Unlike normal ships, which carried their propellant in tanks, Ymir was a big chunk of solid
propellant—ice—inhabited by a sort of parasitical infestation of equipment whose purpose was to
convert that propellant into thrust. Not knowing exactly what he would find on Comet Grigg-
Skjellerup, Sean had come equipped with more than one alternative architecture for putting Ymir
together. If the comet core had turned out to be a loose ball of ice-dust, then he’d have had to scoop
out what he needed and pack it into something like a snowball, giving Ymir a spherical shape with the
reactor embedded in its center. Another option would have been to fashion a long cylinder of ice andplant the reactor in one end of it, then “burn” it forward, consuming the ice en route, like a candle.
What they were seeing now looked more like the third architecture, which was the shard. It suggested
that, upon his rendezvous with Grigg-Skjellerup, Sean had found it made up of at least one fairly hard
and solid crystal that could be relied on to hold itself together structurally during the maneuvers to
come. He had split the shard off from the main body of the comet and planted the reactor system
somewhere near its middle, then embedded the rest of his ship—the part where the humans lived—in
what would become its nose. If the equipment had worked as planned, then executing the “burns”—
i.e., pulling out the control blades to place the reactor into operation and make steam—had been a
matter of sending signals to actuators embedded in the core: motors that would move the rods, valves
that would control the flow of steam and water, and so forth.
Implicit in all of this was a hell of a lot of robot activity, which was why Sean had taken the
extraordinary step of traveling personally to Izzy to clean out Dinah’s supply of them before
proceeding to his rendezvous with Ymir. The reactor had to be fed with ice. Because ice was a solid,
it couldn’t flow through tubes. Robots had to mine ice from the shard and transfer it to a feed system:
a set of augers that would move it into the reactor chamber to be melted and vaporized. A Siwi robot
could move a lot of material in a hurry by embedding its “tail” in the ice and then using a whirring
mill on its “head” to throw off a fountain of fine shavings that could be collected and carried off by
Nats. The long intervals of time between burns could be used to store up a supply of shredded ice in
hoppers that would feed the augers.
Downstream of the engine, robots were also needed to maintain the shape of the rocket nozzle.
This was a long duct with a wide mouth on the aft face of the shard, tapering to a narrow throat near
the reactor. The throat had been constructed on Earth and launched up with the reactor. It was made of
a corrosion-resistant alloy called Inconel. Any other material would rapidly wear out from the hot
steam blasting through it. Conditions in the long spreading bell of the nozzle, however, were more
benign, and so it worked fine for that to be sculpted from ice. Nonetheless, it changed its shape as it
was used. Deeper in, where the exhaust was hot, it grew wider as its walls were melted by the torrent
of steam. Closer to the exit, where the exhaust had cooled to below freezing, it accumulated on the
walls and narrowed the passage. So robots had to scuttle around reshaping the nozzle. This was a fine
task for the Nats that Larz had experimented with in Seattle.
Finally there was a third “crew” of robots living on the exterior surface of the shard, trying to
keep it from falling apart by embedding fibrous reinforcement in the outer layer of ice and wrapping
cables and nets around it, somewhat like a butcher tying up a roast to prevent it from collapsing in the
oven. This was a good match for the capabilities of the Grimmed (steel-armored) robots, which were
mostly Grabbs.
All of these robots needed power, of course. They could store a little of it in batteries, but those
had to be recharged. Some of them collected energy from sunlight; others had to converge from time
to time on one of Ymir’s little nuclear generators to sip electricity.
The general picture was that Ymir would not be anything like the traditional idea of a spaceship,
in the sense of an orderly, symmetrical piece of architecture. It would be more like a flying robotic
anthill, constructed out of a natural found object. The robots crawling around on and in it had general
instructions as to what they were supposed to be doing, but could make their own judgments from
moment to moment to avoid collision with other robots, or from hour to hour as to when they needed
to recharge their batteries.Or that had been the general scheme, anyway. Since there’d been no guessing what Sean would
find, there’d been no way of coming up with any plan worthy of that name. Instead they had sent him
up with tools, resources, and ingenuity. Dinah, Markus, Vyacheslav, and Jiro were about to inherit the
tools and the resources.