How soon will Voyager 1 leave the solar system?S

Thirty-five years ago today, NASA launched its Voyager 1 spacecraft on a mission to photograph Jupiter and Saturn at unprecedented levels of detail. On November 16, 1980, the spacecraft captured the photograph of Saturn you see up top. Four days later, its primary mission was over.

But Voyager 1 had another mission, one that continues to this day: to explore the outer reaches of the solar system. In 35 years, Voyager has put a staggering 11 billion miles between itself and the Sun, soaring through space at speeds approaching 11 miles per second. Today, it is "dancing on the edge" of outer space as it prepares to enter what astronomers call the interstellar medium — but what is this region of space, exactly, where is it, and how do astronomers know we're so close?

Before we address where Voyager 1 is heading (the interstellar medium, or ISM for short), let's quickly review where it's been. Since launching from Earth three and a half decades ago, Voyager 1 has been hightailing it through a bubble of space known as the heliosphere, a region of the solar system dominated by streams of solar wind — charged, subatomic particles that are given off by the sun:

How soon will Voyager 1 leave the solar system?S

As the diagram above illustrates, the ISM is what lies beyond the cosmic bubble of solar wind that surrounds our sun. Now remember that Alpha Centauri, the star system nearest our own, resides over four light years from Earth — that's a distance thousands of times greater than the one Voyager has traveled in the last 35 years. The ISM is the matter that makes up the regions of space interspersed between the galaxy's scattered star systems.

"Voyager tells us now that we're in a stagnation region in the outermost layer of the bubble around our solar system," explained Ed Stone, Voyager project scientist, last December, when the spacecraft started showing signs it was nearing its interstellar destination. "[It's] showing that what is outside is pushing back. We shouldn't have long to wait to find out what the space between stars is really like."

How soon will Voyager 1 leave the solar system?S

The farther from the sun Voyager gets, the less solar wind it experiences, allowing high-energy electrons from the ISM to leak into the heliosphere and "push back," as Stone puts it, on the Sun's solar wind. Using Voyager 1's low-energy charged particle instrument as a sort of windsock for subatomic particles, NASA can get a feel for how the cosmic breezes are blowing at the spacecraft's current position. The point where pressure from the solar wind and the interstellar medium balance is known as the the heliopause. You can think of the heliopause as the cosmic signpost demarcating where our solar system ends, and interstellar space begins.

So how long until Voyager 1 bursts forth from the heliosphere and into the ISM for good? Back in December, Voyager's "windsock" suggested that for the first time, the ISM's wind was blowing back, a sign that it was traversing never-before-encountered cosmic territory. Months later, NASA reported another surge in intensity from the ISM's charged particles. Since then, readings have fluctuated, but the general consensus among NASA scientists seems to be that the day when the first human-made object enters interstellar space is fast approaching.

"The question is, how much further is it to the heliopause?" Stone asked at a lecture, delivered yesterday at JPL. "We don't know," he continued, noting that Voyager 1 travels about a billion miles every three years. "I can't tell you if it's days, months or years," he explained, but "it's hard to imagine that it's going to be too much longer."

In the days (or months, or years) ahead, NASA will wait to receive word from Voyager 1 that there has been a major shift in the direction of the magnetic field lines that surround it. That, note Stone and his colleagues, will be the clearest indication that the spacecraft has "left the bubble" of our solar system, and entered the uncharted reaches of the Milky Way's vast interstellar medium.