The encounter between the meteor and the Aldebaran promptly became a welcome and enduring subject of conversation among the Mars crews. Bergmann the astronomer had drawn Laroche's Vega in the personnel shift and found himself foregathering with the loquacious Billingsley. Meteors in general had become a matter of very considerable importance and Bergmann's sidereal knowledge was much in demand. Heretofore he had not been able to utilize his fine, large telescope, for Mars was still at a greater distance than during his most favorable oppositions and Bergmann was unable to find a refuge from the hail of questions with which he was bombarded.
"I had no idea that these meteors were such dangerous fellows," remarked Billingsley on the day after Aldebaran had been struck. "It would seem that our ships are so tiny compared to all this space that there should be plenty of room for us without any real danger of colliding with the wretched things, you know. D'you think there's much chance of its happening again?"
"I'm afraid there is," answered Bergmann cautiously." I believe, however, that the Aldebaran incident permits us to discount any major danger from collisions of this sort."
"But I say, old chap, what if a bloody great meteor knocks a whole ship to bits and pieces?" bellowed Billingsley.
"That would be very tough luck indeed," answered Bergmann, "but the big fellows are fortunately few and far between in anything as vast as the universe. It's the small fry that give us the headaches.
"Judged by the size of the hole it made in Aldebaran, the meteor must have been about a quarter of an inch in diameter. Not very large, you'd say, but it's quite respectable for a meteor. The risk of a nacelle being hit by another is about once in 10,000 years."
"My dear fellow, you seem shockingly definite," retorted Billingsley with some skepticism, "how on Earth — in space, I mean — can you know that?"
"It's quite simple in principle, although that type of calculation is apt to be somewhat rough. The frequency of shooting stars as we observe them from Earth gives us a point of departure."
"Really, old fellow! How jolly interesting! Won't you elaborate a bit?"
"A meteor one quarter of an inch in diameter is visible as a shooting star of zero magnitude when it strikes Earth's atmosphere and becomes incandescent by air friction.
That is to say that its luminosity is equal to such bright, fixed stars as Vega or Capella. A statistical compilation of the observations of star gazers scattered all over the Earth indicates that about 500,000 shooting stars of zero magnitude and higher strike Earth's atmospheric shell per 24 hours. When this is referred to the tiny globes of our nacelles, it comes out to the figure I gave you: one probable hit every 10,000 years."
"That would seem to settle the matter for the quarter-inch giants, right enough. I shan't worry too much about them. But what about the smaller bits? Our recent visitor went right through two walls of our nacelle, tore and burned my good friend Sherman's undies, and punched two bloody great holes in the hydrazine tank. Can't one of his baby brothers burst through the nacelle some day and knock my breakfast pannikin galley-west?"
"As I told you, it's the smaller ones that give us the headaches, for they become more frequent as they decrease in size, and it is indeed fortunate that their penetration also decreases. This permits us to count on the three-quarter inch walls of the nacelles to protect us from all meteors smaller than the tenth magnitude.
"That size can only be perceived through powerful telescopes when it strikes the atmosphere, for the diameter is not far from one hundredth of an inch. We should call them "grains of meteoric dust" rather than meteors. Should such a grain penetrate the walls of one of our nacelles, the tiny puncture can be repaired with far less fuss than Aldebaran s trouble.
As to probabilities of such punctures, meteors down to the tenth magnitude strike Earth's atmosphere five billion times per day, or ten thousand times as often as those of zero magnitude and larger. The probability of a hit by such a large meteor is, as I said, one in 10,000 years; therefore the likelihood of a hit by a meteoric grain is one per year.
On our three year's trip, we might expect about three punctures per nacelle."
"What about armor plating the nacelles?" asked Billingsley.
"To get effective protection against meteors up to and including the eighth magnitude," answered Bergmann, "we should have to plate our nacelles with one tenth of an inch of steel and that would amount to five tons per nacelle. The planning staff felt that the risk did not justify the sacrifice in payload in order to cut down the chances of puncture to 20 per cent. Whether they were wise or not is a matter of educated guesswork rather than a scientific question. We shall know when we get home."
"That's all very well of the nacelles," pursued Billingsley, "but our propellant tanks are remarkably thin, so I'm told. Aren't we apt to dribble like sieves?"
"It's not as bad as that," went on Bergmann. "The large tanks for the initial maneuver are but one tenth of an inch thick and can presumably be holed by meteors as small as the fifteenth magnitude, which are about one four-hundredth of an inch in diameter. The tanks are roughly the size of the nacelles, and of the same material. Fifteenth magnitude meteors are about one hundred times as frequent as those of the tenth magnitude, so we anticipate roughly one hundred punctures per year. By the time we jettison our presently empty tanks, they'll probably average 70 punctures apiece. We're far more concerned over the smaller tanks for later maneuvers, since they will be exposed to meteoric grain bombardment for a much longer time."
"It seems to me a jolly great miracle that we've a drop of fuel left…"
Bergmann smiled confidently. "You've heard of puncture-proofing, have you not? It was used in automobile tires as early as 1920 and in bulletproof aircraft fuel tanks before that. Our tanks contain a chemical which immediately flows to the tiniest wound and stanches it then and there. Nor is it often called upon to heal anything as large as a 0.50 caliber hole, as it used to do quite regularly when men fought airplanes. It will be 10,000 years before there's another puncture like Aldebaran's, you might remember."
"Really, Bergmann, you're the most accommodating and patient science fellow it's ever been my good fortune to travel to Mars with. They do have meteoric swarms if the tabloid journals are right," persisted Billingsley.
"They are relatively dense, to be sure," answered Bergmann, "but the nice thing about them is that we're on familiar terms and know where to expect them.
"Meteors may be classified as hyperbolic or elliptical. The hyperbolic type enter the solar system from elsewhere and when they enter the solar field of gravitation, they have an initial velocity which tends to increase the closer they approach to the Sun. When their distance from the Sun is equal to the distance of Mars's orbit, they are traveling more than 34.3 km/sec, and at the distance of Earth's orbit more than 42.1 km/sec, for they would have attained this velocity by solar gravity alone, even without any initial velocity.
"Such meteors are 'strangers' and they describe a hyperbola around the Sun and disappear for ever from the solar system, unless they fall into the Sun or onto a planet. We have no way of predicting their appearance, whether as to time or location, for they visit us but once and for a short time. It is assumed that 70 % of meteors are hyperbolic.
"Elliptical meteors must basically be slower than the figures I've quoted, and the whole situation is different. Like planets, they gravitate around the Sun in elliptical orbits which are frequently very eccentric. They cross Earth's orbit at regular intervals and this permits us to become relatively familiar with their paths. The track of this voyage avoids such paths by a safe margin, if our computations have been effective."