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It was the Oort miners—the spotters, the snake handlers, the launchers—who considered themselves the most important part of the project, because they were the ones who started each comet on its long drop from the cloud to impact. That was as far as they went, though. Their original course programming merely aimed the thing in the general direction of the Sun.
That was close enough, for the first three or four years. Then each incoming needed to be caught and corrected, and that was even harder . . . or, at least, hard in a quite different way.
The comets came in from the Oort on a thousand different headings, and all of them were wrong. The right heading was the one that would take it close to the Sun at perihelion—close enough to get maximum deltas from the Sun's gravity, but not so close that the Sun's heat boiled too much of its mass away—and on just the right heading so that it would come away from the Sun on a trajectory that led to Mars. That was the tricky part.
That was the part that was taken over by the control satellites in the co-Martian orbits. They analyzed the trajectories for each incoming comet, and they commanded the burns that would correct those trajectories. It wasn't a onetime deal. At every point along the comet's long fall it needed to be tended, and nursed ever closer to the optimal Sun-swing insertion. As many as five hundred course corrections were necessary to fine-tune that important moment when the Sun's pull would wrench the comet out of its original course and put it squarely in the plane of the ecliptic, at the same time slowing it through the right burns at the right fraction of a second so that it would creep up on Mars at just the right point in Mars's orbit.
That was what the students learned in Phase Four. Some said it was the most important phase in the course. Others—notably the ones in the stations orbiting Mars, as well as the ones in the Oort itself—said it wasn't, but, really, they were all right. There were no parts of the hard, long job of guiding a comet from Oort to Martian impact that were not important.