Voyager 1, launched in 1977, is more than 11.66 billion miles from the sun and is now recognized as the first spacecraft to enter interstellar space. Solar storm aftershocks at the edge of our solar system provided confirmation that Voyager 1 made the passage on August 25, 2012.
According to National Geographic:
“Since an instrument for directly detecting that transition died in 1980, the researchers have had to rely on indirect measures of magnetic and electrical activity from other instruments aboard Voyager 1 to find an answer. One key to identifying this boundary is the difference in the density of charged particles between the solar wind and interstellar space, as it is about 50 times greater in the latter region. Looking at a pair of solar storms that caught up to the spacecraft last October and then again last April, Gurnett’s team reported that measured changes in electrical activity around Voyager correspond to interstellar space. As the storms passed the spacecraft, they triggered spikes in electrical and radio waves that uniquely corresponded in frequency to the spacecraft having entered the more densely charged interstellar space. Based on that increase, the team extrapolated the entry date for Voyager 1 into interstellar space as August 25, 2012.”
Given the estimated lifetime of the plutonium battery aboard Voyager 1, its last signals should be heard on Earth around 2025. The spacecraft will eventually pass within 1.7 light years (about 16.1 trillion kilometers) of another star in 40,000 years. The spacecraft’s twin, Voyager 2, which explored Jupiter, Saturn, Uranus, and Neptune, is also still kicking, now some 9.55 billion miles (15.36 billion kilometers) from the sun on its own journey.