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Exp Astron (2009) 23:809-847 
DOI 10.1007/sl0686-008-9115-8 


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ORIGINAL ARTICLE 

Triple F — a comet nucleus sample return mission 


Michael Kiippers • H. U. Keller • E. Kiihrt • M. F. A’Hearn • K. Altwegg • 

R. Bertrand • H. Busemann • M. T. Capria • L. Colangeli • B. Davidsson • 

P. Ehrenfreund • J. Knollenberg • S. Mottola • A. Rathke • P. Weiss • 

M. Zolensky • E. Akim • A. Basilevsky • E. Galimov • M. Gerasimov • 

O. Korablev • I. Lomakin • M. Marov • M. Martynov • M. Nazarov • 

A. Zakharov • L. Zelenyi • A. Aronica • A. J. Ball • C. Barbieri • A. Bar-Nun • 
J. Benkhoff • J. Biele • N. Biver • J. Blum • D. Bockelee-Morvan • O. Botta • 

J. -H. Bredehoft • F. Capaccioni • S. Charnley • E. Cloutis • H. Cottin • 

G. Cremonese • J. Crovisier • S. A. Crowther • E. M. Epifani • F. Esposito • 

A. C. Ferrari • F. Ferri • M. Fulle • J. Gilmour • F. Goesmann • N. Gortsas • 

S. F. Green • O. Groussin • E. Grim • P. J. Gutierrez • P. Hartogh • T. Henkel • 
M. Hilchenbach . T.-M. Ho . G. Horneck . S. F. Hviid • W.-H. Ip . A. Jackel • 
E. Jessberger • R. Kallenbach • G. Kargl • N. I. Komle • A. Korth • 

K. Kossacki • C. Krause • H. Kruger • Z.-Y. Li • J. Licandro • 

J. J. Lopez-Moreno • S. C. Lowry • I. Lyon • G. Magni • U. Mall • I. Mann • 

W. Markiewicz • Z. Martins • M. Maurette • U. Meierhenrich • Y. Mennella • 

T. C. Ng • L. R. Nittler • P. Palumbo • M. Patzold • D. Prialnik • M. Rengel • 

H. Rickman • J. Rodriguez • R. Roll • D. Rost • A. Rotundi • S. Sandford • 

M. Schonbachler • H. Sierks • R. Srama • R. M. Stroud • S. Szutowicz • 

C. Tornow • S. Ulamec • M. Wallis • W. Waniak • P. Weissman • R. Wieler • 

P. Wurz • K. L. Yung • J. C. Zarnecki 


Received: 1 February 2008 / Accepted: 14 July 2008 / Published online: 7 August 2008 
© The Author(s) 2008 


Abstract The Triple F (Fresh From the Fridge) mission, a Comet Nucleus 
Sample Return, has been proposed to ESA’s Cosmic Vision program. A 
sample return from a comet enables us to reach the ultimate goal of cometary 
research. Since comets are the least processed bodies in the solar system, the 
proposal goes far beyond cometary science topics (like the explanation of 
cometary activity) and delivers invaluable information about the formation of 
the solar system and the interstellar molecular cloud from which it formed. 


M. Kiippers ([31) • H. U. Keller • F. Goesmann • P. Hartogh • M. Hilchenbach • 
S. F. Hviid • R. Kallenbach • A. Korth • H. Kruger • U. Mall • W. Markiewicz • 
M. Rengel • R. Roll • H. Sierks 
Max-Planck-Institute for Solar System Research, 

Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany 
e-mail: Michael.Kueppers@sciops.esa.int 


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The proposed mission would extract three sample cores of the upper 50 cm 
from three locations on a cometary nucleus and return them cooled to Earth 
for analysis in the laboratory. The simple mission concept with a touch-and- 
go sampling by a single spacecraft was proposed as an M-class mission in 
collaboration with the Russian space agency ROSCOSMOS. 

Keywords Comets • Cosmogony • Sample return • Space mission 


1 Introduction 

At the first stage of the formation of the solar system there was the so- 
lar/protoplanetary nebula collapsing from a molecular cloud. The central 
star — our Sun — formed and started to heat the dust/gas mixture. Dust parti- 
cles sank to the mid-plane, accreted, and agglomerated to planetesimals and 
cometesimals, the building blocks of the planets. At the outer fringes of the 
nebula the temperatures were cold enough that ices persisted and volatiles 
condensed before the comets were formed. Investigating the chemical and 
physical properties of this primordial mixture is a key to understanding how 
our solar system formed — and ultimately how life has started. 

The primordial mixture has been preserved — almost unaltered from fur- 
ther processing due to high speed impacts, gravitational compression and 


M. Kiippers 

European Space Astronomy Centre, P.O. Box 78, 

28691 Villanueva de la Canada, Madrid, Spain 

E. Kiihrt • J. Knollenberg • S. Mottola • J. Benkhoff • J. Biele • N. Gortsas • 

G. Horneck • C. Krause • C. Tornow 

German Aerospace Center (DLR), Berlin, Germany 

M. F. A’Hearn 

University of Maryland, College Park, MD, USA 

K. Altwegg • A. Jackel • P. Wurz 
University of Berne, Berne, Switzerland 

R. Bertrand 

CNES, Toulouse, France 

H. Busemann • A. J. Ball • S. F. Green • J. C. Zarnecki 
Open University, Milton Keynes, UK 

M. T. Capria • F. Capaccioni • G. Magni 
IASF/INAF, Rome, Italy 

L. Colangeli • E. M. Epifani • F. Esposito • V. Mennella 

INAF — Osservatorio Astronomico di Capodimonte, Naples, Italy 

B. Davidsson • H. Rickman 
University of Uppsala, Uppsala, Sweden 


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heating — in low density cometary nuclei whose temperature did not exceed 
50 K. Sophisticated analyses of this material in our laboratories will allow 
us to determine the ratio of processed to original interstellar material, and 
to determine the time scales of grain formation. Key questions like ‘How 
important was 26 A1 for heating even small bodies in the first millions of years?’ 
can be assessed by determining the time scale for accretion of cometesimals as 
well as the structure of the cometesimals. Investigation of cometary material 
provides information about the original (primordial) mixture out of which 
the planetesimals and hence planets formed before they were altered in this 
formation process. The proposed Triple F (Fresh From the Fridge), a Comet 
Nucleus Sample Return (CNSR) mission, concentrates on retrieving samples 
of this original mixture to bring them back for analyses that can only be 
undertaken in terrestrial laboratories. There the detailed chemical and isotopic 
composition and the internal structure of ice-mineral cometary grains will be 
measured as well as the granulation of the volatile material. 

The relevance of cometary research goes far beyond the investigation 
of minor bodies, of their physical and chemical properties or even of how 
they came about. The driving quest has always been to learn about the 
composition of the primordial nebula mixture and the formation of our solar 
system. Now is the time to achieve this ultimate goal of the European space 
programme that started more than 20 years ago with ESA’s first planetary 
mission to fly-by comet Halley. The stepwise preparation by cometary fly-bys 
( VEGAs , Giotto , Sakigake, and Suisei fly-bys of lP/Halley [53], Deep Space 


P. Ehrenfreund 

University of Leiden, Leiden, Netherlands 
A. Rathke 

EADS Astrium, Friedrichshafen, Germany 
P. Weiss • T. C. Ng • K. L. Yung 

The Hongkong Polytechnic University, Hong Kong, China 
M. Zolensky 

NASA Johnson Space Center, Houston, USA 

E. Akim • M. Marov 

Keldysh Institute, Moscow, Russia 

A. Basilevsky • E. Galimov • M. Nazarov 
Vernadskij Institute, Moscow, Russia 

M. Gerasimov • O. Korablev • A. Zakharov • L. Zelenyi 
Space Research Institute, Moscow, Russia 

I. Lomakin • M. Martynov 
Lavochkin Association, Moscow, Russia 

A. Aronica • P. Palumbo • A. Rotundi 
University of Naples, Naples, Italy 


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1 fly-by of 19P/Borrelly [46]), impacts {Deep Impact on 9P/Tempel 1, [2]), 
collection of dust (Stardust at 81P/Wild 2, [10]), and a rendezvous {Rosetta 
with 67P/Churyumov-Gerasimenko, [27]) leads to an understanding of comets 
and their spectacular activity. This knowledge now provides a firm basis for the 
design and successful execution of a sample return mission. 

The Rosetta mission will investigate the cometary nucleus from orbit 
and also by instruments placed onto the surface of comet 67P/Churyumov- 
Gerasimenko (CG) by a lander. These investigations will take place in the year 
2014. The physical characteristics of the nucleus of CG will be investigated 
in detail, its coarse chemical composition will be analysed, and the physics 
of its activity examined — a very major step forward in our understanding 
of the chemistry and physics of cometary nuclei. However, many questions 
about the formation of the planetary system will not be answered. How was 
the interstellar (molecular cloud) material metamorphosed into solar system 
compounds, organics and minerals? Did comets contribute to the development 
of life on Earth? Investigations to reveal the physical and chemical processes 
and their time scales during the early stages of planetary formation need 
analyses on ppb levels that cannot be realized by Rosetta. Addressing such 
questions has successfully been demonstrated on meteoritic samples from 
various types of asteroids, the Moon, and Mars. The recent analysis of non- 
volatile material collected during the Stardust fly-by of comet 81P/Wild 2 
shows that microscopic high temperature material formed near the early Sun 
can be found in cometary nuclei that formed at low temperatures, possibly 


C. Barbieri • G. Cremonese • F. Ferri 
University of Padova, Padova, Italy 

A. Bar-Nun • D. Prialnik 
University of Tel- Aviv, Tel Aviv, Israel 

N. Biver • D. Bockelee-Morvan • J. Crovisier 
Paris Observatory, Meudon, France 

J. Blum 

Technical University Braunschweig, Braunschweig, Germany 

O. Botta 

International Space Science Institute, Berne, Switzerland 
J.-H. Bredehoft 

University of Bremen, Bremen, Germany 
S. Charnley 

NASA Ames Research Center, Mountain View, USA 
E. Cloutis 

University of Winnipeg, Winnipeg, Canada 
H. Cottin 

University of Paris, Paris, France 


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as low as 30 K, in the outer reaches of the solar system [42]. Although an 
evolutionary explanation has not yet been ruled out, the structure and the 
chemical heterogeneity of 9P/Tempel 1 as observed by Deep Impact suggest 
that large cometesimals may contain materials from different parts of the 
protoplanetary disk [6]. How well, on what time scales, and how far out was 
the solar nebula mixed before the building blocks of the planets have accreted 
and agglomerated? Many of these physical and chemical processes have been 
revealed by the interpretation of the extremely sophisticated analyses of the 
diverse meteoritic materials. However, a global and conclusive picture and 
time line for the formation process of our solar system has not yet been 
developed. Bringing back a sample from a body that formed on the fringes 
of the planetary system, the temperature of which was always low enough 
to trap compounds as volatile as CO or CH 4 will allow us to investigate an 
end member of the minor bodies of the planetary system. Here we have the 
best chance to understand the relationship between the original interstellar 
material of the collapsing molecular cloud and the processed end-products 
found in the meteoritic samples. The high content of volatiles in cometary 
nuclei shows that little processing occurred during the formation of the comet 
or of the parent body it broke up from. 

