Sunlight intensity — and so a solar panel's power output — falls off with the inverse square of distance from the Sun: double the distance, quarter the power. An RTG instead converts the steady heat of Pu-238 radioactive decay into electricity via the Seebeck effect, so its output barely notices distance at all.
P_solar(d) = P_earth / d² [d in AU]
P_RTG(t) = P_0 · 0.5^(t / 87.7) [t in years, distance-independent]
- Distance slider — crashes the solar curve; leaves the RTG curve flat.
- Mission-years slider — slowly lowers the whole RTG line via Pu-238 decay; the solar curve at a fixed distance never moves in response to it.
Real-world application: Voyager, New Horizons, Cassini and the Perseverance rover's MMRTG all fly RTGs because past roughly the asteroid belt — and in permanently shadowed or multi-decade missions — solar panels simply can't collect enough sunlight to be worth the extra area and mass.