I imagine you would need a ridiculously large solar sail to slow down the mass of an Orion. It's not that slowing down is impossible...you just start throwing

I imagine you would need a ridiculously large solar sail to slow down the mass of an Orion. It's not that slowing down is impossible...you just start throwing bombs the other direction. But by slowing down you increase your travel time, so the 150 year estimate is no longer valid.