The interstellar medium begins where the interplanetary medium of the Solar System ends. The solar wind slows to subsonic velocities at the termination shock, 90 – 100 astronomical units from the Sun. In the region beyond the termination shock, called the heliosheath, interstellar matter interacts with the solar wind.
Voyager 1, the farthest human-made object from the Earth (after 1998), crossed the termination shock December 16, 2004 and may soon enter interstellar space, providing the first direct probe of conditions in the ISM (Stone et al. 2005).
The Voyager 1 spacecraft is a 722 kg (1,590 lb) space probe launched by NASA on September 5, 1977 to study the outer Solar System and interstellar medium. Operating for 35 years, 6 months, and 20 days as of 25 March 2013, the spacecraft receives routine commands and transmits data back to the Deep Space Network. At a distance of about 123.48 AU (1.847×1010 km) as of March 2013, it is the farthest man-made object from Earth and is currently travelling in a previously unknown region of space. It is still unclear whether this region is part of interstellar space or an area within the Solar System.