The scientific rationale for the Triple Emission is outlined in Section 2 on the 
basis of recent results from the cometary missions Deep Impact and Stardust. 
Cometary nucleus properties will be very well understood from investigations 
by Rosetta's Lander. The months spent near the nucleus from the onset of its 


S. A. Crowther • J. Gilmour • T. Henkel • I. Lyon • D. Rost 
University of Manchester, Manchester, UK 

A. C. Ferrari 

University of Cambridge, Cambridge, UK 
M. Fulle 

Trieste Observatory, Trieste, Italy 

O. Groussin 

LAM, Marseille, France 

E. Grim • R. Srama 
MPIfK, Heidelberg, Germany 

P. J. Gutierrez • J. J. Lopez-Moreno • J. Rodriguez 
IAA-CSIC, Granada, Spain 


T.-M. Ho 

ESA-ESTEC, Noordwijk, Netherlands 


W.-H. Ip • Z.-Y. Li 

National Central University, Taipei, Taiwan 
E. Jessberger 

University of Munster, Munster, Germany 


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activity to the perihelion of comet CG will provide extensive experience on 
operating inside the coma. The risks of operations in the vicinity of a cometary 
nucleus will be fully understood and can be minimised during sampling. Ad- 
equate knowledge to provide samples from scientifically significant locations 
will be available. 

Section 3 demonstrates that a sample from a short period (Jupiter-Family) 
comet can be returned by a spacecraft that is considerably smaller than Rosetta. 
The length of the mission is typically 10 years. A spacecraft launched in 
April 2018 by a Soyuz launcher to comet 79P/du Toit-Hartley will return 
1.5 kg of cooled cometary material in April 2028. We chose a conservative 
approach using systems relying on technologies that are either existing or 
to be developed for approved missions like BepiColombo. The same holds 
true for the trajectory calculations of the Solar-Electric Propulsion (SEP) 
driven spacecraft. The Triple F mission is proposed in collaboration with 
ROSCOSMOS (Russian Space Agency) and the total budget is 600 M€, 
equally shared between ESA and ROSCOSMOS. 


2 Scientific goals of a CNSR mission 

2.1 Relevance of comets for solar system formation 

The scientific questions to be addressed in this mission have been raised by 
past space missions to comets, particularly to Halley ( Giotto , Vega), Tempel 1 


G. Kargl • N. I. Komle 
IWF, Graz, Austria 

K. Kossacki 

University of Warsaw, Warsaw, Poland 

J. Licandro 
I AC, Tenerife, Spain 

S. C. Lowry 

University of Belfast, Belfast, UK 
I. Mann 

University of Kobe, Kobe, Japan 

Z. Martins • M. Schonbachler 
Imperial College, London, UK 

M. Maurette 
CNRS, Orsay, France 

U. Meierhenrich 

University of Nice, Nice, France 

L. R. Nittler 

Carnegie Institution, Washington, USA 


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{Deep Impact ) and Wild 2 {Stardust), but also by observations made with 
space telescopes (HST, ISO, SST), and extensive ground-based observing 
campaigns. What is the complement of pristine interstellar organic material 
in comets? Is it possible to reconstruct the physical and chemical history of 
interstellar material in the nebula from observations of its state in comets? 
Does the chemical composition of a comet reflect its formation zone in the 
nebula? Do different cometary materials (dust, ices, organic compounds) orig- 
inate from markedly different environments? If so, what are its implications 
for the physical conditions (temperature, density) of nebular evolution? The 
scientific returns from this mission will allow these questions to be answered 
and permit fundamental progress to be made in advancing knowledge of the 
origin of our Solar System. 

2.1.1 Formation of the solar system from the protosolar cloud 

About 4.6 billion years ago the Solar Nebula formed from a collapsing frag- 
ment of a molecular cloud (MC). Isotopic evidence associates the collapse with 
explosive injection of possibly at least two pulses of material from a nearby 
supernova [7]. In the first phase of this process, which lasted about 10 * * * * 5 years, 
a protostar with a surrounding thick disk formed, deeply embedded in its 
parental MC. During the second phase this disk became thinner and reached 
a size of 100 AU or more in a time of less than 3 x 10 6 years. At the end of 
this phase the solar nebula consisted of a viscous gas-dust mixture. Due to 


M. Patzold 

University of Cologne, Cologne, Germany 
S. Sandford 

NASA Ames Research Center, Moffett Field, USA 

R. M. Stroud 

Naval Research Laboratory, Washington, USA 

S. Szutowicz 

Space Research Center of PAS, Warsaw, Poland 

S. Ulamec 

German Aerospace Center (DLR), Cologne, Germany 
M. Wallis 

University of Cardiff, Cardiff, UK 
W. Waniak 

University of Krakow, Krakow, Poland 

P. Weissman 
JPL, Pasadena, USA 

R. Wieler 

ETH, Zurich, Switzerland 


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the gas-dust interaction the coagulating grains settled to the mid-plane of the 
disk. Contemporaneously, the stellar accretion rate decreased and the proto- 
Sun reached its T Tauri state. Analogues of this ancient star forming process 
are observable in our galaxy today, e.g., in the Orion or Taurus Molecular 
Cloud. In contrast to the Taurus region, the star formation in the Orion 
MC is influenced by intense radiation in the far UV and by external shocks. 
Currently, we do not know whether our Sun has formed in a MC more similar 
to Orion or to Taurus. However, the different conditions prevailing during 
the formation of the solar system have left traces that can be found today 
in cometary material, such as the structure (amorphous or crystallized) and 
composition of the ice and dust fractions as well as pre-solar grains (identified 
by anomalous isotopes) and deuterated compounds or particular substances 
(e.g., PAHs). The Stardust mission recovered a few, probably pre-solar grains 
from comet Wild 2 [57], but the abundance of these grains in comets is as yet 
very uncertain. In the third phase, the dust grains decoupled from the gas, with 
the latter eventually being blown off from the disk. The solar nebula evolved 
to a planet-forming debris disk. This evolution took (3-10) x 10 7 years and it 
is not yet fully understood. Planetesimals accreted to planets with fundamental 
difference inside and outside the snowline. The material that accreted beyond 
this snowline can be found in comets. The composition of comets, as well as 
the differences between comets and the different types of asteroids, provides 
constraints on the solar nebula evolution in phase three. As a prerequisite, one 
needs to distinguish between the influence of the evolving solar nebula on the 
forming comets and their potential subsequent processing. 

2.1.2 Comets as remnants of solar system formation 

As witnessed by the high abundance of volatile ices such as CO in cometary 
nuclei, comets formed and spent most of their lifetime in a cold environment. 
Due to their origin in the coldest part of the solar nebula, comets are the solar 
system objects that underwent the least processing since the formation of the 
solar system from the pre-solar cloud. 

While it is evident that comets consist of the best preserved material from 
the solar nebula, it is a longstanding question to what extent material from 
short period comets evolved since its formation. New results, from the Deep 
Impact mission as well as from laboratory experiments and modeling efforts, 
suggest that we can rule out previous suggestions that cometary materials were 
highly processed to great depths below the surface. 

An aging process operating on a short period comet is collisional processing 
in the Kuiper belt [19]. While such a process would hardly affect the chemical 
and mineralogical properties of the comet, it can be expected to change 
physical properties like strength and density [8]. Recent results suggest that the 
history of short period comets may be less violent than suggested previously. 
For example, it is suggested that the origin of many short period comets is the 
scattered disk, a collisionally much more benign environment than the clas- 
sical Kuiper belt [17]. Also, Deep Impact has shown evidence for primordial 


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layering in comet Tempel 1 [6], suggesting that at least this comet is not a 
collisional fragment. 

After injection into the inner solar system the surface layers of a comet 
are processed by solar heating and sublimation of volatiles. Models of the 
evolution of cometary surface layers disagree about the depth of the surface 
layer that was processed by sunlight. Deep Impact measurements [29, 60] of 
Comet Tempel 1 suggest that the thermal inertia of the surface layer of a comet 
is so low that, in an active area, the timescale of the penetration of the solar 
heat wave is comparable to that of removal of surface material by sublimation. 
A sample of an active region obtained around or shortly after perihelion will 
therefore be largely unaltered by solar heating even near the surface. 

A cometary sample provides the unique opportunity to return pristine ma- 
terial to Earth. After the return of a small sample of the refractory component 
of a comet by Stardust and the upcoming in-depth investigation of a cometary 
surface by Rosetta , the return of a sample of largely unprocessed primordial 
material is the logical next step. 

2.1.3 Potential of Triple F 

The Wild 2 material in the Stardust sample was predominantly fine dust from 
our solar nebula, and not preserved isotopically anomalous pre-solar material 
(e.g., based upon the oxygen isotopic compositions [42]). The dust was mostly 
crystalline, not amorphous, and at least 10% of these crystalline materials 
appear to have originated in the inner solar system, not in the region where 
the Kuiper belt objects were assembled. A cometary sample analyzed with 
the powerful techniques available on Earth will shed more light into the 
composition of a comet and the origin of its components. It would also contain 
much coarser-grained materials and ices and, contrary to the aerogel capture 
of the Stardust samples, the sampled material will suffer little modification. 
Therefore, it will provide a much more complete picture of the processes 
operating in the early solar system. 

Kuiper belt comets carry unique information on materials and processes 
across the entire solar nebula disk. For this reason direct comparison with 
undifferentiated asteroids (formed in the inner part of the disk) is important. 
Kuiper belt comets will also provide new information on the first generation 
planetesimals that formed the primitive asteroids, since in the comets these 
primordial materials are packed in ice and not heated as they were in many 
asteroids. However, to extract such information large returned sample masses 
will be required (hundreds of grams). Stardust collected <1 mg of very fine, 
refractory dust and only traces of volatiles [41, 54]. Large sample masses will 
permit radiometric dating even of minor cometary components, which are 
inherited from many different bodies, including broken up large Kuiper belt 
objects (see [9]). Only large sample masses will permit detailed studies of 
minor, unaltered organic components, including amino acids, and will provide 
sufficiently large quantities of unaltered presolar grains for detailed studies of 
nucleosynthesis and processing in the insterstellar medium. 


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2.2 Comets and life on Earth 

In the endeavor to understand the different steps towards the origin of life 
on Earth and in a wider context in the Universe, one of the prerequisites is 
to identify the premises of emerging life. It is suggested that life emerged in 
water and that the first self-replicating molecules and their precursors were 
organic molecules of growing complexity. It is still an open question, however, 
whether the organic starting material relevant to the origin of life was produced 
in-situ on the primitive Earth or whether it was delivered from space. These 
two processes may not be exclusive but may rather represent complementary 
contributions towards the origin of life. 

Comets represent the most accessible target for acquiring materials formed 
in the outer part of the solar system. How well has this material been preserved 
since the formation of our solar system? What is their inventory of complex 
organic molecules and what was their role in the processes leading to the 
emergence of life on Earth? Answers to these basic questions will also provide 
essential complementary information to the European science-driven aspects 
of the space exploration program with the overarching scientific goal to reach 
a better understanding of the emergence and co-evolution of life with its 
planetary environments. However, it must be stressed that the absence of 
liquid water in comets over long periods of time greatly diminishes, if not 
completely eliminates, the possibility of the existence of living organisms in 
or on comets. 

