For only the second time in history, a human-made object has entered interstellar space. 

On Monday, NASA announced that the Voyager 2 spacecraft had passed into a region of space where it is being bombarded by high-energy particles from outside our Solar System. This indicates that it has passed beyond the heliosphere — a bubble of magnetic fields and plasma that surrounds the Sun.

“For me this is an extremely exciting time for the history of space exploration,” Georgia Denolfo, a NASA astrophysicist, said in a press conference today. “We’re able to look at the galaxy through the clouded lens of our heliosphere, and now we can take a step outside with Voyager.”

Inside the heliosphere, solar wind — the particles of highly charged plasma that stream out from the Sun in all directions — rules. But farther away from the Sun, speedy subatomic particles called cosmic rays start to dominate. The boundary between the two is known as the heliopause, and that’s what NASA says Voyager 2 passed on November 5th.

Voyager 2 isn’t the first spacecraft to pass the heliopause landmark. The spacecraft’s predecessor, Voyager 1, crossed the heliopause in 2012. Both spacecraft launched in 1977, and have steadily ventured into unexplored areas of space since then. (Voyager 2 launched first, and it’s now NASA’s longest-running mission.)

“I think we’re all happy and relieved that the Voyager probes have both operated long enough to make it past this milestone,” Suzanne Dodd, NASA’s project manager for the Voyager program, said in a statement. “This is what we’ve all been waiting for. Now we’re looking forward to what we’ll be able to learn from having both probes outside the heliopause.”

In addition to carrying a fleet of scientific instruments, each spacecraft carries a Golden Record, which contains sounds, images, and information about life on Earth. If there are other spacefaring civilizations out there, and they encounter either probe, the Golden Records could give aliens a glimpse of what life on Earth is like — or, at least, a glimpse of what life was like before 1977.

Both Voyagers still have a long way to go before they encounter any other stars (much less potential life around those stars). Even though they are both in interstellar space, they’re still well within our Solar System.

The edge of the Solar System is delineated by the Oort Cloud, a collection of objects, including comets, that orbits the Sun far beyond Pluto — really far beyond Pluto. Earth is 93 million miles away from the Sun, and the closest members of the Oort Cloud are about 93 billion miles away. NASA estimates that Voyager 2 will reach the near edge of the Oort Cloud in about 300 years, and it could take up to 30,000 years for it to exit the Solar System completely.

Both Voyagers will stop operating long before they reach that frontier. “Both spacecraft are healthy, if you consider them senior citizens,“ Dodd said at the press conference today. While they are operating well, they are constantly losing power and are getting colder as they move away from the Sun’s heat. Dodd estimated that the spacecraft could probably continue to send back data for five to 10 more years, but researchers will have to turn off some of the Voyagers’ scientific instruments to increase their longevity.

“We do have difficult decisions ahead,” Dodd said. “My personal goal would be to get these spacecraft to last 50 years. These spacecraft launched in 1977. If we can get them to 2027, that would be a 50 year mission, and that would be fantastic.”