After 35 years of flight through space, the Voyage 1 probe finally entered interstellar space on August 25, 2012, according to the National Aeronautics and Space Administration (NASA). On that date, at a distance of approximately 12 billion miles (19 billion kilometers) from the Sun, the probe crossed the outermost boundary of the vast solar wind structure called the heliosphere. In popular descriptions of this historic milestone, Voyager 1 is often said to have become the first human artifact to leave the solar system (although the technical accuracy of that description is debatable).
NASA launched twin Voyager space probes in 1977 to examine the giant planets Jupiter and Saturn and other parts of the outer solar system. (Voyager 2, which departed two weeks ahead of Voyager 1, is still only about 9.5 billion miles, or 15 billion km, from the Sun because it detoured past the planets Uranus and Neptune.) Their observations, along with those of other spacecraft such as NASA’s Interstellar Boundary Explorer (IBEX), revolutionized space physicists’ understanding of the heliosphere, a massive bubble of ionized particles that extends far beyond the Sun and planets. The heliosphere consists of gas emanating from the Sun as a fast-moving solar wind. When this material is roughly 100 million times farther from the Sun than Earth is, the energetic outward expansion of the plasma is checked by pressure from the denser (though still diffuse) interstellar medium. The boundary layer between the heliosphere and the interstellar medium is called the heliopause, and astrophysicists and other space scientists have been intensely curious about precisely how far out it is because its size has consequences for models of stellar activity and for how easily harmful cosmic rays from deep space can enter the solar system. See also: Cosmic rays; Interstellar matter; Plasma (physics); Solar wind; Solar magnetic field
In April 2013, a fortuitous burst of solar activity set up vibrations in the plasma surrounding Voyager 1, from which NASA scientists could calculate the plasma’s density. It corresponded to the expected density of the interstellar medium. After reexamining earlier data, the scientists concluded that Voyager 1 passed out of the heliopause on August 25, 2012. NASA scientists reported this discovery online in the journal Science two weeks later, on September 12.
Somewhat paradoxically, even though Voyager 1 is moving through the interstellar medium and in that sense has entered interstellar space, many critics object to saying that the spacecraft has left the solar system. The icy, rocky bodies of the Kuiper Belt and the proto-comets of the Oort Cloud (which has never been observed directly) orbit the Sun at much, much greater distances. The Oort Cloud may extend a third of the way to Alpha Centauri, the nearest star: it will be tens of thousands of years before Voyager 1 (by then inoperative) will emerge from it. See also: Comet; Kuiper Belt; Large Kuiper Belt objects