The Voyager-1 is an unmanned probe launched on September 5, 1977, and is currently the most distant man-made object in the Solar System. It is currently about 90 AU (about 13.5 billion kms) away, and signal from it takes about 13 hours to reach Earth (in comparison, light from the Sun takes about 8 minutes to reach us). Scientists have long speculated about the eventual exit of the Voyager-1 from the Solar System, which would happen when the probe enters into the heliosheath, which is a vast, turbulent expanse where the Sun's influence ends and particles blown off its surface crash into the thin gas that drifts between the stars.

Voyager Graphic (Courtesy: JPL)Now at last, it is happening: scientists at NASA have confirmed that the probe is currently in the heliosheath, and is fast on its way to interstellar space:):).
Scientists do not really know where the actual edge to interstellar space is. According to the current models, the solar wind (matter and radiation constantly blowing away from the Sun) should get weaker and weaker as we move away from the Sun, and at some distance would collide with the sparse gases found between stars. The region where this collision would occur, is known as the termination shock. At the termination shock, the solar wind slows abruptly from a speed that ranges from 1.1-2.4 million km/h and becomes denser and hotter. One of the final tasks of the Voyager probes is to locate the edge of this termination shock (called the heliopause), beyond which is the interstellar space.
The most persuasive evidence that Voyager-1 has crossed the termination shock is its measurement of a sudden increase in the strength of the magnetic field carried by the solar wind, combined with an inferred decrease in its speed. This happens when the solar wind slows down, and thus the density of charged particles from the Sun increase (imagine a slower traffic: the vehicles are bumper to bumper), with a corresponding increase in the magnetic field. This field strength increased by 1.7 times in November 2003, and again by 2.5 times in December 2004 (and is holding steady ever since). This indicates that the solar wind has perhaps reached a minimum speed, and we are soon to reach this elusive boundary of the Sun's dominion:):).