The gas-to-dust mass ratios found for interstellar dust within the Solar System, versus values determined astronomically for the cloud around the Solar System, suggest that large and small interstellar grains have separate histories, and that large interstellar grains preferentially detected by spacecraft are not formed exclusively by mass exchange with nearby interstellar gas. Observations by the Ulysses and Galileo satellites of the mass spectrum and flux rate of interstellar dust within the heliosphere are combined with information about the density, composition, and relative flow speed and direction of interstellar gas in the cloud surrounding the solar system to derive an in situ value for the gas-to-dust mass ratio, R g/d =94 +46−38 . This ratio is dominated by the larger near-micron sized grains. Including an estimate for the mass of smaller grains, which do not penetrate the heliosphere due to charged grain interactions with heliosheath and solar wind plasmas, and including estimates for the mass of the larger population of interstellar micrometeorites, the total gas-to-dust mass ratio in the cloud surrounding the Solar System is half this value. Based on in situ data, interstellar dust grains in the of 10 −14 to 10 −13 g mass range are underabundant in the Solar System, compared to an MRN mass distribution scaled to the local interstellar gas density, because such small grains do not penetrate the heliosphere. The gas-to-dust mass ratios are also derived by combining spectroscopic observations of the gas-phase abundances in the nearest interstellar clouds. Measurements of interstellar absorption lines formed in the cloud around the solar system, as seen in the direction of ǫ CMa, give−207 for assumed solar reference abundances, and R g/d =551 +61 −251 for assumed B-star reference abundances. These values exceed the in situ value, suggesting either grain mixing or grain histories are not correctly understood, -4or that sweptup stardust is present. Such high values for diffuse interstellar clouds are strongly supported by diffuse cloud data seen towards λ Sco and 23 Ori, provided B-star reference abundances apply. If solar reference abundances prevail, however, the surrounding cloud is seen to have greater than normal dust destruction compared to higher column density diffuse clouds. The cloud surrounding the Solar System exhibits enhanced gas-phase abundances of refractory elements such as Fe + and Mg + , indicating the destruction of dust grains by shock fronts. The good correlation locally between Fe + and Mg + indicates that the gas-phase abundances of these elements are dominated by grain destruction, while the poor correlation between Fe + and H • indicates either variable gas ionization or the decoupling of neutral gas and dust over parsec scalelengths. These abundances, combined with grain destruction models, indicate that the nearest interstellar material has been shocked with shocks of velocity ∼150 km s −1 . If solar reference abundances are correct, the low R g/d value towards λ Sco may indicat...

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An analysis of the Helios in situ dust data for interstellar dust (ISD) is presented in this work. Recent in situ dust measurements with impact ionization detectors on-board various spacecraft (Ulysses, Galileo, and Cassini) showed the deep penetration of an ISD stream into the Solar System. The Helios dust data provide a unique opportunity to monitor and study the ISD stream alteration at very close heliocentric distances. This work completes therefore the comprehensive picture of the ISD stream properties within the heliosphere. In particular, we show that gravitation focusing facilitates the detection of big ISD grains (micrometer-size), while radiation pressure prevents smaller grains from penetrating into the innermost regions of the Solar System. A flux value of about 2.6±0.3×10−6 m −2 s −1 is derived for micrometer-size grains. A mean radiation pressure-to-gravitation ratio (so-called β ratio) value of 0.4 is derived for the grains, assuming spheres of astronomical silicates to modelize the grains surface optical properties. From the ISD flux measured on the Helios trajectory, we infer a lower limit of 3 ± 3 × 10 −25 kg m −3to the spatial mass density of micron-sized grains in the Local Interstellar Cloud (LIC). In addition, compositional clues for ISD grains are obtained from the data provided by the time-of-flight mass spectrometer subsystem of the Helios instrument. No clustering of single minerals is observed but rather a varying mixture of various minerals and carbonaceous compounds.

We report on the development of the LISA Technology Package (LTP) experiment that will fly onboard the LISA Pathfinder mission of the European Space Agency in 2008. We first summarize the science rationale of the experiment aimed at showing the operational feasibility of the so-called transverse-traceless coordinate frame within the accuracy needed for LISA. We then show briefly the basic features of the instrument and we finally discuss its projected sensitivity and the extrapolation of its results to LISA.

Interstellar dust grains intercepted by the dust detectors on the Ulysses and Galileo spacecrafts at heliocentric distances from 2 to 4 astronomical units show a deficit of grains with masses from 1 x 10(-17) to 3 x 10(-16) kilograms relative to grains intercepted outside 4 astronomical units. To divert grains out of the 2- to 4-astronomical unit region, the solar radiation pressure must be 1.4 to 1.8 times the force of solar gravity. These figures are consistent with the optical properties of spherical or elongated grains that consist of astronomical silicates or organic refractory material. Pure graphite grains with diameters of 0.2 to 0.4 micrometer experience a solar radiation pressure force as much as twice the force of solar gravity.