties of the regolith, measured upon penetration by accelerometry.
The application of penetrometry - defined here as the measurement of a body's penetration (e.g. force, deceleration, velocity, depth) to derive mechanical and / or structural information concerning the target - to solid surfaces elsewhere in the Solar System has a long and varied flight heritage, stretching back to the earliest era of in situ planetary surface exploration in 1966. We attempt to review all known planetary penetrometry investigations, including basic details and key references for all experiments that were (or are due to be) built and launched, and the results obtained (if any). We suggest a classification scheme for penetrometry investigations and discuss the various motives for planetary surface penetration (scientific, operational, engineering) and the information obtainable by its measurement. Many penetration mechanics investigations are / were opportunistic rather than dedicated instruments, producing useful (but rarely earth-shattering) supplementary information. Penetrometry is perhaps most effective when used on completely unfamiliar surfaces or as one element in a suite of synergistic surface science sensors, especially if measurements can be made at several different locations on the surface. Accelerometry data also allows reconstruction of the trajectory of payload delivery penetrators to determine their position and orientation - information which may be important for interpretation of data from other sensors.
Last October a 3-day workshop on 'Penetrometry in the Solar System' was held in Graz. The discussions reflected growing interest in the use of penetrators - for delivery of a payload into a planetary surface - and in 'penetrometry' more generally - the use of penetrating probes to investigate planetary surface materials. The workshop brought together workers using penetrometry for planetary science with those applying similar techniques on Earth. Topics discussed ranged from the design and dynamics of large 'payload delivery' penetrators - impacting the Moon or Mars at several hundred metres per second - to the use of small probes designed to acquire samples or make measurements of such properties as small-scale texture, thermal and mechanical properties and bulk density. The results of various laboratory and field experiments were also discussed. Two common themes running through much of the discussion were the interpretation (with the help of modelling) of accelerometry (or force) data acquired during penetration in terms of the target's mechanical properties, and the way thermal measurements can best be made - even the possibility of using the impact heating of penetration to measure the target's thermal properties. The formal output of the workshop will be the publication of a proceedings volume by the Austrian Academy of Sciences Press. However a less tangible result is likely to be the increased interaction between workers in this field.
The application of penetrometry - defined here as the measurement of a body's penetration (e.g. force, deceleration, velocity, depth) to derive mechanical and/or structural information concerning the target - to solid surfaces elsewhere in the Solar System has a long and varied flight heritage, stretching back to the earliest era of in situ planetary surface exploration in 1966. We attempt to review all known planetary penetrometry investigations, including basic details and key references for all experiments that were (or are due to be) built and launched, and the results obtained (if any). Particular attention is paid to experiments not covered in detail elsewhere in this volume. We suggest a classification scheme for penetrometry investigations and discuss the various motives for planetary surface penetration (scientific, operational, engineering) and the information obtainable by its measurement. Many penetration mechanics investigations are/were opportunistic rather than dedicated instruments, providing useful (although rarely earth-shattering) supplementary information. Penetrometry is perhaps most effective when used on completely unfamiliar surfaces or as one element in a suite of synergistic surface science sensors, especially if measurements can be made at several different locations on the surface. Accelerometry data also allows reconstruction of the trajectory of payload delivery penetrators to determine their final position and orientation - information which may be important for full interpretation of data from other sensors.
We review a selection of the wide range of previous work, both experimental and theoretical, in predicting the penetration of projectiles and, in particular, in applying measurements during penetration to recover information about the properties of the target. Although we do not pretend to provide anything near a complete list of references or historical summary of penetration mechanics, we aim to summarise the wide range of experimental techniques and theoretical methods that are most relevant for spacecraft application.
SPICE, one of 9 experiments selected to fly on the Netlander mission to Mars, addresses properties of the uppermost few cm of Martian surface materials, focussing on thermal and mechanical properties. Measurements of (lateral) temperature field, thermal diffusivity / conductivity and penetration resistance will be made, within a mass budget of just 100 g. The temperature field will be monitored by means of 16 thin film temperature sensors mounted around the edge of the lander, in contact with the ground. As well as operating in a passive temperature measurement mode, these thermal sensors can also be heated to measure thermal diffusivity and look for condensed volatiles. The penetration resistance of the Martian surface will be measured during the motor-driven deployment of the seismometer anchoring spikes. In addition to monitoring motor parameters (current & voltage) and displacement, force sensors incorporated into the tips of the spikes will monitor the penetration resistance directly. SPICE will use the electronics of the seismometer experiment.
