And an answering voice: Their deaths have changed all of us. Jenny is more human, Wilmer is more alert, and Reza is closer to the borderline between normal and psychotic.
The other three didn’t seem to find Celine’s assumption of leadership as odd as she did. Jenny was rubbing her eyes, as though she had been secretly weeping, but she said quietly, “I think it is obvious what happened. I downloaded software modules for orbiter control from the program library onto spare chips available on the Schiaparelli. I installed those chips in the Lewis, replacing dead elements there. I believed that I had done everything correctly. But when Zoe tried to change the orbiter’s pitch, the software module gave a command that drove the correction the wrong way. The orbiter was entering the atmosphere more steeply after the correction, instead of less steeply. Drag forces and frictional heating on the Lewis increased, rather than decreasing, until temperatures went past hull material limits. If only I had been more careful, and checked—”
“We don’t know that’s what happened.” Celine cut Jenny off smoothly but firmly. “Did you find a software error that could produce an effect like that?”
“No. But I’m still looking. It’s the only thing that could possibly—”
“Not proven. We need to hear from everyone before we attempt an analysis.” Celine turned away from Jenny. “Reza?”
“Well, we may never know exactly what sequence of actions Zoe took.” Reza’s voice was higher than usual, but he picked up before Jenny could speak again. “There was no telemetry for the crucial period, because of ionization radio blackout. But the controls of the orbiters are quite a bit different from the controls of the Schiaparelli or of the Mars landers.”
Speech seemed to have stabilized him, because his voice was more normal when he went on, “I know that, because I’ve had more practice sessions than anyone except Zoe herself. It would be easy, in the heat of the moment, to invert a control command and increase the angle of attack rather than decreasing it.”
“If Zoe had done something like that she would have realized it in a split second,” Jenny said. “She would have made the correction. She didn’t.” Her voice wobbled and rose in pitch. “I tell you, Reza, it’s in the software routines.”
“We’ll discuss software and other possible causes later,” Celine said curtly. “I don’t want to talk about it now. Wilmer?”
She turned to him, without much hope of hearing anything useful. It wasn’t clear that he had even been listening. While Jenny and Reza were talking he had made a peculiar little drawing, and now he was scribbling numbers.
“Oh, it wasn’t the software.” Wilmer grinned like an idiot. “At least, it was, but not at all in the sense that Jenny means.” And then, while Celine glared, he went on, “Do you remember when we first noticed Supernova Alpha, you asked me what else it might do to the solar system? And I said, pretty much nothing, apart from melting ice for a while on the moons of the outer planets.”
“I remember.” With anyone but Wilmer, you would bat them over the head if they chose such an awful time to wander way off the subject. With Wilmer you waited. His digressions always came back to the point.
“Well, I was wrong,” he said. “Stupidly wrong. Wrong in a way that anyone with a year or two of elementary physics could point out.” He turned the sheet, so that they could see his drawing. It was of three concentric circles, with arrows pointing out from the second one toward the outermost. “We all know the pressure/volume/temperature relation for an ideal gas, Pv = RT. A planetary atmosphere satisfies that, almost perfectly. Increase the temperature and leave the pressure the same, and the volume increases linearly. Here’s Earth’s surface.” He pointed to the inner circle. “Above it lies the atmosphere. Pump in heat, an incredible amount of it, from Supernova Alpha. For a couple of months it’s like having two suns in the sky. The temperature of the surface rises. The atmosphere expands. Where does it go? The only place it can go.” He pointed at the arrows between the second and third circle. “Upward. The whole atmosphere swells.”
Wilmer released the sheet of paper with the diagram, and it hovered before him in the free-fall environment of the Schiaparelli’s cabin. “That’s a general comment, but it’s easy to catalog specific effects. First, there will be only a small change at ground level. The atmosphere has expanded, but its total mass remains constant. The surface will experience the same atmospheric pressure, because the whole column of air above it exerts the same downward force.
“But now think about conditions higher up. The atmosphere still becomes thinner with height, but it does so more slowly than it used to. So if you go high enough, the air is more dense than it was at that same height before Supernova Alpha. The drag force and frictional heating on a spacecraft will increase — and they have an exponential dependence on air density.
“The routines that we put into Lewis came from the general software library applicable to our class of orbiters. They provide an explicit calculation for drag force as a function of height, in terms of spacecraft angle of attack, mass, shape, and velocity — and air density. But it’s the air density at a given height as it was before Supernova Alpha — not as it is now — that’s in the equations. An acceptable angle of attack for a ship fifty miles up, moving through the atmosphere as it was two months ago, would be an absolute disaster today. No orbiter could stand the increased drag and heating. We saw what it did to the Lewis.”
Wilmer had credibility and authority on all matters scientific. He also spoke as though what he said was no more than common sense, and quite undeniable. Celine reminded herself that it was the unquestioned acceptance of authority — Zoe’s authority, as head of the expedition — which had led her to remain silent before. She could not afford to do it again.
“You may be right, Wilmer. But you may be wrong.” And when he stared at her in surprise — this wasn’t the old Celine — she went on, “Jenny may be right, there was an error in the way the software was transferred, so Zoe’s action drove the controls the wrong way. Or Reza could be right, it was pilot error on Zoe’s part that destroyed the Lewis. The trouble is, we have no way of knowing which idea is correct. Even if you are right, there’s nothing we can do to prove it.”
“Oh, but there is.” One nice thing about Wilmer, he was too intellectually secure to become upset when he was questioned. He rubbed at the top of his head and went on, “We lost radio telemetry for the critical period, but we have a complete visual record from the Schiaparelli’s big scope. That provides our observables. We can compute trajectories using a variety of different assumptions — that the angle of attack was adjusted the wrong way, or that it was reduced but the drag was already too high for that to help, or any other idea that anyone has. The right model is the one which minimizes residuals between computer and observed values. We can even use the difference between computed and observed data to calculate a density function for today’s atmosphere, one that best matches a computed orbit to the observations. I’m sure Jenny can handle that.”
“Jenny?” Celine looked uncertainly to Jenny Kopal.
“Easily.” Jenny nodded. She seemed like a woman reprieved from a death sentence. “It’s a nonlinear least squares fitting problem, but we have all the routines.”
“So let’s do it.” Celine was about to add, Soon, so we can get out of here and down to Earth. But she was learning. “Take as much time as you need. You tell me when you’re done. Then we’ll discuss what comes next.”