Doing useful work when slowing at star arrival

In my previous post, I had mentioned the possibility of using a sort of 'reverse gravity assist', where a man-made object (a spacecraft) would purposely change the orbit of another object. I had suggested this as a way of flushing space junk from Earth's orbit. Lying awake last night, I thought about how approximately half of the energy expended in interstellar travel would be to slow down upon approach to the destination star system. Then it occured to me: If we are using the energy of our star system (Sol) to accelerate, then we can pump energy into the destination star system upon arrival, to slow down. Better yet, we can change the arrival system to our benefit (with limits, of course). For instance, it is postulated that comets are balls of ice from the very edge of our solar system, some of which have obtain an orbit which brings them close enough to the sun to melt, thus their distinctive appearance. This theory also says that there are other 'snowballs' at the outer solar system that do not have these orbits, and remain hidden from us (for now). Theories of planetary formation have said that the earth is too close to the Sun for water to have formed as the planet formed. If this is so, the water must have arrived after planetary formation - most likely from a collision with a comet. I think that such a collision would be so energetic as to possible cause the earth to crumble, and much rocky matter and cometary water be thrown off. I think what is more likely is that a comet had a trajectory that took it near enough to Earth so that Earth's gravity was able to strip off some of its liquid and gaseous water. Perhaps this was a periodic even, leading to repeated addition of water to earth, or perhaps it was a very massive comet and one such event was all that was needed. Or, perhaps the comet became trapped in the Earth's orbit, and it rained for a thousand years. Whichever actually happened, it is possible that we can perturb a comet in the destination system to contribute water to a dry, rocky planet much like the same mass as earth.

There are, of course, some limitations and pitfalls to such a plan. The first is that we must, before arrival, have a precise and accurate map of the star system and the composition of its massive bodies. The velocity at which we enter this system will be high, and therefore any collision could be catastrophic. We need to be able to sense even pebbles from a distance, or be able to correct course very quickly, or both. Another limitation is that decelerating in this manner may take much more time than the traditional interstellar, fueled deceleration, especially if we want to make the best use of it. Furthermore, the calculations involved in order to best chart the spacecraft's course are computationally expensive. But of the last limitation, that is the one I have the least concern about, as we humans seem to be able to build very fast computers. It is the sensors we need, and it is the map-building and then careful course plotting that will be time consuming.

As I type this, I wonder if we could not attempt to 'throw a snowball' at Mars. That planet will need enough mass to be able to keep its water, so it may have to be a very big snowball.