2.2.1 Organics and prebiotic molecules 

Comets probably contributed part of the carbonaceous compounds during 
the heavy bombardment phase in the inner solar system including the Earth 
4.5-4 billion years ago [18]. Material arriving from outside may have been 
crucial for the evolution of carbon chemistry and subsequently life, since 
the atmosphere and surface of the early Earth were likely not favorable to 
organic syntheses. The terrestrial accretion process itself and the subsequent 
core differentiation as well as the impact events, are important energy sources 
which kept the surface of the early Earth fairly hot [40] and covered with vol- 
canoes. Amino acids — the building blocks of proteins — and nucleic acids have 
been found in several carbonaceous chondrites. The small L-enantiomeric 
excess of amino acids measured in those meteorites indicates that the origin 
of asymmetric amino acid formation is not yet well understood. 

Whereas the organic inventory of meteorites can be investigated in the 
laboratory by use of sophisticated analytical techniques, cometary nuclei — so 
far — evade our direct access. Our current knowledge is based on data from 
the different fly-by missions, and from the Stardust probe that brought back 
cometary grains from the coma of comet Wild 2. An organic component was 
identified in the Stardust samples which is richer in oxygen and nitrogen than 
organic compounds found in carbonaceous meteorites and Halley dust, indi- 
cating a different chemical composition and thus different chemical pathways 


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to its formation [54]. Polycyclic aromatic hydrocarbons (PAHs) have been 
observed in the Stardust samples, such as naphthalene (Ci 0 H 8 ), phenanthrene 
(C 14 H 10 ) and pyrene (C^Hio). From laboratory simulation experiments, the 
existence of more complex molecular structures in comet nuclei is inferred. 
However, so far, we have not yet succeeded in a direct identification of com- 
plex organic molecules in cometary nuclei. One goal of the Triple F mission 
is to study the ratio of simple to complex organic molecules in the cometary 
nucleus. For instance, so far it is not known [21] whether H 2 CO is an original 
nucleus molecule or rather a daughter molecule of POM (polyoxymethylene). 
The same is true for monomeric or polymeric HCN [25]. 

There is strong evidence that amorphous carbon and similar macromolecu- 
lar material account for most of the carbon in the interstellar medium [43]. The 
same trend is observed in meteorites, where macromolecular material accounts 
for more than 80% of the carbon [23]. The link between macromolecular 
carbon in the solar system and the interstellar macromolecular carbon is yet 
to be understood, but it is tempting to assume that such a material is also 
present in comets. Apart from a major fraction of aromatic solid carbon, minor 
abundances of many organic molecules, probably including prebiotic ones, will 
be present in the comet ary nucleus. 

Living organisms are based on (a) left-handed amino acids that form 
proteins (biocatalysts, enzymes); and (b) nucleotide bases, phosphoric acid and 
right-handed ribose (sugar, carbohydrate) that form the genetic material DNA 
and RNA. The detection of life’s precursor molecules in comets would provide 
important constraints for the origin of life on Earth and possibly elsewhere. 
The exact determination of enantiomeric ratios (and isotopic compositions) of 
prebiotic molecules in a sample returned from a comet will provide invaluable 
insights into the place and means of origin of the molecules important to the 
development of living systems. 

2.2.2 Hydrosphere and atmosphere 

Two sources of water on Earth are commonly envisioned: Adsorption of water 
by grains in the accretion disk [16] or delivery by comets and asteroids [15, 45]. 
The D/H ratio of SMOW (Standard Mean Ocean Water) is only half the D/H 
ratio of cometary water, and hence the suggestion that comets are a major 
source of terrestrial water is questionable. So far the cometary D/H ratio is 
estimated from the coma of three long-period and Halley-type comets: Halley, 
Hale-Bopp, and Hyakutake. It is unclear if the ratio is the same in short-period 
comets. Since comets formed from components that were created over a wide 
range of heliocentric distances, the D/H ratio may also vary between different 
ice crystals in the same comet, providing information about the variation of 
D/H in the solar nebula. Therefore, we need to determine the D/H ratio in 
various water ice aggregates and its variation within the comet. 

A further question is related to the formation of the terrestrial atmosphere. 
Preferred gas components used to investigate this question are the isotopes 
of the noble gases and nitrogen. The former are chemically inert and the 


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latter corresponds to the major part of the current atmosphere. We know that 
the composition of the terrestrial atmosphere is not solar. Consequently, any 
realistic formation concept has to consider an evolutionary process starting 
from a primordial atmosphere. The evolution itself is caused by [51]: 

• gravitational escape, probably driven by the Moon-forming impact and by 
the adsorption of intense ultraviolet radiation from the young Sun and 

• planetary degassing. 

Various scenarios describe how Earth could have acquired its primordial 
atmosphere. The primordial atmosphere could have been captured gravita- 
tionally from the gas of the surrounding solar nebula [52], or the atmospheric 
volatiles resulted from gases adsorbed on the infalling planetesimals during 
the accretion phase. The abundances of Ar, Kr and Xe on Mars, Earth and 
Venus suggest that comets could have delivered considerable amounts of 
these gases, along with other volatiles, to these planets at the end of the 
late bombardment period [49]. If one measures the elemental and isotopic 
noble gas ratios (e.g., 4 He/ 20 Ne, 4 He/ 36 Ar, 20 Ne/ 22 Ne, 21 Ne/ 22 Ne, 136 Xe/ 130 Xe, 
129 Xe/ 130 Xe) and 15 N/ 14 N of the material provided by the Triple Emission, the 
potential cometary source, and hence the various scenarios, can be evaluated. 

2.2.3 Potential of Triple F 

A cometary nucleus sample return mission will be a crucial step in the 
investigation of the organic component and isotopic ratios of a cometary 
nucleus. The investigation of large samples with sophisticated analytical in- 
struments in specialized laboratories will allow us to study in detail the variety 
of organic compounds including both large and small organic molecules, 
complex carbonaceous material, the organic-mineral connections in comets, 
and isotopic ratios. 

The Rosetta mission on the way to Comet 67P/Churyumov-Gerasimenko 
carries instruments that will study the in-situ chemical composition of the 
comet nucleus. However, Rosetta is limited in the analysis of complex organics. 
The instrument COS AC [28] on the Rosetta Lander Philae will methylize non- 
volatile compounds such as carboxylic acids as well as amino acids to make 
them visible for gas chromatographic analyses. However, many of the larger 
organic molecules cannot be analyzed with Rosetta ' s in-situ instrumentation. 
One important analysis that is not covered is sugar chemistry. Amino acids 
and their polymers are possibly accessible by derivatisation and GC-MS, but 
sugars and their polymers need methods too sophisticated for the kind of space 
instrumentation used on Rosetta’s lander Philae. Similarly, the analyses of 
isotopic ratios is restricted to light elements and limited by sample size. 

2.3 The mystery of cometary activity 

In spite of substantial observational, experimental, and theoretical efforts, 
comet ary activity is far from being understood. However, knowing how activity 


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works is vital for assessing how the pristine material from which comets formed 
has been processed and possibly altered over time. An answer may be found 
by combining data from the ESA mission Rosetta and the currently proposed 
mission. The latter will deliver complementary, unique, and extremely im- 
portant information regarding a number of basic questions, discussed below. 
Some questions will be answered by conducting direct measurements on 
the retrieved material; others can be derived from these measurements in 
combination with modeling. However, there is no obvious way to settle the 
issue without bringing a sufficiently large and relatively unaltered sample of 
cometary material to Earth for analysis. 


2.3.1 Heterogeneous distribution of active areas across the surface 

The asymmetric shapes of the cometary gas production curves relative to 
perihelion as well as the asymmetric non-gravitational forces perturbing the 
cometary orbits are indirect evidence of discrete outgassing regions on the 
nuclei. More directly, the existence of jets, fans, shells and other structures 
in comae indicate an anisotropic emission of gases and possible “active areas” 
on the surface of the nucleus. The ESA Giotto mission to Comet lP/Halley 
in 1986 provided the first close-up imaging of a cometary nucleus that turned 
out to have complex surface structures with dust emission restricted to a few 
“active regions” covering about 20% of the sunlit side of the nucleus [32, 33]. 
The close encounter with 19P/Borrelly (NASA Deep Space 1 mission, 2001) 
revealed narrow, highly collimated structures similar to those already seen 
in lP/Halley [55]. Analysis of the surface morphology and albedo variations 
suggests that some landforms (e.g. mottled terrain) represent surface subjected 
to extensive sublimation-driven erosion in the past, while other features (e.g. 
bright-appearing slopes of mesas) are probably freshly exposed sources of 
some of the active jets [34]. Observations of Comet 9P/Tempel 1 by the NASA 
Deep Impact mission show differing coma distribution patterns of water and 
carbon dioxide with a high degree of spatial asymmetry [20]. Therefore, the 
mixture of active and inactive areas appears to be common, but the intrinsic 
differences between such surface types are unknown. 


2.3.2 Depth of the water sublimation front in active areas 

It is unclear if coma gas primarily originates from exposed surface ice, or 
from shallow sub-surface regions, but various arguments suggest that in an 
active area we find volatile material within a few cm of the surface. There 
certainly is water ice on the surface of Comet 9P/Tempel 1, as shown by 
spectral absorptions at 1.5 and 2.0 pm [59]. However, the estimated active area 
fraction [14] is at odds with the area fraction actually showing water absorption 
features. Furthermore, the distribution of water just above the surface strongly 
suggests that the bulk of the outgassing takes place along the noon meridian, 
which is on the visible side of the nucleus in a region that must have less than 


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1% surface ice coverage [20]. This implies subsurface sources, unless the far 
(unimaged) side of the nucleus is richer in ice, and/or additional (undetected) 
ice is present on the imaged side. If subsurface sources of ice indeed dominate 
cometary outgassing, chances are still excellent to find sublimation fronts well 
within the sampled 50 cm depth. Laboratory work [30] shows that a thin 
refractory mantle of some cm would strongly quench the outgassing, much 
below the level observed at comets. Both the analysis of the Hale-Bopp data 
[36] and the Deep Impact measurements of surface temperatures on Tempel 
1 [29] show that the thermal conductivity of the surface material must be very 
low (<0.01 W/m K). This also implies that potential dust mantles in active areas 
must be very thin (mm to cm range), otherwise it would not be possible to 
transport enough heat to sublimate the water ice below [35]. 

An inactive area is probably characterized by a deeper refractory-volatile 
boundary. The absence of water absorption features in up range rays of Deep 
Impact ejecta and strong water ice absorption at 3.0 pm in the remaining ejecta 
indicate a stratified surface at the most likely inactive impact site, with an ice- 
rich interior covered by an ~1 m thick layer of dry material [60]. 


