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MELISSA BLOCK, host: It has spent 33 years traveling nearly 11 billion miles from the sun at a rate of about 38,000 miles per hour, sending back spectacular images of the planets and their moons. And now the space probe Voyager 1 is poised to cross the outer edge of our solar system and venture into interstellar space. We say, poised, meaning in about four years.

The astrophysicist Neil deGrasse Tyson joins us to talk about that impending transition. He directs the Hayden Planetarium at the American Museum of Natural History in New York. Welcome back to the program.

Dr. NEIL DEGRASSE TYSON (Director, Hayden Planetarium): Happy to be with you.

BLOCK: This is the farthest any spacecraft has gone by far, right?

Dr. TYSON: Mm-hmm.

BLOCK: NASA announced this yesterday, the Voyager 1 will be leaving the region called the heliosheath（日鞘）. What does that mean?

Dr. TYSON: Well, there's a boundary between what we call the heliosphere, and that's the region of influence that the sun has over its environment. The sun has a wind. You might've heard of that before - the solar wind. That's what makes the aurora borealis（北极光）. These are particles from the sun that are charged, and when they hit the earth's atmosphere they collide with the atmospheric molecules and render them aglow.

Well, this stream of particles doesn't just stop at earth, it goes out to the outer reaches of space. And you reach a point where particles from the sun are no longer distinguishable from random other particles traveling throughout the galaxy. And that's the realistic edge of the sun's influence out there. And that's where Voyager 1 is approaching.

BLOCK: And once Voyager 1 leaves the sphere of influence of the sun and goes out into interstellar space, what do you think astrophysicists like yourself might be learning from it?

Dr. TYSON: Well, first of all, the transition from being within the heliosphere of the sun to being outside of it is itself an interesting boundary. There's still a lot of interesting measurements that can be made there. Once you're completely beyond it, then you're - that's it, the sun is no longer really operating on you.

And once you're outside the zone, it's not clear whether things will change much for you because you're kind of wandering

BLOCK: Will Voyager 1 still be sending information back once it's crossed into interstellar space?

Dr. TYSON: Yes. It has some - while it's not undergoing any kind of propulsion at the moment, it still has a way to generate electricity. And it's that small amount of electricity that it uses to communicate back and forth with us. So there's no reason why we couldn't keep talking to Voyager well into the next decade.

BLOCK: I read this from the former mission design manager of Voyager, calling it the greatest mission of discovery in the history of mankind. What do you think we've learned from Voyager 1 that's so spectacular?

Dr. TYSON: I might be a little biased, but I have to agree that because it wasn't just a mission to one planet, it was a mission to many. It was carefully conceived to be a sort of multi-cushion pool shot from one planet around to another planet. And it was sort of a grand tour of the outer solar system. And then it continued beyond.

For me, it told me that the moons of planets can be as interesting or more interesting than the planets themselves. These planetary missions, on the flybys that they executed, got good close-up images of Europa and Io and Titan and Ganymede. These are the names of moons around Jupiter and Saturn. And when you see them, they have volcanoes and ice sheets and lakes and you say, my gosh, it's not just this big ball of gas out there, they're surrounded by worlds.

And when we think of a world, you think of a place you might want to visit. And so I think of the solar system differently in the era of Voyager than I did before.

BLOCK: Well, Neil deGrasse Tyson, thanks so much.

Dr. TYSON: Happy to be with you again.

BLOCK: Astrophysicist Neil deGrasse Tyson directs the Hayden Planetarium at the American Museum of Natural History in New York. He's also author of the book "The Pluto Files: The Rise and Fall of America's Favorite Planet."

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