“I got it.” He swept across the tarmac, smooth, easy.

“If you want to change the landing site, you just use your attitude control to swivel and point your window, and the PGNS starts recomputing. You can set to ATTITUDE HOLD and just glide for a while, if you like. And you can change your down velocity by—”

“I know it, Ted. I—”

York came on the line. “Ralph. It’s Natalie. I think you ought to pull out.”

“Huh? Why?”

“You’re coming in too fast. Too low.”

He checked over the crude instrument panel. Everything looked fine to him. It was true he was coming in low and fast, but that was intentional; he knew all of Curval’s gassing hadn’t left him a lot of time, and he didn’t want to run out of fuel. “What’s your problem, York?”

“I think you’re going to overload the PGNS.”

“Come on. Everything up here is sweet as a nut.”

“It’s not as simple as that, Ralph.” She started gassing about best-fitting polynomials and higher-order curves, and a lot of other crap that went right over Gershon’s head.

He just tuned her out.

He watched the tarmac roll away under him. He swept along, the PGNS working smoothly; he barely needed to touch his controls. He felt a surge of success, of achievement. Here’s something else I can do, Ma. Another step on the long fucking ladder I’m climbing to Mars.

He’d just let her land, this first time through. He’d made his point; he didn’t want to antagonize Curval too much. Maybe he could persuade Curval to get the MLTV refueled and he could take her up again. Next time he’d try changing the landing site a couple of times.

That big old “three” started to loom up toward him, upside down as he looked at it, slightly obscured by the dust his rockets and the jet were kicking up.

The MLTV tipped itself back, to slow his forward velocity. He checked his numbers; the CRT display evolved smoothly to match what he saw through his Plexiglas screen.

The MLTV started to drop down as the auxiliary thrusters throttled themselves back.

Maybe that dip down was a little sharp.

York was still yammering in his ear. He needed to think. He watched his trajectory and tried to visualize where he was going.

Something was wrong, for sure. He was coming down too quickly.

He let another couple of seconds pass, checking his instinct. Yes: his trajectory was a tight downward curve that would bring him to the ground maybe a hundred yards short of the “three.”

Well, so what? Maybe the PGNS was out by a little; maybe all these damn reticles on the window needed recalibrating. If he came to a smooth halt in midair, but short of his target, he could blame fucked-up equipment…

But he wasn’t coming to any smooth halt. The lift rockets were cutting right back, and he was starting to fall, hard, toward the ground.

York and Curval were both shouting at him.

He watched the ground explode toward him, resolving into unwelcome detail, bits of dirt and dust and concrete ridging highlighted by the low morning sun.

He pushed the button to disable the automatics.

He didn’t waste time trying to straighten up the MLTV’s attitude; instead he just throttled up the turbofan jet and let it push him away from the ground. He felt a surge of acceleration, a good crisp couple of Gs, strong enough to keep him away from Earth’s unwelcome clutch.

He pulled up maybe a hundred feet from the ground. He throttled back the turbofan, and landed softly.

York ran toward the downed MLTV.

Technicians in white protective suits surrounded the trainer. Ralph Gershon had already climbed out. His hair had been compressed flat by his flight helmet, and his face, released from behind the visor, was round and shining with sweat. His eyes were bright red, she guessed from the dose of peroxide he’d taken earlier.

“You asshole, Gershon,” Curval said. “I told you that if you wrecked the trainer—” Curval towered over Gershon, his hands bunched into heavy fists. He started to chew out Gershon.

In a way the anger was justified, York knew; if Gershon, with his gung-ho heroics, had gotten himself killed, or smashed up a key piece of equipment like the MLTV, he could have put the whole program back a long way. York decided Gershon needed the bawling out, and she let it run on for a couple of minutes.

Then she stepped forward, putting herself between the two of them. “Actually,” she said, “none of it was Ralph’s fault.”

Curval turned to face her, still high on his anger.

“It was the landing program. I think it has a bug, Ralph. It nearly killed you.” She turned to Curval. “We can prove it by running Ralph’s trajectory through the sims a couple of times.”

Curval said, “What the hell do you know about programming?”

She sighed. “Not a hell of a lot. But I’m the big-brained, tight-assed college girl, remember? It’s not my field, but I’ve done enough math to know how routines like the PGNS work.

“Look.” She mimed the MLTV coming down. “The PGNS tries to fit a smooth curve between your position and velocity, at any time, and your destination. But it isn’t magic. It’s just math. And it has its limitations.

“The curves the program uses are polynomials. Smooth curves, with wiggles. The higher the order of the polynomial, the more the curve will wiggle about. You don’t have an infinite choice of curves; it’s like trying to fit a template out of a fixed set to suit the job. And the more complicated the data you feed the program, the more the polynomial will have to wiggle to fit your data points. You see?”

“So why is this bad?” Gershon asked with a kind of fake innocence. “And why do I need to know about it?”

She struggled to keep her patience. “Because the program doesn’t know the ground is there. It’s not like a human pilot, Ralph. It’s really pretty dumb. All PGNS is doing is fitting a curve to two positions in space. It doesn’t care how much the curve wiggles in between. And if one of those oscillations happens to carry you down into the ground, and up again—”

Curval whistled. “So because Ralph was flying low and fast—”

“The polynomial solutions, the best the PGNS could come up with, were high order. Full of wiggles.”

“Helicopter experience,” Gershon muttered.

York was confused by the non sequitur. “Huh?”

“Helicopter experience. That’s a nice bird, and it’s easy enough to fly. But it goes against everything, every instinct you build up flying a plane.” He obviously hadn’t listened to her. Or maybe he had taken in what she’d said, as much as he felt he needed to know, and had moved on to his next thought, the next step of his inexorable approach to Mars. “If that’s the way the MEM is going to handle, anybody with a lot of chopper proficiency is going to have an edge. That’s obvious.”

“And you have, I suppose?”

“No. But I will soon.”

His helmet under his arm, he stalked off across the tarmac, short, purposeful, bristling with determination, back toward the MLTV.

Curval scratched the back of his crew-cut head. “What a day. What an asshole.”

Maybe, York thought. But he looks to me like an asshole who is going to Mars.