BepiColombo, ESA's cornerstone mission to Mercury, will include a surface element, most likely a soft-lander, which will perform in situ measurements of the planetary surface, sub-surface and exosphere. Discussed here are possible instrumentation concepts to help answer some of the key scientific questions we have about Mercury. Sensors are proposed for both the lander itself and a "mole" (sub-surface, self-penetrating) device. A mole can take advantage of its unique deployment method to study physical properties at varying depths, for example by observing regolith stratigraphy and measuring profiles of bulk density and thermal and electrical properties. Samples of the regolith can be ingested and analysed, e.g. heated so that any volatiles can be released and identified. Miniature, low-power devices mounted on the lander can be used to evaluate surface properties such as radiation flux and exospheric pressure, giving both ground truth for the orbiters and answering questions on, for example, the dynamics of surface-magnetosphere interactions. Possible instruments capable of recording these data are evaluated, as are the engineering challenges presented by both the extreme environment on Mercury and the harsh power and mass constraints placed on the lander.
Archaeology and planetary science share a surprising number of aspects, including some of their scientific principles and the way solid surfaces and their constituent materials can be analysed. In both cases the surface holds a story about its history that we can try to decipher. While the connections between planetary science and terrestrial field geology and geophysics are more readily apparent, many archaeological techniques are of a scale and simplicity which may perhaps be adaptable for planetary surface exploration (both robotic and crewed missions). Presented here are examples of archaeological techniques and principles and their extraterrestrial analogues. More generally, this comparison illustrates that it is often useful to draw on the terrestrial experience for inspiration.
The SPICE experiment is one of 9 selected to fly on the Netlander mission to Mars. SPICE addresses properties of the uppermost few cm of Martian surface materials, focussing on thermal and mechanical properties. Measurements of (lateral) temperature field, thermal diffusivity and penetration resistance will be used to constrain thermal models of the surface materials and the exchange of volatiles between the ground and the atmosphere. The experiment will also constrain models of the surface material and its structure, in terms of mechanical strength, texture and porosity. The experiment aims to serve as a ground truth for remotely-sensed thermal inertia measurements, and the measurements may also provide useful additional information for other experiments such as the seismometer and the meteorological package.
ESA's fifth cornerstone mission BepiColombo includes a 'Surface Element' to land a scientific payload on the surface of Mercury. The current strawman payload includes a Heat Flow and Physical Properties Package (HP3), focussing on key thermal and mechanical properties of the near-surface material (down to a depth of 2-5 m) and the measurement of heat flow from Mercury's interior, an important constraining parameter for models of the planet's interior and evolution. We present here an overview of the HP3 experiment package and its possible accommodation in a self-inserting 'mole' device. A mole is considered to be the most appropriate deployment method for HP3, at least in the currently-assumed case of an airbag-assisted soft landing architecture for the Mercury Surface Element.
The two successful Lunokhod rovers (1970-71, 1973) travelled a total of 47.5 km over the surface of the Moon (in Mare Imbrium and Le Monnier crater, respectively). Measurements of the physico-mechanical properties of the lunar regolith were made at over 1000 locations (to a maximum depth of 100 mm), by means of the PROP cone / vane-shear penetrometer instrument, which measured both penetration resistance and resistance to rotational shear. The published analyses of this data suggest that further analysis is possible and worthwhile. However, the data is currently in a far from accessible form and would require some effort to convert it to a suitable modern electronic format. The output of further work ought to include: 1) a more thorough statistical analysis of the PROP data, 2) a comparison of the force profile data with penetration models, 3) a better understanding of the metre- to kilometre-scale variations in mechanical properties of the lunar regolith, 4) a better knowledge of the context of the in situ data (location, slope, etc.), 5) a comparison between the Lunokhod 1 and 2 routes, and 6) datasets in an easily accessible electronic form for subsequent study by other workers. It may also be useful to obtain additional contextual information on the Lunokhod routes, using the high resolution imaging capabilities of forthcoming lunar missions.
