all space missions are difficult, since there are too many factors



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送交者: steven 于 2005-7-07, 01:27:04:

回答: That's right. I don't! But I didn't say it's easy. 由 skipper3 于 2005-7-06, 23:36:09:

beyond our control. Let take one aspect of deep impact for example, orbit calculation, it is not as easy as it seems. An object in orbit doesn't stay nicely on its theoretical locus. In orbit prediction, we give an object position a bounding box, which the object could be anywhere inside the box, smaller the box is, harder it requires to compute and control. Observation only gives you a snapshot, and it doesn't always give you the history. Besides, many errors, such as atmospheric disturbance, get introduced by the observation processes. To compute orbit of comets, we need to solve many bodies problem, since the gravitational force of sun and planets all have effect on comets. The observation errors may get magnified by the numeral processing, and the size of the comet, which is generally very small, makes the error margin even thiner.

In space, things don't fly like an airplane. We need to perform Hohmann transfer to move objects from one orbit to another. Hohmann transfer can only take place at the intersection of two orbits, and the velocity has to be just right. The burn control must be very accurate, so that the SV can reach the right velocity at the right point to get in the right orbit. If missed, we won't see deep impact happen in our life time.

Anyway, I am not here to give you lectures in orbital mechanics, and there are many other engineering details such as how to determine the velocity of an SV; or how to make the SV to aim the comet which doesn't sends of any positional telemetry so that we can track. This is a lot harder then shooting a bullet with bullet.



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