“Good burn, Ares,” Young said. “You’re closing at one hundred thirty-one feet per second.”

Stone went through two more corrections, and five sharp braking maneuvers. Then, maybe half a mile from the booster, he took the Apollo on a short, angular inspection sweep. The reaction control systems bit sharply, rattling York against her restraint.

York watched the cluster roll with silent grace past her window.

The booster cluster was squat, pregnant with fuel. Its heart was a fat MS-II booster, a Saturn second stage, modified to serve as an orbital injector. Fixed to the front of the MS-II was an MS-IVB, a modified Saturn third stage, a narrower cylinder. To either side of the MS-II were fixed the two External Tanks, fat, silvery cylinders as long and as wide as the MS-II stage itself. The supplementary tanks carried more than two million pounds of liquid oxygen and hydrogen, propellant Ares would need to break clear of Earth orbit.

The MS-II and its tanks looked like three fat sausages, side by side, with the slimmer pencil shape of the MS-IVB protruding from the center. The rest of the Ares stack — the Mission Module, MEM, and Apollo — would be docked onto the front of the MS-IVB to complete the assembly of the first Mars ship, a needle well over three hundred feet long.

The cluster was oriented so that it was pointing toward the sun; that way, boiloff of the cryogenic propellants inside the tanks was reduced. Shadows of struts and attitude thrusters lay long against the sunlit white-and-silver bellies of the fuel tanks. The booster’s underside was illuminated only by the soft blue and green of Earthlight. She could see the great flaps of the cluster’s solar panels, folded up against the sides of the MS-IVB stage like wings; the panels would be unfurled when Ares was safely launched on its trajectory to Mars. There was the bold red UNITED STATES stenciled against the side of the MS-II, and the finer lettering along the long thin protective flaps masking the solar panels, and the NASA logo; and she could make out the support struts and attachment pins which held the External Tanks in place against the flanks of the MS-II, and the gold-gleaming mouths of the MS-II’s four J-2S engines, upgrades of the engines which had pushed Apollo to the Moon.

To assemble this much mass in Earth orbit had taken all of nine Saturn VB flights over the last five years — half of them manned. The booster stages and their tanks had been flown up and assembled more or less empty, and then pumped full of gas from tanker modules. The cluster was an exercise in enhanced Apollo-Saturn technology, of course, and the essence of its design went all the way back to the 1960s. But NASA had had to develop a raft of new techniques to achieve it: the assembly in orbit of heavy components, the long-term storage of supercold fuels, in-orbit fueling.