A proposed NASA mission to send a space probe out of the Solar System to examine the interstellar medium, the ultra-thin gas between the stars. It involves using a solar sail of 200-m radius to accelerate the probe away from the Sun until it leaves the heliosphere, the bubble around the Sun created by the outward flow of the solar wind. In the course of this journey, the probe would explore the boundaries of the heliosphere to reveal how the Sun interacts with its environment, and directly sample the nearby interstellar medium.
http://interstellar.jpl.nasa.gov/ Outlining of a plan for a future interstellar probe. The site has clearly illustrated articles about the interstellar medium, the outer Solar System, and the interaction between the interstellar medium and the solar wind.
Subjects: Astronomy and Astrophysics.