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MUPUS – a Thermal and Mechanical Properties Probe for the Rosetta Lander Philae


Abstract and Figures

MUPUS, the multi purpose sensor package onboard the Rosetta lander Philae, will measure the energy balance and the physical parameters in the near-surface layers – up to about 30 cm depth- of the nucleus of Rosetta’s target comet Churyumov-Gerasimenko. Moreover it will monitor changes in these parameters over time as the comet approaches the sun. Among the parameters studied are the density, the porosity, cohesion, the thermal diffusivity and conductivity, and temperature. The data should increase our knowledge of how comets work, and how the coma gases form. The data may also be used to constrain the microstructure of the nucleus material. Changes with time of physical properties will reveal timescales and possibly the nature of processes that modify the material close to the surface. Thereby, the data will indicate how pristine cometary matter sampled and analysed by other experiments on Philae really is.
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1Institut f¨
ur Planetologie, Westf¨
alische Wilhelms Universit¨
at, M¨
unster, Germany,
2Institut f¨
ur Planetenforschung, Deutsches Zentrum f¨
ur Luft- und Raumfahrt, Berlin, Germany
3Physikalisches Insitut, Universit¨
at Bern, Bern, Switzerland
4Planetary and Space Science Research Institute, CEPSAR, The Open University, Milton Keynes, UK
5Space Research Centre, Warsaw, Poland
6Institut f¨
ur Weltraumforschung, ¨
Osterreichische Akademie der Wissenschaften, Graz, Austria
7European Space Technology Centre, ESA, Noordwijk, The Netherlands
8Telecommunication Institute, PIT, Warsaw, Poland
(Author for correspondence: E-mail:
(Received 28 February 2006; Accepted in final form 11 October 2006)
Abstract. MUPUS, the multi purpose sensor package onboard the Rosetta lander PHILAE, will mea-
sure the energy balance and the physical parameters in the near-surface layers – up to about 30 cm
depth- of the nucleus of Rosetta’s target comet Churyumov-Gerasimenko. Moreover it will monitor
changes in these parameters over time as the comet approaches the sun. Among the parameters studied
are the density, the porosity, cohesion, the thermal diffusivity and conductivity, and temperature. The
data should increase our knowledge of how comets work, and how the coma gases form. The data may
also be used to constrain the microstructure of the nucleus material. Changes with time of physical
properties will reveal timescales and possibly the nature of processes that modify the material close
to the surface. Thereby, the data will indicate how pristine cometary matter sampled and analysed by
other experiments on PHILAE really is.
Keywords: rosetta, comets, surface, heat flow
1. Introduction and Scientific Goals
Rosetta was sucessfully launched on 2 March 2004 and is expected to start its
rendevous with Comet Churyumov-Gerasimenko in May 2014. The Rosetta Lander
PHILAE will be the first spacecraft to make a soft landing on a comet nucleus.
Scientific observations are to be carried out for a minimum of one week, but might
continue for several months as the comet approaches perihelion. Rosetta’s target
comet Churyumov-Gerasimenko has a period of 6.6 years. its nucleus, with an
estimated size of 3×5 km, is expected to have a rotation period of approx 12 hours.
Space Science Reviews (2007) 128: 339–362
DOI: 10.1007/s11214-006-9081-2 C
Springer 2007
The physics of comets involves the production of the coma by the sublimation of
ices at or close to the surface of the nucleus. The rates of production of coma gases
depend on the energy balance at the surface and in a boundary layer underneath
the surface into which heat is transferred by conduction and vapour transport. The
surface energy balance is
RiHi=Sεσ T4(q+qv) (1)
where Riare the sublimation rates of the coma gases (species i), Hiis the enthalpy
of sublimation of species i(approx. 2.8×106Jkg1for water ice), Sis the insolation
corrected for the surface albedo, εis the surface emissivity (slightly less than 1.0
for a ‘dark’ comet), σthe Stefan-Boltzmann constant, Tis temperature and qand
qvare the conductive heat flux and the heat flow associated with the vapour flow
into or out of the interior, respectively. Temperatures at a dust-covered surface can
exceed the temperature of a sublimating water ice surface (200 K) by more than
100 K near 1AU. The vapour flux into the interior is driven by the gradient in
vapour density that forms in response to the temperature gradient during cometary
day time. The gas flow is likely to be in the Knudsen regime. During night time the
flow may be reversed.
The energy balance in the porous interior of the nucleus reads
where ρand care the density and specific heat of the nucleus ice, ρi,ci, and ui
are the densities, specific heats (at constant volume) and flow velocity of vapour
species iin the interior. Since the vapour pressures are functions of temperature,
the ratio between the two heat transport terms on the right-hand-side (RHS) of
(2) depends on the temperature, the enthalpy of sublimation and on the thermal
diffusivity and permeability of the ice. The thermal properties of the ice are not
precisely known. Depending on pore volume and structure in the ice, thermal con-
ductivity could be 2 or more orders of magnitude lower than solid ice, i.e. 568/T W
m1K1(Klinger, 1981), so that the vapour could contribute considerably to the to-
tal energy transport. Smoluchowski (1982) was the first to point to the importance
of heat transfer via the vapour phase. It has been demonstrated experimentally
through the KOSI (comet simulation) experiments (e.g., Gr¨un et al., 1991) that
heat transfer via vapour was important at temperatures above about 200 K for
water ice (Spohn and Benkhoff, 1990; Steiner, 1990; Benkhoff and Spohn, 1991;
Espinasse, 1991; Steiner and K¨omle, 1991; Steiner et al., 1991; Benkhoff et al.
1995). Spohn and Benkhoff (1990) have outlined a porous medium heat and mass
transfer theory to describe the effects. Benkhoff and Huebner (1995) and Hueb-
ner and Benkhoff (1997,1999) expanded the model and applied it to cometary
MUPUS (’Multi-Purpose Sensors for Surface and Sub-Surface Science’) origi-
nated from the proposal to the then RoLand comet lander by Spohn et al. (1995),
with a related experiment (’SuSI’) also having been proposed for the (ultimately
cancelled) Champollion lander (Benkhoff et al., 1995). The central part of MUPUS
is a thermal probe that, after insertion into the regolith of the comet nucleus up to a
depth of 32 cm, aims to measure both the temperature profile underneath the surface
and the thermal diffusivity and conductivity profiles. The thermal diffusivity and
conductivity will be derived from the time rates of change of sensor temperatures
during active heating cycles. In addition, MUPUS will measure the surface temper-
ature and the mechanical strength of the nucleus material. The temperatures and
the thermal transport parameters will be measured regularly over the life time of
the lander. The mechanical strength of the material will be derived from the energy
spent per unit distance penetrated during the hammered insertion. Profiles of pene-
tration resistance will also be derived from MUPUS accelerometry measurements
in each of the lander’s two harpoon anchors. The anchors also contain MUPUS
temperature sensors.
