The most recent episode of Enterprise, although I thoroughly enjoyed it, had me scoffing at the depiction of normal gravity on such a small celestial body. The men would have weighed between 5 and 40 pounds (depending on the comet’s ratio of rock to ice, ultradensity of the rare material in question notwithstanding), which would make jumping out of a one-meter hole a piece of cake. Even if you do slip, injuring your knee in a one-meter fall would be exceedingly difficult under such meager gravity. Even so, with an injured knee, you should still be able to hop on one foot, bounding ten meters at a time, back toward your shuttlecraft, making for a quick escape. (However, keeping your balance while doing this, I must admit, would be next to impossible – at least for me.) But Enterprise is not the only SF with gravity gaffes. Highly respected, classic SF literature has them, too. In particular, the Ringworld series of novels, by Larry Niven, depicts a civilization that has converted all its solar system’s mass, excluding their star, into a giant ring orbiting their sun. A concentrated, spherical mass, like a planet, usually has an inherently stable orbit. It’s like a marble in a round-bottomed hole. Nudge the marble in any direction, and it will once again seek the bottom of the hole (or orbit about that point) with almost no risk of flying out of the hole. Likewise, if you nudge the Earth, its orbit may become more (or less) elliptical, but only an unspeakably cataclysmic amount of nudging, like a head-on collision with a similarly-sized planet going in the opposite direction, would send the Earth crashing into the sun. This is not so with the Ringworld concept. From a gravitational energy point of view, Ringworld is like a marble on top of a round hill. Balance it carefully, and it will sit there indefinitely, but only as long as nothing interacts with it. But give the marble the slightest nudge, and it will roll off the hill, accelerating relentlessly toward its doom. Likewise, Ringworld, with only the slightest of celestial tugs, will inevitably fall and crash into its own sun. The orbit of a ring about a gravity well is inherently unstable, and I wouldn’t bet a civilization on its long term viability. Then we have the Dyson Sphere, which is an enormous, hollow sphere, with a radius approximately equal to one astronomical unit, constructed about a star. The people live inside the sphere, and none of the star’s light gets out. From a gravitational energy point of view, this is like a marble resting on a flat surface. Nudge the marble in any direction, and it will continue to go in that direction, without acceleration, until it encounters another force. This means that the net force on the sphere due to the sun’s gravity is zero, regardless of where the sun sits inside the sphere. (Of course, this also assumes that the sphere is rigid enough to withstand asymmetrical, or tidal, forces resulting from migration of the sun away from the center of the sphere.) Occupying the flat nether-region between orbital stability and instability, the Dyson Sphere would require thrusters to keep the central sun from creeping too far away from its central location. Slight nudges would need to be dealt with, but only a nudge of equal amount in the opposite direction would be required to stop relative motion between the Dyson Sphere and its central sun. This is not the case with the Ringworld, which continues to accelerate toward the sun after being nudged off the top of its orbital-stability hill. But some other aspects of the Dyson Sphere are intriguing as well. Does the sphere spin? Or does its rigidity simply keep its structure from collapsing into the central sun? Even if it does spin, counteraction of the central sun’s gravity would occur only in a plane. The poles of the sphere would still rely on the sphere’s rigidity, even more so if it is spinning, to avoid collapsing into the central sun. If the sphere is not spinning, then the inhabitants would perceive “down” as toward the sun, making their world vastly different from our own. If the sphere is spinning just enough to cancel the force of the sun’s gravity at the sphere’s equator, then the sphere’s inhabitants, at the equator, would be weightless. And lest you think that the sphere itself would become the “ground” due to its own gravity, rest assured that the inhabitants would be weightless regardless of the thickness of the sphere. The “marble on a flat surface” applies to the gravitational interaction between the sphere and its inhabitants just as it does to the gravitational interaction between the sphere and the central sun. In other words, the inhabitants, as long as they are inside the sphere, will always experience a net zero force from the sphere’s gravitational attraction. (This assumes uniform thickness and density of the sphere, of course.) If the sphere is spinning more than enough to cancel the force of the sun’s gravity at the equator, then the sphere’s interior surface, at the equator, would be “down” and the sun would once again be in the sky as it should be. But this would tax the sphere’s structural integrity immensely. But so what? If a civilization can build a sphere that big, I think it’s safe to assume that they have the engineering prowess to make it unbelievably strong. Regardless, no matter what you do regarding the sphere’s spin, inhabitants at the poles would perceive “down” as toward the sun. If the sphere is spinning, the interior civilization might be broken up into classes with the rich elite at the equator with the sun in the sky, while the downtrodden working class citizens live at higher latitudes and at the poles, with only plate steel for a sky, and the unseen sun forever below ground. However, it could be the case that all the sphere’s inhabitants live only at the equator, and that the higher latitudes are uninhabited. But then why would a civilization build a sphere instead of a ring if they didn’t intend to populate the entire structure? Why, to keep it from crashing into the sun, of course!