In the years 2011-2013 the ESA mission Rosetta will explore the short period comet 46P/Wirtanen. The aims of the mission include investigation of the physical and chemical properties of the cometary nucleus and also the evolutionary processes of comets. It is planned to land a small probe on the surface of the comet, carrying a multitude of sensors devoted to in situ investigation of the material at the landing site. On touchdown at the nucleus, an anchoring harpoon will be fired into the surface to avoid a rebound of the lander and to supply a reaction force against mechanical operations such as sample drilling or instrument platform motion. The anchor should also prevent an ejection of the lander due to gas drag from sublimating volatiles when the comet becomes more active closer to the Sun. In this paper, we report on the development of one of the sensors of the MUPUS instrument aboard the Rosetta Lander, the MUPUS ANC-M (mechanical properties) sensor. Its purpose is to measure the deceleration of the anchor harpoon during penetration into the cometary soil. First the test facilities at the Max-Planck-Institute for Extraterrestrial Physics in Garching, Germany, are briefly described. Subsequently, we analyse several accelerometer signals obtained from test shots into various target materials. A procedure for signal reduction is described and possible errors that may be superimposed on the true acceleration or deceleration of the anchor are discussed in depth, with emphasis on the occurrence of zero line offsets in the signals. Finally, the influence of high-frequency resonant oscillations of the anchor body on the signals is discussed and difficulties faced when trying to derive grain sizes of granular target materials are considered. It is concluded that with the sampling rates used in this and several other space experiments currently under way or under development a reasonable resolution of strength distribution in soil layers can be achieved, but conclusions concerning grain size distribution would probably demand much higher sampling rates.
The first - and possibly deepest - in situ science measurements on the 46P/Wirtanen nucleus will be made by two sensors of the Rosetta Lander's MUPUS experiment. A piezoelectric shock accelerometer (ANC-M) and a resistance temperature sensor (ANC-T) will be mounted in the Lander's harpoon anchor. This will be shot into the surface at about 60 m s-1 on touchdown, reaching a final depth of between a few centimetres and about 2.5 m, depending on the hardness of the ground and the maximum available cable length. Early indications of the strength of the surface material and any distinct layers should prove valuable to subsequent depth-sensitive investigations, including the MUPUS thermal probe, seismic sounding experiments, the sampling drill and composition analyses of the extracted material. Interpretation of the ANC-M data will help to constrain models of the formation and evolution of the material found at the landing site and document the mechanical and structural context of nearby sampled material. We report on the results of recent test shots performed with a prototype anchor into several porous materials: two types of glass foam, H2O ice and CO2 ice. With the help of data from direct shear tests and quasi-static penetration tests, we interpret the processed deceleration data using a cavity-expansion penetration model. Layers of distinctly different strengths can be detected and located, and the deceleration profiles are in reasonable agreement with the profiles obtained by quasi-static tests. The anchor projectile's long sharp tip tends to smear out the boundaries, however. In applying the penetration model we found that the coefficient of sliding friction and the target's volumetric strain have a much stronger influence on the deceleration profile than the initial target density and angle of internal friction. Very small values of volumetric strain (corresponding to high 'drag coefficient') were required to fit deceleration profiles to the measured data for the glass foam, contrary to what we initially expected by inspecting the thin layer of crushed material around the walls of the penetrated channel. We interpret this to mean that such brittle, porous materials as the glass foam (and perhaps highly porous, brittle, cryogenic ice) do not exhibit plastic deformation before failing completely by the crushing of cell walls. The decelerating forces are thus thought to be dominated by momentum transfer to the crushed material and by the crushing strength of the cellular microstructure, rather than by the force required to deform the target plastically. The cavity-expansion model seems to be well-suited to the ice shots, but for the brittle, cellular glass foam, alternative approaches, taking into account the material's microstructure, are needed. As a first step in this direction, a microstructural model linking textural properties of the material (pore and grain size, and relative contact area between grains) is applied to the glass foam data, obtained from quasi-static penetration tests and from direct shear strength tests. It is demonstrated that the dependence of strength on porosity can be well represented by the model suggested. A microstructural model for sintered ices, relating strength properties to porosity and thermal properties, would be useful for interpretation of MUPUS ANC-M data in the context of other physical properties measurements. The work presented here may also have some relevance to the design of future comet landers or penetrators. The harpoon anchor/penetrometer approach could be employed on other minor body landing missions, while the modelling technique is similar in many ways to that appropriate for other penetrometers/penetrators.
