Stone said, “On my mark, three minutes thirty to ignition…. Mark. Three thirty.”

Gershon set the switch to arm the descent engine.

Gershon was ready. He was in charge for the first time in the mission. It gave him a sense of liberation, of power. He could make sure nothing fouled up.

And besides, he’d completed this run a thousand times, in the sims, and in the MLTV trainer. He could do it with his eyes shut.

Sure you could. But this is Mars, pal. Maybe that big old world out there has different ideas.

And this MEM was going to have to function better than any of the test articles that had preceded it.

“I’ve got a 63 for PDI,” Stone said quietly. A relic from Apollo. 63: a computer query about readiness to proceed to PDI, powered descent initiation.

“Do it,” Gershon said. “I’ve got go.”

Stone pressed the PROCEED button. “Ignition.”

“Right on time.”

Gershon felt nothing at first. But the gauges showed him that the descent-stage engine was firing up to 10 percent of maximum, smooth as cream.

Then, after half a minute, the engine reached full thrust.

He still couldn’t hear anything, but the cabin filled up with a grating, high-frequency vibration. It was uncomfortable, something like the sensation of having your teeth drilled at high speed. Different from the sims already.

Challenger slid down U.S. Highway One, braking easily.

“AGS and PGNS agree closely,” Stone said. Stone was acting as the navigator; he was telling Gershon that the redundant-pair primary and abort guidance systems were agreeing with each other. “We’re looking good at three, coming up… Three minutes. Altitude thirty-nine thou five.” That height reading was still only an estimate from the two guidance computers, though; the landing radar had still not acquired its lock. And Stone would also be able to read off heights from the altimeter, although that instrument, working on the pressure of the unfamiliar Martian atmosphere, was experimental, and its data excluded by the mission rules.

“Still go,” Stone said. “Take it all at four minutes… We’re go to continue at four minutes.”

“Rager,” Gershon said tersely.

“The data is good. Thirty-three thou…”

But caution and warning lights were glowing on Gershon’s station. The landing radar should have been working by now; it should have locked onto its own signals bouncing off the ground.

But it hadn’t achieved lock.

“Where’s that goddamn radar, Ralph?” Stone asked.

“Punch it through again.”

“Yeah.” Stone tried.

“Come on, baby,” Gershon said quietly. “Let’s have the lock on.” But there was no change. “Come on.”

“Does talking to it do any good?” York asked drily.

“Shut up, Natalie,” Stone said, distracted.