Rosetta lander in situ characterization of a comet nucleus

Acta Astronautica Vol. 45, Nos. 4-9, pp. 38’2-395. 1999 0 1999 Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0094-5765/99 $ - see front matter PII:SOO94-5765(99)00158-7

Rosetta Lander In Situ Characterization of a Comet Nucleus K. Wittmann’,

B. Feuerbacher’,

S. Ulamec’, H. Rosenbauer*,

J.P. Bibring3, D. Moura4, R. Mugnuolo’, S. diPippo5, K. Szego6, and G. Haerende17

‘DLR, Inst. f. Space Simulation, D-5 1147 Koln, Germany *Max Planck Inst. f. Aeronomy, D-37 189 Katlenburg-Lindau, 31AS, F-91405 Orsey, France, 4CNES, F-3 1055 Toulouse, France, ‘ASI, I-75 100 Matera, Italy, %FKI, H- 1525 Budapest, Hungary, 7Max Planck Inst.f.Extraterr, Physics, D-85740 Garching, Germany.

Abstract:

Rosetta is one of the cornerstone missions within the science program ,,Horizon 2000“ of the European Space Agency (ESA). Its objective is the characterization of comet Wirtanen, which will be reached after 9 years of cruise in the year 2012. As comets are believed to be the most primitive bodies in our planetary system, having preserved material from the early stages of its formation, the Rosetta mission shall result in a better understanding of the formation of the solar system. The Rosetta Lander, part of the Rosetta payload, is contributed to the mission by an international consortium of research institutes. It will perform in situ measurements on the surface of the comet nucleus. The science objectives of the Rosetta Lander can be comprised by: l

l

l

l

determination of the composition of cometary near surface matter: bulk elemental abundances, isotopes, minerals, ices, carbonaceous compounds, organics voladles in dependance on time and insolation. measurement of physical parameters - mechanical strength, density, sound speed, electrical permittivity, heat conductivity and temperature. investigation of topology, surface structure including colour and albedo, near surface structure (strategraphy) and internal structure. the comets interaction with solar wind.

The payload of the Rosetta Lander consists of nine instruments with a total mass of about 2Okg.

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The Rosetta Lander system with an overall mass of about 85kg consists of a light weight structure of carbonfibre material, solar cells to provide power, a thermal control system securing operation without the use of radiactive heaters, a telecommunications system. using the orbiter as relay to Earth and a central computer, serving all subsystems and the payload. The lander will be ejected from the main spacecraft after selection of an adequate landing area from an orbit, about l-5km above the surface of the nucleus. The actual descent strategy is highly depending on the (yet unknown) physical parameters of P/Wirtanen (like mass, shape and rotation period). Thus, a flexiblelandingconcept, which allows the setting of the landing parameters interactively during the mission is required. Landing will take place on a tripod that includes a device that dissipates most of the impact energy and allows rotation of the main structure. At impact, a hold-down thruster and the shot of an anchoring harpoon will avoid rebound from the surface. 0 1999 Published by Elsevier Science Ltd. All rights reserved.

Introduction The Rosetta Lander (figure 1) will be built in a cooperative effort major with contributions from the following European research institutions: DLR, Max Planck Society (D), CNES (F), IAS (F), AS1 (I), KFKI (H), IUL (GB), STIL (IRL): FM1 (SF). A novel approach is applied to manage the Rosetta Lander project by an international decentralized project office, which is supervised by a Steering Committee composed of representatives of the consortium partners. A central budget authority has not been implemented. Thus, each partner is responsible to ensure the necessary funds to provide contributions to the Rosetta Lander system according to a commonly agreed schedule.

Figure I: Rosetta Lander

The launch of the ESA cornerstone mission Rosetta’92 with an Ariane V is foreseen in January 2003, after one swingby at Mars and two at the Earth as well as two asteroid flybys (Rodari and Mimistrobell), the target comet, P/Wirtanen shall be reached in 2011. After several months of remote investigation of the comet, from an orbit with probably between 1 to 50 km altitude above the surface of the comet, the Lander will be delivered, in 2012. Figure 2 shows the trajectory of Rosetta. The paper the overviews scientific objectives and the payload of the Rosetta Lander as well as its system concept.

Figure 2: Rosetta trajectory

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Scientific

Background

Comets are among the most inlcrcsting bodies in our solar s\‘stem. Thq~ are believed to be the most primiti\.c hndics in our planetary system, ha\.ing prcser\,cd material from the early stages of its formation3.4. They consist of material that has been preserved in the Oort-cloud. far away from the sun since the formation of the the solar system. Due to gral-itational effects some of them get injected into orbits. that bring them‘closer to the sun (and the Earth). Once they are in such an orbit. their surfaces may get processed. because of the irradiation of the sun. Our present knowledge of comets is \.ec limited. The closest in\.estigation of a nucleus was achieved during the Giotto flyby at comet P/Halley in 1986.

