Voyager 1 and Voyager 2, launched in 1977, continue to coast outwards from the sun, and both spacecraft are approaching interstellar space. The boundary of interstellar space is defined as the point beyond which the solar wind gives way to stellar winds.
The Voyagers detect the direction of particle flow using low energy charged particle detectors combined with the known orientation of the spacecraft. Voyager 1 is now in a region of space where the velocity of the solar wind is measured at zero. Recently, NASA has begun rolling the spacecraft into different orientations to determine the prevalent velocity of charged particles, and the data are being analyzed now. Scientists expect that within a few years Voyager 1, the most distant of the two, will enter interstellar space.
Voyager 2 tweets regularly at http://twitter.com/#!/Voyager2. Voyager 1 doesn’t seem to have an official account on Twitter.