The MUPUS experiment on the Rosetta Lander will measure thermal and mechanical properties as well as the bulk density of the cometary material at and just below the surface of the nucleus of comet 46P/Wirtanen. A profile of bulk density vs. depth will be obtained by measuring the attenuation of 662 keV gamma rays emitted by a 137Cs source. Compton scattering is the dominant interaction process at this energy, the attenuation depending directly on the total number of electrons along the source-detector path. This in turn is approximately proportional to the column density. We report here on the design of the bulk density instrument and the results of related Monte Carlo simulations, laboratory tests and calculations of the instrument's performance. The 137Cs radioisotope source is mounted in the tip of the MUPUS thermal probe- a 10 mm diameter rod, to be hammered into the surface of the nucleus to a depth of ~370 mm. Two cadmium zinc telluride (CZT) detectors mounted at the top of the probe will monitor the count rate of 662 keV photons. Due to the statistics of photon counting, the integration time required to measure column density to a particular accuracy varies with depth as well as with bulk density. The required integration time is minimised for a material thickness equal to twice the exponential attenuation length. At shallower depths the required time rises due to the smaller fractional change in count rate with varying depth, while at greater depths the reduced count rate demands longer integration times. The former effect and the fact that the first 45 mm of the source-detector path passes not through the comet but through the material of the probe, mean that the first density measurement cannot be made until the source has reached a depth of perhaps 100 mm. The laboratory experiments indicate that at this depth an integration time no less than 348 s (falling to 93.9 s at full penetration) would be required to measure a bulk density of 1000 kgm-3 to 5% accuracy, assuming a source activity of 1.48 mCi (decayed from an initial 2 mCi). Although solutions involving feedback of the measured bulk density into a time-budgeting algorithm are conceivable, a simple approach where equal time is spent per unit depth may be best, providing an accuracy in bulk density of around 5-20%, for 25 mm slices and the expected range of parameters.
The European Space Agency's fifth cornerstone mission BepiColombo includes a 'Surface Element' to land a scientific payload on the surface of Mercury. The current strawman payload includes a heat flow and physical properties package (HP3), focussing on key thermal and mechanical properties of the near-surface material (down to a depth of 2-5 m) and the measurement of heat flow from Mercury's interior, an important constraining parameter for models of the planet's interior and evolution. We present here an overview of the HP3 experiment package and its possible accommodation in a self-inserting 'mole' device. A mole is considered to be the most appropriate deployment method for HP3, at least in the currently-assumed case of an airbag-assisted soft landing architecture for the Mercury Surface Element.
In recent years the exploration of planetary bodies by surface probes has entered a new phase of interest. The mechanical properties of the near-surface layers of cometary and planetary bodies, including their strength, texture and layering, are important parameters needed both for a proper physical understanding of these bodies and for the design of lander missions.
The strength properties of such a surface can be determined either by measuring the penetration resistance encountered by a slowly penetrating tip (quasi-static penetrometry), or by impacting the surface with an artificial body whose mechanical properties and impact velocity are known. At low speeds (up to about 300 m s-l), it is often feasible to measure the force or deceleration during penetration ('impact' or 'dynamic' penetrometry). If the event is better described in terms of hypervelocity impact cratering than penetration, analysis of the resulting crater is performed instead. Other uses of artificial impacts include the generation of seismic waves to aid the calibration of seismometers and the formation of a crater and / or ejecta for analysis or sampling of the target body. Such methods feature in many recent, current and forthcoming planetary missions. Examples include: the Surface Science Package of the Huygens probe, the anchoring harpoon of the Rosetta Lander, various Mars penetrators and landers, NASA's Deep Impact mission and ESA's planned Mercury cornerstone mission BepiColombo.
A short overview of missions (past, present, future) featuring penetrometry experiments or artificial impacts is given. The theory of impact penetration and methods to interpret deceleration profiles in terms of material strength are discussed and applied to data sets obtained from test shots performed with the Rosetta Lander anchoring harpoon.
The SIMONE mission proposal is led by QinetiQ, with scientific aspects led by the Open University's Planetary and Space Sciences Research Institute. Currently the focus of an ESA-funded study, the concept is to help understand the diversity of the NEO population using a fleet of microsatellite-class (~120 kg) interplanetary spacecraft. These will individually rendezvous with specific Near-Earth Objects of interest (together representing a number of important spectral and photometric types) to perform 'up close' measurements of key physical, morphological and compositional features. The Delta-V capability of the solar-electric-propelled spacecraft should make a wide range of objects accessible for rendezvous, or at least a sequence of resonant flybys. The wide-ranging dataset produced will be important for risk assessment and studies of possible mitigation approaches, as well as for science.
Impact processes are now recognised as the primary process driving planetary surface evolution, from planet-shattering collisions to dust particle impacts. On airless bodies such as the Moon and Mercury, impacts generate the fine-grained soil, or regolith. Everything we learn about a planetary surface from remote sensing comes from emission and reflection of radiation with this uppermost layer of material. Understanding the dynamical evolution of solar system regoliths is therefore a vital part of planetary geology.
