The Voyager 1 space probe has reached the edge of the solar system.
NASA says the craft is sending back data showing a sharp increase in charged particles that originate from beyond the solar system.
Voyager 1, along with its sister spacecraft Voyager 2, was launched in 1977 and is now about 18 billion km from the Sun.
It is moving at a speed of about 17km per second and data takes nearly 17 hours to reach NASA's network in California.
The two probes have explored all the planets of the solar system: Jupiter, Saturn, Uranus and Neptune, as well as 48 of their moons.
The plutonium power sources on the Voyager probes are designed to last until 2025.