In 1977, NASA launched the Voyager spacecraft which were destined to leave the Solar System and give man its first glimpse of an interstellar environment. Despite how many years have passed though, neither craft have yet exited the heliosphere, though Voyager 1 may be close. Right now though, it is in a region of space researchers had not anticipated, as reported by NASA.
The heliosphere is a giant bubble of charged particles the Sun blows around the entire Solar System, and its outermost layer, the heliosheath, is considered the boundary of interstellar space. Once one crosses into interstellar space, it is expected that the magnetic field lines will suddenly change direction, as the Sun will no longer be the dominate source. Thus far this has not happened according to Voyager 1's instruments, though the intensity of the magnetic field has been increasing while the speed of charged particles has dropped to zero. Basically while the effects of the Sun are definitely appearing to disappear, the 'signpost' indicating we have left the Solar System, is still ahead of us, which is not what anyone expected.
The researchers who monitor the data from Voyager 1 believe it may just be a matter of months or a couple years before it has finally escaped the Sun's influence. Until the field lines start shifting though, they are not going to celebrate as it was the field lines which had previously pinpointed when the spacecraft passed another milestone in the heliosphere; the termination shock where the solar wind abruptly slows.