'Star Trackers' Help Juno Find Its Way Space navigation is tricky. There's no up or down, no left or right, and no road signs. This device uses stars to help determine if a spacecraft is off course. It takes nifty pictures, too.

'Star Trackers' Help Juno Find Its Way

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In space, there are no road maps. So if you happen to be heading to Jupiter, you better bring your own navigation tools. NPR's Joe Palca has been describing how inventions solve problems as part of his series, Joe's Big Idea. And today he has the story of a device that is critical for NASA's Juno spacecraft to navigate its way to Jupiter.

JOE PALCA, BYLINE: Getting around on Earth is relatively straightforward.


UNIDENTIFIED NARRATOR: On Earth, we move between points which remain fixed relative to each other.

PALCA: But as this 1967 NASA documentary pointed out, finding your way around in space presents a whole new set of challenges, challenges scientists and engineers were going to have to overcome.


UNIDENTIFIED NARRATOR: Eventually, man will set out toward another planet millions of miles distant. The instruments and techniques of modern science will guide his spacecraft with an accuracy and precision inconceivable to earthbound navigators.

PALCA: John Leif Jorgensen has spent a career pursuing that kind of accuracy and precision. He's at the Technical University of Denmark. He says if you want to go to Jupiter, for example, you have to know exactly where Jupiter is, but you also have to figure out where you are.

JOHN JORGENSEN: The problem in space is there's no up and down. So the only real reliable source to find your orientation is where's the sun.

PALCA: But knowing where the sun is isn't enough. You need another reference point. And Jorgensen says fortunately, there are plenty of reference points out there - stars. There are two reasons stars are good for spacecraft navigation. One, they don't move around, and two -

JORGENSEN: It's always night up in space, so we can always see stars.

PALCA: To navigate by the stars, Jorgensen builds instruments called star trackers. A star tracker is essentially a camera with a built-in star catalog. It looks at a patch of sky and picks out stars it knows and tells the spacecraft which way it's pointing, its orientation. So now it knows that Jupiter is, say, ahead and to the right and Earth is below and to the left. And that's important information for a spacecraft like Juno.

JORGENSEN: Know where to point the antenna, the telescopes and stuff like that.

PALCA: But from a navigating standpoint, there's still a problem.

JORGENSEN: It doesn't give you the position.

PALCA: Position is different from orientation. Orientation tells you where you're pointed. Position tells you where you are in space, how far you are from the Earth and the Sun and Jupiter.

JORGENSEN: To find the position when you go deep space, like on Juno, you need to have help from somewhere.

PALCA: The help comes from something called the Deep Space Network. These are three large radio antennas, one in California, one in Australia and one in Spain. They receive radio signals from Juno and use those to figure out where the probe is and how fast it's moving.

JORGENSEN: And that works beautifully for deep space spacecraft. It's just that it is relatively expensive to track a satellite with a big dish antenna. So people have been looking for different ways of navigating. That's a problem you also can solve with the star tracker.

PALCA: Jorgensen is working on an advanced version of the star tracker that can navigate all on its own. That's for the future. NASA is still using the Deep Space Network to guide Juno on its path. There are actually four of Jorgensen's star trackers on Juno, but they aren't being used for navigation. His are extremely high-precision, and they're being used by one of Juno's instruments to know precisely where it's pointing as it maps the planet's magnetic field. After all, if you've gone all that way to Jupiter, you don't want to make an inaccurate map. Joe Palca, NPR News.

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