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Comment by cubefox | Hacker News Reader

Naive question: Today, smartphones use microscopic gyroscopes based on MEMS (micro-electromechanical systems) technology. Would these also work on Hubble? I suspect they'd work fine in space, perhaps while also being more durable (the failing Hubble gyroscopes were only installed in 2009). Though I guess their precision is not good enough for a telescope.

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The accuracy of MEMS gyros is orders of magnitude worse than the gyros on Hubble. I suspect based on this article [0] that Hubble uses hemispherical resonator gyros (HRG [1]). The really high accuracy rate sensors that you find in spacecraft are usually HRGs or Fiber Optic Gyroscopes (FOG).

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

[1] https://en.wikipedia.org/wiki/Gyroscope#HRG

Oh, this made me worried about JWST (as it can't be serviced once the gyroscopes break), but I found this:

> To detect changes in direction JWST uses hemispherical resonator gyroscopes (HRG). HRGs are expected to be more reliable than the gas-bearing gyroscopes that were a reliability issue on Hubble Space Telescope. They cannot point as finely, however, which is overcome by the JWST fine guidance mirror.[18]

https://en.wikipedia.org/wiki/Spacecraft_bus_(James_Webb_Spa...

So Hubble uses gyroscopes that are more precise but less reliable than the HRGs used in JWST.

Interesting, TIL that Hubble uses gas-bearing gyroscopes. I been struggling to find any other spacecraft that use those gas-bearing gyros. The only other one that seems to be using them (that is still operational) is the Chandra X-ray Observatory.

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

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

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...

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