A human-made object, the Voyager probe, crossed into interstellar space for the first time in 2012. Now, its twin, Voyager 2 has traveled 11 billion miles away from Earth, and has also officially entered interstellar space.

So what is interstellar space, exactly? It’s easier to define it in opposition to the type of space in which Earth exists, which is called a heliosphere. Stars like our sun produce a huge amount of energy; the heliosphere is the sphere surrounding a star affected by its heat and magnetism. Interstellar space is the area outside of that. However, the probe is still inside our vastly greater solar system, which is determined by the pull of the sun’s gravity. A large part of the solar system falls outside the heliosphere and scientists expect it could take another 30,000 years until Voyager 2 leaves the outer edge of our solar system.

Since Nov. 5, instruments on board Voyager 2 have seen a significant decrease in the amount of solar wind surrounding the craft, as well as an increase in cosmic rays, suggesting it’s left the heliosphere and reached interstellar space. Currently, the probe’s readings take about 16 and a half hours to reach Earth, and that lag time will only get longer; the probe moves at around 34,000 miles per hour, so by the end of next year, it will be another 290 million miles further away.

Both Voyager probes were launched in 1977, and were originally designed to explore Jupiter, Saturn, Uranus, and Neptune. Though their initial missions were expected to last around five years, both probes have been in commission for 41 years. Scientists anticipate that they could hurtle through space for billions of years; in the case that anyone out there finds them, they’re outfitted with an American flag and a Golden Record which contains sounds and pictures of earth.