In doing these measurements, MUPUS can contribute to an assessment of the
energy balance of the comet nucleus and the physical properties of its material.
Since the thermal conductivity and diffusivity are strong functions of the structure
of the porous ice and the degree to which it has been sintered (Seiferlin et al.,
1995), MUPUS can constrain the microstructure and the degree to which the comet
material has been thermally altered. MUPUS will thus predict at what depth pris-
tine material can be expected. This is important for characterising the context of
the samples extracted by the drill, as well as for our understanding of the thermo-
physics of cometary activity. The scientific objectives of MUPUS are summarised as
To understand the properties and layering of the near-surface matter as these
evolve with time as the comet nucleus spins and approaches the Sun.
To understand the energy balance at the surface and its variation with time
and depth.
To understand the mass flow at the surface and its evolution with time.
To provide ground truth for thermal mapping from the Orbiter, and to support
other instruments on the Rosetta Lander (e.g. SESAME-CASSE).
2. Instrument Description
The MUPUS package consists of three major parts, the penetrator MUPUS PEN
with ist subsystems, the radiometer MUPUS TM, and the anchor sensors MUPUS
ANC. Their positions on the lander are coloured red in Figure 1. The MUPUS main
electronics are integrated into the Common Electronics Box on the lander, together
with the main electronics of other experiments and lander subsystems.
Figure 1. MUPUS instrument package configuration overview. The penetrator is placed on the sur-
face away from the lander by a deployment arm (not shown). Both anchors of the lander harbour
a temperature sensor (ANC-T) and an accelerometer (ANC-M). The 4-channel IR sensor TM is
mounted near the top of the lander housing. The penetrator is equipped with a depth sensor (PEN-M),
thermal sensors that measure the temperature profile (TP) and a thermal diffusivity / conductivity
profile (THC). The hammering device and the front-end electronics are mounted in the housing (thick
cylinder) on top of the penetrator tube (thin cylinder).
The main part and the most complex instrument in the MUPUS experiment suite
is the thermal probe MUPUS PEN (Figure 2). A fibre compound tube with a
metal tip will be inserted into the ground about one meter away from the lander
by a deployment device and a hammer mechanism (Seiferlin et al., 2002). The
hammer mechanism is accommodated together with heated front end electronics
in a gold plated cylinder housing at the top of the penetrator and will remain above
the surface. The total length of the probe is restricted by the vertical height of the
lander on the balcony of which MUPUS PEN was required to fit. The hammer and
electronics housing is about 10 cm high, leaving 33 cm for the tube and 3 cm for
the tip. The length of the tube is several times the thermal skin depth for rotation
rates of the nucleus between 10 and 100 h and thermal conductivities less than that
of compact ice. We consider a skin depth of only a few cm as most likely.
Until its descope in mid-2001, MUPUS PEN also incorporated a gamma ray den-
sitometer (Ball et al., 2001). It was designed to measure the attenuation of gamma
rays emitted by a 137Cs source in the tip of the penetrator. During penetration the
Figure 2. MUPUS PEN: The right image shows the MUPUS PEN, mounted on the lander (launch
and cruise phase configuration). A stepper motor (lower centre, between the two black support tubes)
will uncoil two metal profiles (brass-coloured) from their stored position on two spools (left and
right of the stepper motor) and thus deploy the penetrator to a distance of about 1 m from the lander.
A hammer (in the gold-plated cylinder on top) will insert the probe (thin, brownish tube) into the
regolith. The tube carries 16 thermal sensors that can be used to measure the temperature. These
sensors can also be used to heat the surrounding comet regolith (2 such sensors are shown in the
left panel) for a measurement of the thermal conductivity. A densitometer photon source and two
densitometer detectors (top, next to the hammer housing) were planned to be integrated (left image).
The densitometer had to be descoped because of the lack of funding.
changing flux of transmitted gamma rays would have been measured by two semi-
conductor detectors mounted on opposite sides of the front-end electronics. The
sensor was descoped because of technical problems arising ultimately from fund-
ing difficulties in the early stages of the project. Unfortunately, the vertical profile
of bulk density will now have to be inferred by other means. For example, a bulk
density profile might be retrievable via microstructural models of the sub-surface
material, constrained by other physical properties measurements as well as compo-
sitional information. Such measurements include: mechanical properties measure-
ments made by the MUPUS thermal probe during hammering, anchor deployment
and sampling drill operation; thermal diffusivity measurements made by the MU-
PUS thermal probe; permittivity measurements made by the SESAME-PP exper-
iment; acoustic wave propagation measurements made by the SESAME-CASSE
experiment (Seidensticker et al., this volume); surface thermal inertia measure-
ments made nearby by MUPUS-TM, and compositional measurements made by
the Philae geochemical experiments. Larger-scale bulk density can also be assessed
by the orbiter radio science experiment and by the CONSERT radio transmission
tomography experiment (Kofman et al., this issue).
2.1.1. The Thermal Probe
The tube has a radius of 10 mm and a wall thickness of 1 mm. The mantle of
the hollow tube is made of cyanato-ester with fibreglass, which has a thermal
conductivity of 0.5 Wm1K1. This material choice provides a good compromise
between the structural requirement of being stiff enough to penetrate the surface,
and the thermal requirement to minimize vertical heat flow along the probe. A more
important factor for this heat pipe effect is, however, not the probe itself but the
sensors and their conductors which are made of Titanium and copper, respectively.
Though they are only a few micrometres thick, their total conductance is about
twice that of the fibre compound tube. The heat pipe effect of the probe can be
illustrated by a comparison of the total conductance of the probe with estimates
of that of the cometary material that occupied the volume within the tube before
insertion. For a thermal conductivity of the replaced porous cometary material of
0.1 Wm1K1the probe conducts about 5 times as much heat as the material it
replaced. In addition, the thermal effect of the zone of compacted material around
the inserted probe needs to be taken into account.
