Limits by Larry Niven

It stirred the hair on my neck.
    “Me too,” Dot added. She glared at me, the enemy.
    I made one more try. They’d had more time to think about it than I did, but the thrust figures were right there, scrawled in an upper corner of the diagram. “Now pay attention. You can’t possibly use the attitude jets on the solar panels for that long. They work by squirting dust through a magnetic field, throwing it backward so the reaction pushes you forward. Okay, you’ve got free solar power, and you can get the acceleration. But where can you possibly get enough dust?” I saw Jack’s guilty grin, and finished, “Holy shit!”
    Jack nodded happily.
    “Why not?” Jill asked. “We won’t need solar flare shielding around Ceres. On the way we can keep what we do have between us and the Sun, while we grind up the surplus.”
    They meant it. They were going to make dust out of the radiation shields and use that.
     
    In theory it would work. The panel engines didn’t care what was put through them; they merely charged the stuff up with electricity gathered from the solar cells and let the static charge provide the push. A rocket is nothing more than a way to squirt mass overboard; any mass will do. The faster you can throw mass away, the better your rocket.
    At its simplest a rocket could be a man sitting in a bucket throwing rocks out behind him. Since a man can’t throw very fast that wouldn’t be a very good rocket, but it would work.
    But you have to have rocks, and they were planning on using just about all of ours.
    It was a one-way mission. They’d have to find an asteroid, and fast, when they got to the Belt; by the time they arrived they’d be grinding up structure, literally taking the Shack apart, and all that would have to be replaced.
    It would have to be a special rock, one that had lots of metal, and also had ice. This wasn’t impossible, but it wasn’t any sure thing either. We knew from Pioneer probes that some of the asteroids had strata of water ice, and various organics as well; but we couldn’t tell which ones. We knew one more thing from the later probes, and The Plan was geared to take advantage of that.
    The Skylark—newly named by McLeve, and I’ve never known why he called it that—would head for Ceres. There were at least three small-hill-sized objects orbiting that biggest of the asteroids.
    A big solar flare while they were out that far would probably kill the lot of them. Oh, they had a safety hole designed: a small area of the Shack to huddle inside, crowded together like sardines, and if the flare didn’t last too long they’d be all right—Except that it would kill many of the plants needed for the air supply.
    I didn’t think the air recycling system would last any three years either, but Jill insisted it was all right.
    It didn’t matter, I wasn’t going, and neither was Jack; it was just som e thing to keep Jill happy until the shuttle came.
     
    There was more to The Plan. All the nonessential personnel would go to Moonbase, where there was a better chance. Solar flares weren’t dangerous to them. Moonbase was buried under twenty feet of lunar rock and dust. They had lots of mass. There’s oxygen chemically bound in lunar rock, and if you have enough power and some hydrogen you can bake it out. They had power: big solar mirrors, not as big as ours, but big. They had rocks. The hydrogen recycles if it’s air you want. If you want water, the hydrogen has to stay in the water.
    We figured they could hang on for five years.
    Our problem was different. If Moonbase put all its effort into survival, they wouldn’t have the resources to keep sending us rocks and metal and hydrogen. Hydrogen is the most abundant element in the universe; but it’s rare on the Moon. Without hydrogen you don’t have water. Without water you don’t have life.
    I had to admit things were close. We were down to a shuttle load a month from Earth; but we needed those. They brought hydrogen, vitamins,