An ice-giant orbiter spends roughly a decade in cruise before it even arrives — Uranus at ≈19 AU, Neptune at ≈30 AU — usually bending its path past Jupiter (and sometimes Saturn) to steal orbital energy via gravity assists. The RTG that will power the whole mission starts decaying from the moment it's fuelled, so every one of those cruise years is already eating into the power budget the science phase will have to live on.
P_RTG(t) = P_0 · 0.5^(t / 87.7) [t in years since launch]
P_available = P_RTG(t) − P_bus [bus ≈ 110 W housekeeping]
if Σ P_instruments > P_available → shed lowest-priority instruments first
- Initial power margin — sets P₀ (how many RTG modules launched). A generous margin buys a longer viable tour before the bus itself can no longer be kept alive.
- Instrument checkboxes — request instruments during the extended tour; only what the current budget can afford actually accrues instrument-hours.
- Moon flybys — scheduled opportunities during the tour that need a short power spike; missed if the bus is already power-starved.
Real-world grounding: Voyager 2 is still the only spacecraft to have flown past Uranus (1986) and Neptune (1989) — brief flybys, not orbiters. Every serious Uranus Orbiter & Probe concept since has had to design around exactly this squeeze: a decade-plus cruise burning down RTG output before the multi-year science tour even starts.