The effect of a thermal ’short circuit’ through the probe is minimised by using
a thin, hollow probe of a low-conductivity material, but significant enough that it
needs to taken into account in the data analysis. However, remaining perturbations
can be reduced by an inversion-type data evaluation method that was developed by
the MUPUS team (Hagermann, 1999; Hagermann and Spohn, 1999). This method
makes use of the damping and lagging effect which temperature signals from the
surface experience as they penetrate to greater depths. More details about this
method can be found in the appendix.
Figure 3. A new type of thermal sensor has been developed by the MUPUS team. A 20 μm thin
layer of titanium on a Kapton substrate is used to measure the temperature (electrical resistance is
proportional to the temperature). The same titanium cell can be actively heated in order to perform
thermal diffusivity and conductivity measurements (upper panel). 16 such cells and all required
electrical connections (thin lines) are laser-sputtered on one Kapton sheet (lower panel). The cell
dimensions grow from top (left in image) to bottom. The Kapton sheet is rolled and glued to the inner
tube wall of MUPUS PEN.
2.1.2. The Thermal Sensors
The PEN tube carries a Kapton sheet that is glued to its inner wall onto which the
thermistors are vapour deposited. The technology used for manufacturing the ther-
mal sensors was developed by the MUPUS team for the experiment. It is described
in Gregorczyk et al. (1999).
The Kapton sheet is glued to the inner wall of the tube in order to protect the
thermal sensors from mechanical strain upon insertion. There are sixteen meander-
shaped titanium sensors with temperature-dependent electrical resistance (see Fig-
ure 3). The number of sensors matches the number of independent data channels
of the electronics. The depth intervals covered by these sensors increase from the
top to the tip of the tube, from 1 cm to 4 cm length. This configuration has been
chosen in order to allow a denser coverage of the thermal profile near the surface
where the temperature gradient is expected to be steeper than at greater depth. The
resistance of the titanium is a function of temperature similar to the temperature
dependence of the resistance of PT100 type sensors. PT100 sensors and their use for
temperature measurements are standardized under the IEC 751 norm, its European
counterpart EN 60751 and the German DIN 60751.
The temperature dependence is given by
R(T)=R0(1 s1·T+s2·T2) (3)
where R0is a reference resistance of 100 Ohm at a reference temperature of 275.73
K, Tis the temperature difference to the reference temperature. For a PT100 and
other platin-based sensors, s1in eq. 3 is 3.9083×103K1, and s2=−5.775×107
The linear coefficient s1for other bulk metals is typically in the order of 4×103
K1, while s2is typically two or more orders of magnitude smaller. The relation
holds for a temperature interval of 110 K to 375 K. For the MUPUS-type Titanium
sensors, s1and s2have to be determined individually for each sensor, because tab-
ulated values for bulk metals cannot be applied to thin films of vacuum-deposited
metal. s1was found to be about 2×103K1with small variations between individ-
ual sensors, which is about half the value of that for PT100. After vacuum-deposition
on the Kapton sheet, the sensors have been tempered and cured. This procedure
has been repeated after integration of the Kapton sheet into the hollow tube. The
long-term stability of the sensor’s characteristics is unknown. In-flight calibration
will be required.
The 16 sensors thus allow measuring a temperature profile that extends over the
length of the tube (32 cm). Because of the small mass of the sensors the reaction
time of the sensors to changes in temperature is small, typically a few seconds.
The specific thermal timescale for a sensor being separated from the medium by 1
mm (i.e. the thickness of the probe wall) of a material with a thermal diffusivity of
about 106m2s1is one second. The surface (proportional to heat flux) to volume
(proportional to total heat capacity) ratio of a MUPUS-type sensor is 100 times
better than that for a conventional ceramics-sealed PT100, and reacts to changes in
temperature faster by about the same factor. The sensitivity of the bare sensors is
slightly worse than that of PT100 standard sensors because of the difference in s1
in Equation (3). The effective resolution of the flight instrument is limited by the
performance of the 16 bit AD converter to about 12 meaningful bits, covering about
200 K, which corresponds to about 0.05 K (see also Marczewski et al., 2004). The
temperatures are measured by applying a constant current of 20 mA and measuring
the voltage drop across the resistors. All PEN sensors are connected to the current
source through one shared conductor and individual sensor wires inside the probe
structure, and two external PEN cables. The number of wires is limited to 18 in order
to minimize undesired heat losses along the cables. Several options are available
for the measurement sequence, but a default operation is defined for long term
operations, consisting of a temperature scan every 20 sec.
2.1.3. Expected Performance
Initial tests of the probe performance were obtained by heating individual sensors
in vacuum with background temperatures between -160 C and -100C as well
as under ambient conditions with the PEN probe immersed in different media.
These tests showed that the influence of the heating on the neighbouring sensors is
moderate and that mainly the immediate neighbours are affected. The temperature
increase in immediately adjacent sensors will depend on the thermal conductivity
of the nucleus surface layer but is expected to be relatively small. Under worst
conditions in vacuum we measured a temperature rise of the neighbour sensor by
about 50% of the temperature of the heated sensor depending to some extent on the
ambient temperature. In solid ice the increase was 5%, in snow and sand 10–15%
and in a Teflon cylinder with a thermal conductivity of 0.25 W/m K about 10%.
Additional tests with a model of the MUPUS penetrator in terrestrial soil showed
that the penetrator was more sensitive to weak energy fluxes than the commonly
used method of heat flux plates (Marczewski et al., 2004). These authors have
reported on a series of test experiments with the MUPUS thermal probe.
In order to demonstrate and illustrate the performance of MUPUS PEN mea-
suring the temperature profile of the uppermost layers of a comet, we studied two
different data sets that are related to cometary thermal evolution, and come from
two quite different sources:
1. In recent years, the International Space Science Institute ISSI, located in Bern,
gathered comet modellers (one of them was J. Benkhoff, co-author of this paper)
in a group and invited this group several times to workshops. One goal of this
4-year program, coordinated by W. Huebner, was to compare individual model
codes, improve them and finally develop a set of standard thermophysical comet
models. The used model code is a 1-dimensional, multi-component (e.g., water,
CO) finite difference code which solves a set of coupled mass- and heat diffusion
differential equation, thus including heat transport by the vapour phase. We
selected a model comet with the following key parameters: (a) A spherical
model comet in an orbit of the Rosetta target comet P/Wirtanen, (b) the spin axis
is assumed to be perpendicular to the orbital plane, (c) porosity is simulated by a
pipe network (pore radius 10mm), (d) the nucleus consists of water ice, several
minor volatile components, and dust, (e) molecular flux in the pores, (f) low heat
conduction (Hertz factor 0.001).
