ARI SHAPIRO, HOST:

NASA's probe, Juno, has traveled for five years over more than 1.5 billion miles, and now it is about to arrive at Jupiter. Later tonight, the probe is supposed to fire its main engine for 35 minutes, slowing it down just enough for Jupiter's gravity to capture the probe and swing it into orbit around the giant planet.

Mission control for Juno is at NASA's Jet Propulsion Laboratory in Pasadena, Calif. That's where NPR science correspondent Joe Palca is, who joins us on the line to tell us more about the mission.

Hey there, Joe.

JOE PALCA, BYLINE: Hey there.

SHAPIRO: Well, you've watched a lot of NASA missions unfold over the years. What's the feeling at JPL like right now?

PALCA: Well, it's this incredible feeling of extreme excitement like we've finally arrived, and total tension, like, oh, my gosh, is this really going to work?

SHAPIRO: Yeah. It sounds like it's a very delicate procedure. The probe fires its rocket for 35 minutes. It's way out there in the solar system. What if things go wrong?

PALCA: Well - so yes. So there is a very complex ballet that the spacecraft has to go through. You know, right now its solar panels are pointed toward the sun and its antennas are pointing toward the Earth - its main antenna. And it's got to sort of roll over and point its engine in the direction it's going 'cause it's trying to break with its engine, and that means no solar power and just a low-gain antenna. And so it also has to spin faster to stabilize it so it doesn't wobble around and it has to fire the engines. And the engines have worked before but not at Jupiter before.

And so all this has to happen, and then afterwards it has to roll back over and point back toward Earth and get the sun back on the solar panels or it'll run out of juice, and that's another thing they have to worry about.

SHAPIRO: When will we know if everything went according to plan and this actually worked?

PALCA: Well, this is the crazy thing. It takes 48 minutes for a radio signal to come from Jupiter to the Earth. So in fact this entire thing will have happened before we ever hear about it on Earth because it's 35 minutes for the rocket and 48 minutes for the radio signal. But as the engine is firing, there will be tones coming from the spacecraft that will say, hey, I've switched on the engine.

And they can also measure the Doppler shift - this is a slight change in frequency of the radio signal coming from the spacecraft - and that will tell them that it's slowing down. So they'll get a tone at the start and a tone at the end. And then they'll relax, although they still have to turn back toward Earth with the solar panels - or, back toward the sun with the solar panels. So there's that.

SHAPIRO: So after they've popped the champagne and celebrated if it gets successfully into orbit around Jupiter, what do scientists hope to learn from this probe?

PALCA: Well, Jupiter is the big - well, I call it the big kahuna of the solar system. It's huge, and if you understand Jupiter, you begin to understand how the solar system fit together. They want to know more about its radiation environment. It's got a very harsh radiation environment. It's got an aurora. It's got a very powerful magnetic field. It's got a very powerful gravitational field. They have instruments that'll probe what the interior composition is. And so there's a lot of data that will tell them, first of all, how Jupiter formed, but also they think this will tell them something about how all the planets in the solar system might have come into being.

SHAPIRO: Are we going to get good photos of Jupiter out of this?

PALCA: There will be a camera on board. It will take great pictures, but it's not considered a scientific instrument. It's actually more to engage the public. In fact, this is interesting - the public will be able to vote to say which way you want to point this Juno cam and what you want it to take a picture of, and that's what NASA will point it at.

So it's a cool mission, and it will have fantastic pictures, up-close pictures of Jupiter, but it's not so much a science instrument. That's not a priority for the scientists.

SHAPIRO: That's NPR's Joe Palca speaking with us from Juno mission control at NASA's Jet Propulsion Laboratory in Pasadena, Calif.

Thanks Joe.

PALCA: You bet. Transcript provided by NPR, Copyright NPR.