Crushing strength of porous ice-mineral bodies-relevance for comets

In laboratory investigations with fluffy, highly porous ice and ice-dust bodies a new mechanism could be identified which strengthens the porous bodies. The process takes place under isothermal conditions and leads to the formation of ice bridges between the ice (dust) particles. It is driven solely by the dependence of the partial pressure of water vapour on the curvature of the particles. This mechanism is generally called “sintering”. A theory for the crushing strength of a porous ice and ice-dust agglomerate is developed which describes the experimental results on isothermal changes is strength due to sintering quantitatively well. The relevance for the evolution of comets is discussed.     

Asteroid surface materials: Mineralogical characterizations from reflectance spectra

The interpretation of diagnostic parameters in the spectral reflectance data for asteroids provides a means of characterizing the mineralogy and petrology of asteroid surface materials. An interpretive technique based on a quantitative understanding of the functional relationship between the optical properties of a mineral assemblage and its mineralogy, petrology and chemistry can provide a considerably more sophisticated characterization of a surface material than any matching or classification technique for those objects bright enough to allow spectral reflectance measurements. Albedos derived from radiometry and polarization data for individual asteroids can be used with spectral data to establish the spectral albedo, to define the optical density of the surface material and, in general, to constrain mineralogical interpretations.Mineral assemblages analogous to most meteorite types, with the exception of ordinary chondritic assemblages, have been found as surface materials of Main Belt asteroids. C1- and C2-like assemblages (unleached, oxidized meteoritic clay minerals plus opaques such as carbon) dominate the population (80%) throughout the Belt, especially in the outer Belt. A smaller population of asteroids exhibit surface materials similar to C3 (CO, CV) meteoritic assemblages (olivine plus opaque, probably carbon) and are also distributed throughout the Belt. The relative size (diameter) distributions for these two populations of objects are consistent with an origin by sequential accretion from a cooling nebula (C2 as surface layers, C3 as interior layers or cores). Based on information from meteoritic analogues and on qualitative models for the behavior of these materials during a heating episode, it seems unlikely that these C2- and C3-like asteroidal bodies have experienced any significant post-accretionary heating event either near surface or in the deep interior.The majority of remaining studied asteroids (20) of 65 asteroids exhibit spectral reflectance curves dominated by the presence of metallic nickel-iron in their surface materials. These objects are most probably the several end products of an intense thermal event leading to the melting and differentiating of their protobodies. These thermalized bodies are concentrated toward the inner part of the Asteroid Belt but exist throughout the Belt.The size of the proto-asteroid has apparently exercised control over the post-accretionary thermal history of these bodies. The available evidence indicates that all asteroids larger than about 450 km in (present) diameter have undergone a significant heating episode since their formation. The post-accretionary thermal history of the asteroidal parent bodies was apparently affected by both distance from the Sun and body size.The C2-like materials which dominate the main asteroid belt population appear to be relatively rare on earth-approaching asteroids. This suggests that most of these Apollo-Amor objects are not randomly derived from the main belt, but (a) may derive from a single event in recent time (10⁷ yr), (b) may derive from a favorably situated source body, (c) may derive from a particular, compositionally anomalous region of the belt, or (d) may derive from an alternate source (e.g. comets).     

International cooperation in remote sensing for global change research: political and economic considerations

Activities concerning international cooperation in space-based remote sensing for global change research have for the most part focused on technical/functional aspects such as data harmonization, research project coordination, and sensor selection and deployment. Until fairly recently, little attention has been directed towards the various political and economic constraints which may act as ‘stumbling blocks’. A review of the contemporary international remote sensing milieu as it relates to the global change research agenda is presented. Several important political and economic conditions affecting this area are identified: (1) data access and pricing policies; (2) national security concerns; (3) developed/developing nation relations; and (4) inconsistent political/financial commitments. A set of recommendations is offered to enhance international cooperation in the use of space-based remote sensing data for global change research.     

