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| Nucleus properties of Comet 67P/Churyumov-Gerasimenko estimated from non-gravitational force modeling |
| By considering model comet nuclei with a wide range of sizes, prolate ellipsoidal shapes, spin axis orientations, and surface activity patterns, constraints have been placed on the nucleus properties of the primary Rosetta target, Comet 67P/Churyumov-Gerasimenko. This is done by requiring that the model bodies simultaneously reproduce the empirical nucleus rotational lightcurve, the water production rate as function of time, and non-gravitational changes (per apparition) of the orbital period (Delta P), longitude of perihelion (Delta omega tilde), and longitude of the ascending node (Delta Omega). Two different thermophysical models are used in order to calculate the water production rate and non-gravitational force vector due to nucleus outgassing of the model objects. By requiring that the nominal water production rate measurements are reproduced as well as possible, we find that the semi--major axis of the nucleus is close to 2.5 km, the nucleus axis ratio is approximately 1.4, while the spin axis argument is either 60+/-15 or 240+/-15 degrees. The spin axis obliquity can only be preliminary constrained, indicating retrograde rotation for the first argument value, and prograde rotation for the second suggested spin axis argument. A nucleus bulk density in the range 100-370 kg/m^3 is found for the nominal Delta P, while an upper limit of 500 kg/m^3 can be placed if the uncertainty in Delta P is considered. Both considered thermophysical models yield the same spin axis, size, shape, and density estimates. Alternatively, if calculated water production rates within an envelope around the measured data are considered, it is no longer possible to constrain the size, shape, and spin axis orientation of the nucleus, but an upper limit on the nucleus bulk density of 600 kg/m^3 is suggested. |
| Publication date: 01 Aug 2005 |
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| Asteroid target selection for the new Rosetta mission baseline: 21 Lutetia and 2867 Steins |
| The new Rosetta mission baseline to the comet 67P/Churyumov-Gerasimenko includes two asteroid fly-bys. To help in target selection we studied all the candidates of all the possible scenarios. Observations have been carried out at ESO-NTT (La Silla, Chile), TNG (Canaries), and NASA-IRTF (Hawaii) telescopes, in order to determine the taxonomy of all the candidates. The asteroid targets were chosen after the spacecraft interplanetary orbit insertion manoeuvre, when the available total amount of $\Delta V$ was known. On the basis of our analysis and the available of $\Delta V$, we recommended to the ESA Science Working Group the asteroids 21 Lutetia and 2867 Steins as targets for the Rosetta mission. The nature of Lutetia is still controversial. Lutetia's spectral properties may be consistent with a composition similar to carbonaceous chondrite meteorites. The spectral properties of Steins suggest a more extensive thermal history. Steins may have a composition similar to relatively rare enstatite chondrite/achondrite meteorites. |
| Publication date: 01 Jan 2005 |
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| Water production rate of comet 67P/Churyumov-Gerasimenko |
| The Solar Wind Anisotropies (SWAN) instrument is a scanning Lyman-alpha imager on board the Solar and Heliospheric Observatory (SOHO) spacecraft. Since becoming operational in January 1996 SWAN has been producing full sky Lyman-alpha maps which are primarily used to study the interaction between solar wind and the interplanetary neutral hydrogen. In addition to that SWAN images can be used to study the hydrogen coma of comets down to about a visual magnitude of 12. After the retargeting decision of the Rosetta mission the SWAN archive was checked for possible occurrences of 67P/Churyumov-Gerasimenko. Five values were obtained for the 1996 apparition but none for the 2002 apparition because of degraded instrument sensitivity and larger observing distance. The observations suggest a perihelion water production rate of about 8x10^27 per second and possible post perihelion increase of activity. |
| Publication date: 01 Oct 2004 |
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| ESA SP-1276: ESA's Report to the 35th COSPAR Meeting |
Scientific editor: A. Gimenez
Editor: A. Wilson
The report for the 35th COSPAR Meeting covers, as in previous issues, the missions
of the Scientific Programme of ESA in the areas of astronomy, Solar System
exploration and fundamental physics. This year's COSPAR Meeting occurs only
weeks after the Saturn-orbit insertion of the Cassini spacecraft - carrying Europe's
Huygens probe to explore the atmosphere of Titan - and at the same time as the
launch of the second satellite of the Double Star project. |
| Publication date: 15 Jun 2004 |
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| Rosetta - A Race Against Time |
| The International Rosetta Mission was approved as a Cornerstone mission within ESA's Horizon 2000
Science Programme in November 1993. Even at this early stage, it was envisaged that the ambitious mission would be scheduled for launch in the 2003 timeframe and a number of comet rendezvous opportunities were identified. Although the original target, Comet Schwassman Wachmann 3, has since been superseded by another periodic intruder into the inner Solar System, Comet Wirtanen, there has been little shift in the original launch schedule. |
| Publication date: 01 Nov 2002 |
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| Rosetta Rises to the Challenge |
