After this final planetary flyby, the twins focused on finding the outskirts of the solar system. In December 2004, magnetic data indicated that Voyager 1 may have passed ‘termination shock’, where the solar wind slows from millions of miles per hour to 250,000 miles per hour (400,000 km/h) in response to external pressures from the interstellar plasma. Later, the team said that 8.7 billion kilometers from the Sun, Voyager 1 had entered the heliosheath, the region beyond the terminal shock. Voyager 2 entered this region in August 2007.
In August 2012, following an increase in galactic cosmic ray measurements, Voyager 1 crossed the heliopause, where the pressures of the solar wind and interstellar plasma balance each other. At that time, the probe was about 11 billion miles (18 billion km) from Earth. Voyager 2 followed suit in November 2018.
“We can now answer the question we’ve all been asking,” Voyager project scientist Ed Stone said at a press conference called to announce the discovery. “Are we there already? Yes we are!”
Forty-five years later, Voyager 1 is 14.6 billion miles (23.5 billion km) and Voyager 2 about 12 billion miles (19.3 billion km) from home, speeding more than 38,000 miles per hour (61,000 km/h) and 34,000 miles per hour (55,000 km/h) respectively. One-way radio traffic extends to just under 10 p.m. for Voyager 1 and 6 p.m. for Voyager 2. Power levels are so low that only half their instruments remain on. Over the years, scientists have come together to discuss the best way to keep the spacecraft running for as long as possible, shutting down systems as needed. The team hopes the pair will remain functional until 2025, but no one knows for sure.