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The design of Juno and the science measurements that we wanted to make were all taken into consideration at once when we were trying to figure out how to do this the most efficient way. 

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Juno has three solar arrays that stick out. It could have had four, or five, or any number.

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We looked for something that was efficient and could be packaged 
inside of a rocket. That was one of the first requirements.

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We also wanted to carefully make it so that the science instruments could look out between the 
solar arrays. We knew we were going to have to see a certainÉ what's called a field of view.

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I wanted to be able to look up and down the magnetic field lines as we flew 
over Jupiter, so I could see the particles that were creating the aurora.

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When we come off of the launch vehicle, we're spinning at about 1.4 rpm.

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When we do our burns, we spin up to about 5 rpm to give us a little more stabilization, 
and when we're going around the planet, we're spinning at about 2 rpm.

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Juno spins like a propeller, where the propeller's kind of 
facing the sun because they're all solar powered

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and we want to have each of the instruments be able to look out between the solar arrays 
and see Jupiter - or wherever they need to look in order to do their science objectives.

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There's two basic reasons why we want to have a rotating spacecraft.

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One is really simple: it's just stable that way. If you spin something, it stays spinning.
It's like a gyroscope. We call it a simple spin or spinning spacecraft.

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The other reason is we can use a spinning spacecraft to let each instrument get its turn to see Jupiter.

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If I had only one sensor looking in one direction, because I was spinning twice a minute, 
you'd think I'd be able to look in all directions every half a minute.

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But I'm moving so fast going over Jupiter that in fact I'm going to pass the field lines that are making 
the aurora and I might only be looking up, or only looking down, or looking sideways by mistake.

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And so by putting three sensors around, looking all the time between the solar arrays, we ensure that no matter how fast we go across, we're looking in the right direction to make our measurements.