And he found a problem with some of the module’s seventeen chargeable battery regulator modules. One of them, number fifteen, had malfunctioned days before, and then number three went off-line. All that cut the power available in the module by around two hundred watts. Houston thought there might be a low-voltage trip occurring somewhere, which was switching out the regulators too often, and Stone had to go around the module firing up systems and calling out power consumption numbers to the specialists in the back rooms behind the MOCR.

It was slow, dull, almost mindless work. The routine stuff really ground you down; it was a hazard of long-duration missions. But all of it was essential to keep this handcrafted bucket of bolts flying. [Hr:Min:Sec] 15:49:01

At last he could get to his interplanetary navigation.

He headed down to the wardroom, to the picture window there, and dug out his optical kit.

The kit contained a one-power telescope and a twenty-eight-power sextant. The sextant was a chunky little gadget with an eyepiece and a calibrated semicircular dial, to measure angles between stars. They were nice devices, compact, heavy things of brass, which Stone enjoyed handling. If he was going to take away any one souvenir of the flight, then to hell with a Mars rock; it would be this little kit.

Stone set to work by the picture window.

First he measured the apparent size of the sun’s disk, which would give him a good measure of how far the craft was from the sun, and then he measured the angles between Venus and a fixed star, and the Earth and a fixed star. Those three basic measurements would fix him in three-dimensional space. He would finish up with a few more redundant measurements.

He’d found he couldn’t hold the optical equipment steady in the microgravity. But the way around that was to set the gadgets floating in the air, as well aligned as possible; then he could bring his eye up to the eyepiece and make an accurate reading.

The first TCM — Trajectory Correction Maneuver — had taken place ten days after leaving Earth. At that time the flight path had been pretty badly misaligned. The trajectory planners back in Houston had sent corrective burn parameters chattering up the line to Ares, and the MS-II stage’s maneuvering propulsion system — two modified Lunar Module engines — had applied a hefty velocity change of twenty-five feet per second. But, according to the latest data, the craft was still on a slightly divergent trajectory. Today Stone would check the craft’s position and velocity, and the track would be recomputed; and tomorrow, if they could get it right, the second TCM burn would fix the trajectory’s small remaining anomaly.