2.3.3 Physical, chemical, structural, and mechanical properties of near-surface 
material on a microscopic scale 

The short- and long-term evolution of cometary material depends on its 
microphysical properties and the illumination conditions. To understand the 
outgassing processes and the physical properties that distinguish an active from 
an inactive region, thermal models must be supplied with physical, structural 
and mechanical parameters, such as heat conductivity, heat capacity, porosity, 
size distributions of grains and pores, pore connectivity, tensile strength, 
chemical composition et cetera. Such parameters change with surface loca- 
tion, depth, time, and in some cases, temperature. They can only be measured 
accurately and systematically in a sample of sufficient size in well-equipped 
laboratories. Such detailed information on microphysics is vital for explaining 
phenomena occurring on a global scale, over extended periods of time. 


2.3.4 Conditions for dust mantle formation 

Activity of comets at the same heliocentric distance of succeeding orbits is 
relatively constant. However, laboratory work on comet analog material has 
shown that ice and dust mixtures irradiated by solar light are quickly depleted 
in their water content at the surface by forming an insulating dust mantle, 
and activity drops quickly. Even taking into consideration that the presence 
of Earth’s much higher gravity may cause results not directly applicable to 
comets, it remains true that the gas pressure of the volatiles is much lower 
than the Van der Waals forces between particles [37]. How then is activity 
maintained over time? 


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2.3.5 Intrinsic exothermal processes 

Cometary outbursts and distant activity are common phenomena. Exothermic 
reactions, such as crystallization of amorphous ice [5, 38], are perhaps the 
strongest candidate for delivering energy for driving outbursts and dust blow- 
off. While it is difficult to avoid crystallization of amorphous ice during the 
transport from the comet to Earth, amorphous ice may be detected indirectly 
by a temperature increase in the sample during crystallization. 

2.3.6 Potential of Triple F 

Rosetta with its 21 experiments will provide important input to solve the ques- 
tions listed above by investigations of Comet 67P/Churyumov-Gerasimenko. 
Etowever, there are some limitations that will be overcome by the proposed 
mission: 

• Laboratory measurements of microphysical parameters can be made in 
substantially larger detail and more systematically than in situ. 

• The Lander Philae will go down to only one (probably inactive) area. 
Triple F will visit several regions with different morphologies and out- 
gassing levels, exploring the reasons for diversity. 

• The size of samples investigated with Philae (10 to 40 mm 3 ) is limited. The 
volume of samples returned will be orders of magnitudes larger and the 
samples will cover a wider depth range. 

• The sample analysis is not limited by the technology and resources at 
the time the spacecraft is developed — analyses can be made using the 
full capability of terrestrial laboratories and new analyses can be made of 
curated samples as questions and techniques evolve. 


3 The mission concept 

3.1 Overview 

The baseline mission foresees a launch in April 2018. The spacecraft driven by 
Solar-Electric Propulsion (SEP) will fly-by Earth in 2019 and arrive at the tar- 
get comet 79P/du Toit-Hartley in mid-2023. During approximately 6 months 
of operations at the comet the spacecraft will get samples from three surface 
locations of the cometary nucleus by touch-and-go sampling. To prepare 
sampling and to enhance the scientific return, the comet will be investigated 
by remote sensing instruments and in situ dust and gas measurements. The 
sampling devices are corers that will be driven into the cometary surface once 
the spacecraft touches the ground. Ground contact will be for a few seconds 
only. After retrieving the samples, return travel to Earth will begin in early 
2024. The samples are cooled down to 133 K during the complete return travel, 
except for 2 h at re-entry into Earth’s atmosphere when the temperature will 


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increase to 163 K. The spacecraft will return to Earth and deliver the re-entry 
capsule in April 2028, after a total mission duration of 10 years. 


3.2 The target comet: 79P/duToit-Hartley 

A mission to Comet 79P/du Toit-Hartley (hereafter 79P) was selected as the 
baseline because of the relatively low AV of approximately 10.3 km/s required 
for the round trip and because it offers a launch opportunity at the beginning 
of the Cosmic Vision timeframe. Five other comets with A V between 10.0 km/s 
and 11.3 km/s were identified. 

Comet 79P was detected by D. du Toit from South Africa in 1945. After its 
first observed perihelion passage it was lost for several decades. Finally it was 
recovered by M. Hartley in 1982, after a probable splitting event in 1976. 79P 
was observed again by professional astronomers during its last two orbits in 
1995 and 2003. From observations of its inactive nucleus at large distance from 
the Sun the radius of 79P is estimated to be 1.4 ± 0.3 km [39]. 

The perihelion distance of 79P is currently 1.23 AU, but it will decrease to 
1.12 AU by its perihelion in 2023 when it will be visited by Triple F. Its aphelion 
distance of 4.8 AU and orbital inclination of 3° are both relatively low, making 
79P a good target for a sample return mission. 


3.3 Launcher 

For the Triple F mission a Soyuz launch from Kourou is considered. For 
Soyuz both, direct escape and escape from Geostationary Transfer Orbit 
(GTO) using a propulsion module are attractive. Various escape scenarios are 
possible. As the baseline, we consider use of a propulsion module and a lunar 
gravity assist which results in an escape mass of 2,000 kg and an excess velocity 
at escape of 1,100 m/s. 


3.4 The interplanetary trajectory 

The baseline trajectory for the Triple Emission is outlined in Fig. 1. An optimal 
solution was found for a launch date in April 2018. The round trip mission 
duration for this option is 10 years. The stay time at the comet is 7 months 
which provides sufficient time for a characterization of the comet and the 
sampling. 


3.4.1 Transfer 

Even with a state of the art electric propulsion system a direct transfer to 
Jupiter-family comets such as 79/P is not possible within reasonable time. A 
generically applicable strategy to reduce the AU for the transfer to a comet is 


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Fig. 1 Transfer (a) and 
return (b) trajectories to 
comet 79P/du Toit-Hartley 
(dimensions in AU). 
Thrusting periods are shown 
in red 


Projection of the trajectory onto the etfiplie plane 



xjAU] 


b 

Projection of the trajectory onto the ecliptic plane 



the return to Earth for a gravity assist. The basic sequence for the transfer to 
the comet is the following: 

• Launch into GTO 

• Raise apogee to lunar crossing orbit by chemical propulsion stage (requires 
~700 m/s from GTO) 

• Perform a Lunar Gravity Assist 

• Leave Earth and thrust for increasing eccentricity of heliocentric orbit and 
for targeting Earth 

• Come back to Earth after little more than 1 year and perform an Earth 
Gravity Assist. 


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For the return trajectory it needs to be taken into account that the re-entry 
velocity into the Earths atmosphere shall not exceed a certain value. For the 
proposal a maximum reentry velocity of 13 km/s is chosen that lies within 
the range that was already demonstrated by previous missions (e.g. Stardust, 
Genesis). Hence the re-entry velocity is limited to 13 km/s. In order to comply 
with this limit, the return trajectory requires a braking maneuver that adds to 
the roundtrip A V. 

3.4.2 Comet approach 

Due to the low-thrust transfer the spacecraft will approach the comet at a 
very low relative velocity. The rendezvous will be achieved by a series of 
small maneuvers. In order to ensure collision avoidance during this phase 
(with the comet’s gravity field still badly known), the maneuvers will never 
directly target the comet but always some point outside of its cross section. 
This strategy is also foreseen for Rosetta. However, the rendezvous maneuver 
sequence for Triple F is less risky due to the much lower relative velocity. 


3.4.3 Re-entry 

For the re-entry the Triple F spacecraft targets a hyperbola with its perigee at 
the Earth’s surface. Approximately 5 h before the perigee the re-entry capsule 
is deployed. 

The re-entry capsule will follow the desired re-entry trajectory entirely 
passively and without maneuver capability. After the deployment of the re- 
entry capsule the spacecraft conducts an orbit maneuver that puts itself on an 
Earth fly-by trajectory in order to avoid destruction. 

3.5 Spacecraft 

The basic concept of Triple F is simple: A single spacecraft will be sent to the 
comet, acquire the sample, and return. Due to the low gravity environment, the 
propellant penalty for this concept compared to a mothercraft-lander system 
is minimal and one can avoid the complexity of having to rendezvous the 
mothercraft with the lander in order to transfer the sample. The preferred 
sampling concept is that of a touch-and-go sampling. The whole maneuver 
will typically take about 1 h. Limiting factors are the rotational period of the 
comet and the need that the whole approach sampling and take-off sequence 
has to be carried out in daylight. The conclusion that touch-and-go sampling 
with the whole spacecraft is the preferred operational scenario had already 
been reached for the Hayabusa asteroid sample return mission [22] and also 
during the Asteroid Sample Return Technology Reference Study [1] for ESA. 
A major advantage is that the thermal control of the spacecraft does not have 
to be adjusted to the “hot” surface and that all resources of the main spacecraft 
are available. 


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Fig. 2 Triple F spacecraft 
configuration concept 



high gain antenna ion thrusters 


A major challenge is to reconcile the large wingspan of the solar arrays of 
approximately 15 m each, that is required by the electric propulsion system, 
with the need to avoid simultaneous ground contact of both solar panels (with 
the spacecraft continuing to move downwards) when landing. Our concept uses 
a 1 degrees-of-freedom (DOF) rotational mechanism so that when moving, 
the solar array follows an imaginary cone. To maximize ground clearance, the 
solar array will have to be turned in the morning/evening orientation when 
landing which is compatible with the desired landing scenario. The option 
yields optimal ground clearance and makes use of an existing solar array drive 
mechanism. The disadvantage of the concept is the motion of the centre of 
mass of the spacecraft introduced by the solar panel movement. However, 
this appears not critical, since it will be compensated by the thruster pointing 
mechanism of the electric propulsion system. 

This thruster pointing mechanism is mandatory anyway in order to minimize 
the angular momentum build-up that would otherwise result from shifts in the 
spacecraft’s center of mass due to fuel consumption and antenna pointing. 
Landing legs are foreseen to guarantee clearance of the attitude control 
thrusters from the ground. 

A 2 DOF articulated high gain antenna (HGA) is proposed to ensure 
permanent radio link to the spacecraft during thrust phases and to allow 
simultaneous remote observations and data downlink at the comet. 

The re-entry capsule needs to be placed on a panel of the spacecraft 
that is permanently in shadow. This panel also accommodates the sampling 
mechanism. The resulting spacecraft configuration is shown in Fig. 2. 

3.5.1 Solar electric propulsion 

The mission is based on the simultaneous operation of two RIT22 or T6 
thrusters. The total thrust time of nearly 20,000 h per thruster is well within 
the 25,000 h expected lifetime capability of the BepiColombo thruster. In 
addition a third thruster is foreseen for redundancy. The architecture of the 
propulsion system corresponds to that of BepiColombo hence extensive use of 
the BepiColombo heritage could be made. 


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3.5.2 Power , telecommunications and data handling 

For the Triple F mission a solar array with an area of 60 m 2 is foreseen. 
Considering aging and losses in the power processing units this corresponds to 
an available power of approximately 12,650 W at 1 AU. Of this power typically 
500 W (+100 W margin) will be required for the bus leaving approximately 
12,050 W to the electric propulsion system enabling a thrust level of 350 mN at 

1 AU. The available power will decay with heliocentric distance, R , roughly as 
R~ lJ due to the improved solar cell efficiency at lower temperatures. Ample 
power is available during coast phases of the transfer and during the comet 
proximity operations. In particular, an off -pointing of the solar arrays from the 
Sun direction that may be necessary to achieve sufficient ground clearance of 
the solar array during the touch-and-go operations is fully compatible with the 
available power. The minimal thrust level of the RIT22 and T6 ion engines 
of 30 mN is reached at 3.2 AU heliocentric distance. Beyond this distance the 
electric propulsion system needs to be switched off. 

