ARUN RATH, HOST:
Last year this time, we told you to mark a date on your calendar - July 14, 2015.
ALAN STERN: It's Bastille Day. To celebrate, we're storming the gates of Pluto.
RATH: Alan Stern is the principal investigator for NASA's New Horizons mission to Pluto. The day he's been waiting for is finally almost here. The New Horizons spacecraft will make its closest encounter with Pluto on Tuesday. We've had a long wait. It's been more than nine years since the spacecraft left the Earth.
(SOUNDBITE OF NASA NEW HORIZONS SPACECRAFT LAUNCH)
UNIDENTIFIED ANNOUNCER: Three, two, one - we have ignition and lift-off of NASA's New Horizons spacecraft.
RATH: Since it took off in 2006, it has traveled more than 3 billion miles. To prime us for New Horizon's big moment, we turned to Carolyn Porco. She's a planetary science rock star. In the ‘80s, she worked on the legendary Voyager missions that explored the outer planets, and she's the head of the imaging team for the Cassini probe that currently orbits Saturn. Porco says she does not consider Pluto a planet, but not to get hung up on that, because Pluto is well worth the visit.
CAROLYN PORCO: It's been this reminder at the raggedy edge of the solar system of how big the solar system is, and it's a place we really haven't been. That's why this encounter is so special.
RATH: This will be the first venture into the Kuiper belt, a region on the outer limits of our solar system. Porco says it's like a window into the past. On Tuesday, New Horizons will zip past Pluto, snapping photos and collecting data to transmit back to Earth. Porco expects to see a hazy atmosphere, maybe some icy volcanoes, similar to what Voyager 2 saw on Neptune's moon Triton. But, she'd rather be surprised.
PORCO: I hope that I'm entirely wrong and that there's nothing on the surface of Pluto that looks anything like Triton because that would be the most exciting - that would really be the, you know, the exploratory fix, you know, that we all - we all want where we could feel like, wow, this was worthwhile, you know? We're seeing something we've never seen before.
RATH: New Horizons has already started sending images back, teasing scientists with previously unknown features like a dark, whale-shaped band on the surface. That was spotted more than 3 million miles away. On Tuesday, the New Horizons spacecraft will be only 6,000 miles from Pluto.
PORCO: It's one of those events that is sacred in my business of exploring planets where we have been completely ignorant one moment and then the very next moment, we are aware. And it's like, you know, the doors have been blown off the house, the windows have been blown off, and we suddenly can see what there is to see. And it's living in one universe one moment and living in another one the next.
RATH: So get ready to celebrate this Tuesday, but be patient. It takes a little while to get the view from 3 billion miles away. Transcript provided by NPR, Copyright NPR.