Lesson 6 of 6 · Rotation and stability
Keep a spacecraft pointed where it belongs
9 minute lesson · 3 checks along the way
Your goal for this lessonDistinguish attitude sensing from actuation and evaluate a simple reaction-wheel exchange without treating spin as automatic stability.
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Pointing and traveling are different tasks
means a spacecraft’s orientation. Turning a telescope toward a star changes its attitude; it does not by itself redirect the center of mass along a new orbit. A spacecraft can travel rapidly while holding a nearly fixed pointing direction, or spin while its center of mass follows the same trajectory. Guidance must distinguish where the vehicle is going from where its instruments are looking.
is one possible design choice. Juno rotates so fixed instruments sweep their fields of view through space; other spacecraft aim instruments steadily at one target. Stability depends on mass distribution, the chosen rotation axis, flexible structures, and disturbances. Merely saying that an object spins does not prove that every pointing direction remains steady or that a disturbance will correct itself.
Sense the error, then apply a torque
A control system compares measured orientation and rotation with the desired state. measure rotation, and star sensors help establish direction against the sky. Sensors inform the controller; an actuator produces the . The Hubble Space Telescope uses reaction wheels and magnetic torquers for this physical control. Its rate-sensing gyroscopes are not interchangeable with the larger wheels that turn the observatory.
A motor changes a ’s , while the spacecraft body changes by the opposite amount if external torque is negligible. Different moments of inertia mean the body and wheel need not rotate at equal speeds. The motor consumes electrical energy, even though the combined angular momentum can remain zero. This is the rotational counterpart of two objects recoiling with opposite linear momenta.
Check 1 of 3
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Work through a decision
Choose a wheel response using one frame
Consider a fictional spacecraft initially at rest about one axis, with negligible external torque. Its wheel has 2 kg m² and its body has 20 kg m² about that axis. After a motor command, the wheel rotates at +5 rad/s measured in the original . Its angular momentum is therefore +10 kg m²/s, so the body must have −10 kg m²/s.
Dividing that body by 20 kg m² gives a body of −0.5 rad/s in the same inertial frame. The wheel’s speed relative to the rotating body is different: +5 minus −0.5 equals +5.5 rad/s. Specifying the frame avoids mixing a motor’s relative-speed reading with an inertial conservation calculation. These invented values demonstrate the principle; they are not Hubble specifications.
Check 2 of 3
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A stable design still needs limits and evidence
A reaction wheel cannot absorb accumulating angular momentum without limit. When environmental torques drive its speed toward an operating limit, the spacecraft needs another way to exchange angular momentum with its surroundings. Hubble’s interact with Earth’s magnetic field to help reduce wheel speeds. Thrusters offer another approach on other spacecraft by sending angular momentum away with expelled material.
Ulysses provides a useful caution about flexible hardware: after a boom was deployed, heating-induced flexing contributed to unwanted wobble, and engineers developed thruster corrections. Spin did not make the spacecraft immune to disturbances. The STS-125 photograph shows Hubble’s physical observatory after servicing, but a sharp photograph cannot establish hours of pointing stability. That requires timed attitude measurements, sensor performance, and control-system evidence.
The Hubble Space Telescope after STS-125 servicing, separating from Atlantis on May 19, 2009, in NASA image S125E011810. The rear structure, antennas, and solar arrays are visible; internal reaction wheels and rate sensors are not exposed in this view. The preserved source frame includes its image identifier.
Look closely
Find the telescope body and projecting hardware. What would distinguish one well-exposed photograph from evidence that Hubble kept a target centered for a long observation?
See what to notice
Pointing stability needs calibrated orientation measurements over the relevant interval and an account of sensors and actuators. The exterior photograph supports the observatory’s physical identity and servicing context, but does not expose or measure its internal control performance.
Check 3 of 3
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Put it together
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