![]() ![]() Of course, slowing the wheel will make the spacecraft turn in the opposite direction, and you don't want that once the spacecraft is pointed in the right direction. At some point you must slow them down and start over. Now, you can't spin the wheels faster forever. If you've ever ridden the Magic Teacup ride at Disneyland, then you get the idea! Each wheel weighs 10 kg (22 pounds), and can be spun as fast as 6,000 rpm (revolutions per minute). In effect, it pushes the wheel faster or slower and, as a result, the spacecraft gets pulled in the opposite direction. ![]() If the spacecraft needs to turn in one direction, it changes the speed of one of the wheels in the opposite direction. These wheels can be spun at variable speeds using electric motors. These are literally spinning wheels - four in total: one for each rotational axis plus a spare in case one of the three isn't working. Mars Reconnaissance Orbiter therefore has devices called reaction wheels. Slow and steady turns are required to take high-resolution images of Mars from orbit. While the reaction control system thrusters allow the spacecraft to turn quickly, they're not good at slow and steady turns. That's okay as long as navigators can measure it and take this change into account in their subsequent calculations. If one of the thrusters in any of these couples fails, the other one can do the job almost as well, but the single thruster will add some lateral velocity. The reaction control thrusters are arranged in "couples" - pairs of thrusters that together can spin the spacecraft without giving it any lateral velocity. They also work with the reaction wheels in a special operation described below. The reaction control system thrusters are good at making quick turns, and are used for getting to new orientations quickly. They are unlike the other larger thrusters, which are used to change the linear velocity of the spacecraft - that is, its speed along a line. These thrusters change the speed of the spacecraft's rotational (spinning) motion. ![]() That's where the control devices come in. Once the spacecraft is armed with knowledge of where it is pointed and how fast it is turning, the question becomes how to change these things. ![]()
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