Composition of the Volatile Material in Halley's Coma from In Situ Measurements

The investigation of the volatile material in the coma of comets is a key to understanding the origin of cometary material, the physical and chemical conditions in the early solar system, the process of comet formation, and the changes that comets have undergone during the last 4.6 billion years. So far, in situ investigations of the volatile constituents have been confined to a single comet, namely P/Halley in 1986. Although, the Giotto mission gave only a few hours of data from the coma, it has yielded a surprising amount of new data and has advanced cometary science by a large step. In the present article the most important results of the measurements of the volatile material of Halley's comet are summarized and an overview of the identified molecules is given. Furthermore, a list of identified radicals and unstable molecules is presented for the first time. At least one of the radicals, namely CH₂, seems to be present as such in the cometary ice. As an outlook to the future we present a list of open questions concerning cometary volatiles and a short preview on the next generation of mass spectrometers that are being built for the International Rosetta Mission to explore the coma of Comet Wirtanen. This revised version was published online in June 2006 with corrections to the Cover Date.     

Quantum Field Theoretic Description of Matter in the Universe

Quantum field theory at finite temperature and density can be used for describing the physics of relativistic plasmas. Such systems are frequently encountered in astrophysical situations, such as the early universe, supernova explosions, and the interior of neutron stars. After a brief introduction to thermal field theory the usefulness of this approach in astrophysics will be exemplified in three different cases. First the interaction of neutrinos within a supernova plasma will be discussed. Then the possible presence of quark matter in a neutron star core and finally the interaction of light with the Cosmic Microwave Background will be considered.     

A Statistical Glint/Radar Cross Section Target Model

We review the details of the glint (angular scintillation) problem in electromagnetic scattering. These results are employed to develop a statistical glint and radar cross section (RCS) target model featuring the correct glint probability density function, the correct time correlations of RCS and glint, and the correct cross correlation between RCS and glint. This model is suitable for simulation applications, and an implementation scheme for a glint/RCS signal generator is included.     

Large light X-ray optics: basic ideas and concepts

One of the main guidelines for future X-ray astronomy projects like, e.g., XEUS (ESA) and Generation-X (NASA) is to utilize grazing-incidence focusing optics with extremely large telescopes (several tens of m² at 1 keV), with a dramatic increase in collecting area of about two order of magnitude compared to the current X-ray telescopes. In order to avoid the problem of the source's confusion limit at low fluxes, the angular resolution required for these optics should be superb (a few arcsec at most). The enormous mirror dimensions together with the high imaging performances give rise to a number of manufacturing problems. It is basically impossible to realize so large mirrors from closed Wolter I shells which benefit from high mechanical stiffness. Instead the mirrors need to be formed as rectangular segments and a series of them will be assembled in a petal. Taking into account the realistic load capabilities of space launchers, to be able to put in orbit so large mirror modules the mass/geometric-area ratio of the optics should be very small. Finally, with a so large optics mass it would be very difficult to provide the electric power for an optics thermal active control, able to maintain the mirrors at the usual temperature of 20 °C. Therefore, very likely, the optics will instead operate in extreme thermal conditions, with the mirror temperature oscillating between −30 and −40 °C, that tends to exclude the epoxy replication approach (the mismatch between the CTE of the substrate and that of the resin would cause prohibitively large deformations of the mirror surface profiles). From these considerations light weight materials with high thermal–mechanical properties such as glass or ceramics become attractive to realize the mirrors of future Xray telescopes. In this paper, we will discuss a segments manufacturing method based on Borofloat^TM glass. A series of finite element analysis concerning different aspects of the production, testing and integration of the optics are also presented as well.     

The infra-red background generated by pregalactic stars and their remnants

We examine whether the IR background reported by Matsumoto (1983) could have been generated by stars or black holes in the pregalactic era, z=10100.     

Ultrastructural aspects of otoliths and sensory epithelia of fish inner ear exposed to hypergravity.

The present electron microscopical investigations were directed to the question, whether alterations in the gravitational force might induce structural changes in the morphology of otoliths or/and inner ear sensory epithelia of developing and adult swordtail fish (Xiphophorus helleri) that had been kept either under long-term moderate hypergravity (8 days; 3g) or under short-time extreme hypergravity (10 minutes up to 9g). The otoliths of adult and neonate swordtail fish were investigated by means of scanning electron microscopy (SEM). Macular epithelia of adult fish were examined both by SEM and transmission electron microscopy (TEM). The saccular otoliths (sagittae) of normally hatched adult fish revealed an enormous inter- (and even intra-; i.e. left vs. right) individual diversity in shape and size, whereas the otoliths of utricles (lapilli) and lagenae (asterisci) seemed to be more constant regarding morphological parameters. The structural diversity of juvenile otoliths was found to be less prominent as compared to the adults, differing from the latter regarding their peculiar crystalline morphology. Qualitative differences in the fine structure (SEM) of otoliths taken from adult and larval animals kept under 3g in comparison to 1g controls could not be observed. The SEM and TEM investigations of sensory epithelia also did not reveal any effects due to 3g stimulation. Even extreme hypergravity (more than 7g) for 10 minutes did not result in distinct pathological changes.     

Probability Density of the Maximum Likelihood Elevation Estimate of Radar Targets

The probability density of the maximum likelihood estimate of elevation angle of a radar target in the presence of multipath is calculated. For detectable signals that have low signal-to-noise ratios, the density is a mixture of a Gaussian density and a delta function at the horizon.