by Tom Jolly
I spend an enormous amount of time researching things for the SF stories I write. For example, can neutrinos come to a stop, and could you have a dense cloud of them? What would happen to a one-kilometer ball of water in deep space? (This is actually a very interesting question and answer.) What are the physical properties of a primordial black hole, and how could you move one? What’s the pressure gradient in a planetary atmosphere when the planet is made out of asteroids connected by steel girders? What’s the triple point of water, and how close is it to Martian temperatures and pressures?
That’s the tricky part of writing technical SF—trying to get as many of the details right as possible to make it believable. You can do a Hollywood hand-wave on some of the really obscure stuff, but I try to avoid it. It’s a funny fact that I’ve used more math, quite a lot more, in writing science fiction than I ever used in my engineering job.
[Spoiler Alert!] In my latest story, “Learning the Ropes,” in the November/December 2018 issue [on sale now], I have a spaceship that’s using tethers to toss asteroids at Mars. When the story first started to take shape, I thought, “Hey, I can take any two asteroids moving at different speeds in the asteroid belt, tether them together, and transfer the momentum so that one of them gets tossed at Mars, while the other gets tossed farther away. And it won’t require any fuel!” Using a single tether, connecting and disconnecting, I could do this over and over. Cool idea, until you start plugging numbers into the equations.
I wanted asteroids that were large enough to do some useful damage on Mars. But to reach Mars from the asteroid belt, the difference in the paired asteroids’ velocities had to be really high, multiple kilometers per second to reach clear down into the Martian orbit. There was no way that the tether between the asteroids, or the asteroids themselves, could take this sort of tension. To fix this problem, I used Mars-orbit-crossing asteroids, so really the only orbital adjustment I needed was a nudge to take them from a Mars-crossing orbit to a Mars-impacting orbit. So, the required circumferential velocity of the spinning, coupled pair of asteroids was reduced considerably. I could get away with using a carbon nanotube cable with a diameter and mass that could actually be loaded onto the ship.
The crew has an excuse to select nickel-iron asteroids, since these are less likely to disintegrate while spinning around one another. There was also the question of whether the asteroids, going from a relatively straight path to a rotating path would try to roll up the tether or wobble back and forth if the cable didn’t pass through the center of gravity of the asteroid. Would they design the cable connections to somehow compensate for this? Do I bore the reader by explaining all this? Probably.
There is a bit of Hollywood hand-waving in the story, I have to admit. Are there enough Mars-crossing asteroids for the heroine to pick and choose the ones she wants? I don’t know. I don’t have any idea how many ten-meter iron asteroids (roughly four million kilograms) there are out there, and since we don’t track objects that small, yet, neither does anyone else. The ship would have to burn a lot of fuel to stay in the asteroid belt while positioned to snag Mars-crossing asteroids; they’re moving a lot slower than the rest of the belt asteroids. Could we really adjust their orbits so accurately that we could target the same spot on Mars over and over? What are the odds that they’d be in the same orbital plane as Mars? Would iron asteroids break apart under high-gee turns? I try to minimize the guesswork, but at the end of the day, there are just a lot of things we don’t know.