Juno’s solar panels have a very large postcode

I keep a paper model of Juno on a shelf. The arrays occupy almost all of the useful shelf space. The central spacecraft looks like the fitting supplied to hold the panels together.

The real geometry is less frivolous. JPL’s arrival press kit gives more than 60 square metres of array surface and about 500 W of output at Jupiter, compared with roughly 14 kW at Earth’s distance from the Sun. Those are design figures from the mission documentation, not a live reading from the spacecraft. ^[1]

The square is the expensive part

For a first estimate, sunlight falls with the inverse square of distance. At 5.2 astronomical units, the fraction of the sunlight available at 1 AU is:

Take the quoted 14 kW near Earth and multiply by that fraction. You get about 518 W. This is pleasingly close to 500 W, but the agreement does not mean we have independently designed Juno’s power system. It tells us that the dominant geometric effect is the one we expected.

Real cells have temperature-dependent behaviour. Their performance changes with radiation exposure. The panels must point appropriately, the spacecraft must avoid unwanted shadow, and power has to survive wiring and conversion before an instrument gets it. A launch-date power figure is not automatically an end-of-mission guarantee.

JPL’s explanation of the solar design explicitly connects the large array area to the inverse-square loss and improvements in solar-cell efficiency. There is an engineering history behind those wings; it is not just a decision to buy more of the same panel. ^[2]

No line painted across the Solar System

At 10 AU, an otherwise identical panel receives about one hundredth of the sunlight it would at 1 AU. At 30 AU, about one nine-hundredth. Those numbers get ugly, but neither is zero. Whether a solar design is useful depends on the load, area, mass, deployment, pointing and the periods when illumination is unavailable.

I would not infer from Juno that every outer-planet spacecraft ought to use solar power. Nor would I call Jupiter a fundamental solar-power boundary. The relevant comparison is between complete mission designs that satisfy the same requirements.

The model has acquired a slight droop in one wing. I suspect the shelf humidity was omitted from its qualification programme.