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Phase Five was where it all paid off. By the time a comet had completed its long drop to the Sun, made its perihelion run, and been vectored to its final Mars approach, its course was set and its relative velocity down to a few kilometers per second. It was almost home . . . but that last step could be a killer.
That was where the stations in Mars orbit came in.
For the last few days of the comet's life they had it under their control. The control got easier as the comet caught up with the planet, because the distances diminished and the response time became shorter and shorter; burn corrections were actuated almost as soon as commanded. But at the same time the fine-tuning became more urgent. The comet couldn't splash down just anywhere on Mars's old crust. It couldn't come within five hundred kilometers of a deme or an industrial outpost. It couldn't strike the Valles Marineris or Olympus Mons or any other of a hundred Martian surface features, because they were, it had been decreed, sites of significant value. In short, the comet had to land where it was meant to land, with a circular error probability of no more than two hundred kilometers—and that included 99 percent of its fragments.
So the last hours of a comet's life were by far its busiest. Its final impact had to be set. Its Augensteins had to be blasted loose and sent into safe orbits for retrieval—certainly no antimatter could enter the Martian atmosphere, however thin that still might be. The comet's demolition charges had to go off in just the right sequence, with just the right strength, to convert the impact into a violent shower of pieces, not one single destroyer. And there was no room at all for error.