Near the middle of last year, the planet-hunting satellite, Kepler, looked like it would never Keple again. On May 11, 2013, the second of Kepler’s four reaction wheels went out, which made precision pointing, a requirement for Kepler’s scientific mission, nearly impossible. Some of this site’s posts have covered Kepler’s calls for ideas about how to use the gimpy satellite. And a fantastic idea finally emerged for how to compensate for Kepler’s non-working reaction wheels.
Before we get to the solution (highlighted a ways below), there may be some of you out there wondering what exactly a reaction wheel (also known as a momentum wheel) is, and why it’s so important to satellite pointing/aiming. This requires a lengthy, English-based, explanation, with little math (since that’s my weakness). The main thing to understand is a reaction wheel is a part of a satellite’s navigation, guidance, and control system. It is actually in the “control” part, since the reaction wheel is responsible for moving a satellite.
But the resulting movement is not what you’d expect. It’s not the kind of movement that changes a satellite’s orbital path. It doesn’t move a satellite forwards or backwards in a particular direction. But a reaction wheel helps a satellite aim or look in a particular direction. It helps maintain, or change, a satellite’s orientation. In other words, a reaction wheel affects a satellite’s attitude. Confused yet? Maybe it’s beer time again?
Let’s use a kayak in the middle of a calm lake as an example. It’s a cheap kayak, because to save money, you bought one without a rudder. But it’s well balanced, and should be easy to control. You’re sitting in the kayak, facing the front of the kayak. You are aimed north. Let’s also assume you are in excellent shape with great core muscles. Have you ever noticed what happens when you turn your body? If not, then you’re about to be enlightened.
Pretend you’re holding the kayak’s paddle in both of your hands while sitting in the kayak. Keeping the paddle parallel to the water, turn your upper torso to the right (facing east) while still sitting in the kayak. What happens to the kayak when you do this? It should start pointing to the left (west). If you do the opposite and try to face west, the kayak will attempt to point east. Congratulations, you’ve just become a reaction wheel!
The video above helps show this action/reaction in a kayak. So, you’re moving the kayak to face different directions, but not forwards, back, or to the sides. What you’re doing is changing the kayak’s attitude along the horizontal, or “xy” plane. Also, you might have noticed your movements don’t have to be huge to move the kayak. This is the essence of how a reaction wheel moves a satellite. It also happens to be a great example of Isaac Newton’s Third Law of Motion: “To every action, there is always opposed an equal reaction.”
Reaction wheels help with pointing. But how do they work? If you’ve ever played with a gyroscope, or ridden a bicycle or motorcycle (particularly a horizontally opposed BMW), you’ve dealt a little bit with how reaction wheels work. Reaction wheels are weighted wheels (amount of weight will vary), which are designed to spin, normally very fast (thousands of revolutions per minute–rpm). When a reaction wheel spins at a particular speed (this too will vary), it resists any external force, such as solar wind, micro-gravity, etc., staying very stable.
But if you were to increase the speed of the wheel using small electric motors, the satellite, in which the reaction wheel is housed, starts spinning in the opposite direction. If the reaction wheel’s speed is slowed, the satellite responds by changing attitude in the other direction. With electric motors, space operators can control just how much or how little the satellite will spin, by adjusting the speed of the reaction wheel. And satellites typically have three other reaction wheels, all mounted perpendicular to each other. This system, when paired with a computer and other sensors, provides very refined satellite attitude movement in any direction.
Such an attitude control system is very complicated compared to small thrusters, which could also be used to change a satellite’s attitude. But thrusters require fuel, and there are no gas stations out in space (yet!). A satellite like Kepler would quickly run out of fuel by constantly firing thrusters to point accurately, unless the designers increased fuel capacity, thereby increasing fuel weight. And increasing weight of a satellite on a rocket requires the rocket to also have more fuel, which means more weight. But ultimately, it all means more money. So reaction wheels provide a neat solution/compromise for Kepler and other satellites–until they no longer work.
Going back to Kepler then–there are only two reaction wheels left working of the original four. This means the attitude can only be changed along the axes of those two wheels, right? In Kepler’s case, the team came up with a great, ingenious solution, one which used something that’s already out in space to help get some control along another of Kepler’s axes–solar pressure.
There is apparently enough solar pressure to push against Kepler’s solar panels and keep it fairly stable when the satellite is positioned “just so.” From what I understand, a thruster is required to fire occasionally to help keep the satellite stable, but it seems to work. Weird, right? But it’s apparently working well enough for NASA to propose using Kepler for a different, but equally useful, mission–K2 (read abstract here). It will still be planet hunting, but instead of one specific region, Kepler will be looking out along the Solar System’s plane at different regions in space.
In essence, Kepler will still be planet-hunting for a while longer. All it took was a change in attitude.