Hubble’s Slews Without Fuel: Wheels and Earth’s Field Do the Pointing

Hubble steers by trading angular momentum among four internal reaction wheels and using magnetic torquer bars to transfer excess momentum to Earth’s magnetic field, a propellant-free attitude-control method that avoids contamination. The system provides three-axis redundancy, and slews are deliberately slow (about 14 minutes for a 90-degree turn) to ensure accuracy, with gyros sensing motion and wheels applying the torque. Magnetic unloading replaces thrusters (which would eject exhaust), keeping the telescope stable for observations; servicing has allowed extensions, but the platform cannot raise its orbit on its own.