[Image above] Juno’s solar panels are seen here as the spacecraft approaches Jupiter. Juno has the largest solar array flown by NASA on any mission other than the International Space station (ISS). There are three 30-foot-long (9-metre) solar arrays packed with 18,698 individual solar cells. (Image courtesy of NASA/JPL-Caltech)
The almost 19,000 cells on Juno are outfitted on three arrays the length of a “big rig” truck trailer. In Earth orbit, the solar panels would generate 14 kW of electricity, but when Juno settles into its orbit around Jupiter they will only output a weak 400W. It is fortunate that the scientific instrumentation and onboard computer are very energy-efficient.
The Sun’s rays reach distant Jupiter
Being this far from the Sun only allows a meager solar intensity of only 3.4 to 4.1% of what the Earth receives. To make things more difficult in powering Juno, the solar arrays operate at extreme low temperatures.
Let’s do a bit of analysis using the Stefan-Boltzmann Law (Refs 1,2) to analyze the conversion of solar radiation to heat. We can calculate the flat-plate equilibrium temperature ( Teq) using the Stefan-Boltzmann equation:
where α is the solar absorptivity, I the solar intensity, ε the front and back thermal emissivity (assuming two-sided emission), and σ the Stefan-Boltzmann constant, 5.67 x 10 -8 W/m 2K4.
Solar intensity drops off as the square of the distance from the Sun. If we look at Table 1, we can see the solar intensity at Jupiter, along with the calculated equilibrium temperature, for an assumed absorptivity and emissivity that is typical of current generation high-efficiency solar cells.
Table 1: The solar intensity at Jupiter, and the equilibrium flat-plate temperature of a solar array (Ref 2)
Temperatures calculated for absorptivity 0.92; front and back side thermal emissivity of 0.85, and cell efficiency of 25%.
The low solar intensity