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| ||||| Saturday, March 09, 2002 - 06:42 pm With the Hubble orbiting the earth at 27,000 km/h, I was wondering whether the Hubble needs to constantly manoeuvre around in space so that it stays pointing at whatever it's viewing. And then of course when it's captured that picture, it needs to re-align to another target. Does it have small thrusters or something to manoeuvre it? Are these working almost constantly to keep the telescope on target? And have they been refuelled? | |
| ||||| Saturday, March 09, 2002 - 07:32 pm Hubble does not use rocket motors to orient itself. It uses an object called a Reacton Wheel. We replace one of these on the second space walk day. It looks like a large oval shaped object. Image you are floating in a boat with a large weight. If you rotate the weight in a counter-clock wise direction (looking from up high), your boat and you will rotate in a clock-wise direction. This action conserves your total momentum since there are no external forces on your boat. This manner of manuevering is what HST uses with its four reaction wheels, called RWAs. By using only electrical energy, HST can point in any direction and do this often without penalty. Another way HST uses electrical energy only to orient is with its torquer bars. These are the diagonally mounted bars on HST's light shield. They form magnets that use the Earth's magnetic field to exert a force on HST. We use these to regularly 'unload' excess momentum in the RWAs. That is a story for a different message (if you care to know more). http://www.edcheung.com/job/sm3b/sm3b.htm | |
| ||||| Saturday, March 09, 2002 - 07:46 pm Amazing. Please tell me more. | |
| ||||| Saturday, March 09, 2002 - 08:00 pm Also, I don't know how long an exposure is normally needed, but surely the Hubble would have moved through quite a bit of space in it's 94 minute (??) orbit of the earth -- and hence the direction to the target would have altered, possible dramatically. So during an exposure would these RWAs be constantly active keeping the telescope alligned? Also, there would need to be consideration given to whether the Earth itself was going to obscure the later stages of a long exposure picture or measurement. | |
| ||||| Saturday, March 09, 2002 - 08:02 pm Back to the boat analogy. Suppose the water is swirling around. So to keep your boat steady, you spin your weight in the opposite direction. However, to get any torque from your weight, you need to ACCELERATE the weight. Thus simply rotating it at a constant speed will not exert any torque on the boat. Since the water is exerting a constant torque on your boat (it is swirling), you need to apply a constant acceleration to your weight. Pretty soon, your weight is spinning so fast that you run out of ability to go any faster. To enable you to continue to point the boat, you need to 'unload' this momentum, and find a way to zero the speed of your weight again. This is the same with HST. The real-world motor in the RWA cannot ramp its speed to infinity. At some point, it reaches some safe limit (you don't want this wheel to fly apart), and you need to 'unload' the momentum stored in it. To do this, you need to find a way to apply a constant torque onto HST. This source is the torquer bars on the light shield. We energize them, and using the Earth's magnetic field, it causes a torque on HST that allows the wheels to slow down to their normal operating range. The RWAs and torquer bars are a great example of physics at work on a space craft. No wonder it was my favorite subject in high school (back in Aruba). http://www.edcheung.com/job/sm3b/sm3b.htm | |
| ||||| Saturday, March 09, 2002 - 08:07 pm An exposure can indeed take hours. Using the Fine Guidance Sensors, and the RWAs, HST stays locked solid onto its target. It is a beautiful thing. The FGS are cameras in their own right. When an exposure is setup, guide stars are chosen to assist in keeping HST locked onto its target. By keeping the stars always in a certain location in HST's field-of-view, we know that we are locked onto a certain part of the sky. Like I said, it is a beautiful thing. http://www.edcheung.com/job/sm3b/sm3b.htm | |
| ||||| Saturday, March 09, 2002 - 08:28 pm Thanks Edward. I appreciate your assistance with this -- it's helped me understand the incredible complexity and sophistication of the system and people involved. | |
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