I've been asked in to help with the gravity science experiment.
Measuring Jupiter's gravity depends on measuring
changes in the spacecraft velocities that goes around Jupiter.
What we are looking is for variations in that acceleration rate
that tells us something about the change in the gravitational field
of Jupiter as it gets closer to the surface
associated with the mass structure inside.
And so what we're trying to do, then, is measure the spacecraft's velocity
as a function of time very, very accurately.
And we do that by measuring the Doppler shift of the radio signal.
So if you've every heard a fire engine going by
with its siren as it's approaching you it's high pitched (makes a high noise)
and when it goes by it goes (makes a low noise)
Right? That Doppler shift, that change in pitch
tells you about the velocity of the fire engine relative to you. So the instrument that we use
is sort of a combination of the listening on the ground to the radio signal
and the radio signal transmitted by the spacecraft.
So the biggest part of the instrument, of course
is the 34-meter diameter radio antenna
at Goldstone, California.
Then on the spacecraft there's a high gain antenna
that's pointed at the earth.
So the radio instrument measures the voltage coming in
from the antenna from the earth
and then it transmits the voltage that goes fed out to the radio signal to send back to the earth.
And then the gravity measurement tells us something about density variations
which then can tell us something about whether there's storms on the outside
or whether they penetrate all the way through,
and whether there's some hint of a structure at the center
other than just compressed gas of the hydrogen.