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Many parameters of Rosetta’s target comet, P/Wirtanen such as shape, mass, size, rotation period and axis, or surface properties, will not be known exactly before the spacecraft has started its investigation. Too late to change the design of the lander. This means, the lander has to be designed extremely flexible, able to cope with the wide range of possible comet properties. The range is indicated in tables 1 and 2. radius (km) density (g/cm’) rotation period (h) surface acceleration (m/s’)

minimal 0.5 0.2

typical 076 0.7

mZFiiFJ 1.5 175

3

10

240

3 lo-5

7.1o-4

Table 1: Assumedproperties nucleus

minimal typical temperature at 3 AU (K’, temDerature at 1 A’U(K) cond. heat (Wm.’ K”) tensile strength (Pa) porosity mantle thickness (m) i

110 120 IO”

lo-’

10’

10’

80% lo”

40% 0.1

:F

of

--

--max. 200 ?m --270(a) 0.1 10’ --- 10% 10 --

Table 2: Assumed properties oj surface material (i: inactive, a: active areas) Figure 3: The nucleus of comet PLHalley, as seen by Giotto Spacecraft (HMC) One of the results of this misxic)n \+:I‘; lhe fact that cometar). acti\,it\ appc’;~rh 10 hc restricted to local regions \+i~h ch;lt;lclc%htic scales of some hundred IN~ICI->~.

The orbital parameters of PIWirtanen are well determined. Table 3 shows some of the relevant data6. perihelion aphelion inclination orbital Deriod

1.08 AU 5.15 AU I l”7’ 5.5 a

Table 3: Orbital parameters PYWirtanen

of comet

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Scientific

Objectives

It is the general task of the scientific investigations carried by the Rosetta Lander to get a first in-situ analysis of primitive material from the early solar system, to study the structure of a cometary nucleus, reflecting growth processes in the early solar system and to provide ground truth for Rosetta orbiter instruments”‘. The scientific objectives of the Rosetta Lander can be comprised by: The determination of the composition of cometary surface matter: bulk elemental abundances, isotopes, minerals, ices, organics carbonaceous compounds, volatiles - in dependence on time and insolation. The investigation of the structure, physical, chemical and mineralogical properties of the cometary surface: topography, texture, roughness, mechanical, electrical optical and thermal properties. The investigation of the local depth structure (stratigraphy), and the global internal structure.

Scientific Payload

payload includes an imaging system (with sensors for panorama imaging, downlooking and also microscopic devices), evolved gas analysers to investigate the volatile components of the comet surface material (combined gas chromatograph and mass alpha-proton-x-ray spectrometer), an fluorescence spectrometer, a magnetometer, a simple plasma monitor, instrumentation to investigate the physical properties of the surface material (dielectric-, acoustic-, mechanicaland seismic-, thermal parameters) and a radiowave experiment to investigate the global internal structure of the nucleus.

Lander Design The design of the Rosetta Lander is based on the old RoLand concept, a smaller lander, which has been originally proposed to ESA for the Rosetta Mission. The overall mass, including those parts, remaining after separation on the Orbiter shall stay within 85 kg.

The payload, preliminarily selected for Rosetta Lander is listed in table 4. This Instrument APX-spectrometer (evolved COSAC MODULUS gas anal.) CIVA (imaging system) ROLIS ROM[Ap (magnetometer/ plasma monitor) SESAME (seismic sensor, acoustic- prop. permittivity) MUPUS (temperature, physical properties) CONSERT (radiowave experiment) x

Principal investigator R. Rieder H. Rosenbauer C. Pillinger J.P. Bibring S. Mottola U. Auster, I Apathy I (merge of 2 instru1 mentproposals) I D. Miihlmann, H. Laakso, I. Apathy (merge of 3 instr. proposals) T. Spohn W. Kofman

Table 4: Selected instruments

Responsible __

-_

(PI-)lnstitute

__ _ -

.

.-.

Max Planck 1. t. Chemistry (D) Max Planck I. f. Aeronomy (D) Open University (UK) IAS (F) DLR (D) IMax Planck Inst. f. Extraterr. I Physics (D), KFKI (H)

.

1 _ I DLR (D), FM1 (SF), KFKI (H) I

Mass (kg) 1,U 4;3 3.6 3,4 0.45 1.5

University of Mtinster (D)

2.2

CEPHAG (F)

1.5 17.95

of the

Rosetta Lander

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Structure The Rosetta Lander structure \vill consist of a carbonfiber sandwich ground plate. an experiment platform and a polygonal sandwich construction. the hood. co\ering the \varm area and carrying the solar will be generator. All components manufactured in high-modulus carbonfiber material. h4ost instrument electronics and subsystems will be sheltered by the hood. One part of the ground plate. however. will not be underneath the hood, forming a “balcony” and providing space for external instruments and subsystems. like the drilling and sampling system or .unfoldable sensors that have to get into direct contact with the comet surface (e.g. the APX-spectrometer).

throughout payload environment thermally underneath dissipation compartment

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the mission. All subsystems and elements warm requiring will be mounted on the insulated experiment platform the hood. An average power of 2 W inside the warm as well as special absorber

units with a very high ol/& ratio will prevent the lander from freezing, assuming periods which are not considerably than the days.

night longer

When approaching the sun, in a distance closer than 2 AU to the sun, overheating becomes a problem. Depending on the actual surface properties of the nucleus, operations until 1 AU may be possible, but ca:ulot be guaranteed.

