If, indeed, it were working at all. And since much of the action—the rendezvous with the comet and the "burn" of the nuke-powered, steam-fueled engine—had occurred while it was on the far side of the sun, this had been very much in doubt until a couple of months ago, when astronomical observations had proved conclusively that Comet Grigg-Skjellerup had changed its course—something that could only have happened as the result of human intervention. The comet was headed right for them. It would have triggered mass panic on Earth had Earth not already been doomed. Since then they had watched its orbit converging slowly with that of Earth, and plotted the time when it would disappear against the sun once more as it reached L1. The reactor would then have to be powered up again, as a huge "burn" would be needed to synchronize Ymir ’s orbit with Earth’s and pilot it through L1 to a long ellipse that would bring it their way.