Phobos crossed the sky in four-and-a-quarter hours, so she didn’t have to wait long. It had risen as a half moon, but now it was gibbous, almost full, halfway to the zenith, moving at its steady clip across the coagulating sky. She could make out a faint point of light inside the gray disk: the two little domed craters, Semenov and Leveykin. She held the radio transmitter out and tapped in the ignition code, MANGALA. It was like using a TV remote.
A bright light flared on the leading edge of the little gray disk. The two faint lights went out. The bright light flared even brighter. Could she really perceive the deceleration? Probably not; but it was there.
Phobos was on its way down.
• • •
Back inside Cairo, she found that the news had already spread. The flare had been bright enough to catch people’s eyes, and after that they had clumped together around the blank TV screens, by habit, and exchanged rumors and speculation, and somehow the basic fact had gotten around, or been worked out independently. Nadia strolled past group after group, and heard people saying “Phobos has been hit! Phobos has been hit!” And someone laughed, “They brought the Roche limit up to it!”
She thought she was lost in the medina, but almost directly she came to the city offices. Maya was outside: “Hey Nadia!” she cried, “Did you see Phobos?”
“Yes.”
“Roger says when they were up there in year One, they built a system of explosives and rocketry into it! Did Arkady ever tell you about it?”
“Yes.”
They went in to the offices, Maya thinking aloud: “If they manage to slow it down very much, it’ll come down. I wonder if it’ll be possible to calculate where. We’re pretty damn close to the equator right here.”
“It’ll break up, surely, and come down in a lot of places.”
“True. I wonder what Sax thinks.”
They found Sax and Frank bunched before one screen, Yeli and Ann and Simon before another. A UNOMA satellite was tracking Phobos with a telescope, and Sax was measuring the moon’s speed of passage across the Martian landscape to get a fix on its velocity. In the image on the screen Stickney’s dome shone like a Fabergé egg, but the eye was drawn away from that to the moon’s leading edge, which was blurry and streaked with white flashes of ejecta and gases. “Look how well balanced the thrust is,” Sax said to no one in particular. “Too sudden a thrust and the whole thing would have shattered. And an unbalanced thrust would have set it spinning, and then the thrust would have pushed it all over the place.”
“I see signs of stabilizing lateral thrusts,” his AI said.
“Attitude jets,” Sax said. “They turned Phobos into a big rocket.”
“They did it in the first year,” Nadia said. She wasn’t sure why she was speaking, she still seemed out of control, observing her actions from several seconds behind. “A lot of the Phobos crew was from rocketry and guidance. They processed the ice veins into liquid oxygen and deuterium, and stored it in lined columns buried in the chondrite. The engines and the control complex were buried centrally.”
“So it is a big rocket.” Sax was nodding as he typed. “Period of Phobos, 27,547 seconds. So it’s going. . . 2.146 kilometers per second, approximately, and to bring it down it needs to decelerate to. . . to 1.561 kilometers per second.
So, .585 kilometers per second slower. For a mass like Phobos. . . wow. That’s a lot of fuel.”
“What’s it down to now?” Frank asked. He was black-faced, his jaw muscles pulsing under the skin like little biceps— furious, Nadia saw, at his inability to predict what would happen next.
“About one point seven. And those big thrusters still burning. It’ll come down. But not in one piece. The descent will break it up, I’m sure.”
“The Roche limit?”
“No, just stress from aerobraking, and with all these empty fuel chambers. . . .”
“What happened to the people on it?” Nadia heard herself ask.
“Someone came on and said it sounded like the whole population had bailed out. No one stuck around to try and stop the firing.”
“Good,” Nadia said, sitting down heavily on the couch.
“So when will it come down?” Frank demanded.
Sax blinked. “Impossible to say. Depends on when it breaks up, and how. But pretty soon, I’d guess. Within a day. And then there’ll be a stretch somewhere along the equator, probably a big stretch of it, in big trouble. It’s going to make a fairly large meteor shower.”
“That will clear away some of the elevator cable,” Simon said weakly. He was sitting beside Ann, watching her with concern. She stared at Simon’s screen bleakly, showed no sign of hearing any of them. There never had been word of their son Peter. Was that better or worse than a soot pile, a dotcode name coming up on your wristpad? Better, Nadia decided. But still hard.