WEBVTT

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I've been asked in to help with the gravity science experiment.

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Measuring Jupiter's gravity depends on measuring

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changes in the spacecraft velocities that goes around Jupiter.

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What we are looking is for variations in that acceleration rate

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that tells us something about the change in the gravitational field

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of Jupiter as it gets closer to the surface

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associated with the mass structure inside.

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And so what we're trying to do, then, is measure the spacecraft's velocity

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as a function of time very, very accurately.

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And we do that by

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measuring the Doppler shift of the radio signal.

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So if you've every heard a fire engine going by

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with its siren as it's approaching you it's high pitched (makes a high noise)

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and when it goes by it goes (makes a low noise)

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Right? That Doppler shift, that change in pitch

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tells you about the velocity of the fire engine relative to you.

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So the instrument that we use

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is sort of a combination of the listening on the ground to the radio signal

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and the radio signal transmitted by the spacecraft.

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So the biggest part of the instrument, of course

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is the 34-meter diameter radio antenna

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at Goldstone, California.

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Then on the spacecraft there's a high gain antenna

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that's pointed at the earth.

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So the radio instrument measures the voltage coming in

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from the antenna from the earth

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and then it transmits the voltage that goes fed out to the radio signal to send back to the earth.

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And then the gravity measurement tells us something about density variations

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which then can tell us something about whether there's storms on the outside

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or whether they penetrate all the way through,

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and whether there's some hint of a structure at the center

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other than just compressed gas of the hydrogen.