Voyager 1 was the first spacecraft to leave the Solar System’s heliosphere.
NASA launched Voyager 1 on 5 September 1977 to study Jupiter and Saturn. In August 2012, measurements showed that it had crossed the heliopause, the boundary where the Sun’s solar wind is overcome by interstellar space. NASA announced the result in 2013 after analyzing the data.
Voyager 1 and Voyager 2 were designed for a rare planetary alignment that enabled gravity assists. Voyager 1 flew close to Jupiter and Saturn, while Voyager 2 continued on to Uranus and Neptune. Voyager 2 crossed the heliopause later, in 2018.
The phrase “left the Solar System” can be confusing. Voyager 1 entered interstellar space but remains gravitationally bound to the Milky Way and has not passed beyond the distant Oort Cloud. Its power comes from radioisotope thermoelectric generators, whose output gradually declines.