Interstellar Dust Inside and Outside the Heliosphere

In the early 1990s, after its Jupiter flyby, the Ulysses spacecraft identified interstellar dust in the solar system. Since then the in-situ dust detector on board Ulysses continuously monitored interstellar grains with masses up to 10−13 kg, penetrating deep into the solar system. While Ulysses measured the interstellar dust stream at high ecliptic latitudes between 3 and 5 AU, interstellar impactors were also measured with the in-situ dust detectors on board Cassini, Galileo and Helios, covering a heliocentric distance range between 0.3 and 3 AU in the ecliptic plane. The interstellar dust stream in the inner solar system is altered by the solar radiation pressure force, gravitational focussing and interaction of charged grains with the time varying interplanetary magnetic field. The grains act as tracers of the physical conditions in the local interstellar cloud (LIC). Our in-situ measurements imply the existence of a population of ’big’ interstellar grains (up to 10−13 kg) and a gas-to-dust-mass ratio in the LIC which is a factor of > 2 larger than the one derived from astronomical observations, indicating a concentration of interstellar dust in the very local interstellar medium. Until 2004, the interstellar dust flow direction measured by Ulysses was close to the mean apex of the Sun’s motion through the LIC, while in 2005, the data showed a 30◦ shift, the reason of which is presently unknown. We review the results from spacecraft-based in-situ interstellar dust measurements in the solar system and their implications for the physical and chemical state of the LIC.