Satellite InferenceAI Inference in OrbitGet in touch NODE SIZING / POWER AND HEAT
Every watt of compute arrives as sunlight and leaves as heat. The simulator opens on our 100 kW node, its solar arrays, battery and radiators drawn to scale and run through one orbit on the hardest day of the year; the scenarios show what changes with another orbit, another year's technology or another way through Earth's shadow. Then follow the heat itself, from the GPUs through the coolant to the radiator, through a real orbit and a whole year.
100 kW of compute in the dawn–dusk, 650 km orbit with 2030 technology. Arrays are sized for the worst day of the year at end of life (fixed arrays facing the orbit normal); every watt of compute leaves as heat through two-sided radiators, 15% larger than the sizing for average conditions, shown glowing in proportion to the heat they carry. The battery keeps 40% of the GPUs computing through the longest shadow. Drag the node to turn it.
01 / WHAT DECIDES THE SIZE
Sunlight carries 1,361 W per square metre. Cells convert 22–30% of it, and the array is sized at end of life on the day with the least light. In a dawn–dusk orbit, arrays fixed to face the orbit normal stay well lit all year, so the model needs no array drive.
In vacuum, heat leaves only as infrared radiation, which grows with the fourth power of temperature. A hotter coolant loop sheds more heat per square metre, so the radiator shrinks; Earth's own infrared and reflected sunlight load the side that faces it.
In a dawn–dusk orbit the longest shadow is about 20 minutes on 90 days a year, so pausing all compute would cost about 4% of the year. Our node keeps 40% of its GPUs computing through it on a 45 kWh battery and computes 97.6% of the year. Orbits that spend up to 35 minutes of each pass in shadow need about 290 kWh of battery to keep 100 kW computing, or lose about a third of the year.
02 / HEAT THROUGH THE ORBIT
The sizing above uses averages. Below, the same 100 kW node flies its orbit minute by minute: the Sun, Earth's shadow, Earth's infrared glow and reflected sunlight are computed for the date, and the GPUs, the coolant and each part of the radiator warm and cool with them. A valve sends part of the coolant around the radiator and heaters add heat, to hold the coolant reaching the GPUs at 40 °C.
If every GPU stops in shadow, the coolant cools even with the heaters at their 10 kW limit: on 21 June, the longest shadow of the year, the GPUs swing by 41 K, and the node enters shadow 1,323 times a year. Our node keeps 40% of its GPUs running instead: their heat holds the coolant without heaters and the swing falls to 12 K, for a larger battery.
Sized from average conditions, the 154 m² radiator holds the 45 °C limit with 6% of its area to spare on a typical June day, but needs 1% more under NASA's hottest Earth for polar orbits and 11% more once its white paint is contaminated as on POSA-I. Mirror tiles need 7% more in the hot case. Our node carries 15% more: 177 m².
In a 10:30 sun-synchronous orbit the node enters a 34-minute shadow on every orbit: 5,495 a year against 1,323 in the dawn–dusk orbit, each one a temperature cycle for the GPUs and a charge cycle for the battery.
03 / METHOD AND LIMITS
Sizing is a port of node sizing in the Satellite Inference engineering model v3.1, fed by the same orbit model as the orbit simulator. Load: compute divided by 95% conversion efficiency, plus 8% for housekeeping and thermal control. Arrays: III-V cells (30%) in 2026 and silicon (22%) later, 90% packing, 88% end-of-life factor, 100, 150 and 250 W/kg. Battery: housekeeping and the share of compute kept running, through the longest shadow, at 92% path efficiency and 40% depth of discharge in a dawn–dusk orbit (25% elsewhere). Radiators: two-sided panels edge-on to the Sun, emissivity 0.9, fin efficiency 0.9, Earth infrared 237 W/m² and an albedo load on the nadir face. Structure, avionics and propulsion add 12–20% and propellant 5%. Automated tests check it against the engineering model: a 10 kW node in the dawn–dusk orbit with 2026 technology needs about 43 m² of array, 15 m² of radiator and a 0.7 kWh battery, for 411 kg.
The heat balance follows our node through its orbit, with the radiator sized for today's coolant limits: GPUs on cold plates (0.02 K/W per 1 kW GPU, measured on a commercial plate), a pumped loop sized for a 20 K rise, a radiator in 12 segments along the flow, and an ideal valve and heaters. The Sun and the conical Earth shadow come from the same astronomy as our other simulators. Earth infrared and albedo use NASA's design values for polar orbits (NASA/TM-2001-211221), with albedo integrated over the sunlit part of the visible Earth and corrected for low sun. An independent calculation (JPL's DE421 ephemeris through Skyfield, a sampled solar disc for the shadow, adaptive quadrature for the view factors and SciPy's LSODA solver) agrees on temperatures to 0.11 K, on heater energy to 0.04% and on shadow entry and exit to the second; energy balances to one part in a million. Automated tests check this on every build.
Coolant limits are those of today's liquid-cooled GPU racks: supply up to 45 °C, return up to 65 °C (QCT, GB200 NVL72 liquid cooling, 2025). Our 2030 and 2035 sizing assumes radiators 10 to 20 K warmer, which needs GPUs that accept warmer coolant than that. Not modelled: conduction between radiator panels, arrays blocking the radiator's view, battery and avionics temperatures, and pump and valve dynamics. A communication tool, not spacecraft design. The SI-P1 pathfinder is a hosted payload that draws about 200 W at peak from its host spacecraft; these sizes and temperatures apply to future SI Nodes. Coatings: AZ-93 and Z-93P white paints flown on MISSE and POSA-I; quartz mirror tiles from the Spacecraft Thermal Control Handbook. Coolants: CoolProp 8.0 and NIST. Cold plate: Martinez, Caceres and Ortega, Journal of Electronic Packaging, 2024.