The solar array sizing is entirely driven by the power demand of the 
electric propulsion system. Also the power system design is determined by 
the requirements of the electric propulsion system: A 50 V bus is needed for 
the propulsion system while the rest of the spacecraft is powered by a 28 V bus 
derived from the 50 V bus. The power system architecture can be based on the 
concept of BepiColombo. 

An X-band telecommunication system based on BepiColombo with a trans- 
mit power of 27.5 W is foreseen for telemetry, tracking and telecommand and 
science data downlink. An additional Ka-band link for Doppler measurements 
is proposed in order to allow a precise determination of the comet’s gravity 
field even at low angles between the Earth and the Sun as seen from the 
spacecraft. For the HGA a design similar to that of Mars Express and Rosetta 
with a diameter of 1.6 m is foreseen. This system will provide a data rate of 

2 kbps over a distance of 5 AU. 

The two drivers for the data handling system are on the one hand the 
Guidance, Navigation and Control (GNC) during the touch-and-go sequence 
and on the other hand the demands of the remote sensing instruments, e.g. data 
compression. For the GNC the requirements are reduced by the fact that the 
gravitational acceleration due to the comet is small, and hence, the approach 
will be slow. Consequently processors like the LEON III that will be available 
in the Cosmic Vision timeframe will be more than sufficient for the GNC. 
Probably even a current ERC32 with 14 Mips would fulfill the requirements. 
The requirements for the handling of the instrument data will depend on the 
processing capabilities that are already incorporated within the instruments. 


3.5.3 Guidance, navigation and control 

Three-axis stabilisation is the only viable control concept in order to fulfill the 
pointing needs during thrust phases and proximity operations. The attitude 


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control system consists of sensors and actuators. A sun sensor is foreseen 
for initial attitude acquisition after launch and safe mode. Star trackers and 
an inertial measurement unit are used for attitude determination. Standard 
reaction wheels of 12 Nms capacity and a set of 12 (+12 redundant) 10 N 
thrusters are foreseen as actuators. In all nominal modes the attitude control 
of Triple F relies on reaction wheels. For safe mode the attitude control will 
rely on the thrusters. The thrusters are also used for wheel desaturation and 
orbit control. 

The major challenge for the guidance navigation and control (GNC) are 
the comet proximity operations, including the touch-and-go on the comet. 
Years before the launch of the Triple F mission, extended experience of op- 
erating in the near nucleus cometary environment will have been accumulated 
by Rosetta. 

The rendezvous with the comet takes place at heliocentric distances below 
2 AU. Hence, the spacecraft will be confronted with significant outgassing 
from the comet during its proximity operations. Due to the gas and dust flux 
from the comet, stable orbits may not exist and a different strategy must 
be devised. Attractive options are hovering, which was the nominal mode 
of Hayabusa, or an eclipse-free terminator orbit (low cometary activity), the 
latter being the baseline in Agnolon [1]. For the terminator orbit, regular 
eccentricity control must be conducted in order to compensate disturbances 
by the cometary environment. These correction maneuvers will have to rely 
on optical navigation based on landmark tracking or limb recognition. Due 
to the high operational effort of a ground controlled GNC during proximity 
operations, the implementation of autonomous GNC seems preferable. Also 
for the hovering strategy, a certain level of autonomous GNC for the spacecraft 
is required in order to avoid a strong deviation from the nominal position due 
to changes in the cometary environment. 

Hazards due to the activity of the comet need to be considered. The gas 
streaming from the nucleus exerts a pressure on the solar panels that pushes 
the spacecraft away from the comet. However, even for active Jupiter-family 
comets the average gas pressure at heliocentric distances larger than 1 AU is 
of the order of the gravity attraction. When the samples will be collected at a 
heliocentric distance of about 2 AU it will be considerably less. This can easily 
be compensated by the SEP. The second hazard is fine dust leaving the nucleus; 
the importance of that hazard depends on the sticking properties of the dust 
particles and the ill-constrained dust size distribution. Our strategy to avoid 
high dust fluxes on the solar panels is twofold. Firstly, the solar panels will be 
directed in a way that the area pointing towards the comet is minimized (orbit 
near terminator). Secondly, when operating close to the comet, times (local 
noon) and regions of high dust activity will be avoided. The dust flux will be 
continuously monitored by the cameras and the in-situ dust instrument. Due to 
the experience from Rosetta we will have a good understanding of the expected 
dust environment and the knowledge how to operate close to a comet. There is 
ample time (7 months) to characterize the inhomogeneous activity of 79P and 
to devise a safe strategy before touch down. 


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Ballistic descent from a low orbit is our baseline strategy for the GNC during 
the touch and go, following the conclusion of Agnolon [1]. He proposes to 
have the comet detection as well as the approach and orbit capture controlled 
by the ground, using radio navigation. The spacecraft would autonomously 
control the orbit and perform the landing and ascent maneuver, based on visual 
navigation with wide angle cameras. The duration of descent is estimated to 
range between 30 and 90 min, the landing precision on the order of 10 m. 

The only major difference between the study by Agnolon [1] and Triple F 
as proposed here is the much larger size of the solar array due to the electric 
propulsion system. Avoiding the danger of ground contact with the solar arrays 
has already been discussed above. With this issue under control the GNC 
of Triple F can be based on the findings of Agnolon [1] and does not pose 
challenges beyond those identified there. 

3.5.4 Thermal control 

The Rosetta spacecraft is currently demonstrating the thermal control of a 
spacecraft over a range of heliocentric distances from 1 to approximately 
6 AU. The thermal control of Rosetta is achieved by louvers which open at 
high temperatures and close at low temperatures. This simple concept is also 
applicable to the Triple F mission. More refined concepts could allow mass 
savings in the thermal control subsystems if required. In particular the louvers 
could be replaced by radiators and heat switches, which are currently being 
developed for the Exomars Rover in ESA’s Aurora Programme. 

A particular challenge of the Triple F thermal control is to ensure a 
cryogenic temperature of the sample. In Section 3.6.1 it is argued that a 
temperature of at most 135 K during transfer and 170 K for the reentry 
is desirable to maximize the science return. We designed a simple thermal 
control that will perform reliably during the 4 years return transfer and 
keeps the temperature below 133 K in interplanetary space and 163 K during 
re-entry. 

The sample container will be transferred to the reentry capsule soon after 
sample acquisition. The re-entry capsule will be kept in the shadow of the 
spacecraft during the complete return transfer. The capsule will have three 
zones at different temperatures which are well thermally decoupled from 
each other. 

The desired temperature of the sample container can be maintained with 
a modest radiator size of 0.4 x 0.4 m 2 and a conductivity coefficient between 
the cold and the intermediate zone of 0.03 W/K. This is challenging due to the 
requirement to have a safe mechanical interface between the zones, but well 
within reach. 

After the detachment of the re-entry capsule from the spacecraft the cooling 
of the sample container can no longer rely on the radiator of the return capsule 
because it may be exposed to sunlight due to the approach trajectory of the 
capsule towards Earth. Hence, during the final approach, the reentry and on 
ground before recovery of the capsule the sample container will be cooled from 


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the heat capacitor to which it is coupled. This concept is feasible because the 
temperature on the backside of the ablative heat shield will be quite modest. 
For a preliminary assessment a conservative value of 523 K was assumed 
taking into account the values for Stardust. Allowing for a temperature rise 
of the sample container of 30 K to 163 K and assuming a re-entry duration of 
15 min (14 min for Stardust ), followed by 2 h on ground before recovery, a heat 
capacitor of 5 kg water and a conductivity coefficient of 0.26 W/K between the 
backside of the heat shield and the cold zone is sufficient. Hence, the cooling of 
the sample during reentry is well compatible with the structural requirement 
of the re-entry capsule. 


3.5.5 Re-entry capsule 

There is a trade-off between the re-entry velocity of the return capsule and the 
braking AV of the Triple F spacecraft on its return transfer. We have taken 
a conservative approach in limiting the entry velocity to 13 km/s. A detailed 
analysis may well reveal that a higher re-entry velocity is feasible and more 
mass efficient and could hence lead to a better overall system performance. 
In any case an ablative heat shield is considered mandatory. It is the most 
lightweight option and it facilitates the cooling of the sample because it leads 
to a rather low temperature of the backside of the heat shield. For the final 
stage of the descent a parachute is foreseen in order to avoid high mechanical 
loads on the sample at touchdown. 


3.5.6 Operations concept 

The mission operations concept for Triple F shall ensure that the long mission 
duration of 10 years does not become a driver for the operational cost. Hence, 
a minimal number of ground contacts and a high level of spacecraft autonomy 
during transfer are desirable. For the mission the operational experience of 
the BepiColombo electric propulsion system will be available and hence it is 
assumed that also during thrust phase the spacecraft operations can be largely 
autonomous and ground contact can be reduced to once every week. 

During the coast arcs of the transfer, ground contact will be infrequent — 
typically once per fortnight — because no specific telecommand or navigation 
needs arise during this phase. For the period around the Earth gravity assist 
and before the Earth re-entry permanent ground coverage and the use of 
delta-differential one-way ranging is desirable to achieve the best possible 
targeting accuracy. 

For the comet rendezvous one ground station is sufficient and no large 
baseline tracking techniques are required. This low level of operational activity 
is possible due to the low approach velocity relative to the comet. 

During the comet proximity operations the use of two or three ground 
stations is desirable in order to maximize the science return of this phase. 
However, the GNC strategy will be used to ensure safe operations of the 


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spacecraft when no ground contact is possible and hence a permanent oper- 
ational attention during the comet proximity phase is not mandatory. 

The mission will use ESA 15 m ground stations for launch and early orbit 
operations as well as for apogee raising sequence for escape from GTO. ESA 
35 m ground stations will be used for deep space communications during all 
other phases of the mission. 

3.6 Sampling, storage and analysis of samples 

3.6.1 Requirements on sampling 

Number of samples and sampling locations on the comet The purpose of the 
mission is to return at least three samples from different locations on the 
surface of the comet. Samples will be taken from places with different levels of 
activity and at different “geographical” locations (with respect to the spin axis 
and, therefore, insolation), as surveyed by the payload instruments. Ehghest 
priority will be given to a sample from an area where ice is visible, or where 
activity has been observed. Then, an inactive area will be sampled, to compare 
active versus inactive regions. Finally, one sample will be taken from a (polar) 
region that sees little sunlight and therefore has experienced relatively little 
heating since the comet reached the inner solar system. Once the topography 
of the comet is known from the monitoring phase, additional criteria for the 
choice of the sampling site may become important. For example, it would be 
interesting to have a sample from a smooth area similar to the ones imaged 
on Tempel 1 [61], because there is the possibility that this material could have 
erupted from the subsurface. 

