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[Music throughout]

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[Moving Roman: Reaction Wheels]

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While thrusters are great for moving
a spacecraft around,

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they aren't as good or efficient

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at controlling where the spacecraft points.

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Instead, rotating wheels and conservation
of angular momentum

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make it possible to turn in any direction
without any outside influence.

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Most space telescopes,

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from NASA's Hubble to Webb,
and everything in between, 

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use components called reaction wheels to
change where they are pointing.

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The reaction wheels run on electricity,
unlike thrusters,

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which are less precise
and can run out of fuel.

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The reaction wheels are basically
flywheels driven by electric motors.

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They spin to store angular momentum.

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By slowing or speeding the rotation of a given wheel,

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changing the amount of momentum,

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a computer can precisely adjust
how the spacecraft points

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around its center of mass in one plane.

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With three wheels set at specific angles,

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a satellite can control its pitch,

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roll, 

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and yaw, 

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and then hold that position
without any change.

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NASA's Nancy Grace Roman Space Telescope
has added

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its set of reaction wheels to the
main spacecraft body,

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or bus.

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Roman has six reaction wheels,

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rather than the necessary three, 

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to give it more angular momentum for faster pointing,

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as well as complete redundancy

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should any one wheel fail.

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Each of Roman's wheels is 18 inches across

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— about the size of an extra large pizza —

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weighs roughly 45 pounds,

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and spins up to 4000 rpm.

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As long as the solar cells
provide them with electricity,

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the reaction wheels could repoint
Roman indefinitely,

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except that they have one function
requiring thrusters.

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The reaction wheels counteract
the pressure from sunlight,

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which exerts a tiny, but steady, force

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that, over time, would push Roman out of alignment
with whatever it is observing.

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The wheels can exactly offset that pressure
by slowly spinning faster.

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When a wheel finally reaches its maximum
rotation speed,

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it needs to “unload”, or slow down,

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so that it can begin to freely vary its speed again.

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Roman engineers gently fire specific
thrusters,

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causing the maxed-out reaction wheels

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to compensate the rotational
force by slowing down.

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Thrusters are also critical

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for any action that physically moves Roman.

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Reaction wheels can only rotate the spacecraft
in place,

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they can't cause a change in location.

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Fortunately, Roman's extensive

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set of thrusters will be well fueled
and fully up to the task,

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positioning the spacecraft
and unloading momentum

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so that the reaction wheels can keep it
perfectly pointed at the stars.

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[Music fades]

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[NASA]