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Diverting an object may be the most appealing option. The idea is to gently push the object, a little at a time — for years or even decades — and move it into an orbit that won't strike the earth. Suppose we find an object that will strike the earth within twenty years. We give ourselves ten years to change its speed. This includes time to explore the object, then build and install a system to move it, then a year to run the system and change its speed. In the ten years remaining, we want the changed speed to be enough to move the object away from the earth. A ten centimeter/sec change in velocity will probably be enough. Further calculations show the force and energy required. We still have to figure out whether it's best to mount some kind of engine on the object, or use an explosion to push it away. To do that, we'll have to explore the object. If it's just a pile of rubble, held together loosely by gravity, it'll be hard to mount an engine on it. We might have to build a structure, dozens or hundreds of meters across, to spread the force of the engine across a large part of the object. It's even possible we'll have to try out our procedure first, on an object that's much smaller (and not headed towards the earth). If that works, we build a larger engine and try it on a larger object. After several such iterations, we would be able to try out the actual system on the actual meteor. Building a rocket motor that will operate for a few hours or days is one thing. Building one that will operate steadily for a year is something else. We may have to develop new types of rocket motors (or ion engines or something else) to get something that works. Or we may have to replace the motor every few weeks, until we've changed the speed enough. We don't necessarily have to build a motor, either. The pressure of sunlight on an asteroid may be enough to divert it from its orbit, given enough time. This principle is called the Yarkovsky effect. If we have decades of warning, we may be able to attach a solar sail to the asteroid, and let the pressure of sunlight move it. Or we might be able to paint the asteroid. If we paint it white, it will reflect most of the sunlight that falls on it; this will push the asteroid more than if it absorbed most of the sunlight; that may be enough. We would, of course, want to test this on an asteroid that's not moving toward the earth, to be certain we could make it work. In any case, once we find a large object that will hit the earth, we'll certainly have incentive to develop the technology to divert it, if we decide that's the way to go. We don't have the technology (at present) to do exploration and experimentation with robot probes. Astronauts would have to do it. By the time we find such an object, we might have the robot technology. Since we'll probably have decades of warning, we'll probably have time to develop the technology. |
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