Dimensions of the sample container As discussed in Section 2.3, a sample 
depth of 50 cm will be sufficient to find water ice in an active region. The 
diameter of the sample container is driven by the desired sample mass and 
by the requirement that sampling change the sample properties as little as 
possible. Determination of the formation history of the comet by radiometric 
dating requires several hundred grams of material (Section 2.1). For a density 
of 500 kg/m 3 , a cylinder with a length of 50 cm and a diameter of 5 cm can 
collect 500 g of cometary material. 

Any volatiles (including organic and inorganic molecules) that are present 
in the subsurface should be sampled in such a way as to avoid structural or 
compositional changes. Therefore, non-destructive sampling is important for 
the further analysis of the cometary material. The maximum allowed stress 
during sampling must not exceed the tensile, compressive, and shear strength 
of most of the sample. As we expect the cometary surface material to be 
extremely fragile, care must be taken to ensure that the sampler will not 
compress or otherwise alter the sample. We consider a sampling tube with a 
thin wall as the best non-invasive means to fulfill this criterion. Figure 3 shows 
an x-ray image of a cylindrical specimen of high-porosity (85% porosity) non- 
volatile cometary analogue [8] which was sampled from a 2.5 cm diameter body 


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Fig. 3 X-ray image of a 
sample taken from a 2.5 cm 
diameter high-porosity dust 
agglomerate by means of a 
tube sampler. The diameter 
of the sample is 7.5 mm. 
Although the compressive 
strength of the sample is as 
low as 500 Pa, the sample 
structure is preserved at 
distances >1 mm from the 
sampler wall 



by means of a plastic tube (inner diameter 7.5 mm, wall thickness 0.25 mm). 
Although the outer edge shows a sawtooth shape, which stems from the 
manual operation of the sampler, the overall morphology of the sample is 
unaltered. In particular, the porosity of the sample is unchanged at distances 
>1 mm from the sampler wall. The sample itself is extremely fragile with 
a compressive strength of 500 Pa and a tensile strength of 1,000 Pa. For a 
sample diameter of 5 cm, only a small fraction of the sample will be altered 
by interaction with the walls of the sampling tool. 

Temperature of the sample during return to Earth The preservation of the 
sample micro-structure during sampling, cruising phase, re-entry into Earth’s 
atmosphere and landing is vitally important for part of the mission science. 
The storage temperature of the samples is driven by the requirement that (at 
least) the most abundant volatile species, i.e. water ice, will be preserved in 
solid form. The relation between the gas mass and the initial solid phase mass 
as a function of the reservoir temperature P R is: 

m/ Mi C e = '!'/('!' - 1) * (Pym g ) / (p ics k B T R ) 

In the above equation is the fraction of the probe volume filled by solid 
material, Py is the vapour pressure, m g the mass of a water molecule, and pi ce 
the density of the ice. We used the Goff Gratch equation (Smithsonian Met. 
Tables, 5th ed., pp. 350, 1984) to calculate Py(P). The calculations show that 
up to a temperature of 190 K and a porosity of 0.7, the fraction m/Mi ce is 
always below 2 * 10 -7 . A temperature T < 200 K will guarantee that the ice 


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will stay in cubic form (if cubic ice is present close to a cometary surface) and 
that sintering will be inefficient in solidifying ice-ice contacts. 

The equilibrium calculation shows that during the whole return trip only a 
negligible fraction of the water ice will be in gaseous form. However, equilib- 
rium is maintained by a large number of sublimations and recondensations. In 
the porous cometary material the gas molecules may move before recondensa- 
tion, resulting in structural changes. As a worst case scenario, we assume that 
sublimation and subsequent recondensation always cause modification to the 
sample. In this case, the requirement is that the sublimation timescale (into 
vacuum) is larger than the travel time from the comet to Earth. 

Figure 4 shows that a temperature of 135 K is sufficient to preserve the ice 
quantitatively (at the 90% level) in solid form over a mission time of 5 years. A 
temperature increase to 170 K during 2 h of atmospheric re-entry is acceptable. 

The sample temperatures fulfill the requirement of preservation of the 
micro-structure of the sample. This does not necessarily mean (and it is 
not required for the scientific goals of the mission) that the sample remains 
completely unchanged. For example, due to the much higher vapour pressures 
of other cometary volatiles, a quantitative retention of minor ice species in 
solid form may require still lower temperatures. Also, should amorphous ice 
be present in the samples, the temperatures may not be sufficiently low to avoid 
its transformation into crystalline ice. However, maintaining substantially 
lower temperatures would be technically challenging. Therefore, we decided 
to identify volatile ices and amorphous water ice without trying to maintain 
them in their original state. Since the sampling device is sealed, a mass- 
spectroscopic analysis of the gas composition will unambiguously determine 
the total contents of all volatile species. Thermal probes on the sampling 
device will detect the temperature increase associated with the exothermal 
transformation of amorphous ice into crystalline ice. 

Strength of the sampled material Unfortunately neither Deep Impact nor 
other comet observations measured the strength of cometary material. Various 


Fig. 4 The percentage of 
sublimated water ice 
molecules is shown for a time 
span of 5 years ( red curve) 
and of 2 h ( blue curve) 



Tin K 


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analyses of the impact cloud created by Deep Impact resulted in different 
upper limits for the strength of Comet Tempel 1 on scales of meters. The 
highest upper limit is 65 kPa [31]. The few observational constraints we have 
for comets and cometary meteoroids as well as theoretical considerations and 
laboratory measurements [8] for weakly bound aggregates lead us to estimate 
the quasi-static tensile (or shear) strength of cometary material in the dm- 
to m-range to be on the order of 1-10 kPa, while the compressive strength 
is estimated to be on the order of 10-100 kPa. We require the sampling 
mechanism to be able to sample materials with strength of up to 1 MPa. This 
corresponds to the highest values measured for sintered water ice. 

3.6.2 Concept of the sampling mechanism 

Several methods of sampling material from a small body have been discussed 
in Agnolon [1]. Here we show the preliminary design of a simple mechanism 
that allows rapid touch-and-go sampling (duration <2 s) for the expected 
conditions of low or moderate surface strength of a comet. It was devel- 
oped at the Polytechnic University of Hong Kong based on heritage from 
Beagle 2 [47]. 

A sampling mechanism with three corers is foreseen that can be activated 
separately in three different regions of the nucleus (Fig. 5). The device will 
use a tubular coring system with a length of 50 cm and a diameter of 50 mm 
that is propelled by a spring mechanism into the cometary surface. Since the 
bottom of the sampling cylinder is open during sampling, a shutter mechanism 
is needed that keeps the sample in the corer during its transport from the 
cometary surface to the spacecraft. 

The acquired sample is directly delivered into a cooled transport con- 
tainer inside the re-entry capsule. Cross-contamination can be avoided by 


Fig. 5 Global architecture of 
the sampling mechanism 



Comet 


Re-Entry 

Capsule 

Transport Container 
(cooled I 

Sampling Mechanism 
with 3 Corers 


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Fig. 6 Penetration depth 
plotted as a function of time 
for three different cohesive 
compression strengths. A 
friction constant of 0.1 N/m 
is assumed 



t in ms 


the complete separation of the samples in the cooling compartment. The 
shutter mechanism is activated by the same cable mechanism that retrieves the 
coring tube. 

The suggested sampling tube will be a Titanium cylinder with a wall 
thickness of 0.5 mm. Its total weight is about 200 g. Such a cylinder can be 
accelerated by the springs to a velocity of approximately 12 m/s (spring system 
has flight heritage from the Rosetta lander eject system). Calculations show 
that the device can penetrate into material of strength of up to approximately 
1 MPa (see Fig. 6). The penetration depth is calculated from the linear 
momentum balance equation taking into account the cohesive force of the 
material, friction along the surface of the cylinder, gravitational force, and the 
increase of the moving mass due to shoved material. The calculations represent 
a lower limit to the actual penetration depth because the cutting force of the 
sharp edge of the cylinder is not considered. Should the need arise to sample 
still stronger materials, a projectile gas generator (flight heritage from the 
Rosetta lander anchor subsystem) could accelerate the same tube to more than 
90 m/s, allowing sampling even of compact water ice. 

3.6.3 Sample analysis strategy 

Handling of samples on Earth, distribution and preservation The future return 
of extraterrestrial materials by ESA requires a European repository facility. 
In the present case, it will perform sample separation of gaseous volatiles, ices 
(including their trapped gases), organics and solid minerals. The gases in the 
sample containers would be transferred into reservoirs for further analyses. 
Organics will be present in icy and solid fractions. New techniques are required 
for separation of these phases under conditions similar to those on the comet. 
The facility will also coordinate the sample distribution. It will store the 
major part of the material for posterity, in anticipation of improved future 
analysis techniques. This facility might be associated with a scientific institution 
that already has required analytical techniques, experience in micro-sample 


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handling including microtome and in situ focused-ion-beam lift-out, as well 
as the preparation of thin sections. Opening of the containers, sampling and 
storing of the different phases will be performed in a clean environment, 
with the sample in vacuum to avoid adsorption of terrestrial atmospheric 
contaminants. 

Examination of the samples with state-of-the-art analytical techniques is 
necessary to accomplish the major mission goals outlined in Section 2. To 
maximise scientific output and minimise sample consumption, many tech- 
niques will be applied to the same material. Sample requests and allocation 
(a few percent of the recovered material within the first 5 years after return) 
should be controlled by a standing committee (similar to NASA’s “CAPTEM” 
committee or its “Meteorite Working Group”). For a fast publication of 
scientific results, a small fraction (<1%) of the returned material will be 
distributed rapidly to pre-selected research institutions. 

Analyses The amount of material returned with Triple F (100 s of grams) 
will exceed by far that returned with Stardust (^mg). Moreover, the sampling 
technique (Section 3.6.2) will not mix the material intimately with any collector 
material. These unique mission elements will allow a large number of exami- 
nations that were not possible with the Stardust samples. 

Initial non-invasive sample inspection could be achieved with x-ray tomog- 
raphy, providing sub-micrometer resolution and full 3D structural information. 
The measurements will reveal information about porosity, grain size, pore size, 
stratigraphy and (to some extent) elemental distribution within the sample. Ini- 
tial mineralogical and petrologic analysis of most of the mineral grains should 
entail Scanning Electron Microscope (SEM) imaging and Energy Dispersive 
X-ray (EDX) analyses on all particles, enabling sample requestors to analyse 
the most suitable samples. Quantitative x-ray spectrometric elemental analyses 
should be performed on each grain to establish a library of compositions, done 
by the repository facility for consistency in data. Large particles (>10 pm) 
should be analyzed with synchrotron X-Ray Diffraction (XRD). For smaller 
particles, microtome slices could be prepared for TEM analysis. Relatively 
non-destructive spectroscopic analyses including IR, Resonant Raman and 
UV spectroscopy will follow, prior to further analyses by more or completely 
destructive techniques, such as, e.g., Secondary Ion Mass Spectroscopy (SIMS) 
or conventional mass spectrometry. 

A major goal of all mineralogical analyses will be the search for high 
temperature phases and hydrated minerals. The presence of the former would 
add to the evidence of large scale mixing in the nebula [10, 63], whereas the 
latter would potentially indicate extended warm periods on the comet in the 
past. Moreover, as yet unknown or fragile minerals might be present that 
were not preserved in primitive meteorites or Wild 2 particles due to sampling 
technique or more severe heating on the asteroidal parent bodies. 

