The Voyager 1 and the Voyager 2, a pair of NASA probe spacecraft that launched all the way back in 1977, are still out there and plugging away. According to a recent press release from NASA, the Voyager 1 is officially “the first human-made object to venture into interstellar space.” NASA’s data indicates that the Voyager 1 is currently approximately 12 billion miles from our sun, while the Voyager 2, not to be outdone, is about 9.5 billion miles out.

The news of the groundbreaking achievement only reached NASA recently, though the governmental space organization estimates that the probe has actually been traveling “through plasma, or ionized gas” for roughly an entire year now. Such conditions are a hallmark of the space between stars, and indicate to NASA that the Voyager “is in a transitional region immediately outside the solar bubble.”

So why did it take NASA a year to determine whether or not the Voyager 1 had crossed into interstellar space? Quite simply, the craft is obsolete. After spending 36 years floating through space, first past Jupiter and Saturn and then much, much further out, the Voyager 1 understandably lacks some of the modern day technology that could have made it possible for NASA to make a firm judgment sooner. For instance, the craft does not have a functional plasma sensor (NASA didn’t mention whether it had ever had a plasma sensor), so scientists had to get creative to determine where the craft was.

That meant gauging pressures and taking advantage of serendipitous solar events to determine whether the Voyager 1 had indeed entered interstellar territory. Pressure wasn’t a dead giveaway: the craft first noted an increase of pressure all the way back in 2004, and NASA has been working to determine the relative location of the craft ever since. Unfortunately, for the bulk of those years, the process was little more than educated guesswork.

NASA got a lucky break in March 2012, when a coronal mass ejection – what NASA classified as “a massive burst of solar wind and magnetic fields” – radiated from our sun through the entire solar system and beyond. When the coronal burst reached the Voyager 1 – a trip that took a whole 13 months – NASA scientists noticed that the plasma around the spacecraft was vibrating “like a violin string.” NASA then used a wave measuring device on the craft to determine the pitch of the oscillations, which indicated the relative density of the plasma.

What were their findings? According to the press release, the Voyager 1 was traveling through plasma that was 40 times denser than what scientists had noticed in 2004, suggesting that the probe had crossed the last layers of the heliosphere and entered interstellar space – a first for human spacecraft.