On August 20th, 1977 NASA launched Voyager 2 for a record flight that is still going. Shortly after, on September 5th, the identically built Voyager 1 followed. The initial aim of the mission was to obtain more information about Jupiter and Saturn — planets which were still largely unexplored at the time. Thanks to the long-lasting plutonium batteries, both spacecrafts are still active.

Weighing 825kg (1,818 pounds) on Earth, the Voyager probes are among NASA's biggest success stories. Both still regularly send reliable data from space. They're moving farther and farther away from Earth, but the radio connection is expected to work until 2030.

On August 25th, 2012 Voyager 1 crossed the heliopause — one of the borders of our solar system. There, the interstellar space of our galaxy, the Milky Way, begins. Voyager 1 is the furtherst human-made object from Earth.

A short time later Voyager 2 followed. On November 5, 2018, the sister probe dived into interstellar space. The evaluation of the measurements has now yielded surprising results and thrown some historical theories overboard.

The solar system has different borders: The first is the "termination shock." That's where the solar winds slow down dramatically. After the heliosphere comes the heliopause. That's the edge of the space bubble where solar flares shield us from interstellar rays. Until now, the assumption was that the winds gradually decreased.

But the comparative measurements of the two sister probes have shown that there is a very sharp boundary in the interior of our solar system. And the temperature of the interstellar medium is significantly higher than expected. The researchers suspect that the heliosphere could push a kind of bow wave through the interstellar medium in front of it, but this has yet to be measured.

Aside from the interstellar discoveries, the spacecrafts had much more to absorb. Voyager 1 sent this image of Jupiter on January 1, 1979. It took a total of 17,477 images of the planet and its four moons in total. The existence of the thin ring system surrounding Jupiter was detected for the first time through these images.

Voyager 1 also documented atmospheric flows on Jupiter, as seen in this picture. After the Jupiter flyby, Voyager 1 reached a speed of 16 kilometers per second.

Voyager 2 sent this full-color photo of Saturn back to Earth. The probe reached the sixth planet in our solar system in 1981. In outer space terms, this photo is a real close-up — it was taken from a distance of just 21 million kilometers (about 13 million miles).

The distant probes are monitored and controlled as closely as they can be by the control center of the Voyager mission at the California Institute of Technology in Pasadena, seen here in 1980. Today, the equipment is much more modern. But NASA regularly has to consult with the engineers who designed and built the Voyager spacecraft — even though they are long retired.

In the event the probes encounter life on their endless journey, they have these shiny gold discs along for the ride. The records contain pictures and sounds of people, animals and nature on Earth. In case the aliens don't own a record player, a needle and detailed instructions are provided.