The artist’s impression above shows the approximate current positions of both Voyager spacecraft relative to the Sun and other surviving spacecraft. Note that while Voyager 1 left the solar system in 2012 above the orbital plane of the planets, Voyager 2 left the solar system on the 10th of December 2018 below the orbital plane of the planets.
Launched from Cape Canaveral aboard a Titan-Centaur rocket on the 20th of August 1977, Voyager 2 was originally designed and intended to be operational for only five years. Its primary objective during the proposed mission lifetime was to explore the outer gas planets, and to date, it is still the only space probe that has visited the outer gas giants. To the surprise of both its designers and controllers however, Voyager 2 showed no signs of deterioration after the expiration of its original mission, and like its twin, Voyager 1, the craft was set on a course towards outer space, which it has now (almost) reached.
Nevertheless, it is not entirely accurate to say that Voyager 2 has left the solar system. At its current distance of 18 billion km (11 billion miles) from Earth and traveling at a speed of 55,230 km/h (34,321 m/ph), the craft has now reached the outer edge of the heliosheath, which is a bubble of space around the Sun that is flooded with solar particles carried by the solar wind.
While Voyager 2 was traveling through the bubble of outward-flowing plasma, one of its instruments, the Plasma Science Experiment, measured the speed, temperature, density, and pressure of the solar plasma on a continuous basis. As a practical matter though, the solar wind can carry hot plasma only up to a point, this point being where incoming material from true outer space meets with the plasma streaming outwards from the Sun.
On the 10th of December 2018, however, the instrument detected a sharp decline in the density, pressure, and temperature of the surrounding solar plasma, which was interpreted by mission controllers to mean that the spacecraft is now outside of the heliosheath, and is now measuring the composition of the interstellar medium. Incidentally, from this distance, it takes radio communication signals 16.5 hours to reach Earth from the craft, whereas light from the Sun reaches Earth in only 8 minutes and around 20 seconds.
That said, Voyager 2 has a long journey ahead of it before it leaves the solar system proper on its way towards true interstellar space. Provided it does not collide with anything, it will only reach the inner edge of the Oort Cloud, an agglomeration of icy bodies around the solar system, and that is still under the Sun’s gravitational influence, in about another 300 or so years.
Beginning at about 1,000 AU from the Sun, the Oort Cloud is thought to extend outward to about 100,000 AU, which means that at its current speed, Voyager 2 will need about another 3,000 or so years to cross the Oort Cloud. Voyager 2 is therefore still very much within the solar system, but as far as is known it is far beyond the orbits of all known planets, including the dwarf planets that populate the Kuiper Belt.
In addition, provided that Voyager 2 is not intercepted by an alien race, it faces a lonely journey. For instance, on its current trajectory it will approach a small red dwarf star known as Ross 248 to within about 1.7 light years in about 40,000 years’ time. At this time, Ross 248 will also (briefly) be the closest star to the Sun, at a distance of only 3.02 light years.
Still moving outward, Voyager 2 will approach the bright star Sirius to within about four light years, which will happen in about the year 289,000 AD. About 100,000 years later, Voyager 2 will swing close by the stars delta Pav and GJ 754, and from this point onwards, Voyager 2 will be well into its first orbit of the Milky Way galaxy’s center- which it should complete about 250 million years from now.
Sadly, though, and although both Voyagers are still in contact with their controllers, the nuclear power plants that supply power to the craft are not expected to last beyond another six years or so. Even so, having exceeded their expected life times by several decades, the Voyager craft represent the longest continuously running space exploration program in human history.