High-precision mass spectrometric analyses of all fractions of returned 
cometary material will have highest priority. Deuterium enrichments will 
unequivocally prove the extraterrestrial origin of the material [44]. H, C, and 


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N isotopic anomalies in the organic fraction will localise interstellar material 
[11] that has survived the formation of the solar system. The O isotopic 
compositions in various extraterrestrial materials exhibit heterogeneities [12], 
implying spatial or temporal variations during the condensation of the first 
solar system material. Large samples to be returned with this mission will 
allow the investigation of refractory elements such as Mo, Zr and platinum 
group elements, which are important to assess heterogeneity and degree of 
large-scale mixing in the solar system. These analyses require high-precision 
isotopic measurements, obtainable with MC-ICPMS or TIMS. The samples 
will also be investigated for stable isotope fractionation effects, e.g. in Mg and 
Fe., which will provide information on the conditions during which cometary 
material formed. 

The isotopic compositions of presolar stardust grains indicate that they 
formed in the dust ejecta of previous star generations [62]. Their abundances 
in extraterrestrial matter reflect thermal processing in the early solar system 
and on parent bodies. Thus, the abundance of presolar stardust in returned 
comet samples, identified by SIMS isotopic mapping, could be a sensitive 
probe of cometary thermal history. Only a few grains have been identified 
in comet Wild 2 dust [57], suggesting a lower abundance than in cometary 
IDPs [48]. However, sampling might have introduced severe bias. The more 
gentle collection method of this mission will allow presolar grains to be more 
readily preserved, providing better estimates of the presolar grain abundance 
in comets. Presolar grains might retain coatings acquired while present in the 
ISM, but that would not have survived the heating that other extraterrestrial 
material has experienced. 

All cometary matter (ices, organics and minerals) is exposed at various 
times to irradiation. As the cometary surface will continuously be renewed, 
the most recent effects of space weathering and the production of cosmogenic 
nuclides can be examined. Structural changes due to such irradiation, e.g., 
amorphisation of ices, minerals and carbonaceous matter might be visible. For 
these studies, noble gases in various grains will be studied, the depth profiles 
of various elements, as well as the response to irradiation of carbonaceous 
material with Resonant Raman and IR spectroscopy. The latter techniques 
will also be used to find evidence for the irradiation of carbonaceous matter 
in interstellar space prior to its incorporation into the comet. Chemically, 
isotopically or structurally distinct rims around cometary grains would help 
to assess the conditions prevailing in the protosolar cloud. The composition of 
micron-sized grains will be measured, e.g., with the Time of Flight SIMS [58] 
technique, which provides high sensitivity at a very small scale. This technique 
also detects organic compounds in situ and provides information about the 
elemental distributions in mantles around cometary grains. 

The volatile (noble gases, H, C, N, O, S) content of cometary ice is 
controlled by the conditions prevailing during its formation, either in the 
dense interstellar cloud or in the outer solar nebula [50]. Hence, it is of high 
priority to determine the relative volatile abundances in the various cometary 
ice grains. Moreover, the isotopic compositions of noble gases, especially Xe, 


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should be determined, both in solid grains and in icy agglomerates, which 
requires particular, highly sensitive techniques including resonance ionization 
[26]. Gas abundances of CH 4 , CO, C0 2 , N 2 , NH 3 , etc. are extremely important, 
e.g., in order to assess the form in which N has been trapped in the ice, and for 
comparison with spectroscopic results. 

Analyses of organic matter found in ice and associated with mineral 
grains will provide insights on the early history of our solar system and 
the extraterrestrial delivery of organic compounds that occurred on early 
Earth (see Section 2.2). Hence, most important is the search for biologically 
relevant compounds, such as amino acids or nucleobases, which have been 
detected in meteorites. In addition, the detection of chiral excesses in these 
compounds will be essential, as will the comparison with meteoritic organ- 
ics. Analytical techniques include liquid chromatography with fluorescence 
detection (HPLC-FD), liquid chromatography-mass spectrometry (LC-MS), 
gas chromatography-mass spectroscopy (GC-MS) and ionization techniques 
such as Matrix Assisted Laser Desorption Ionization (MALDI), solid state 13 C 
NMR (Nuclear Magnetic Resonance), XANES (X-ray Absorption Near-Edge 
Structure) and EPR (Electron Paramagnetic Resonance). 

3.7 Payload 

Overview 

Apart from the sampling device, the mission requires a basic payload to 
fulfill the following objectives: 

1. Characterize the comet, mainly its activity level and topography, with 
sufficient spatial resolution to identify appropriate sampling sites 

2. Mapping of the nucleus to derive important chemical and physical proper- 
ties that provide the context for the samples 

3. Monitor the environment of the spacecraft to avoid hazards, mainly due to 
cometary dust 

An overview of the payload is given in Table 1. The total mass of that 
payload is less than 20 kg. 

Cameras 

Triple F will carry a complement of four camera systems, designed to ad- 
dress different scientific requirements of the mission. For increased reliability, 
except for the high resolution camera each of the cameras consists of two 
identical, cold redundant units, for a total of seven camera heads. The first 
camera (high resolution imager, HI) provides the global monitoring of the 
nucleus during comet approach and during phases when the spacecraft is 
further than several tens of km from the nucleus. The second camera (the 
Mapping Imager — MI) will consist of a moderate angular resolution framing 
imager. This instrument will be responsible for global mapping from distances 
between 5 and 20 km, corresponding to a resolution between 0.5 m and 2 m. 


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Table 1 Summary of the payload (objective as defined in the text) 


Instrument 

Objective 

Mass (kg) 

Power (W) 

Remarks 

Cameras 

1,2,3 

4.2 

10 


Near-IR imager or spectrometer 

1,2,3 

2.0 

10 


Thermal IR instrument 

1,2 

3.5 

7 


Mass spectrometer 

2 

4.0 

10 

Will also examine 





small sample 

Dust in-situ monitoring experiment 

3 

< 5.0 

20 


Radio science investigation 

2 




Penetrometer 

2 

0.1 



Permittivity probe 

2 

< 0.3 

< 1 

On sampling device 

Thermal probe 

2 

< 0.1 

< 0.2 

On sampling device 


The third imager is a miniature wide-angle Fast-Framing Camera (FFC) which 
will be active during the descent, and ascent phases. This instrument, by 
acquiring image sequences at a rate in excess of 5 Hz, will document (and 
possibly guide) the descent of the orbiter with increasing accuracy. In this way 
the FFC will also provide local context by imaging the neighborhood of the 
sampling site with a resolution up to about 1 mm. The fourth camera will be 
a Close-Up Imager (CUI) which will monitor the complete sample acquisition 
and storage sequence during touch-down. In order to be able to observe the 
sample in the shadow of the S/C, the CUI will be equipped with a miniature 
LED illumination device. 

The field of view of the HI will be 1.5°, imaged on a 2,048 x 2,048 APS 
array. The specifications of MI, FFC, and CUI follow those of the cameras in 
Agnolon [1]. 

Near-IR imaging spectrometer 

The main scientific objectives of the infrared spectrometer are the following: 

• to support the selection of the sampling sites by searching for surface 
ice and monitoring the gaseous activity on the nucleus and its spatial 
distribution 

• to determine the nature of the solids on the nucleus surface (composition 
and structure of ices, dust and characterisation of organic compounds) 

• to identify the gaseous species in the coma 

While it is highly probable that the design of the near-IR instrument will 
benefit from the technological progress of the future years, we can refer to 
heritage from instruments already existing or in development. 

The proposed instrument could be a simplified version of VIHI, a hyper- 
spectral imager being developed to fly on ESA’s BepiColombo. The VIHI 
channel concept is based on a collecting telescope and a diffraction grating 
spectrometer ideally joined at the telescope focal plane where the spectrome- 
ter entrance slit is located. The image of the slit is dispersed by the diffraction 
grating onto a two-dimensional detector. A single 256 x 256 thinned infrared 
array detector shall be used which can achieve high quantum efficiency even 
in the visible domain. The instantaneous acquisition on the two-dimensional 


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detector consists of the slit image diffracted by the grating over the selected 
spectral range (1-3.5 pm). The complete image is built in time by subsequent 
acquisitions (push broom mode). The final result is a three dimensional data 
set, in which each pixel has a spectrum associated with it. The instantaneous 
field of view will be 250 prad and the field of view 3.7°. The instrument 
is housed in a single box that contains the optical system (Telescope plus 
Spectrometer), the calibration unit, shutter, Focal Plane Assembly (housing 
the detector and the TE Cooler) and proximity electronics. The operating 
temperature will be 170 K. 

Thermal IR imaging spectrometer/radiometer 

Measuring the thermal emission from the cometary surface would support 
the selection of the sampling site and provide valuable information about 
physical and mineralogical characteristics of the target. The temperature of 
sublimating water ice on the surface is about 200 K near perihelion. A thermal 
infrared radiometer (TIR) could be very useful in detecting possible locations 
of surface ice (and therefore activity) by their low temperature. Furthermore, 
the TIR spectral range provides a further excellent opportunity to characterize 
the mineralogical state of the comet ary surface. 

From the discussion above we propose an integrated imaging TIR spectrom- 
eter/radiometer, which works in the 5-40 pm range, be included in the payload 
of the Triple F mission, an instrument similar to MERTIS on BepiColombo 
but with a larger FOV (11.5°) and covering a broader wavelength range for the 
spectrometer part. Because of the sensitivity limitations of thermal detectors, 
the spectrometer is designed to provide spectra in 240 spectral channels with 
good S/N mainly for surface temperatures above 300 K whereas the radiometer 
is optimized for accurate measurements of the emitted flux in two broadband 
channels to cover the whole range of relevant temperatures from 100-400 K. 

The focal length of the instrument will be around 5 cm. The spatial resolu- 
tion will be 0.7 mrad for the spectrometer (7 m from a distance of 10 km) and 
5 mrad (50 m from 10 km) for the radiometer. A bolometer and a thermopile 
will be the detectors for the spectrometer and radiometer, respectively. The 
total dimension of the instrument will be about 14 x 16 x 12 cm. 

Mass spectrometer 

It is well known that comets contain a wealth of volatile and super-volatile 
material including radicals like CO, CH 2 , and CH 4 (e.g., [3, 24]). The goal of 
the mission is to bring back original, unprocessed material. However, taking 
a sample from the comet and bringing it back to the Earth cannot be done 
without some disturbance to the material, in particular sublimation of volatiles. 
Therefore, it is mandatory to have a mass spectrometer on board, which 
can analyze the most volatile or reactive compounds in situ in order to get 
the starting condition of the sample. The measurement would be twofold: 
the mass spectrometer would analyze the natural outgassing of the comet 
in the approach and prelanding phase. When acquiring the main sample a 
second much smaller sample will be acquired and placed in front of the mass 


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spectrometer. The mass spectrometer can then analyze the composition of this 
second sample during the journey back to the Earth over a long time (until the 
sample has fully evaporated). This will require two separate inlets to the ion 
source which can easily be achieved because the direction of the neutral gas 
flow is not critical. The mass spectrometer will not have the same resolution 
and sensitivity as laboratory mass spectrometers, but it will allow us to assess 
the initial condition of the sample and to deduce the chemical alterations the 
material has undergone from the comet to the laboratories on Earth. 

