>450,000 Earth years away with current technology With current tech spread thinly an alpha emitter like Po (2% mass ejected at 5% c) on the backplate and you’l

>450,000 Earth years away with current technology With current tech spread thinly an alpha emitter like Po (2% mass ejected at 5% c) on the backplate and you’ll get 100km/s with 10% payload. Adding an electromagnetic “nozzle” to axially redirect sideway alphas would make it past 200km/s, ie. less then 20k years. Spread thinly enough for the decay leftover - Pb - to be ejected too and you can get to something like 600 km/s, just 6000 years.