"I am afraid we have not appreciated the speed at which we have been travelling," remarked the doctor. "Phobos is very slow in overtaking us;" and he was just beginning to slacken speed still more, when he suddenly cried out,—
"Here she is ahead of us now! We have overtaken her, instead of waiting for her to catch us!"
And, true enough, we were gradually approaching a small brownish mass, feebly illuminated on its outer half by the sun, and more faintly still on its inner half by reflected light from Mars.
And how shall I describe that queer little toy-world which we were gradually overtaking? Imagine, if you can, a little island, less than a third the size of the Isle of Wight, tossed a few thousand miles into space, and circling there rapidly to avoid falling back upon the greater sphere. Imagine that flying island devoid of soil, of trees or vegetation, of water or air, of everything but barren, uncrumbled, homogeneous rock, and you have some idea of the unadorned desolation of Phobos, into which we were slowly sailing, or falling. There was not even the slightest trace of sand or scraps of rock, such as time must have abraded from even the hardest surfaces, but the reason for this soon became apparent.
The doctor feared steering directly against her as we approached, lest we should land with a crash. We had already reached her and were travelling along her inner side. Although we were very near her, she seemed to have very little attraction for us. Then he turned very much closer, but as soon as the influence of the rudder was released, we seemed to leave her instead of falling upon her as we expected. We were still travelling faster than she was, and had we steered directly against her, we would have crashed and bumped against her protuberances. Still there seemed to be no other way to make a landing. In order to estimate the amount of such a shock, the doctor calculated, from the best information he had of her size and a guess at her density, that she would attract the projectile and its entire load with a force of only two pounds. That was not enough to cause any very great shock, and he decided to take chances at once, before we had entirely passed her. He turned the rudder hard over toward the satellite, and we came against her with scarcely any crash, but with a bumping and grating that continued until the rudder was eased back. Then, to our great surprise, we did not remain on the surface, but rose from it and sailed inward towards Mars.
"Something wrong here!" exclaimed the doctor. "She has no attraction for us."
"Well, how do you explain this?" I asked. "You say the whole projectile weighs only two pounds toward Phobos, when, just a short time ago, I weighed nearly eight pounds myself on the scales."
"True enough!" he cried; "the gravity of Mars must be dominant." He began figuring rapidly, and then exclaimed: "We weigh one hundred and thirty pounds toward Mars, and only two pounds toward the satellite. Small wonder that we could not make a landing, with Mars pulling us away sixty-five times harder than Phobos attracted us! But this is very strange! I remember no mention of this in any of the astronomical writings, and it is as easily calculable on Earth as it is here. Moreover, this must cause everything that is loose upon Phobos to fall upon Mars. The great planet is tugging at everything the satellite has with a force sixty-five times stronger than her own!"
"Now, I am afraid those figures won't do, Doctor," I put in. "For, if what you say is true, what prevents the whole satellite from tumbling into Mars at once?"
"She would do so were it not for centrifugal force. The speed with which she whirls around the planet must just balance the force with which he attracts her, and thus she is kept in her orbit. But stones and loose things on this side of her centre are attracted more strongly by Mars than they are repelled by the whirling, so they must all have fallen to the planet. That is why the surface was perfectly barren. If Phobos always keeps the same side turned toward Mars, there may be rocks and soil on the outer side, and we could land there with a positive current; but we could not see the great planet, as I had hoped."