ESA Science & Technology - Publication Archive

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.

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.

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.

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.

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.

Proceedings of the 3rd Integral Workshop, Taormina 1998

Saturn's large moon Titan is unique among planetary satellites in that it possesses a thick atmosphere and a haze layer that is opaque to visible light. This haze is believed to be composed of organic compounds produced by the photolysis of methane. It has been suggested that the photochemical products of methane photolysis, primarily ethane, would "rain out" over time and may produce reservoirs of liquid hydrocarbons on Titan's surface. Such material would appear very dark, with an albedo =<0.02 (Khare et al. 1990, Bull. Am. Astron. Soc.22, 1033). Such low-albedo regions have not been previously detected on Titan's surface. Here we report observations of Titan at a resolution of 0.04 arcsec (0.02 arcsec/pixel) using the technique of speckle imaging from the 10-m Keck I Telescope. By observing Titan at specific infrared wavelengths which are windows through its atmosphere, we have made both an albedo map of Titan's surface at 1.6 and 2.1 µm and an estimate of Titan's haze optical depth at these wavelengths. We clearly distinguish low-albedo features (reflectance <0.05) on Titan's surface.

The structure of the dissipation region in collisionless magnetic reconnection is investigated by means of kinetic particle-in-cell simulations and analytical theory. Analyses of simulations of reconnecting current sheets without guide magnetic field, which keep all parameters fixed with the exception of the electron mass, exhibit very similar large scale evolutions and time scales. A detailed comparison of two runs with different electron masses reveals very similar large scale parameters, such as ion flow velocities and magnetic field structures. The electron-scale phenomena in the reconnection region proper, however, appear to be quite different. The scale lengths of these processes are best organized by the trapping length of bouncing electrons in a field reversal region. The dissipation is explained by the electric field generated by nongyrotropic electron pressure tensor effects. In the reconnection region, the relevant electron pressure tensor components exhibit gradients which are independent of the electron mass. The similarities of the gradients as well as the behavior of the electron flow velocity can be derived from the electron trapping scale and the electron mass independence of the reconnection electric field. A further model which includes a significant guide magnetic field exhibits almost identical behavior. The explanation of this result lies in a Hall-type electric field which locally eliminates the magnetizing effect on the electrons of the guide magnetic field. The resulting electron dynamics is nearly identical to the one found in the model without guide magnetic field. --- Remainder of abstract truncated ---

This article is based on a talk about the history of our universe given by Professor Gustav Andreas Tammann to the Pro ISSI.

Observations of outflow velocities in coronal holes (regions of open coronal magnetic field) have recently been obtained with the Solar and Heliospheric Observatory (SOHO) spacecraft. Velocity maps of Ne⁷⁺ from its bright resonance line at 770 angstroms, formed at the base of the corona, show a relationship between outflow velocity and chromospheric magnetic network structure, suggesting that the solar wind is rooted at its base to this structure, emanating from localized regions along boundaries and boundary intersections of magnetic network cells. This apparent relation to the chromospheric magnetic network and the relatively large outflow velocity signatures will improve understanding of the complex structure and dynamics at the base of the corona and the source region of the solar wind.

A chorus generation mechanism is discussed, which is based on interrelation of ELF/VLF noise-like and discrete emissions under the cyclotron wave-particle interactions. A natural ELF/VLF noise radiation is excited by the cyclotron instability mechanism in ducts with enhanced cold plasma density or at the plasmapause. This process is accompanied by a step-like deformation of the energetic electron distribution function in the velocity space, which is situated at the boundary between resonant and nonresonant particles. The step leads to the strong phase correlation of interacting particles and waves and to a new backward wave oscillator (BWO) regime of wave generation, when an absolute cyclotron instability arises at the central cross section of the geomagnetic trap, in the form of a succession of discrete signals with growing frequency inside each element. The dynamical spectrum of a separate element is formed similar to triggered ELF/VLF emission, when the strong wavelet starts from the equatorial plane. The comparison is given of the model developed using some satellite and ground-based data. In particular, the appearance of separate groups of chorus signals with a duration 2-10 s can be connected with the preliminary stage of the step formation. BWO regime gives a succession period smaller than the bounce period of energetic electrons between the magnetic mirrors and can explain the observed intervals between chorus elements.

Presented at International Conference On Environmental Systems, July 1998, Danvers, MA, USA, Session: Satellite, Payload, Instrument and Launch Vehicle Thermal Control. Abstract: A study of the first in-orbit temperatures of Huygens shows that the probe will very likely survive thermally all vacuum cruise and coast phases. Calculated heat fluxes, mass flows of Titan's atmosphere into and out of the probe and temperatures give confidence also for the mission phase proper in 2004, i.e., the 2.5 h descent into Titan's -200 °C atmosphere. Basotect foam bags insulate the probe from this atmosphere. These bags and their fixation had to be drastically modified between Titan test on STPM (May 95) and on FM (June 96). The mission phases, thermal requirements, thermal design, tests with the probe, special tests for the foam bag development and their modification are presented.

The spectral response of a 100 x 100 micron² tantalum based superconducting tunnel junction to 5.9 keV x-ray photons from a ⁵⁵Fe source has been studied. In full illumination the energy resolution for the Mn K_alpha line complex is 56 eV, dominated by spatial nonuniformity in the response of the detector. When illuminating selectively a 5-10 micron diam spot in the center of the detector, the energy resolution improves to 22 eV, corresponding to 15.7 eV for the individual Mn K_{alpha 1} and Mn K_{alpha 2} lines. This exceeds the predicted theoretical energy resolution of 7.3 eV for this type of device by only a factor of ~ 2.

Photon counting experiments at wavelengths ranging from near infrared to x-ray with niobium based superconducting tunnel junctions with aluminium trapping layers are presented. Single photons can be detected up to a wavelength of 1 micron. The response in the ultraviolet to near-infrared region is characterized by a good energy linearity (< 2.5%), a capability to handle event rates up to ~ 3 kHz, and moderate energy resolving power (E/DeltaE = ~7 for E = 4 eV). The x-ray response at 6 keV is characterized by anomalously high signals compared to the low energy response, a severe energy nonlinearity and a relatively poor energy resolution of ~ 140 eV, full width at half maximum.

We report Hubble Space Telescope NICMOS observations of the central region of NGC 5128 at 2.2 mm and in Paa. The continuum images show extended emission typical of an elliptical galaxy and a strong unresolved central source we identify as the nucleus of the galaxy. Its position is consistent with ground-based IR and radio data, and with the peak of reddening found with the first wide field planetary camera. In Paa , we detect a prominent elongated structure, centered on the nucleus, extended by ~2" at a position angle of ~33°, and with a major to minor axis ratio of ~2. We interpret this as an inclined, ~40 pc diameter, thin nuclear disk of ionized gas rather than a jet-gas cloud interaction. We do see several weaker Paa features, some of which may be circumnuclear gas clouds shocked by the X-ray/radio jet. The disk is one of the smallest ever observed at the nucleus of an active galactic nucleus (AGN). It is not perpendicular to the jet but is consistent with being oriented along the major axis of the bulge. If it represents the warped outer portion of an accretion disk around a black hole, we conclude that even on the scale of a few parsecs, the disk is dominated by the galaxy gravitational potential and is not directly related to the symmetry axis of the AGN.