The formational and evolutionary history of natural satellites, their mineralogical composition and other phenomena of scientific interest are discussed. Key scientific questions about IO, Ganymede, Callisto, and Europa are posed and the measurements and instruments required for a Galilean satellite lander in the 1980's are described.

Using the low-resolution mode of the Space Telescope Imaging Spectrograph (STIS) aboard the Hubble Space Telescope (HST), we obtain spectra from 20 spatially-resolved, ultracool dwarfs, eighteen of which belong to nine very low-mass binary systems with known angular separations in the range 0.37 arcsec-0.098 arcsec. For these dwarfs, we derive

Aims: Fossil record methods based on spectral synthesis techniques have matured during the past decade, and their application to integrated galaxy spectra has fostered substantial advances in the understanding of galaxies and their evolution. Yet, because of the lack of spatial resolution, these studies are limited to a global view, providing no

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There is no agreement on the thermal structure of the solar chromosphere. While results of the CO observations and 3D MHD simulations suggest very cool structures in the upper atmosphere, SUMER observations of UV spectral lines is interpreted as signature of a full-time hot chromosphere. We tried to look for cool structures in the solar

We have derived a primordial helium abundance of Yp=0.2477+/-0.0029, based on new atomic physics computations of the recombination coefficients of He I and of the collisional excitation of the H I Balmer lines together with observations and photoionization models of metal-poor extragalactic H II regions. The new atomic data increase our previous