f distress which shut Blades up.

To everybody's relief, they reached Central Control about then. It was
a complex of domes and rooms, crammed with more equipment than Blades
could put a name to. Computers were in Chung's line, not his. He
wasn't able to answer all of Warburton's disconcertingly sharp
questions.

But in a general way he could. Whirling through vacuum with a load of
frail humans and intricate artifacts, the Sword must be at once
machine, ecology, and unified organism. Everything had to mesh. A
failure in the thermodynamic balance, a miscalculation in supply
inventory, a few mirrors perturbed out of proper orbit, might spell
Ragnarok. The chemical plant's purifications and syntheses were
already a network too large for the human mind to grasp as a whole,
and it was still growing. Even where men could have taken charge,
automation was cheaper, more reliable, less risky of lives. The
computer system housed in Central Control was not only the brain, but
the nerves and heart of the Sword.

"Entirely cryotronic, eh?" Warburton commented. "That seems to be the
usual practice at the Stations. Why?"

"The least expensive type for us," Blades answered. "There's no
problem in maintaining liquid helium here."

Warburton's gaze was peculiarly intense. "Cryotronic systems are
vulnerable to magnetic and radiation disturbances."

"Uh-huh. That's one reason we don't have a nuclear power plant. This
far from the sun, we don't get enough emission to worry about. The
asteroid's mass screens out what little may arrive. I know the TIMM
system is used on ships; but if nothing else, the initial cost is more
than we want to pay."

"What's TIMM?" inquired the _Altair's_ chaplain.

"Thermally Integrated Micro-Miniaturized," Ellen said crisply.
"Essentially, ultraminiaturized ceramic-to-metal-seal vacuum tubes
running off thermionic generators. They're immune to gamma ray and
magnetic pulses, easily shielded against particule radiation, and
economical of power." She grinned. "Don't tell me there's nothing
about them in Leviticus, Padre!"

"Very fine for a ship's autopilot," Blades agreed. "But as I said, we
needn't worry about rad or mag units here, we don't mind sprawling a
bit, and as for thermal efficiency, we want to waste some heat. It
goes to maintain internal temperature."

"In other words, efficiency depends on what you need to effish," Ellen
bantered. She grew grave once more and studied him for a while before