You Are Here, Voyager: This artist's concept puts huge solar system distances in perspective. The scale bar is measured in astronomical units (AU), with each set distance beyond 1 AU representing 10 times the previous distance. Each AU is equal to the distance from the sun to the Earth. It took from 1977 to 2013 for Voyager 1 to reach the edge of interstellar space.
"We have been cautious because we're dealing with one of the most important milestones in the history of exploration,” said Voyager Project Scientist Ed Stone of the California Institute of Technology in Pasadena. “Only now do we have the data -- and the analysis -- we needed."
Basically, it was needed more data on plasma, which is ionized gas, the densest and slowest moving of charged particles in space. The plasma is the most important marker and show us that Voyager 1 is inside the solar bubble, known as the heliosphere, or in the interstellar space and surrounded by material ejected by the explosion of nearby giant stars millions of years ago.
The two Voyager spacecraft were launched in 1977 and Voyager 1's plasma instrument, which measures the density, temperature and speed of plasma, stopped working in 1980. The science team detected the pressure of interstellar space on our heliosphere in 2004, they focused on the direction of the magnetic field as a proxy for source of the plasma. Since solar plasma carries the magnetic field lines emanating from the sun and interstellar plasma carries interstellar magnetic field lines, the directions of the solar and interstellar magnetic fields were expected to differ.
Then on April 9, 2013, it happened: Voyager 1's plasma wave instrument picked up local plasma oscillations. The oscillations increased in pitch through May 22 and indicated that Voyager was moving into an increasingly dense region of plasma. This plasma had the signatures of interstellar plasma, with a density more than 40 times that observed by Voyager 2 in the heliosheath.