Gyroscopes

Gyroscopes are mounted disks that spin so that their axes can turn freely and maintain a constant orientation in space. Consequently, they play an important role in space travel as they are used to stabilize spacecraft and keep them pointed in a specific direction. Any changes in a spacecraft's orientation detected by onboard gyroscopes can be used by guidance systems to make adjustments. This ability to retain a particular position is vital. For instance, it allows controllers to orient a spacecraft so that a communications antenna is pointed toward a receiving antenna and so that solar panels are pointed toward the Sun.

Gyroscopes are the heart of the space shuttle's Inertial Measurement Units (IMUs). IMUs measure the shuttle's attitude and velocity , and this information is used by the shuttle's navigation, guidance, and flight control systems for steering and control.

The basic principle of the way in which gyroscopes provide stabilization for spacecraft has not changed since gyroscopes were invented in the nineteenth century by Jean Foucault. The physics of gyroscopes involves the conservation of angular momentum . A spinning top can be used to illustrate how a gyroscope works. If one pushes a spinning top so that it tilts, the top will right itself. This ability to retain position is used in space to ensure that satellites and spacecraft remain in the proper orientation and do not tumble out of control.

Late in the twentieth century the importance of gyroscopes to space missions was demonstrated on two occasions. In November 1999 the science missions of the Hubble Space Telescope had to be put on hold when one of its three gyroscopes failed. The National Aeronautics and Space Administration (NASA) had to send a rescue mission to the Hubble in which space shuttle astronauts made a special space walk to replace the defective gyroscopes. In June 2000 NASA was forced to perform a controlled re-entry of the fully functional Compton Gamma Ray Observatory because a gyroscope had failed in December 1999. The Compton Observatory still had two working gyroscopes, but NASA was concerned that if one of those gyroscopes failed, NASA controllers would not be able to control the descent of the spacecraft. Because the Compton Observatory was one of the largest objects ever placed in space (about 17 tons), NASA felt that it would be prudent to bring Compton down while it was sure it could fully control the observatory's attitude during the deorbiting maneuvers, ensuring that it did not hit populated areas of Earth.

Gyroscopes are essential to any space mission. As with any element of a spacecraft, certain traits—smaller, lighter, longer life span, reduced power consumption—are desirable. This has been a driving force in the development of new gyroscope technology.

The major change in gyroscopes since their discovery has been the shift from mechanical to electronic devices. Nonetheless, gyroscope units used in space can weigh up to 9 kilograms (20 pounds). In 1999 NASA's Jet Propulsion Laboratory developed an experimental gyroscope on a chip. The new device measures 4 millimeters by 4 millimeters (about the size of a shirt button) and weighs less than 1 gram. It will be some years before these devices are used on spacecraft.

see also Guidance and Control Systems (volume 3); Navigation (volume 3).

Salvatore Salamone

Bibliography

Feynman, Richard P. The Feynman Lectures on Physics. Reading, MA: Addison-Wesley, 1970.

Halliday, David, and Robert Resnick, with the assistance of John Merrill. Fundamentals of Physics. New York: Wiley, 1988.