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Several consequences of\ud the passage of the solar system through dense interstellar molecular clouds are\ud discussed. These clouds, dense (more than 100 cm-3), cold (10–50\ud K) and extended (larger than 1 pc), are characterized by a gas-to-dust mass\ud ratio of about 100, by a specific power grain size spectrum (grain radii\ud usually cover the range 0.001–3 micron) and by an average dust-to-gas number\ud density ratio of about 10-12. Frequently these clouds contain\ud small-scale (10–100 AU) condensations with gas concentrations ranging up to\ud 10 5 cm-3. At their casual passage over the solar system they exert\ud pressures very much enhanced with respect to today’s standards. Under these\ud conditions it will occur that the Earth is exposed directly to the interstellar\ud flow. It is shown first that even close to the Sun, at 1 AU, the cloud’s\ud matter is only partly ionized and should mainly interact with the solar wind by\ud charge exchange processes. Dust particles of the cloud serve as a source of\ud neutrals, generated by the solar UV irradiation of dust grains, causing the\ud evaporation of icy materials. The release of neutral atoms from dust grains is\ud then followed by strong influences on the solar wind plasma flow. The behavior\ud of the neutral gas inflow parameters is investigated by a 2-D hydrodynamic\ud approach to model the interaction processes. Because of a reduction of the\ud heliospheric dimension down to 1 AU, direct influence of the cloud’s matter\ud to the terrestrial environment and atmosphere could be envisaged. Key words. Interplanetary physics (heliopause\ud and solar wind termination; interplanetary dust; interstellar gas

Consequences of the Solar System passage through dense interstellar clouds