Spacecraft like the Hubble Space Telescope and the International Space Station must be able to twist and turn to aim their science instruments toward different points in space as they orbit the Earth. A common way to do that is to use a set of Control Moment Gyroscopes, or CMG's, like the one seen here that was built by NASA during the 1980s. Inside the core shell is a big, heavy gyroscope that spins at thousands of revolutions per minute. By tipping the rotor in one direction or another, the spinning wheel causes a gyroscopic torque that changes the spacecraft's orientation, or attitude. The system works best when there is more than one CMG in use. Note that the CMG only changes a spacecraft's attitude, not its altitude or the shape of its orbit. Rocket engines are still needed for that job.