There was a whole slew of ways for the ground to keep track of a spacecraft. The faster the craft was receding from Earth, the more its radio carrier frequency was shifted, like a whistle on a speeding train. Or, to fix distance, an uplink modulation pattern — a brief digital code — could be transmitted to the spacecraft and sent straight back. The delay in receiving the copied signal would tell the specialists how far away the craft was from Earth. And on Ares another radio method was being run, an experiment involving the change in angle between Ares and a quasar, a radio source in the background sky.

But even all those techniques in combination weren’t accurate enough to position Ares; the combined accuracy was only maybe half the precision required.

The answer was to use equipment on board the spacecraft itself.

In fact Ares had its own automated optical sensors. There were two sun sensors — little cadmium sulfide photoresistors — placed on the solar cell array. And there was a star tracker, a lens with an image dissector tube. But the automatic system just wasn’t too smart. Every few days, the star tracker got fooled by bright particles, bits of junk floating along with the spacecraft in its orbit around the sun.

So — just as had sailors on Earth’s oceans for millennia — Phil Stone had to navigate his ship by the stars.

He found himself humming as he worked. He knew he was good at this. He’d practiced the techniques in planetariums on the ground, and in Moonlab; he could get a measurement true within a few thousandths of a degree, and he took a lot of satisfaction from the basic craft of it.

When he’d finished he packed away the instruments and drifted back up to his control station. He ran the numbers through the computer, to figure out the position for himself. He would feed the raw data down to Houston, of course, but it kind of satisfied him to know that he could do this independently.

Stone liked to visualize the mission trajectory, to figure out where he was.

The energy expended by the injection booster — although monumental in human terms, the result of five years’ fuel haulage to orbit — was so low on the cosmic scale that the craft’s trajectory barely diverged from Earth’s. The Ares stack, having pushed itself away from Earth, was scooting alongside the home planet in its orbit like a dog beside its master.

The first results looked good; the spacecraft was pretty much where it was supposed to be, and he figured that the TCM-2 burn would only have to be a few feet per second.

When he was done he indulged himself. He doused the wardroom’s lights and sat in the warm darkness close to the picture window, surrounded by the hum of fans.