The model calculations were carried out as follows: a homogeneously mixed
body at a constant initial temperature (T =20K) and a constant mass den-
sity distribution is considered. Due to heating of the body and sublimation of
the volatile components, the initially homogeneous body differentiates into a
multi-layer body (if it contains more than one volatile component), where the
deepest layer has the original composition. The subsurface temperatures are cal-
culated every 15 Minutes for several orbits. The underlying model is described
in Benkhoff (2002) and Prialnik et al. (2004).
2. From ca. 1987 to 1992, the German Research Foundation (DFG) sponsored a
research program in which several German and International teams cooperated
in oder to simulate comets, (see Gr¨un et al., 1991, for example) or at least some
physical processes with relevance to comets, in a large space simulator – basically
a vacuum chamber equipped with a LN2 cooling facility and an arrangement
of Xenon lamps to simulate solar insolation. In each of the KOSI (“Kometen-
Simulation” =Comet Simulation) experiments, a sample of porous ice-dust
mixtures was filled into a sample container of typically 30 cm diameter and 15 cm
depth, then cooled down in vacuum and finally exposed to artifical sunlight. We
(Benkhoff, Spohn and Seiferlin) were responsible for the thermal measurements
during these experiments. The selected KOSI-9 test was special in one respect:
there were 3 subsequent insolation periods of more or less similar intensity
profile (the first one a bit shorter but all with the same intensity profile) and a
total experiment duration of about 70 h, in order to simulate natural day/night
cycles. Differences between the temperature profiles for the 3 phases can be
explained by texture modification like sintering and recrystallization of water
ice, which would be more effective in warm layers. This KOSI 9 experiment was
described by Seiferlin et al. (1995).
Temperature profiles from both sources were processed in the same way to
simulate a measurement done with MUPUS:
1. The temperature profiles, as they were available, contained fewer data points in
z-direction than MUPUS would measure. The profiles were therefore projected
onto an array of sufficient length, and gaps between existing data points were
filled by a polynomial fit. In the time domain, an artificial equidistant data set
with sufficient resolution was generated by interpolating between existing time
steps (i.e. subsequent data points).
2. Because the individual MUPUS sensors are between 10 and 40 mm long, they
record the average temperature along their length. The fitted temperature profile
obtained in step 1 was projected onto the geometry of the MUPUS sensors in
such a way that each sensor was assigned a temperature equivalent to the average
temperature of the stretch covered by the sensor considered. After this step, the
temperature profile consisted of 16 temperature values, just as they would be
recorded by MUPUS.
3. In this step, the quality of the data was degraded and limited to 10 bit resolution,
which is slightly less than provided by the MUPUS flight electronics.
4. Moderate random noise was then added to simulate random errors such as may
be caused by varying thermal contact to the medium, calibration uncertainties,
electromagnetic noise etc.
5. The temperature profile as a function of time was then converted into a colour
coded image. The colour palette contains only 256 colours corresponding to
8 bit resolution. Thus, the resolution of the data as represented by the colour
maps is degraded in comparison with the expected results from MUPUS. The
simulated time step is 5 minutes according to the the nominal measurement cycle
Figure 4 shows a simulated measurement of the comet temperature history as was
proposed by the ISSI working group. Figure 5 shows the KOSI 9 data set. Because
the KOSI 9 record covered only 15 cm depth, the simulated MUPUS measurement
was also truncated at 15 cm. The results suggest that if the ISSI and the KOSI results
Figure 4. Simulated measurement with MUPUS. Temperatures are color coded in Kelvin. Time is
horizontal, depth is vertical. The time step (i.e. measurement interval and pixel resolution in the
image) is 5 minutes. The maximum depth is 32 cm. Black horizontal lines indicate sensor edges.
The temperature data processed for this image were taken from a numerical model of comet nucleus
temperatures developed in the framework of an ISSIworking group, and were provided by J. Benkhoff.
See text for further description.
are representative of the near surface temperature profiles in a cometary nucleus,
the MUPUS probe should be suitable to record it. The total penetration depth and
the number and spacing of sensors seems appropriate and, considering the limited
resources on the Rosetta Lander PHILAE, satisfactory.
2.1.4. Transient Thermal Properties Measurements
The titanium resistor cells may also be heated by applying electrical power to them
of up to 1 W at 12 V. The temperature increase caused by the heating is a function of
the known power and the thermal diffusivity (resp. conductivity) of the material that
surrounds the heated cell(s). In a typical measuring cycle, the heating is applied for a
Figure 5. Simulated measurement with MUPUS. Temperatures are color coded in Kelvin. Time is
horizontal, depth is vertical. The time step (i.e. measurement interval and pixel resolution in the image)
is 5 minutes, total duration is 70 hours. The maximum depth is 15 cm. Black horizontal lines indicate
sensor edges. The temperature data processed for this image were taken from the results of the KOSI
9 experiment (see text for further detail). Three day/night cycles are visible. The discontinuity at the
afternoon of the third day is caused by a data gap in the original data set.
given time interval (5 minutes under standard conditions) and is then interrupted for
a temperature measurement for a few milliseconds. The temperature measurement
thus affects the heating only insignificantly. After switching off the heating power,
the temperature relaxation can additionally be measured.
The heating power is controlled by the filling ratio of the pulses, varying from
zero to the maximal available power (approx. 1 W) with 12-bit resolution. Under
standard operational conditions, heating will use up to 1/4th of the maximal avail-
able power (approx. 0.2–0.3 W). Heating will be applied to one or more sensors
simultaneously. For a measuring a conductivity depth profile, heating will be ap-
plied consecutively to sensors with increasing depth. The heating and the associated
temperature measurements can be repeated every hour.
The evaluation of the data to obtain diffusivity and conductivity profiles is not
straightforward, but Banaszkiewicz et al. (1997) have derived appropriate mathe-
matical tools.
