Houston grimaces. “The necessary dwell time is just too long, Mac. Too long.” He sighs. “It’s always been the toughest requirement in the whole system’s architecture, if you ask me. The Soviets have got their fast-burn boosters down to sixty seconds, so most of their ICBMs will only be in boost phase for that minute, and half that time they’ll be in the atmosphere where the lasers won’t do much. So for our purposes we’re talking about a window of thirty seconds.”

He scribbles down the figures on the tablecloth as he talks, nervously, without looking at them, as if they are his signature or some other deeply, even obsessively memorized sign. tB = 30.

“Now, during that time we’ve got to locate the ICBMs, track them, and get the mirrors into the correct alignment to bounce the lasers. Art’s team has got that down to around ten seconds, which is an incredible technical feat, by the way.” He nods mulishly, writes tT = 10. “And then there’s the dwell time, the time the beam has to be fixed on the missile to destroy it.” He writes tD = , hesitates, leaves the other side of the equation blank.

“You told the Air Force we could pulse a large burst of energy, right?” McPherson asks. “So the damage is done by a shock wave breaking the skin of the missile?”

Dan nods. “That’s right.”

“So dwell time should be short.”

“That’s right! That’s right. Dwell time should be on the order of two seconds. That means that each laser station can destroy N missiles, where,” and he writes:

“However,” Dan continues carefully, looking down on the simple equation, one of the basic Field-Spergels that he has to juggle every day, “dwell time in fact depends on the hardness of the missile, the distance to the target, the brightness of the laser beam, and the angle of the incidence between the beam and the surface of the missile.” He writes down H, B, R, and 0, and then, obsessively, writes down this equation too, another Field-Spergel:

“And we’ve been getting figures for hardness of about forty kilojoules per square centimeter.” He writes H = 40 KJcm2. “Our lasers have twenty-five megawatts of power hitting ten-meter-diameter mirrors at wavelength two point seven nanometers, so even with the best angle of incidence possible, dwell time is,” and he writes, very carefully:

tD = 53 seconds.

“What?” says McPherson. “What happened to this pulse shock wave?”