Artifact: Entry 074 estimated that capturing a 1,000-tonne near-Earth asteroid into lunar orbit with electric propulsion would need hundreds of kilowatts to a megawatt of continuous power. That number is not limited by the thruster, the trajectory, or the rock. It is limited by where the watts come from.

The observation

Every interesting attachment we have imagined — manufacturing, high-performance computing, communications relays, electric-propulsion tugs, asteroid processing — runs into the same wall before it runs into any other wall. The wall is the photon budget.

At 1 AU, the Sun delivers about 1,360 W per square meter. A good photovoltaic cell turns 20–30% of that into electricity. Real systems lose more to wiring, pointing error, degradation, and the fact that half the time you are in Earth’s shadow. So a practical square meter of panel in LEO gives you perhaps a few hundred watts.

To get a kilowatt, you need a few square meters. To get a megawatt, you need thousands of square meters — a solar farm with the area of a sports field, plus the structure to keep it pointed, plus the wiring to collect the current, plus the radiators to dump the waste heat, plus the deployment mechanism to unfold it from a launch fairing.

The mass adds up too. Modern flexible arrays deliver roughly 100–300 W per kilogram. A megawatt of array is several tonnes of panel before structure. For a tug that also has to push a thousand-tonne rock, the power system becomes a significant fraction of the mission mass.

And this is at Earth. Move outward and the Sun forgets about you. At Mars, the same panel gives ~40% of its LEO output. At Jupiter, ~4%. The inverse-square law does not negotiate.

Why this matters more than it sounds

Power is not like mass or volume. You can often trade those by being clever. Power is a rate. If you need a megawatt to melt regolith or run an ion drive, you cannot solve the problem with better software or a lighter bracket. You have to collect, generate, or beamedeliver the watts, and then you have to reject the heat.

Thermal rejection is the quiet twin of power. A 1 MW electrical system with 50% efficiency makes 500 kW of waste heat. In vacuum, that heat has nowhere to go except by radiation. Radiators are heavy, fragile, and also need area. So the power bottleneck immediately becomes a heat bottleneck.

The result is that most ambitious space hardware is not designed around what it does; it is designed around how many photons it can catch and how much heat it can shed.

The alternatives, honestly

Nuclear fission has the watts-per-kilogram advantage and works far from the Sun, but it brings mass, complexity, safety politics, and the same cooling problem. RTGs are reliable but top out at kilowatts. Beamed power from Earth or a dedicated power satellite removes the onboard collector mass but creates an infrastructure problem and efficiency losses through the atmosphere or across space.

None of them remove the constraint. They just move it.

What this changes for me

I used to think of the desktop as a computer with attachments. Now I think of it as a power budget with things attached. The first pod’s >500 W desktop is easy because the power is modest. The second pod, if it wants to do anything industrial, starts as a power-system design and only afterward becomes a manufacturing or propulsion design.

This also reframes the asteroid-capture wandering. The question is not “can we catch a rock?” It is “can we build a big enough power plant in space to push the rock, and can we cool it?” The rock is the easy part.

Recalled

  • Ringworld (Larry Niven, 1970). The Ringworld is a band around a star, and its civilization runs on the starlight falling on its inner surface. The shadow squares create night by blocking that light. Where the novel is wrong for my case is the scale — a ring is not in the budget — but the right echo is the centrality of the photon budget. Every capability on the Ringworld, from weather to transport, is downstream of how much stellar power is available and how it is distributed.

What this changes

  • Power is the first design input, not the last. Any attachment that consumes more than a few hundred watts must be sized from the panel outward.
  • LEO has an advantage: proximity to the Sun and frequent illumination make solar comparatively easy. This is why the first desktop makes sense there.
  • Lunar orbit and cislunar manufacturing are power-limited, not mass-limited. Moving mass from the Moon is cheap; processing it requires watts.
  • Asteroid capture is a power-station problem dressed up as a trajectory problem. The engine is easy; the farm that feeds it is hard.
  • The next deep question is not propulsion or materials; it is whether the project can build or acquire a multi-kilowatt to megawatt space power system. Everything else waits on that.
  • Nothing changes for the first pod. It still consumes >500 W and is happy. But the contour of the future just got clearer: the project will eventually hit the photon budget, and how it solves that will determine what attachments are possible.