| Few enterprises are more difficult or hazardous than space travel. Yet, even when compared with the achievements of its illustrious predecessors, ESA's Rosetta mission to orbit Comet Wirtanen and deploy a lander on its pristine surface must be regarded as one of the most challenging ventures ever undertaken in more than four decades of space exploration. |
| Publication date: 01 Nov 2002 |
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| Why 'Rosetta'? |
| On the night of 12-13 January 2003, one of the most powerful rockets in the world will blast off from Kourou spaceport in French Guiana. On top of the giant Ariane-4, cocooned inside a protective fairing, will be the Rosetta comet chaser, the most ambitious scientific spacecraft ever built in Europe. |
| Publication date: 01 Nov 2002 |
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| ESA SP-1259: ESA's Report to the 34th COSPAR Meeting |
Scientific editor: B. Foing
Editor: A. Wilson
The report to the 34th COSPAR Meeting covers the missions of the Scientific Programme of ESA in the areas of astronomy, Solar System exploration and fundamental physics. |
| Publication date: 01 Oct 2002 |
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| Rosetta's Comet Chaser |
| In November 1993, the International Rosetta Mission was approved as a Cornerstone Mission within ESA's Horizons 2000 science programme. Since then, scientists and engineers from all over Europe and the United States have been combining their talents to build an orbiter and a lander for this unique expedition to unravel the secrets of a mysterious mini ice world - a comet. |
| Publication date: 15 Sep 2001 |
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| Bulletin of the American Astronomical Society, Vol 31, No. 4 (DPS) |
| Several instruments have been described at the annual meetings of the
EGS (European Geophysica Society) and the DPS (Division for Planetary Sciences)
The numerous abstracts which were published can be found in:
Bulletin of the American Astronomical Society, Vol 31, No. 4 (DPS) |
| Publication date: 01 Oct 1999 |
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| Coma Composition and Evolution of Rosetta Target Comet Wirtanen |
| Comet 46P/Wirtanen is the target comet of the ROSETTA mission. Here, we give an overview of the information currently available on this comet from remote-sensing observations. Main emphasis is put on the description of the coma in terms of morphology, composition and evolution. We also summarize the current knowledge of the basic properties of the nucleus, in particular its size and rotational properties. |
| Publication date: 01 Oct 1999 |
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| Rosetta goes to Comet Wirtanen |
| The International Rosetta Mission, approved by the Science Programme Committee of the European Space Agency as the Planetary Cornerstone Mission in ESA's long-term programme Horizon 2000, will rendezvous in 2011 with Comet 46P/Wirtanen close to its aphelion and will study the nucleus and the evolution of the coma for almost two years until it reaches perihelion. |
| Publication date: 01 Oct 1999 |
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| Activity and nucleus properties of 46P/Wirtanen |
| Astronomical observations of size and of related outgassing rates seem not to be compatible for the nucleus of comet 46 P/Wirtanen, the target comet of the ROSETTA mission. This possible disagreement has caused speculations about peculiar properties of this comet nucleus. It is shown by model calculations which also takes into account vertical heat fluxes into the nucleus that there is a possibility to combine the results of astronomical observations within a model of a freely sublimating ice surface of this comet with an outgassing area of about half the dayside surface. The resulting half-size parameter (i.e. the radius of an equivalent sphere) can be shown to be of about R H (725±230) m, and the nucleus is shown to have an active area of about half of the dayside surface, i.e. of about 25% of the total surface. |
| Publication date: 09 Aug 1999 |
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| Numerical simulation of the dust flux on a spacecraft in orbit around an aspherical cometary nucleus - I |
| This study is the first investigation of the dust collection by a spacecraft orbiting a cometary nucleus, which is based on a physically consistent ab-initio model of the dust distribution in the vicinity of an aspherical comet nucleus. The homogeneous bean-shaped nucleus of Crifo & Rodionov (1997a) is used, with updated parameter values adapted to comet 46P/Wirtanen, target of the Rosetta mission, but the conclusions of the study have a general significance. |
| Publication date: 01 Jul 1999 |
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| All comets are born equal |
| The infrared emission of various comets can be matched within the framework that all comets are made of aggregated interstellar dust. This is demonstrated by comparing results on Halley (a periodic comet), Borrelly (a Jupiter family short period comet), Hale-Bopp (a long period comet), and extra-solar comets in the beta-Pictoris disk. Attempts have been made to generalize the chemical composition of comet nuclei based on the observation of cometary dust and volatiles and the interstellar dust model. Finally, we deduce some of the expected dust and surface properties of comet Wirtanen from the interstellar dust model as applied to other comets. |
| Publication date: 01 Jun 1999 |
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| Constraints on comet 46P/Wirtanen dust parameters provided by in-situ and ground-based observations |