We foresee using a time of flight mass spectrometer with an electron bom- 
bardment ion source. Such an instrument can easily reach a mass resolution 
m/dm of 1,500, high sensitivity and mass ranges up to >300 amu. The design 
could be based on the heritage of Rosetta-ROSINA/RTOF [4]. 

However, RTOF is too heavy to be included in the present mission. This can 
easily be solved by omitting the requirement that the mass spectrometer has to 
be built with an ultra-high vacuum enclosure and be launched sealed under 
vacuum as was done for ROSINA/RTOF. This requirement was necessary 
in the case of the Rosetta mission to study the comet far away from the Sun 
when cometary activity is minimal. RTOF has two independent channels (ion 
sources and detectors) that can be reduced to one for the present mission, 
thus saving not only detector and ion source but also electronics. Therefore, it 
would be possible, with current technology, to build a reflectron-time of flight 
instrument with a mass budget of 4 kg. The dimensions of the instrument will 
be about 20 x 20 x 50 cm. 

Dust in-situ monitoring experiment 

The S/C will have to operate deep inside the comet coma, both during 
the monitoring phase and during the descent for sampling. Therefore, it is 
important to characterize the dust distribution in the coma in order to control 
and guarantee safety for mission operation and S/C health. Therefore, an in 
situ dust instrument is essential as a “security device” to measure the dust flux 
on the S/C. 

The GIAD A experiment on board Rosetta will study the cometary dust en- 
vironment of 67P and will accomplish unprecedented in situ primary scientific 
measurements [13]. A similar experiment on board the Triple Emission will 
fulfill the aforementioned security requirements and will in addition provide 
scientific data as it shall provide real time data on dust flux of “direct” and 
“reflected” grains, dust velocity distribution, dust evolution in the coma, 
dust changes vs. nucleus evolution and emission anisotropy, determination of 
dust-to-gas ratio, and identification of non homogeneous dust emission fea- 
tures from the surface (active areas, jets). 

The instrument is designed to measure momentum, scalar velocity and mass 
of single grains. From the detection of particles vs. time information about 
dust abundance and spatial distribution vs. physical and dynamic properties 
is derived. Its two stages of detection form a cascade: an Optical Detection 


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System (ODS) and an Impact Sensor (IS). When a grain crosses the ODS, it is 
optically (scattered/reflected signal) detected, and its physical (shape and size, 
mainly) and chemical (through the optical constants) properties are measured. 
For each grain impacting onto the IS, the momentum of the incident grain is 
measured. With this system it is possible to measure abundance and dynamic 
properties of grains present in the comet coma. The ODS gives a first estimate 
of the speed of each crossing grain, and the speed of the grain is measured 
again, but with more accuracy, from the time-of-flight between ODS and IS. 
In this way for each detected grain, speed, time-of-flight, momentum and, 
therefore, mass are measured. The field of view of the ODS + IS system is 
about 40°. In addition, a network of microbalances (MBSs), with field of view 
of about 40° each, point in different directions to monitor flux of submicron 
and micron particles from 10“ 10 to 10 -4 g. The instrument is equipped with 
controlling electronics to drive the sensors and to transmit telecommunication 
to and from the spacecraft. The size of the instrument is about 23 x 25 x 30 cm. 

Radio science investigation 

The Radio Science Experiment RSE will use the radio signals transmitted 
from the onboard radio subsystem at the carrier frequencies X-band and 
Ka-band in the two-way radio mode (X-band uplink). The goal is to derive 
perturbing forces acting on the spacecraft by measuring the Doppler shift of 
the carrier signals caused by additional changes in relative velocity. Perturbing 
forces of interest may be: 

• the gravity acceleration of the cometary nucleus which would reveal mass, 
bulk density (together with a volume estimate from the camera) 

• the outgassing from the cometary nucleus which would reveal the com- 
bined gas and dust production rate (although the major contribution will 
come from the gas). 

The measurements require a highly stable two-way link where the stability 
of the X-band uplink is derived from the ground station’s hydrogen maser. The 
spacecraft receives the X-band uplink and transponds it back to Earth phase- 
coherently at two simultaneous downlink frequencies at X-band and Ka-band. 
Although the Ka-band downlink seems to be much noisier than the X-band 
downlink, it is possible to correct for the plasma noise in order to achieve a 
clear and four times stronger Doppler signal at Ka-band than at X-band. This 
allows us the detection of very small perturbations in the <10 pm/s range. X/X 
and X/Ka transponders are available on the market and have been flown on 
ESA missions (SMART-1). 

These measurements require that the spacecraft performs attitude changes 
using the reaction wheels only during the gravity observations. Any attitude 
changes by thruster firing would destroy the observations. 


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The mass determination shall be an iterative process starting during the 
approach phase. Each mass determination at a certain error level will allow 
manoeuvring the spacecraft closer to the nucleus. 

Comet Penetrometry Experiment (CPEx) 

Penetrometry is the use of a penetrating probe to measure mechanical 
properties of the target material. A comet nucleus sample return mission 
offers a rare opportunity to make contact with undisturbed cometary sub- 
surface material. Compared with previous missions, Triple F will provide 
measurements at multiple locations on the target comet. Penetrometry at the 
sampling locations will be useful for two main reasons: 

Firstly, it can support sample collection by means of dynamics measure- 
ments performed during sampling. Questions that may be addressed include: 
Where did the material actually come from? What was its undisturbed vol- 
ume? What is the strength and texture of the undisturbed material? To 
what extent was it disturbed, crushed and mixed during sampling? Secondly, 
penetrometry can generate unique ground-truth science. The penetration 
resistance of cometary material is sensitive to its origin and modification. 
Furthermore, measurements with depth can detect layering, e.g. the presence 
of low-cohesion material overlying a sintered layer at the water ice sublimation 
front. European heritage in this technique is strong, including sensors flown on 
Huygens and Philae. 

Although the primary requirement for sampling limits the possibilities 
for dedicated geotechnical measurements, a great deal may still be learned 
from a set of simple measurements. The solution we propose for Triple F 
is to incorporate small sensors to monitor dynamics during touchdown and 
sampling. Assuming a short-duration (few s) touchdown and a coring tube 
driven quickly (10 m/s initial speed) into the ground, we propose the following: 

• Displacement sensor to monitor the relative position of the sampling tube 
and spacecraft. This could be implemented by means of an optical bar- 
code technique. Stripes affixed to the core tube would be interrogated by a 
small light source and photodiode. Mass: ~5 g. Data: 1 bit at 50 kHz during 
sampling, i.e. 50 kbit/s. Heritage: HP 3 DACTIL tether length sensor. 

• Microphone-acoustic vibrations in the sampling mechanism should be 
sensitive to the texture of the sampled material and its variation with depth. 
A small piezoelectric or strain gauge sensor mounted on the sampling 
system at a location affording good acoustic coupling could fulfill this 
role. Mass: ~2 g. Data: 12 bits at 50 kHz during sampling, i.e. 600 kbit/s. 
Heritage: COTS aerospace components. 

• If possible, a single-axis accelerometer mounted near the upper end of 
the coring tube. This would measure the deceleration, and thus force, 
encountered by the sampling tube. Mass ~2 g. Data: 12 bits at 50 kHz 
during sampling, i.e. 600 kbit/s. Heritage: COTS aerospace components, 
Philae MUPUS ANC-M, Huygens SSP ACC-I, Huygens HASI ACC, HP 3 
DACTIL. 


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• Harness and electronics: 100 g for the displacement, microphone and 
accelerometer sensors. 

Permittivity probe 

A permittivity probe measuring the electric properties of the sample be- 
tween 0.1 Hz and 10 kHz is well suited to monitor changes in internal structure 
of the sample which are associated with sublimation processes of H 2 0. The 
electrical permittivity of ice is unique amongst the rock forming minerals in 
the low frequency range, as the dielectric constant is of the order of 100 under 
static conditions, thus, more than an order of magnitude higher than for rocky 
materials, and decreases with increasing frequency to a value of 3.1 for infinite 
frequency. 

The permittivity probe used to monitor the sample closely follows the 
design of a mutual impedance probe. Ringsector-like electrodes printed onto 
a Kapton foil are attached at different depths to the inner wall of the sample 
container. A current generator is connected to two selectable transmitter 
electrodes, and the generated voltage is sensed as a function of frequency 
by a number of receiver electrodes. By using different transmitter/receiver 
geometries across the sample an electrical “image” of the sample can be 
created. The total mass of the permittivity probe including electronics and 
harness is estimated to be less than 300 g and the average power consumption 
during operation is less than 1 W. 

Thermal probe on sampling device 

The knowledge of the original cometary temperature profile at the time of 
sampling measured as a function of depth would be of considerable value for 
the interpretation of the sample measurements. Because the titanium sample 
container has a several orders of magnitude higher thermal conductivity 
than the cometary soil, contact measurements cannot easily be used for that 
purpose. A possible solution is, therefore, based on the use of radiometric 
sensors (e.g. thermopiles as will be used for measuring the surface temperature 
at the Rosetta PHILAE landing site of comet 67P/Churyumov-Gerasimenko, 
[56]) which are mounted in different depths at the inner wall of the sample 
container. The short time constant of these sensors of about 200 ms makes the 
measurement of the temperature profile during the first seconds after insertion 
of the sampling device possible. This is before the temperature of the low 
conductivity cometary material can effectively adjust to the wall temperature. 
In addition, continuing the measurements after sampling would allow inferring 
thermal properties of the fresh sample. Furthermore, in case that a significant 
amount of heat is released inside the sample in the process of crystallization of 
originally amorphous ice, monitoring of the sample temperature as a function 
of depth could prove the existence of this process. Assuming 25% amorphous 
ice in the lower parts of the sample, the crystallization heat would increase the 
temperature by approximately 20 K, which is easily detectable by the sensors. 

For the reasons described above the implementation of several (around 
8-16) thermopile sensors into the sampling device could provide valuable 


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information about the initial temperature profile of the sample. The resources 
needed by this instrumentation are minor, basically an electronics comprising 
of multiplexer, amplifier and ADC (assuming that another DPU controls the 
data acquisition) with a mass of about 100 g (including harness) and a power 
consumption of less than 200 mW. 


4 Summary 

A Comet nucleus sample return mission is a unique means to provide valuable 
information about 

• The formation of the solar system from the protosolar nebula 

• The role comets played for the development of the hydrosphere and life 
on Earth 

• The way cometary activity works 

Recent results from the Deep Impact mission show that the cometary 
surface is of low strength and that cold, relatively unprocessed material can 
be found at shallow depths of a few tens of cms. Therefore, the return of a 
cooled sample from an active Jupiter-family comet is possible as a medium- 
sized mission with a total duration of approximately 10 years. 

The Triple Emission has not been chosen for the first cycle of Cosmic Vision. 
Nevertheless, we believe that it is a compelling mission concept that should be 
considered for future space exploration programs. 

Open Access This article is distributed under the terms of the Creative Commons Attribution 
Noncommercial License which permits any noncommercial use, distribution, and reproduction in 
any medium, provided the original author(s) and source are credited. 


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