Selected Articles from the
January 2001 Odyssey
Editor: Terry Hancock
Other Articles from this issue:
- Reflections on a Desert Night, by Steve Bartlett
- Challenger Disaster Shaped a Generation, by Bathsheba Marcus
A Matter of Attitude
By Robert Gounley
One of my favorite cartoons shows two weary scientists glumly staring at a partially assembled rocketship. Outside the laboratory we see their boss shouting, "What's taking so long? This isn't brain surgery, you know!"
A stimulating email discussion about the future of Space Station Mir brought this cartoon to mind. Citing potentially hazardous consequences if problems controlling station attitude continued, one writer concluded that Mir should be promptly ejected from Earth orbit. Another writer replied with a novel remedy and closed by saying, "Control is easy!"
Perhaps it is, compared to some things. (Brain surgery, perhaps?) But rather than take sides, I'd rather explain what it means to control spacecraft's attitude and how it can be simple in principal, yet challenging in practice.
A host of spacecraft problems begin with bad attitude? not attitude in the sense of personality, but attitude, signifying how a satellite is oriented. In some way, most parts of a spacecraft depend on being aimed in the correct direction. Some must point at something to work, like an antenna toward a radio signal; others must avoid pointing at something, like a camera at the Sun. Even the basic framework of a spacecraft may be designed to preferentially allow sunlight to warm a particular side.
To illustrate, imagine yourself in orbit about the Earth. Trouble breathing? OK, put yourself inside a spaceship orbiting the Earth. You'll need power for life support, so add solar wings. Let's say you plan to watch the Super Bowl, so youíve bolted a satellite TV dish antenna outside. A video camera floats by a window, taking pictures of the Earth below. To get home, a rocket nozzle points out the rear. This bare-bones personal spaceship contains basic parts common to many Earth-orbiting satellites: solar panels for power, an antenna for communications, a camera (or some sort of sensor) for observations, and a rocket engine for changing orbits.
Feeling comfortable? Go ahead and relax in front of the TV. Take a nap, if you like. Save your energy for whatever may come.
Your first sign of trouble will probably be static on the TV screen. Rubbing the sleep from your eyes, you realize the satellite dish must have drifted away from its signal. While you consider going outside to tilt the dish, the lights flicker. Shadows from the rest of the spaceship are creeping down one of the solar panels and the resulting power loss threatens imminent blackout. Meanwhile, unfiltered sunlight is streaming through the window, turning the cabin interior into a sauna. The smell of smoke alerts you that your video camera is now blind and parts are melting.
This is too much. Best go home and plan a better trip. Unfortunately, the rocket engine hurls your craft in the wrong direction. It has truly been a bad day.
If everything pointed in their correct directions none of this would have happened. But, you say, everything was aimed the right way. How did our spaceship get spun around?
Even in vacuum of space, miniscule forces disturb our well-ordered plans. The most basic comes from gravity itself. In orbit, the speed you travel determines how high you fly above the Earth. Change your altitude and you change your speed. As long as your spaceship remains together, all parts travel at the same speed. But what would they do if all the parts were free to move separately?
Think of a barbell in orbit, tilted so that one weight is slightly closer to the Earth. The nearer weight feels Earth's gravity more strongly. If it were not attached to the higher weight, it would travel in a slightly faster orbit than its twin would. The rod connecting them prevents the weights from slowly drifting apart, but together these weak forces can have a profound effect on the whole object. The minute differences in gravity between opposite ends of the barbell (referred to as the gravity gradient) causes it drift like a compass needle. In the end, the bar points straight up/down --- one weight as close to Earth as it can get, the other as far away as it can get. As the spacecraft travels around the Earth, the barbell rotates to keep its orientation, completing a full rotation every orbit.
This is why the Moon keeps one face towards us. Below the lunar surface are regions of higher density rock. Although considerably less lopsided than our barbell, gravity still draws one side closer to the Earth while the other remains a perpetual "far side".
Sometimes spacecraft are designed to take advantage of this force. They always point downward. However, other forces must be considered that can disturb this relatively stable arrangement.
If your spaceship's orbit is sufficiently low, Earth's atmosphere will have an effect. At that altitude, the atmosphere is thin nitrogen/oxygen plasma with an air pressure too small to be measured. Even so, the speed of a spacecraft colliding with the atoms can have a profound effect. If an uncontrolled spacecraft is long and smooth, the airstream will try to steer it like weathervane. Irregular spacecraft (like those with exposed booms, antennas, and solar arrays) may be coaxed into spinning like a pinwheel. Worse, the irregularities could trigger chaotic motion, leaving the spacecraft drifting along like a tumbling tumbleweed.
Sunlight exerts gentle pressure on everything it contacts. On Earth, this effect is too small to consider, but in space the small forces accumulate. If the Sun's view of the spacecraft isn't evenly balanced, solar pressure will try to tilt it to one side.
Even magnetism counts. If your spaceship has any trace of magnetism, Earth's magnetic field will try to point it like a compass needle. For this reason, satellite manufacturers either avoid magnetic materials altogether or, after carefully mapping the spacecraft's magnetic field, mount permanent magnets so the fields cancel each other out.
Fortunately, all these forces are quite small and take time to become troublesome. An experienced spacecraft designer can build in measures to counteract them. But, as in most matters dealing with space, cutting too many corners can be costly.
Take the example of a certain Japanese satellite built with stainless steel fuel tanks: Confident their spacecraft contained only nonmagnetic materials, the manufacturer eliminated the costly step of magnetic mapping. Unfortunately, the act of forming the stainless steel into hollow spheres changed the metal's crystalline properties, the usually nonmagnetic material became slightly magnetic. Once in orbit, the Earth's magnetic field constantly tilted the satellite away from its intended attitude and the mission became an untimely loss.
Now that we know what can tilt our spacecraft, how do we push back when there's nothing solid to push against? For that matter, how can we tell where the right direction is?
But these are subjects for another time.
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