Power System

dehonstration

nsetta Lander model

Thermal Control System One of the major challenges of the Rosetta Lander mission is the thermal control of the lander. which has to operate on a comet nucleus with unknon-n relation period. in distances bctwccn 7 and 1 ;I( i fi-om the sm. Special effort has to bc talicn for thcrnial insulation to keep tlic 1cniperalurc inside the lander in a range bct\~ccn -55”~’ and ~.70”(’

Since the use of RTGs (radio thermal generators) is unacceptable, power will be generated using solar arrays, whic:h will cover the hood of the lander (see figure 4). In addition a 1 kWh primary battery will provide power during the first days of Lander operations. At 3 AU only about 10% of the solar input is av.ailable, compared to the insolation near earth orbit. Thus, power is a very precious resource. About 4 W will be available on a .daily average for the scientific instruments at maximum solar distance. The experiments will operate in sequence and (including the possibility for some exceptions) only during daytime. In order to cover peak demands in power as well as to allow limited night time with a operation, secondary batteries, capacity of 100 Wh will be used.

Landing Gear / Anchoring Tile Rosetta Lander will gear which is formed by a is connected to the mechanism that allows

System

rest on a landing tripod. The tripod structure by a rotation of the

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complete lander above its legs (similar to an office chair) on one hand, but also will dissipate most of the kinetic impact energy during landing at the comet surface. This mechanism will work on the basis of a motor that acts as a generator and converts the impact energy into electric energy. A breadboard model of such a device has been developed and tested . At touchdown, a vertical cold gas thruster will be fired to avoid rebound, and, additionally, an anchoring harpoon will be shot into ground. The three systems: (a) damping in the landing gear, (b) retrothruster and (c) harpoon support the landing in a redundant way. The anchoring harpoon device will consist of a harpoon projectile, a tiring device and a cable retensioning mechanism. After tiring, a motor tensions the cable (with a coil) and the lander will be firmly pulled to the surface. The anchoring is especially important in order to allow drilling.

The present drill is based on a two degree of freedom actuation with a maximum (linear) force not exceeding 10 N.

Descent and Landing Scenario After the Rosetta spacecraft is injected into an orbit around P/Wirtanen, there will start an extensive mapping phase of the nucleus, after which the landing site for the lander can be chosen. In a distance of about 3 AU from the sun the Lander will be separated from the orbiter with an eject mechanismn that allows to adjust the separation Av in a range between 0.05 and 0.5 m/s and descend to the comet’s surface, stabilised with an internal flywheel and supported by a cold gas thruster.

Figure7: Landers descent scenacio Figure 6: Anchoring system Drill Mounted on the balcony of the Lander there will be a drilling and balling system, which shall serve the evolved gas analysers and the microscopic camera (part of CIVA) and bring material from at least 20 cm depth to the sensors. The major design difficulties are the uncertain surface properties on one hand and the potentially very low anchoring force of the complete lander on the other hand.

By a careful selection of orbiter attitude, orbit and separation Av , a scenario can be found, where the rotation of the comet can be compensated and the Lander reaches the surface perpendicularely (with negligable lateral speed).

Operations In a first experiment sequence of about 5 days all instruments will be operated at least once to perfomr a basic set of in-situ measurements. Power for this first sequence will be available from primary batteries and

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the solar generator. During the subsequent long term measurement programme the scientific experiments on the Lander will generally operate in sequence. This procedure is possible due to the longevity of the Lander and respects the limited power availability. The Lander system and payload operations control will be performed at CNES and DLR, with direct connection via ESOC. The radiolink will be realized with the orbiter, functioning as a relay station. Data rates between the Lander and Rosetta will be 16 kbit/s. This allows considerable adjustments of the experiment performance, as well as general operability like e.g. data bottleneck for rotation. The transmission are the visibility of the orbiter from the landing site and the data rate from Rosetta to earth. Operations will also include regular health checks of the system, subsystems and instruments during cruise, as well as the preparations for separation (like adjustment of the eject mechanism or spin-up of the flywheel).

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References [l] Rosetta, Comet hTucleus Sample Return, ESA-SP-1125,199l [2] Schwehm G., Rosetta The comet rendezvous mission: in ESA-SP-1179, p.2830,1995 [3] Whipple F-L., The con7etary I7ucZeu.s: current concepts; Astron. Astro-phys., 187, p.852-8, 1987 [4] Greenberg J.M., What are comets made of? A n7odel based on interstellar dust, in Comets (WilkeningL.L., ed.), University of Arizona Press, 1982 [5] Keller H.U., et al. Contet P/lYal-ley’s )7ucleus al7d actkip. Astron. Astrophys. 187, p.807-23, 1987 [6] Jorda L. and Rickmann H., Comet P/Wirranen, sun7mary qf ohscn~atiomsl duta. Planet. Space Sci., Vo143. p. 575-9, 1995 [7] Ulamec S., et al., RoLand: A long term Lander for the Rosetta Mission; Space Technol., Vo1.17, p.59-64, 1997