peered out of the port next to Holt's.
"There's your inner tube, Lussigny," Holt heard one of them say. "I hope you'll enjoy your stay in it. It will seem like a second home to you before you're through." Holt glanced at the speaker from the corner of his eye. He was somewhat elderly, and had obviously neglected to have his gray hair cut. Intelligent gray-blue eyes twinkled behind rimless glasses. His companion's body seemed at least half a head longer than that of the speaker, although they hung at different angles beside the port. The companion's hair was bleached white and contrasted delightfully with the youthful curiosity with which he stared at the enormous wheel.
"It's certainly impressive," remarked the bleached giant, "but I'll have to admit that I don't quite understand it. Can't you tell me something about it before we connect?"
"I don't know quite as much about it as I'd like to," answered the man with the glasses, "but perhaps the colonel here wouldn't mind helping us out a bit?"
Turning to Holt, he introduced himself.
"My name's Hansen, Knut Hansen, of the Palomar observatory, and this is Doctor Francis Lussigny of M.I.T., who's visiting Lunetta for the first time. I'm sure you're an old spaceman who won't mind lecturing a couple of Earth-lubbers for a while…"
"I'm Colonel Holt. It'll be a pleasure."
"Oh, you'll be the Holt who was with Fitzgerald on the Moon! Of course, your picture was much in the papers at the time. You must feel at home in Lunetta."
"Well, I was on duty here for several months."
"How about a quick review of the most important things one should know about Lunetta?
It would be so nice if I didn't have to bother the people here with foolish questions."
"O.K. Let's start with the orbit. Lunetta's orbit is almost exactly circular with respect to the Earth's center 8,110 kilometers away. That is to say, Lunetta flies in a circle at an altitude of 1,730 kilometers above the surface. Her velocity must be 7.07 kilometers per second in order that her weight may exactly counterbalance the pull of gravity by its centrifugal force in the orbit. Right now, our ship is momentarily at the same speed and we therefore follow Lunetta in her orbit without any need for power."
"What is the angle of the orbital plane to the plane of the equator?" Lussigny asked.
"It's exactly 66.5 degrees and exactly vertical to the ecliptic."
"Why was that angle chosen? Wouldn't it have been more practical to lay the orbit in the plane of the equator or in that of the ecliptic?"
"There would have been certain advantages if it had been done that way," said Holt, "but a compromise had to be made. You see, Lunetta was established during the last war and was primarily an observation post and a bomb-dropping station against Russia. That meant that her orbit must pass over some of the higher northern latitudes. Furthermore, to have the plane of the orbit perpendicular to that of the ecliptic affords the advantage that Lunetta is over daylighted ground during the longest possible time averaged over a year.
The ground must be daylighted for effective observation or effective bomb-dropping."
"That I do not understand," said Lussigny curtly.
"If you will plot the angle of the Earth's polar axis in its orbit around the Sun, you'll see that the North Pole points most nearly towards the Sun on June 21 st, while the South Pole does so on December 21 st. This is the reason that Lunetta can make her girdle around the Earth on those two days exactly above the limits of dawn and dark. This means that the Sun never sets on the Earth directly below Lunetta. On the other hand, at the equinoxes Lunetta has night under her for one of the two hours which she requires to circle the Earth, and day for the other. Of course, you realize that the equinoxes are the most unfavorable periods for the operations of Lunetta so far as tactical considerations are concerned, for active work can only go on half the time. But averaged over the year, Lunetta's orbital plane permits her to be over daylighted areas 75 % of the time; that's easy to see."
Holt pointed over at Lunetta, where a small circular door had opened in the bottom of the hub. A man in what seemed to be a diving suit stepped out into nothingness and seemed to float there. Suddenly, the distant figure began to move mysteriously towards the Sinus as though propelled by some invisible force. A thin line leading back to the hub of Lunetta followed him.
"He's bringing us the securing line," said Holt. "In ten minutes we'll have completed the contact operation."
"How does the fellow move himself?" questioned Lussigny.
"He's got a little reaction pistol in his hand. It's fed from a tank strapped to his back. Very simple, really. The tank holds hydrogen peroxide which is chemically dissociated in the pistol. This generates steam, which, spurting from the pistol, produces a couple of pounds of thrust. He can move himself in any direction by pointing the pistol just opposite from where he wants to go."
"Quite a business, quite a business, this pushing yourself around in space with pistols! But tell me, what is the diameter of this Lunetta anyway?
"It's exactly 200 feet, and it rotates once every 20 seconds on its axis. This rotation produces an acceleration at the rim equal to one third of the gravity on Earth, namely 0.3g.
The crew, who spend most of their time in the rim, therefore feel themselves pressed against the outer wall of the periphery with one third of the force to which they are accustomed to press against the Earth at home. A spring balance would show that they weighed but one third of their normal weights." "Three-tenths g?" asked Lussigny in amazement. "Why do they not increase the RPM and produce an acceleration fully equivalent to the 1.0 g to which they are accustomed?"
"That seems quite reasonable at first thought," said Holt, "but there's an excellent reason for not going all the way with our synthetic gravity. In view of the relatively small diameter of Lunetta, her rotation tends to produce what are called Coriolis forces. If you are standing on the floor of the annular living space, which is, of course, where a nail would come through if you think of Lunetta as a tire being punctured, your head is pointed directly at the center of the hub. When you move peripherally in either direction your head must go more slowly then your feet to keep your body in line with the synthetic gravity. But since you are unaccustomed to such walking, because on Earth you must wander many miles before there is an appreciable convergence of the gravitational pull, here, where it happens rapidly you tend to fall in the direction of your movement. It turned out that for this particular diameter, a centrifugal acceleration at the rim of 0.3g was about right. It seems to be well above what we call the comfort level in the space business and yet the Coriolis forces are not high enough to affect equilibrium."
"That's certainly interesting… But how did you get that enormous wheel up here at all?"
"That wasn't so difficult. It consists, you know, of ten hollow sections of rubberized fabric. These were folded up small on Earth and brought up here on vessels like the one we are in. What am I saying — it was the old Jupiter class that did the job. Then men inspace suits like this fellow out there assembled the sections to form a closed ring. Finally it was blown up with compressed air, like an automobile tire."
"Why that's wonderful! And the hub?"
"It's the same idea. The hub consists of a drum, likewise of rubberized fabric, inflated to the same pressure as the rim."
"What's the purpose of the spokes?"
"Please notice that there's a black sphere on the upper end of the hub. That is a steam boiler. It is heated by the parabolic mirror beside it. The diameter of this mirror is 10.6 meters and it concentrates the solar rays on the generator, permitting steam to be constantly drawn off from it. This steam drives a turbogenerator located within the Central Station, as the hub of Lunetta is called. Thus the whole structure is supplied with 35 kilowatts of electrical energy.