2.1.5. The Hammer
The difficult task to emplace a sub-surface probe into a medium of unknown hard-
ness (but not harder than ca. 2 MPa) is performed by a mechanical hammer, es-
pecially designed for MUPUS. The hammer works like a mechanical diode. A
conventional 22 μF capacitor is charged to several 100 V. The stored electrical
energy is discharged through a coil that generates a strong magnetic field. A small
mass (30 g) is accelerated by the magnetic field into the opening inside the coil and
hits the penetrator tube with its maximum speed of 8 m/s, thus causing a powerful
hammer stroke onto the tube. The friction exerted on the tip and the tube and some
weak force extended by the deployment device take most of the rebound following
the hammer stroke. Together, hammer stroke and rebound absorption cause a net
forward movement of the penetrator into the comet nucleus regolith. Both the tube
and the hammer mass are connected by weak springs to the hammer housing. These
springs act to bring both back to their starting positions and prepare the motor for
the next hammer stroke. The energy of the hammer strokes can be adjusted to the
requirements set by the hardness of the nucleus material .
The probe also includes a sensor (PEN-M) to monitor the vertical displacement
of the probe as it is inserted. It starts close to the tip and slides up the probe; the
displacement is sensed electrically in the manner of a potentiometer. The assembly
carrying the displacement sensor also houses an electromagnet to hold the penetrator
and one of the electrodes of the SESAME-PP (Permittivity Probe) experiment
(Seidensticker et al., this issue).
Compared to alternative insertion methods, e.g. a pyro, the mechanism has
several advantages:
the total energy is only limited by the power supply offered by the hosting
spacecraft, while a pyro has a maximum stored energy defined at design time,
insertion is done in small steps and may be interrupted to take measurements
once a given depth is reached,
the advancement of the penetrator is small enough such that the insertion effi-
ciency (depth reached per dissipated electrical energy) can be measured. From
these data a cohesion profile of the penetrated layers can be derived.
Whenever erosion of the cometary surface material re-exposes parts of the pene-
trator, the hammer can be restarted to compensate for the material loss at the top by
inserting the penetrator accordingly. The expected surface erosion may well reach
1 m per comet orbit. This is about 3 times the total length of the penetrator tube.
2.1.6. Front-end electronics
To keep the thermal losses of the lander small and to reduce the harness between
the penetrator and the lander, a front-end electronics package has been included
in the hammer housing. The front-end electronics package communicates with the
electronics on board the lander via digital signals only, using a serial interface. The
cable connection between the lander and the front-end electronics is routed through
the central rotation axis of the lander, below the baseplate. This configuration
ensures that the lander can rotate after PEN deployment without being locked by
a direct, tense cable connection to the balcony, for example. The design of the
front-end electronics proved to be very demanding, since it has to operate in a
low temperature environment, while the small size and mass of the housing make
an effective thermal design difficult. Figure 6 shows the layout of the MUPUS
electronics. A detailed thermal model of the Front-End electronics can be found in
Seweryn et al. (2005).
2.1.7. The Deployment Device
Measuring the nucleus energy balance in the near-surface layers requires a thermal
probe that will be placed far enough away from the lander’s shadow. This require-
ment made it necessary to design and develop a complex mechanical deployment
device for the MUPUS PEN. The deployment device (compare Figures 1, 2 and
Figure 6. Layout of the MUPUS electronics and its integration into the Philae lander. MUPUS’s main
electronics components are the internal electronic box and the PEN front-end electronics.
7) will place the penetrator normal to the ground about1mawayfrom the lan-
der. The value of1mwasdetermined from model calculations that investigated
the subsurface temperature field underneath a lander on a slowly spinning comet
nucleus. An example of the results is shown in Figure 8. Because the lander is
intended to rotate after MUPUS PEN deployment, the deployment device must be
separated from the penetrator and retracted after deployment. A cable then provides
communications and power to the penetrator while the lander can rotate freely. The
design solution solves both problems efficiently: two spools hold one metal strip
each that is coiled flat in its stored configuration. For deployment, a stepper motor
pulls these approximately one meter long metal profiles forward from the spool
Figure 7. The MUPUS PEN and deployment device during a deployment test.
and the metal profiles attain their naturally bent shape. Their C-like cross section
provides sufficient stiffness to hold the penetrator in the low-gravity environment
and supports the PEN driving, which is especially important at the initial insertion
phase. After PEN insertion, the PEN is released from the deployment device and
the metal strips retracted onto the spools.
The MUPUS Thermal Mapper TM (see Figure 9) is an infrared radiometer that
consists of a set of 4 IR (thermopile-type) sensors designed to measure the brightness
temperature at the very surface, averaged over its field of view. In its normal mode,
MUPUS TM will have the MUPUS penetrator in the center of its field of view,
thereby adding an important data point to the temperature profile. Fragile, fluffy
layers such as a dust mantle on the cometary surface with low thermal conductivity
may cause a strong temperature gradient with depth and a significant temperature
drop within a few mm below the surface. These first few mm cannot be resolved with
the thermal sensors on MUPUS PEN even where the sensors are closely spaced. In
Figure 8. Temperature field as a function of radius and depth underneath a circular lander on a slowly
rotating nucleus (20h period) made of porous water ice whose matrix is 20 times less heat conductive
than compact ice. Representing a worst case, this model shows that the temperature profile underneath
or even near the lander is not representative of the undisturbed profile. Tecorresponds to sunrise.
Solid isotherms indicate the deviation in Kelvin from the temperature profile without a lander shadow.
addition to the surface temperature, a direct estimate of the thermal conductivity of
the nucleus surface layer can be derived from the temperature measurements of TM
and the uppermost temperature sensor of the penetrator. Furthermore, the thermal
inertia of the nucleus surface at the landing site can be determined from an analysis
of the TM data.
The third component of MUPUS is the set of sensors implemented in PHILAEStwo
anchors MUPUS ANC (compare Figure 1). The anchors have been described by
Thiel et al. (2001, 2003). Two types of sensors have been implemented:
Figure 9. MUPUS TM: 4 thermopile-type IR sensors and a small front-end electronics package are
mounted in a small housing fixed to a diagonal strut above the lander balcony. The box is tilted such
that the MUPUS penetrator will be in the field of view of the sensors if the lander balcony is pointing
in the right direction. TM will thus provide the temperature at the very surface at the PEN location.
rANC-T, a PT100 temperature sensor to monitor the local temperature at the final
resting place of the anchor after it has been shot into the regolith of the comet
nucleus. ANC-T provides an additional location for temperature measurement,
laterally displaced by approximately 1 m from the MUPUS PEN sensors, and
probably also at a greater depth. (It is likely that the deepest in situ measurements
by PHILAE will be those of the anchor sensors.) A sufficiently high sampling rate
in the first few minutes after deployment for ANC-T may yield constraints on the
thermal diffusivity and conductivity of the cometary material (Paton, 2005).
rANC-M, a miniature shock accelerometer (ISOTRON 2255B-1 from Endevco)
monitoring the acceleration and deceleration of the anchoring projectile while it
is fired into the ground.
