incident solar radiation, dumping it through to its black outsidesurface, and on into the vastness of space. This half of the bigballoon was the spherical collector mirror, facing, through the clearplastic of its other half, the solar disk.
Well inside the balloon, at the tip of the ruby barrel that was itsheart, were located the boiler tubes that activated the self-centeringinertial orientation servos which must remain operational at alltimes. If the big mirror were ever to present its blackened rearsurface to the sun for more than a few minutes, the rise intemperature would totally destroy the entire project. Therefore, theseservos had been designed as the ultimate in fail-safe, fool-proofcontrol to maintain the orientation of the mirror always within onetenth of one degree of the center of Sol.
Their action was simplicity itself. The black boiler tubes wereshielded in such a way that so long as the aim was dead center on thesun they received no energy; but let the orientation shift by afraction of a degree, and one of these blackened surfaces would beginto receive reflected energy from the mirror behind it; the liquidnitrogen within would boil, and escape under pressure through a jet insuch manner as to re-orient the position to the center of the trackingalignment.
Since the nitrogen gas escaped into the balloon, the automaticpressure regulator designed to maintain pressure within the balloonwould extract an equal quantity of gas, put it back through thecooling system on the back side of the mirror, and return it as liquidto the boiler.
These jets were so carefully and precisely balanced that there wasvirtually no "hunting" in the system.
The balloon itself was attached to its anchor tube by a one hundredmeter cable that gave free play to these orientation servos. Theanchor point was the exact center of the black outside surface of themirror-half of the balloon; and beside that anchor point was the airlock to the control center, to which Steve was now going.
From the control room, a column extended up through the axis of theballoon for thirty-five hundred feet--and most of the surface of thiscolumn was covered with the new type, high power ruby rods, thirtyfeet long and one-half inch in diameter, mounted in tubular trays ofreflective material which took up sufficient space to make each rodoccupy two inches of the circumference of the tube on which it wasmounted.
These ruby rods were the heart of the power system, converting therandom wave fronts of noncoherent light received from the mirror intoa tremendous beam of coherent infrared energy which could be bundledin such a pattern as to reach Earth's surface in a focal pointadjustable from here to be something between twenty-two feet indiameter to approximately one mile in diameter.
The banks of rods were so arranged that each of the one hundredsections comprising the three thousand feet of receptive surface atthe focus of the mirror formed a concentric circle of energy beams;each circle becoming progressively smaller in diameter, so that theenergy combined into one hundred concentric circles, one within theother, as it left the rods; but these circles were capable of thenecessary focusing that could bring them all together into a singlesmall point near Earth's surface.
* * * * *
The beam leaving the rods represented three hundred seventy-fivemillion watts of energy, tightly packaged for delivery to Earth. Butthis was only a small fraction of the solar energy arriving at the bigmirror.
The remainder, the loss, must be dumped by the black surface at theback; and to account for the loss in the rods themselves, to preventtheir instantaneous slagging into useless globules of aluminum oxide,their excess loss energy must also be dumped.
A cooling bath of liquid nitrogen therefore circulated over each rodand brought the excess heat to the rear of the big lens, where it,too, could be dumped into the blackness of space beyond.
For all its size and complexity, Hot Rod was only a trifle over sixper cent efficient; but that six per cent of efficiency arriving onEarth would be highly welcome to supplement the power sources thatstatistics said were being rapidly depleted.
The spherical shape of the mirror itself, one of the easiest possiblestructures to erect in space, had dictated the placement of the rodsthrough its center since there was no single focal point for theentire mirror surface.
But it had also added a complication. From this position, the rodscould have been designed to fire either straight forward or straightback.
However, due to the hollow nature of the thirty-five hundred footlaser barrel; the necessity for access to the rods from inside thatbarrel; and the placement of the control booth at its outside end, thefiring could only be forward, straight towards the sun on which themirror was focused.
But to be useful, the beam must be able to track an ever-movingtarget.
This problem had been solved by one of the largest mirror surfacesthat man had ever created--flat to a quarter of a wave-length oflight, and two hundred fifty feet in diameter, the beam director, fromthis distance looking as though it were a carelessly tossedlooking-glass from milady's handbag, anchored one diameter forward ofthe big power balloon.
For all its size, this director mirror had very little mass.Originally it had been planned to be made of glass in much the samemanner as Palomar's 200-inch eye. But this plan had been rejected onthe basis of the weight involved.
Instead, its structure was a rigid honeycomb of plastic; surfaced by alayer of fluorocarbon plastic which had been brought to its finalpolish in space, and then carefully aluminized to provide a highlyreflective, extremely flat surface.
This mirror was also cooled by the liquid nitrogen supplied from theback side of the big mirror. Necessarily so, since even its bestreflectivity still absorbed a sufficient portion of the energy fromthe beam it deflected to have rapidly ruined it if it were notproperly cooled.
The several tons of ruby rods in the barrel, with their clear sapphirecoatings, were far more valuable than any gems of any monarch that hadever lived on Earth. Synthetic though they were, Steve Elbertson, theproject's military commander, knew they had been shipped here atfantastic cost and were expected to pay for themselves many thousandsof times over in energy delivered.
* * * * *
As yet, the project had had no specific target; nor had it been fullyoperational as of midnight yesterday.
But this "morning" for the first time the terrific energy of the laserbeam would be brought to bear on the Greenland ice cap--three hundredseventy-five million watts of infrared energy adjusted to aneedle-point expected to be twenty-two feet in diameter at Earth'ssurface, delivering one million watts per square foot, that should puta hole a good way through the several thousand feet of glacier therein its fifteen minutes of operation, possibly even exposing the barerock beneath, and certainly releasing a mighty cloud of steam.
Focused to this needle sharpness, the rate of energy delivery was manyorders of magnitude higher than that delivered by man's largestnuclear weapons only a few yards from ground zero.
Today's test was primarily scheduled as a test of control in aimingand energy concentration. Careful co-ordination of the project byground control was vital, so that no misalignment of the beam couldpossibly bring it to bear on any civilized portion of Earth's surface.For, fantastic as this Project Hot Rod might be as a source of powerfor Earth, Major Elbertson knew that it was also the most dangerousweapon that man had ever devised.
Therefore, the scientists were never alone in the control booth,despite the mile-long security records of each. Therefore, he and hismen were in absolute control of the men who controlled the laser.
Therefore, too, Steve told himself, as the time came when there wouldbe a question of command between himself and Captain Nails Andersen,science advisor to the U.N. and commander of Space Lab One, his ownsecret orders were that he was to take command--and the rank thatwould give him that command was already bestowed, ready foractivation.
Nails Andersen, Steve reminded himself with amusement, had originatedthe laser project; had fought it through against the advice of morecautious souls; and had, through that
project, attained command of thespace lab, and the rank that made that command possible, all in thename of civilian science.
But not command of the laser project, Steve told himself.
Not of the most dangerous military weapon ever devised--dangerous andmilitary for all that it was a civilian project, developed on theexcuse that it would power Earth, which was rapidly eating itself outof its power sources.
Not in command of that, Steve told himself. Nobody but a military mancould properly protect--and if necessary, properly use--such power.
Those were his secret orders; and he had the papers--and the authorityfrom Earth--to back him up. And orders to shoot to kill withouthesitation if those orders were questioned.
Meantime, today's peacetime experiment would bring forcibly to theattention of Earth both the power for good and the power fordestruction of the laser which he commanded.
Project Hot Rod was manned twenty-four hours a "day." The new shift ofscientists--the ones who would