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.