Many spacecraft have small jets that push them into different positions in space. Hubble has no jets because the exhaust gas from jets could damage its delicate mirrors. Instead, Hubble uses momentum to move. When Hubble needs to move to a new target, engineers on Earth radio a signal to the HST flight computer. The flight computer then activates the Reaction Wheels.
Reaction wheels are heavy fly wheels that spin.
As they spin, the momentum from their motion causes the telescope to move. There are four Reaction Wheels. By spinning each one at a certain speed and in a certain direction, engineers can point the telescop e anywhere they want.