The anchors will be accelerated pyrotechnically within a few milliseconds at the
timeimmediatelybeforelandertouchdownto reach a launch speed of approximately
100 m s1. A peak acceleration value of about 90000 m s2(i.e. ca. 9000 g) is
Figure 10. Example of an anchor test shot. The target sample is a sintered CO2ice crust with softer
material underneath. The shot holes and the anchor cables are visible.
expected. After a very short free flight period the anchor will penetrate the regolith
and be decelerated by the resistance of the nucleus material. The main challenge
concerning the selection of an appropriate accelerometer was the high dynamic
range that it has to cover. On the one hand, ANC-M will have to survive the high
amplitude acceleration phase and on the other hand it will have to be able to resolve a
much smaller amplitude deceleration signal expected when the anchor will penetrate
soft porous ice layers with low cohesion. With the 14-bit resolution of the flight unit
and a sampling frequency of 50 kHz an accuracy of about 1.5 g can be achieved.
The chosen sensor type thereby will allow to record the full acceleration phase
(which is important for calibration purposes) while still giving a reasonable signal
for sintered porous ices. Judging from comet analogue samples, a sintered water
ice crust is to be expected for the nucleus of Comet Churyumov-Gerasimenko.
2.3.1. Dynamic Penetrometry Tests with MUPUS ANC
Penetrometry tests have been performed to check the performance of the anchoring
harpoon and to calibrate the dynamic strength measurements. We show results of a
shot into a CO2ice sample in Figures 10 and 11. Two shot holes are visible together
with parts of the anchoring cables in Figures 10. At the impact sites, small craters
with diameters larger than those of the penetration channels are visible. Note that
the projectiles were accelerated with a cold gas system. The maximum acceleration
in this experiment was lower than the acceleration expected for the PHILAE anchors.
In Figure 11 the deceleration profile measured by the shock accelerometer in one
of the anchors is displayed together with a fit calculated with the similarity model
described by K¨omle et al. (2001). To calibrate the data in terms of strength/cohesion,
a quasi-static strength measurement was performed close to the impact site. This
measurement consisted of a slow (40 mm s1) penetration of a cylindrical rod
Figure 11. Upper panel: Accelerometer signal recorded from the impact close to the centre of the
target. Lower panel: Strength profile obtained by an independent quasi-static measurement with a
conventional force cell.
with a 60tip into the sample, whereby the resistance force was measured by a
conventional load cell that was mounted at the top of the rod. The results of the
quasi-static measurements are shown in the lower panel, together with a simple
fit to obtain a strength profile. The main vertical structure of the layer seen in
the quasi-static test, namely a strong surface crust and a slightly consolidated part
below, is also well represented in the deceleration data. This demonstrates that
the dynamic penetrometry method, together with an appropriate model of the tip
geometry, should be able to detect at least variations and discontinuities in the
vertical strength profile. More detailed discussions of the penetrometry tests have
been discussed by Kargl et al. (2001) and K¨omle et al. (2001).
3. Resources
The following table gives an overview of the required resources. Considering the
complex mechanics that are required to deploy and insert the penetrator, the overall
Figure 12. Direct (“true”) temperature field (left, PEN marked with dotted lines and sensor locations
marked by small rectangles) and “evolution” of the inverted temperature field with increasing order
k of the solution in normalized coordinates. The dashed vertical line in the rightmost plot indicates
the location of the estimate of the undisturbed profile.
experiment mass is very moderate. The data volume contains 2 Mbit of accelerom-
eter data (sampled during anchor shots at the very beginning) and a very low data
volume for the remaining science data (temperature and thermal properties data).
If needed, the data rate can further be reduced by configuring larger time intervals
between measurement scans. The average power consumption can only be esti-
mated because it is partly determined by the heating power needed to keep the
front-end electronics inside the operating range, and, thereby, dependent on the
actual ambient conditions on the nucleus.
Mass (total) 2.35 kg
ID +PEN 0.65 kg
DD +DS 0.85 kg
Main electronics 0.6 kg
TM 0.12 kg
Harness 0.13 kg
Volume (envelope on balcony) 565 ×160 ×188 mm
Power (total) 2.2 W
Main Electronics 1.2 W
TM 0.2 W
Data rate 180 kBit/ha
aAverage for nominal long-term operations (measurements every 20 sec)
4. Conclusions
The combination of MUPUS sensors will provide us with temperatures, thermal
properties and cohesion data for the uppermost 32 cm of a comet nucleus as a
function of time. Using MUPUS data, we will be able to determine the energy bal-
ance at one location of the nucleus as it approaches the Sun, thereby gaining insight
into the way comets work. A very important aspect of the MUPUS research is to
observe changes of physical properties in situ, and determine the specific timescales
of processes modifying cometary matter. Thermal and mechanical properties can
be interpreted in terms of microstructure parameters of the ice, such as particles
with variable texture, contact area between individual grains, and a chain-like par-
ticle structure which might be expected from a low-density material as snow (e.g.,
Keller and Spohn, 2002 and references therein). Having in mind that one of the main
goals of the mission is the search for pristine material, thought to be a record of the
formation of the solar system, it is of extraordinary importance to understand how
much the material analysed by COSAC, PTOLEMY and APXS has been modified
in the geological time record of the nucleus.
The authors, the MUPUS team, appreciate very much the co-operation with the
whole lander team. Markus Thiel, Max-Planck-Institut f¨ur Extraterrestrik, Garch-
ing, is responsible for the design of the lander’s anchors, and was always very
cooperative when the integration of the MUPUS sensors and their tests were con-
cerned. The MUPUS project and MUPUS team members are supported by various
grants, some of which are: Marek Banaszkiewicz: Grant No 2 PO3C.009.12 p/05;
Andrew Ball: PPARC (Particle Physics and Astronomy Research Council), The
Austrian Academy of Sciences, and the Royal Society. German contribution: DLR
grant WE 150 OH 9503 7-ZA. Austrian contribution: FWF Projects P12416 and
5. Appendix
The principles of temperature and thermal property measurements are very simple,
but ensuring that the measured temperature is representative of the environment
temperature can pose great difficulty. The very existence of any thermal probe
influences the temperature field, and the MUPUS PEN too can have a significant
influence on the sub-surface temperature field. If its thermal diffusivity is much
larger than that of the ambient material, heat conduction through the penetrator rod
can result in a distortion of the temperature gradient, resembling a thermal short
circuit. This is a common effect of all heat flow experiments employing penetrators.
