"silvered" with a greenish reflective surface
inside that reflected only that light that could be utilized by the
ruby rods at its long focal center; and that absorbed the remainder of
the incident solar radiation, dumping it through to its black outside
surface, and on into the vastness of space. This half of the big
balloon was the spherical collector mirror, facing, through the clear
plastic of its other half, the solar disk.

Well inside the balloon, at the tip of the ruby barrel that was its
heart, were located the boiler tubes that activated the self-centering
inertial orientation servos which must remain operational at all
times. If the big mirror were ever to present its blackened rear
surface to the sun for more than a few minutes, the rise in
temperature would totally destroy the entire project. Therefore, these
servos had been designed as the ultimate in fail-safe, fool-proof
control to maintain the orientation of the mirror always within one
tenth of one degree of the center of Sol.

Their action was simplicity itself. The black boiler tubes were
shielded in such a way that so long as the aim was dead center on the
sun they received no energy; but let the orientation shift by a
fraction of a degree, and one of these blackened surfaces would begin
to receive reflected energy from the mirror behind it; the liquid
nitrogen within would boil, and escape under pressure through a jet in
such manner as to re-orient the position to the center of the tracking
alignment.

[Illustration]

Since the nitrogen gas escaped into the balloon, the automatic
pressure regulator designed to maintain pressure within the balloon
would extract an equal quantity of gas, put it back through the
cooling system on the back side of the mirror, and return it as liquid
to the boiler.

These jets were so carefully and precisely balanced that there was
virtually no "hunting" in the system.

The balloon itself was attached to its anchor tube by a one hundred
meter cable that gave free play to these orientation servos. The
anchor point was the exact center of the black outside surface of the
mirror-half of the balloon; and beside that anchor point was the air
lock to the control center, to which Steve was now going.

From the control room, a column extended up through the axis of the
balloon for thirty-five hundred feet--and most of the surface of this
column was covered with the new type, high power ruby rods, thirty
feet long and