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Comment by bordakt | Hacker News Reader

You are thinking of the sensor in smartphones, in this case it is a spinning disc that uses gyroscopic torque to rotate the spacecraft.

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No, these are actually referring to the gyroscopic sensors on the spacecraft. This article [0] makes it a little bit more clear. While there are actuators that use gyroscopic torque, referred to as Control Moment Gyros (CMG) Hubble uses reaction wheels for pointing the spacecraft.

[0] https://science.nasa.gov/mission/hubble/observatory/design/h...

So there are two different technologies that are both called "gyroscope"? One a sensor detecting rotation, the other an actuator inducing rotation?

The sensor you’re describing is usually called an accelerometer.

No, accelerometers detect acceleration in linear direction (up/down, left/right, forward/backward), while gyroscopes detect rotation (pitch, yaw, roll). Smartphones have both, to detect all six degrees of freedom.

Edit: Apparently, for smartphones, all these sensors are integrated into a single MEMS chip, one sensor for each degree of freedom, three accelerometers and three gyroscopes: https://youtube.com/watch?v=9X4frIQo7x0

BTW, smartphone acceleromoters are also packaged for hobbyists; a board like https://www.adafruit.com/product/2019 can be easily used to make any number of applications when coupled with a microcontroller. https://learn.adafruit.com/adafruit-mma8451-accelerometer-br...

And the reason accelerometers can be used for sensing orientation here on Earth is, of course, because of the constant force of gravity :)

Interesting point. I guess this is much less precise than gyroscopes though.

No, the comment you're replying to is confused. These gyroscopes are entirely used for determining Hubble's orientation. Reaction wheels effect the actual rotation, and Hubble's are fine.

I think the confusion is a result of imprecise language talking about how they are used to slew the telescope. They are "used" but as feedback, in conjunction with other systems. Also, there have some prominent reaction wheels failures on other missions, and that contributes to the confusion.

Thanks, this makes sense. I guess I should look up the difference between gyroscopes and reaction wheels next.

Per your name: https://m.youtube.com/watch?v=n_6p-1J551Y&pp=ygUG

FWIW I think 2 of the Hubble's 4 reaction wheels are dead, which also requires some interesting workarounds

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