Especially in cases where measurement errors play a significant role, using
forward modelling to explain the data is a way to pick one model that appears to
be physically realistic, but this model is not necessarily the only one fitting the
data. Forward modelling techniques demonstrate the existence of a solution, but
the problem of uniqueness of the solution remains unsolved.
The calculation of an unperturbed temperature profile from perturbed data with
associated errors is a classical problem of inverse theory. It differs from the clas-
sical forward heat conduction problem in that it does not require boundary con-
ditions, but solves for them. In the case of an inverse heat conduction problem,
we have temperature histories at a number of points inside the volume and try
to calculate the boundary conditions, which involves estimating the temperature
throughout the whole volume (e.g. Stolz, 1960). Hagermann and Spohn (1999)
have successfully developed a numerical inversion scheme to find the undisturbed
temperature profile. Their algorithm uses the time-slope dependent extrapolation
of measured temperature histories (Kurpisz, 1991). The algorithm approaches the
undisturbed temperature profile by adding time-dependent extensions to a stationary
In normalised cylindrical coordinates rand zand normalised time t, the temper-
ature response R of the i-th temperature sensor equals the normalised temperature
at the sensor location
With the time derivatives
dtk,k=1,2, ... (A.2)
we can find the solution
ij R(k)
where the ψ(k)
ij are recursive solutions of
ij =ψ(k1)
ij .(A.4)
His the linear system describing the thermal system of the penetrator and the
surrounding material. Figure 12 shows how the result of the transient temperature
field is constructed from a crude stationary solution. The method proved to be robust
in most realistically conceivable scenarios (Hagermann and Spohn, 1999).
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Unmanned in-situ exploration is an important technique to study the physical and mechanical parameters of lunar composition and evolution. The impact penetrator is an effective device for in-situ detection of the lunar soil profile at predetermined depth. Because of the lack of real lunar soil samples, it is very difficult to study and evaluate the performance of the impact penetrator. In order to truly reflect the interaction between the impact penetrator and lunar soil particles, a simulation model of the lunar soil body was established by means of discrete-element analysis, and the model parameters were matched and verified by the experimental method. Based on this model, the interaction behaviors between the penetrators with different head configurations and the lunar soil body were simulated. The stress field distribution in the lunar soil body and particle movement patterns during the penetrating process were revealed, which reflects the working principle and performance of the penetrator. The numerical simulation on the interaction process between the impact penetrator and lunar soil particles provides a feasible and effective method for the design and optimization of the penetrator, which will contribute to the development of lunar subsurface in-situ exploration technologies.
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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 roughly a mole length of 40 cm. 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.
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The Japanese MMX sample return mission to Phobos by JAXA will carry a rover developed by CNES and DLR that will be deployed on Phobos to perform in situ analysis of the Martian moon’s surface properties. Past images of the surface of Phobos show that it is covered by a layer of regolith. However, the mechanical and compositional properties of this regolith are poorly constrained. In particular, from current remote images, very little is known regarding the particle sizes, their chemical composition, the packing density of the regolith as well as other parameters such as friction and cohesion that influence surface dynamics. Understanding the properties and dynamics of the regolith in the low-gravity environment of Phobos is important to trace back its history and surface evolution. Moreover, this information is also important to support the interpretation of data obtained by instruments onboard the main MMX spacecraft, and to minimize the risks involved in the spacecraft sampling operations. The instruments onboard the Rover are a Raman spectrometer (RAX), an infrared radiometer (miniRad), two forward-looking cameras for navigation and science purposes (NavCams), and two cameras observing the interactions of regolith and the rover wheels (WheelCams). The Rover will be deployed before the MMX spacecraft samples Phobos’ surface and will be the first rover to drive on the surface of a Martian moon and in a very low gravity environment. Graphic Abstract
The NASA InSight mission payload includes the Heat Flow and Physical Properties Package HP^3 to measure the surface heat flow. The package was designed to use a small penetrator - nicknamed the mole - to implement a string of temperature sensors in the soil to a depth of 5m. The mole itself is equipped with sensors to measure a thermal conductivity as it proceeds to depth. The heat flow would be calculated from the product of the temperature gradient and the thermal conductivity. To avoid the perturbation caused by annual surface temperature variations, the measurements would be taken at a depth between 3 m and 5 m. The mole was designed to penetrate cohesionless soil similar to Quartz sand which was 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 a mole length of 40 cm. The failure to penetrate deeper was largely due to a few tens of centimeter thick cohesive duricrust that failed to provide the required friction. Although a suppressor mass and spring in the hammer mechanism absorbed much of the recoil, the available mass did not allow a system that would have 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. It was found in addition that the Martian soil provided unexpected levels of penetration resistance that would have motivated to designing a more powerful mole. It is concluded that more mass 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.
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In situ observations of comet Halley provided the first photographs of a cometary nucleus and yielded information about its environment, including the emitted gas and dust. The relation between these measurements and properties of and processes on the nucleus is established by theoretical modelling, while laboratory experiments may provide some of the physical parameters needed. In addition, laboratory tests can stimulatenew ideas for processes that may be relevant to cometary physics. Processes to be studied in detail by large-scale laboratory experiments may include: (1) heat transport phenomena during sublimation of porous ice-dust mixtures, (2) material modification and chemical fractionation caused by the sublimation processes, (3)buildup and destruction of dust mantles, (4) detailed studies of gas release from mixtures of volatile ices, and (5) the investigation of ice and dust particle release mechanism. The KOSI-team (Kometensimulation) carried out sublima- tion experiments with ice-mineral mixtures in a large Space Simu- lator. During initial experiments, cylindrical samples of 30-cm diameter and 15-cm thickness were irradiated with up to 2700-W/m2 light energy. The samples constited of water-ice or water- and CO2-ice mineral mixtures. The experiments showed the importance of advection for heat transport into the interior. It was found that the sublimation of CO2 advances into the sample at a higher speed than that of water vapor release. Therefore, emission of volatile gases responded to insolation changes with a time lag of several hours. The ratio of the emitted gas species, as well as the dust- to-gas ratio, differs significantly from the values within the sample. A partly permeable refractory mantle of minerals and carbonaceous material developed with time. Dust and ice particle emission has been observed to occur from irradiated dirty ices as well as from dust mantles.
