Editing Openai/691a2d9f-c044-8012-bccf-942bfce6334d (section)

===== Each “comet” (ice block + shell) with a nuclear engine on its tail can be used in three phases: =====
# Injection phase (inner / outer Solar System) - Tug / tail engine pushes it from its source orbit onto a Sun-grazing trajectory. - This is your big Δv spend.
# Oberth / perihelion phase (near the Sun) - You optionally fire the engine again when it’s moving fastest, to exploit the Oberth effect and boost outbound energy. - This is where you get “cheap” Δv per unit propellant.
# Outbound correction phase (leaving the Solar System) - After perihelion, the block is now on a hyperbolic escape trajectory. - The same nuclear engine, with a much smaller remaining propellant load, is used for: - small course corrections - tightening the asymptotic direction - keeping it inside a desired outbound corridor for some decades/centuries.

You absolutely do not need that engine to work for 1,000+ years.
You just need it long enough to trim the trajectory until errors stop growing catastrophically, then you let it coast.