Ion engines need a lot of power, but scale down quite well to lower powers and lower thrust. And even with an ion engine, you'd only be running it for a few yea

Ion engines need a lot of power, but scale down quite well to lower powers and lower thrust. And even with an ion engine, you'd only be running it for a few years at the beginning of a mission. Meanwhile, one of your major power draws is a radio, which needs higher power as distance to Earth increases, later in the mission. Finally, nuclear thermal power supplies have decreasing power budgets over time. It seems reasonable to me that there's a combined power budget with focuses on ion propulsion for the first several years of a probe's lifetime, disabling the ion drive and shifting the (reducing) power budget to comms once target velocity is achieved.