| A dust environment working group was encouraged by ESA to provide coma dust environment models useful to plan the ROSETTA operations around the nucleus of short period comet 46P/Wirtanen. Among the many parameters describing the dust released from the nucleus surface, special care was devoted to the dust size distribution. Its present uncertainty makes all environment models sensitive, mainly, to which actual size distribution is adopted. In fact, it must be stressed that no other cometary dust parameter can be derived, such as dust loss rate or dust to gas ratio, if the size distribution remains undetermined. This paper will focus, therefore, on the available information on cometary dust size distributions, starting from the in situ measurement cornerstone provided by the GIOTTO-DIDSY results. Available ground-based observations are then reviewed, in order to disentangle the real sensitivity of them to this quantity; the size distribution is always embedded together with other dust parameters, and its influence on the published results is often forgotten. |
| Publication date: 01 Jun 1999 |
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| Lyman-alpha observations of comet 46P/Wirtanen with SWAN on SOHO: H2O production rate near 1997 perihelion |
| The SWAN instrument on board SOHO is a Lyman-alpha photometer able to map the sky intensity with a resolution of 1°, primarily devoted to the study of the large scale distribution of solar wind from its imprints on the interplanetary sky background. In addition SWAN was extensively used to map the Lyman-alpha emission of several comets since launch in December 1995. Here we report observations of Comet 46 P/Wirtanen near perihelion. From the recorded Lyman-alpha intensity the H2O production rate was derived for 45 observations from 21 December 1996-17 May 1997, with a peak of 1.6±0.4×1028 mol/s just before perihelion. This should help to constrain the physical models of 46 P/Wirtanen for Rosetta mission planning purposes. |
| Publication date: 01 Jun 1999 |
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| Modelling the circumstellar coma of comets: objectives, methods and recent results |
| We investigate whether the modelling of the immediate vicinity of an active nucleus -currently unobservable- can, as the modelling of the outer, observable coma, be based on unrealistic simple assumptions such as those of nucleus and dust grains sphericity. We point out the inconsistency of models based on such assumptions, which, to manage compatibility with the observations, have to introduce additional assumptions that conflict with the previous ones, such as the existence of active areas of the nucleus. We argue that, while the outer coma models being phenomenological in nature, can perhaps tolerate such inconsistencies, the circumnuclear coma models must be predictive, having to obviate the lack of observational data, and therefore must exclude implausible and ad hoc assumptions, and advocate only well-understood physical processes and duly validated modelling methods. We describe the first steps of development of a predictive circumnuclear coma model, and present a set of results obtained with parameters fitted to comet 46P/Wirtanen, the target of the Rosetta mission, but of a quite general significance. |
| Publication date: 01 Jun 1999 |
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| Models of P/Wirtanen nucleus: active regions versus non-active regions |
| In this paper we present the results of numerical models of cometary nucleus evolution, developed in order to understand which are the processes leading to the formation of active and non-active regions on the cometary surface. The used numerical code solves the equations of heat transport and gas diffusion within a porous nucleus composed of different ices--such as water (the dominant constituent), CO2, CO- and of dust grains embedded in the ice matrix. By varying the set of physical parameters describing the initial properties of comet P/Wirtanen, the different behaviour of the icy and dusty areas can be followed. |
| Publication date: 01 Jun 1999 |
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| Numerically improved thermochemical evolution models of comet nuclei |
| An improved unidimensional model of the heat transport and gas diffusion within a porous cometary nucleus is presented, in which the time-dependent gas diffusion equation is coupled with the heat diffusion equation to describe the energy transport due to sublimation and recondensation of volatiles, but is solved independently using a different discrete time step. Also, the erosion of interfaces within the nucleus, due to the sublimation of ices and the removal of dust, is now treated by means of a continuous adaptation of the discrete grid to the interfaces positions, removing numerical stability problems associated with the variation of structure and composition of the discrete layers. The results of this model are then compared with those of another unidimensional model which does not make use of the above-mentioned numerical methods, both computed for the same set of physical parameters describing comet P/Wirtanen, and the effects of the different modelling assumptions on the results are discussed. A new bidimensional model of the heat transport within a porous comet nucleus is presented, and its results are compared with those obtained from the above-mentioned unidimensional model (modified to include the same physics of the bidimensional model). The ability of bidimensional models to better describe the effects of variations in the local physical conditions on the comet activity is then discussed. |
| Publication date: 01 Jun 1999 |
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