"It could be an inertial-measurement malf," said Andy Mercer, the other mission specialist seated beside Emma. "Take it off-line."

"No! It might be a broken data bus!" cut in Emma. "I say we engage the backup."

"Agreed," snapped Kittredge.

"Going to backup," said Hewitt. She switched to computer number five.

The vector reappeared. Every one heaved a sigh of relief.

The burst of explosive charges signaled the separation of the empty fuel tank. They couldn't see it fall away into the sea, but knew another crisis point had just passed. The orbiter was flying free now, a fat and awkward bird gliding homeward.

Hewitt barked, "Shit! We've lost an APU!" Emma's chin jerked up as a new buzzer sounded. An auxiliary power unit was out. Then another alarm screamed, and her gaze flew in panic to the consoles. A multitude of amber warning lights were flashing. On the video screens, all the data had vanished.

Instead there were only ominous black and white stripes. A catastrophic computer failure. They were flying without navigation data. Without flap control.

"Andy and I are on the APU malf!" yelled Emma.

"Reengage backup!" Hewitt flicked the switch and cursed. "I'm getting no joy, guys. Nothing's happening -- "

"Do it again!"

"Still not reengaging."

"She's banking!" cried Emma, and felt her stomach lurch sideways.

Kittredge wrestled with the joystick, but they had already rolled too far starboard. The horizon reeled to vertical and upside down. Emma's stomach lurched again as they spun right side up. The next rotation came faster, the horizon twisting in sickening whirl of sky and sea and sky.

A death spiral.

She heard Hewitt groan, heard Kittredge say, with flat resignation, "I've lost her." Then the fatal spin accelerated, plunging to an abrupt and shocking end.