Two of the most famous robotic spacecraft ever launched, the Voyager probes, continue to beam back scientific data more than thirty three years after they were launched – not bad considering that they were only designed with a five year life-span.  Launched several weeks apart atop powerful Titan-Centaur rockets, they took advantage of a unique positioning of the planets to use a series of gravity assist flybys to boost them up to extremely high speeds.  They are still the fastest man-made objects in the universe – Voyager 1 is travelling at over 60 000 km/h! 

Several months ago, Voyager 1 reached a special milestone in its epic voyage:  It began passing through the Heliosheath, which marks the boundary of the Solar System where Interstellar space begins.  At this point, for the first time since launch, particle detectors on the probe reported that the the Solar Wind had stopped completely.  The science team at NASA’s Jet Propulsion Laboratory (JPL) took additional measurements every month for four months, to ensure that Voyager 1 wasn’t merely passing through a local eddy in the wind, before announcing yesterday that the measurements can only mean one thing:  The wind has stopped because this is the point where it collides with the interstellar medium,

The solar wind is a continuous stream of charged particles blasting outwards from the Sun in all directions at extremely high speeds.  Although it is thinner than the hardest vacuums we can create on Earth, it still has measurable density, and collides with the interstellar medium (an even sparser gas filling the space between the stars).  The zone where the solar wind collides with the interstellar medium defines the heliosheath.  Voyager 2, with its lower speed, is only expected to enter the heliosheath in the next few years.  Scientists are eagerly waiting for both probes to leave the Heliosheath and enter the interstellar medium, to confirm whether our current knowledge of the space between the stars holds true.