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?”