by Athena Cai
Since liftoff on April 24, 1990, the Hubble Space telescope has allowed us to take leaps in what we understand about the cosmos, and is still making contributions today. After I bought a telescope on Craigslist this summer, I began to wonder just how NASA keeps the Hubble steady and accurate enough to take pictures of bodies in deep space when it had taken me a few nights’ practice just to see a clear image of the moon through my eyepiece. Unlike the Hubble, my telescope this summer was not orbiting the Earth while hurtling through space. Ethan Siegel of Forbes compares the accuracy of the Hubble telescope to “shining a laser beam onto a quarter and hitting George Washington’s eye, without fail, from a distance of 14 kilometers away.”
The answer to achieving this astounding stability and accuracy is in gyroscopes.
A gyroscope is a wheel or disk mounted to spin rapidly about an axis and also free to rotate about one or both of two axes perpendicular to each other.
Satellites and the International Space Station also use gyroscopes to find and maintain the appropriate attitude so crucial to their proper functioning.
Gyroscopes have two functions in the spacecraft:
- Providing attitude reference signals to AACS computers
- As control moment gyroscopes (CMGs), which are “fairly massive attitude control devices at the output of AACS computers”
Rigidity in Space
What does it mean for gyroscopes to ‘provide attitude reference signals’? When drifting in space where gravity is too weak to hold you down, it is hard to maintain a sense of direction. Where is up and where is down? When gyroscopes spin they act on a property called rigidity in space. This means that it will “remain in a fixed position in the plane in which it is spinning”. Consequently they won’t drift in random motion in 0G. We can thus use the relatively stable gyroscope as reference in the attitude of the spacecraft from which we can collect and input data into the AACS computers. Aircraft instruments and the iPhone 11 and 11 Pro cameras also use gyroscopes and rigidity in space.
Newton’s Third Law
When walking to your kitchen, your movement is only possible because you have something to brace yourself against: The Earth. This is because of Newton’s Third Law: When you exert a force on an object an equal and opposite force is exerted back on you. Out in space there is nothing for spacecraft to exert a force on in order to move or turn. Spacecraft use control moment gyroscopes (CMGs), also known as reaction/momentum wheels, in combination with thrusters to provide motion. How?
Because of Newton’s Third Law, the force in rotating the control moment gyroscope known as torque creates an equal and opposite force. This force is what rotates the spacecraft. In other words, in order to rotate the spacecraft in one direction, you spin the gyroscope in the other direction. In order to cover all three axes of rotation, three CMGs can be placed in three different axes; the International Space Station uses four.
I think it’s beautiful that we are using properties inherent to devices so simple as the gyroscope to lead research in astronomy. Gyroscopes are found both in the hands of children as toys and aboard the ISS as vital tools.