The processes acting on this topmost layer of regolith are collectively known as "space weathering". Recent discoveries show that a process of vapour deposition can explain many of the observed properties; as well as breaking larger rocks into finer ones, the kinetic energy of impacting micrometeorites can heat both impactor and target material so as to melt and vapourise them. This melt and vapour undergoes fractionation during transport and condensation and favours the production of sub-microscopic iron particles coating regolith grains. This iron plays the dominant role in the optical and magnetic properties of the regolith.
A model of planetary surface evolution is presented here. This model takes into account the effects of meteoritic impacts, sputtering due to solar wind ions and temperature-gradient effects. The planet Mercury is used to demonstrate the model. These results are discussed in the context of the BepiColombo mission to Mercury and a novel instrument proposal for making in situ measurements of regolith physical properties.
Recent interest in the astrobiological investigation of Mars (and eventually Europa) has culminated in the only planned astrobiology-focussed robotic mission to Mars - the UK-led European Beagle 2 mission to be carried to Mars by the ESA Mars Express spacecraft in 2003. Beagle 2, like most proposed Mars missions will be primarily investigating the surface and near-surface environment of Mars. However, the results from the Viking Mars lander (1976) indicated that the Martian surface is saturated in peroxides and super-oxides, which would rapidly degrade any organic material. Furthermore, recent models of gardening due to meteoritic impacts on the Martian surface suggest that the depth of this oxidising layer could extend to depths of 2-3 m. Given that the discovery of organic fossilised residues will be the primary target for astrobiological investigation, this implies that future robotic astrobiology missions to Mars must penetrate below these depths. The need to penetrate into the sub-surface of Mars has recently been given greater urgency with the discovery of extensive water ice-fields as little as 1 m from the surface. We review the different technologies that make this penetration into the sub-surface a practical possibility on robotic missions. We further briefly present one such implementation of these technologies through the use of ground-penetrating moles - the Vanguard Mars mission proposal.
We think of 'planetary robotics' as being required when a planetary mission is not achievable with a vehicle's 'natural' vector (e.g. position, velocity or orientation). It is a way of getting sensors to the environment, bringing samples from the environment to the sensors, or modifying the environment in some way (e.g. digging). A robotic capability on a planetary or satellite surface (and within an atmosphere or even underground) can greatly enhance possibilities for the scientific exploration of our Solar System. We review the basic types of measurement sought and discuss the need for mobility. We outline the common constraints faced by robotic solutions and highlight the wide range of interesting but challenging environments to be faced. The robotic solution to a particular problem is often very sensitive to the measurement or sampling strategy needed to address the chosen scientific goals. However, new robotic technologies may inspire the scientific community to create new experiment designs not previously considered, so the process can be iterative. A high degree of integration between system and payload aspects is often needed. We describe some particular environments of interest and potential mission scenarios (including the future exploration of Titan), highlighting the huge variety of potential payload experiments, ranging in size from tiny sensors weighing <1 g up to multi-kg sample analysis packages. These of course have widely varying demands in terms of robotics.
A number of academic engineering research groups around the UK have become increasingly interested in the applications of robotics or robotics techniques to solving problems in space engineering. Although these groups have sprung up independently and have worked in essentially independent areas, they are seeking to form themselves into a network offering a diverse range of expertise within the UK with the capability of developing complete space robotic systems. Space robotics is an area in which the UK has dabbled in the past, but for the first time, the UK offers a solid base of expertise in mobile robotics and associated space engineering which would enable the UK to contribute to European space robotics projects funded by ESA and/or national agencies. To that end, following the inaugural meeting of the Space & Planetary Robotics Network, an extended group of interested parties will be putting forward an application to EPSRC for Network funding.
This paper discusses test results obtained in both laboratory and terrestrial environment conditions for the "Multipurpose Sensors for Surface and Sub-Surface Science" Thermal Probe (MUPUS-TP), which has been developed for the European Space Agency Rosetta cometary rendezvous mission. The probe is intended to provide in situ long-term observations of the thermal evolution of the comet nucleus and will measure a thermal conductivity profile with time in the top 30 cm of the comet nucleus. The basic operating principles of the probe are briefly described, including typical test results gathered in terrestrial snow and soil. The tests in snow provide verification of the probe as a useful tool for monitoring the metamorphism of snow on the Earth. The tests in soil are intended to demonstrate the probe's suitability as an alternative to other methods of energy measurement currently practiced in soil physics research. The tests of the probe in the natural environment of the Earth provide a demonstration of the behavior of the instrument in the presence of complex energy exchange processes before it is used on the comet.
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