First calibration measurements with the dust impact detector DIDSY-IPM

The IPM detector consists of two separate impact ionization detectors, one of them covered by a 2.5 μm thick plastic film and a piezoelectric sensor mounted to the back of the joint impact plate. First impact tests, with iron projectiles in the mass range 10^?15 to 10^?9 g and in the speed range 1 km/s to 70 km/s, were performed with the calibration (FS) and the flight (F) model of this detector. The charge yield at 69 km/s impact speed (flyby speed of GIOTTO) has been extrapolated from the data and amounts to 400 Coulombs per gram. This corresponds to a preliminary sensitivity threshold for the impact ionization detector of about 6×10^?17 g. The penetration limit introduced by the plastic film is about 10^?14 g for iron particles. Only the biggest particles used for the test produced signals at the piezoelectric sensor. If one assumes an energy dependence of the piozoelectric signal, a preliminary sensitivity threshold of about 10^?13 g at 69 km/s can be established.     

Some aspects of temperature sounding of the atmosphere

Based on measurements with the interferometer spectrometer SI-1 on board of Meteor satellites the following problems are investigated: the systematic effect of the spectral response function in deriving effective radiation temperatures; the selection of ‘optimal’ spectral channels within the 15 μm region, and effects of additional molecular absorbers on the determination of the temperature profile. It can be shown that using broad-band window channels (half width ?20cm^?1) the derived surface temperature is significantly different from the surface temperature derived from narrow spectral band channels (e.g. for the HIRS window channel 8 this difference is about 1° C). Simulations of some combinations of spectral channels including one proposed as an optimal approach show no significant difference in the derived temperature profile. Neglecting the ozone absorption of the 14.1 μm band leads to a change in radiance of about 1.5 mW/m²sr cm^?1 and to a systematic error of the derived temperature of about ?2° C in the troposphere.     

An attempt for objective analysis of TIROS-N images

An attempt for digital processing of analog images taken in the infrared channel (10.5–11.5 μm) from TIROS-N is presented. The data digitized to 8-bits are normalized by using SR Data Manipulator. The investigated period is November 3–4, 1979. The geographic rectification moves the data into a polar stereographic map. To estimate the cloud heights empirical relationships are applied. The following characteristics of the cloud field are produced: the cloud amount, the dominant cloud-type, the daily variation of IR brightness temperature and the cloud texture. The results are visualized on a colour display.     

Hze-particles measured during IML-1 mission

Three stacks of CR-39 plastic nuclear track detectors were exposed to cosmic radiation during IML-1 mission. They were mounted with their axes parallel to the shuttle's axes to allow measurements of LET-spectra for different impinging directions. First results of this experiment are reported.     

Cosmic Rays Beyond the Knee

We briefly review the available data on cosmic rays beyond the `knee, i.e., over the energy range from 10¹⁵ to more than 10²⁰ eV. We discuss the observational status of the field, review some of the current attempts to explain the origin of these particles, and briefly survey the prospects of future measurements.     

Daytime O/N<Subscript>2</Subscript> Retrieval Algorithm for the Ionospheric Connection Explorer (ICON)

The NASA Ionospheric Connection Explorer Far-Ultraviolet spectrometer, ICON FUV, will measure altitude profiles of the daytime far-ultraviolet (FUV) OI 135.6 nm and N₂ Lyman-Birge-Hopfield (LBH) band emissions that are used to determine thermospheric density profiles and state parameters related to thermospheric composition; specifically the thermospheric column O/N₂ ratio (symbolized as \(\Sigma\)O/N₂). This paper describes the algorithm concept that has been adapted and updated from one previously applied with success to limb data from the Global Ultraviolet Imager (GUVI) on the NASA Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) mission. We also describe the requirements that are imposed on the ICON FUV to measure \(\Sigma\)O/N₂ over any 500-km sample in daytime with a precision of better than 8.7%. We present results from orbit-simulation testing that demonstrates that the ICON FUV and our thermospheric composition retrieval algorithm can meet these requirements and provide the measurements necessary to address ICON science objectives.