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We describe the radio science experiment proposed for the Rosetta cometary mission. The experiment consists in the transmission of electromagnetic waves between the landers and the orbiter through the comet to study its internal structure. In the paper, the electromagnetic model of the comet is presented and used to evaluate the potentiality of the experiment. Various modellings of the radio wave propagation are discussed. Finally, a description of the experiment and the instrument is made.
Numerical methods are presented for solving an inverse problem of heat conduction: Given an interior temperature versus time, find the surface heat flux versus time. The analysis is developed specifically for spheres; it applies to other simple shapes. The system is treated as linear, permitting use of the superposition principle. The essence of the method is the numerical inversion of a suitable direct problem: Given a surface heat flux versus time, find an interior temperature versus time. Care is required in selecting a time spacing for, if it is chosen too small in relation to the conditions, undesirable oscillation results. Simplifying suggestions are presented, and the use of the methods are illustrated by examples.
Mole devices are low velocity, medium to high energy, self-driven penetrators, designed as a carrier of different sensors for in situ investigations of subsurface layers of planetary bodies. The maximum insertion depth is limited by energy of single mole's stroke and soil resistance for the dynamic penetration. The principle of operation of a mole bases on the interaction between three masses: the inserted cylindrical casing, the hammer, and the rest of the mass, acting as a support mass. Additionally, the driven spring should act on the hammer and the support, and the return spring should act on the support and the casing. A new mole penetrator "KRET" has been recently designed, developed, and successfully tested at Space Research Centre PAS in Poland. This approach takes advantage of the MUPUS penetrator (a payload of Philae lander on Rosetta mission) insertion tests knowledge. Two aspects were critical in development of a new mole: first one is related to the reliability of the mechanisms whereas the second one to the dynamic properties of the penetrator. The specific technological problems i.e. the abrasion and fatigue of the latch, the shape of the tip and proportions between three masses are shown in the paper as an illustration of those aspects. The proper design was confirmed by the operational tests in the testbed system.
Conference Paper
This is the report to COSPAR on Polish space research and space application projects covering period 2003–2007. The Report was prepared by the Committee on Space Research of the Polish Academy of Sciences. The authors of the contributions to this Report are the chairpersons of four commissions of the Committee: Space Physics, Satellite Geodesy, Remote Sensing, and Astronautics and Space Technology. The report describes scientific activity in space physics, in space applications for planetary geodesy and remote sensing, and the activity in the field of astronautics and space technologies.
The thermal evolution of ice-methanol-CO2-dust specimens in the KOSI comet-simulation experiments is described. In KOSI, 30-cm-diameter 15-cm-thick ice samples are exposed to Xe-lamp irradiation at 77 K in the vacuum chamber of the DLR Koeln large space simulator. Temperature profiles through the specimens are obtained using copper-constantin thermocouples and PT-100 thermistors; the results are presented in graphs and compared with theoretical predictions. The water-ice and CO2 sublimation fronts are characterized by temperatures of 210 and 125 K, respectively, and by the presence of temperature-gradient discontinuities. Heat transfer by water-vapor flow and conduction through the matrix amounted to about 50 percent and about 5 percent of the insolation heat, respectively. Also calculated were: matrix thermal diffusivity = (2-13) x 10 to the -8th sq m/sec and sample permeability = 0.01 sq m/sec.
Conference Paper
KOSI comet simulation experiments have shown strong evidence for significant modifications of the texture of porous ice within typical experiment durations of a few ten hours. Initially, the textures of the fluffy ice samples were obviously not in their equilibrium states. During the cause of the experiments the textures modified through growth of the bond sizes between individual grains which caused significant increases in compressive strength and intrinsic thermal conductivity. The near-surface layers of cometary nuclei may evolve similarly as they are heated during perihelion passage. As a consequence, the heat wave may penetrate into increasingly deeper layers to depths greater than generally expected.
This study is concerned with energy transfer in porous water ice at large Knudsen numbers of the sublimated gas. We present a formula for the effective thermal conductivity, λeff, which is found to be valid for a wide range of material parameters, in particular also for high porosity materials, where other expressions for λeff published in the literature lose their validity. It is found that above ∼ 190 K λeff increases strongly with temperature and depends mainly on the average grain size, while at low temperatures it is primarily controlled by the value of the Hertz-factor. The influence of the porosity is relatively weak. The reliability of our expression for λeff has been checked by comparing computed temperature profiles with the temperature recordings of comet simulation experiments. The predicted temperatures are in good agreement with the experimental results. Finally, our model is applied to a typical surface layer of a cometary nucleus. A strong temperature dependence of the effective thermal conductivity in cometary water ice is predicted.
A major goal of comet research is to determine conditions in the outer solar nebula based on the chemical composition and structure of comet nuclei. The old view was to use coma abundances directly for the chemical composition of the nucleus. However, since the composition of the coma changes with heliocentric distance, r, the new view is that the nucleus composition must be determined from analysis of coma mixing ratios as a function of r. Taking advantage of new observing technology and the early detection of the very active Comet Hale-Bopp (C/1995 O1) allows us to determine the coma mixing ratios over a large range of heliocentric distances. In our analysis we assume three sources for the coma gas: (1) the surface of the nucleus (releasing water vapor), (2) the interior of the porous nucleus (releasing many species more volatile than water), and (3) the distributed source (releasing gases from ices and hydrocarbon polycondensates trapped and contained in coma dust).* Molecules diffusing inside the nucleus are sublimated by heat transported into the interior. The mixing ratios in the coma are modeled assuming various chemical compositions and structural parameters of the spinning nucleus as it moves in its orbit from large heliocentric distance through perihelion. We have combined several sets of observational data of Comet Hale-Bopp for H2O (from OH) and CO, covering the spectrum range from radio to UV. Many inconsistencies in the data were uncovered and reported to the observers for a reanalysis. Since post-perihelion data are still sparse, we have combined pre- and post-perihelion data. The resulting mixing ratio of CO relative to H2O as a function of r is presented with a preliminary analysis that still needs to be expanded further. Our fit to the data indicates that the total CO release rate (from the nucleus and distributed sources) relative to that of H2O is 30% near perihelion.
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.