Published online by Cambridge University Press: 27 February 2018
The local cloud in which the Sun is embedded participates in a general flow from the direction of the Sco-Cen association (Weaver, 1979) while the Sun is moving in the direction of Hercules. The relative motion is well determined by the neutral gas flow (Cox and Reynolds, 1987). A prominent cone of gravitationally focused neutral helium indicates the downwind direction to within ±3°. The average flow speed outside the solar system, 26 km/s, is determined to within ±1 km/s. Measurements from the Ulysses spacecraft (Witte et al., 1993) confirmed this flow. Since interstellar matter contains dust as well as gas, efforts have been made to detect the dust flow through the solar system. However, until recently these efforts have been unsuccessful and modeling of the flow has been controversial.