Hubble’s reaction wheels

To change its orientation and to correct for drifts Hubble does not use propellants. Their fumes could contaminate Hubble’s observations and limit its lifetime.

Instead, Reaction Wheels steer the telescope, using the elegant principle of Newton’s third law: if one of the wheels turns clockwise, Hubble will turn counterclockwise.

This animations shows two reaction wheels, like the ones installed in the NASA/ESA Hubble Space Telescope.