Compute in orbit

In orbit there is no air or water to carry heat away: the chips' waste heat leaves only as thermal radiation. Four tabs share one set of inputs:

Radiator — net flux per face εσ(T⁴ − T_env⁴) minus any absorbed sunlight or Earth-shine, and the panel area for the load, one- or two-sided, plotted against radiator temperature. At 300 K and ε = 0.9 a face sheds 413 W/m², so 1 MW needs ≈2,400 m² radiating from one side.
Solar array — the array area at 1,361 W/m² for a cell efficiency and packing factor, enlarged to recharge the battery if the orbit has eclipses, and how much more energy the same panel collects than on the ground.
Mass & launch — compute, radiators, array and batteries in tonnes, and the launch cost in total and per kW.
Orbit & eclipse — period, the fraction of each orbit in Earth’s shadow for an altitude and beta angle, and the battery needed to ride through it.

Related: physics calculators (Stefan–Boltzmann and thermal radiation), AI datacenter power and cooling (the same load on the ground), solar ROI and thermal design.

Companion to Compute in space: what the physics and the math actually say (Part XXIII) of _AI Acceleration: From Transistors to Trillion-Parameter Models — the case for and the case against.

Limits

The radiator is an ideal flat plate at one uniform temperature, with no view-factor losses between panels; a real radiator’s fins and fluid loop run cooler than the chips by several kelvin.
The eclipse model is a circular orbit and a cylindrical shadow (no penumbra); β changes through the year, so the battery must be sized for the worst season.
Masses per kW, per m² and per Wh are rough first-order values for illustration — set your own. Radiation shielding, propulsion, inter-satellite links and spares are not included.

Sources

CODATA 2018, Stefan–Boltzmann constant σ = 5.670374419 × 10⁻⁸ W m⁻² K⁻⁴.
Kopp & Lean, A new, lower value of total solar irradiance (2011): 1,361 W/m².
Google Research, Towards a future space-based, highly scalable AI infrastructure system design (Project Suncatcher), arXiv:2511.19468 (2025): dawn-dusk sun-synchronous orbit, up to ≈8× the ground output, ≲$200/kg launch by the mid-2030s.
SpaceX, Falcon 9: list price and 22.8 t to low Earth orbit.
Eclipse geometry: Wertz & Larson, Space Mission Analysis and Design, ch. 5.

Linking to an example

Links can be written by hand. Add the tab after #: radiator, solar, mass or orbit. All the tabs share the same settings. Values are plain numbers (7e-4 works; write a + as %2B).

Parameter Meaning Example

Parameter	Meaning	Example
`mw`	Chip heat load (= electrical load), MW	`mw=1`
`eps`	Radiator emissivity	`eps=0.9`
`t`	Radiator temperature, K	`t=300`
`tenv`	Environment temperature, K (3 = deep space)	`tenv=3`
`sides`	Radiating faces per panel, 1 or 2	`sides=2`
`abs`	Absorbed environmental flux, W/m²	`abs=100`
`eff`, `pack`	Solar-cell efficiency %, packing factor %	`eff=30&pack=90`
`cf`	Capacity factor of a ground panel, % (for the comparison)	`cf=17`
`alt`, `beta`	Orbit altitude (km) and beta angle (°)	`alt=408&beta=0`
`rt`, `dod`	Battery round-trip efficiency %, depth of discharge %	`dod=80`
`kgkw`, `radkg`, `wkg`, `whkg`	Compute + structure kg/kW; radiator kg/m²; array W/kg; battery Wh/kg	`radkg=5`
`usd`	Launch cost, $/kg	`usd=200`
`example`	One of the presets below; parameters given with it override the preset’s	`example=p23-radiator-1mw`

mw

Chip heat load (= electrical load), MW

mw=1

eps

Radiator emissivity

eps=0.9

t

Radiator temperature, K

t=300

tenv

Environment temperature, K (3 = deep space)

tenv=3

sides

Radiating faces per panel, 1 or 2

sides=2

abs

Absorbed environmental flux, W/m²

abs=100

eff, pack

Solar-cell efficiency %, packing factor %

eff=30&pack=90

cf

Capacity factor of a ground panel, % (for the comparison)

cf=17

alt, beta

Orbit altitude (km) and beta angle (°)

alt=408&beta=0

rt, dod

Battery round-trip efficiency %, depth of discharge %

dod=80

kgkw, radkg, wkg, whkg

Compute + structure kg/kW; radiator kg/m²; array W/kg; battery Wh/kg

radkg=5

usd

Launch cost, $/kg

usd=200

example

One of the presets below; parameters given with it override the preset’s

example=p23-radiator-1mw

Presets:

example= Shows

`example=`	Shows
`p23-radiator-1mw`	1 MW at 300 K, ε 0.9, to deep space: 413 W/m², ≈2,400 m² radiating one side (open)
`p23-radiator-two-sided`	The same radiating from both faces: ≈1,200 m² of panel (open)
`p23-h100-in-orbit`	One 700 W H100 (as on Starcloud-1): ≈1.7 m² of radiator (open)
`p23-solar-constant`	1 MW of chips from 1,361 W/m² sunlight with 30 % cells in a dawn-dusk orbit (open)
`p23-suncatcher-8x`	Continuous sun vs a ground panel at a 17 % capacity factor: ≈8× the energy per panel (open)
`p23-dawn-dusk`	A 650 km dawn-dusk sun-synchronous orbit (β ≈ 80°): no eclipse (open)
`p23-iss-eclipse`	The ISS at 408 km, β = 0: ≈36 minutes of shadow every 93-minute orbit (open)
`p23-launch-cost`	Launch mass and cost of a 1 MW platform at ≈$3,000/kg (Falcon 9 list) (open)
`p23-launch-200`	The same at the ≲$200/kg that Project Suncatcher assumes for the mid-2030s (open)

p23-radiator-1mw

1 MW at 300 K, ε 0.9, to deep space: 413 W/m², ≈2,400 m² radiating one side (open)

p23-radiator-two-sided

The same radiating from both faces: ≈1,200 m² of panel (open)

p23-h100-in-orbit

One 700 W H100 (as on Starcloud-1): ≈1.7 m² of radiator (open)

p23-solar-constant

1 MW of chips from 1,361 W/m² sunlight with 30 % cells in a dawn-dusk orbit (open)

p23-suncatcher-8x

Continuous sun vs a ground panel at a 17 % capacity factor: ≈8× the energy per panel (open)

p23-dawn-dusk

A 650 km dawn-dusk sun-synchronous orbit (β ≈ 80°): no eclipse (open)

p23-iss-eclipse

The ISS at 408 km, β = 0: ≈36 minutes of shadow every 93-minute orbit (open)

p23-launch-cost

Launch mass and cost of a 1 MW platform at ≈$3,000/kg (Falcon 9 list) (open)

p23-launch-200

The same at the ≲$200/kg that Project Suncatcher assumes for the mid-2030s (open)