This actually provides an interesting constraint: in order to launch science missions on the Falcon Heavy, they will have to be cheaper, relatively speaking. And generally, the cheaper a spacecraft is, the smaller it is. The smaller it is, the faster it can be sent into the outer solar system, with the potential to significantly reduce the time required to reach its destination.

In the first decade of the 21st century, NASA's Mars Exploration Program had a program called Mars Scout, which was intended to provide frequent, low-cost mission opportunities to the Red Planet. The idea was that this would allow NASA to react quickly to new scientific discoveries by allowing scientists and industry to propose mission concepts to follow on new discoveries and hypotheses from its other, more expensive missions.

The outer solar system has never had a similar program for a variety of reasons, with one being the length of time required to reach the destinations themselves. It's hard to have a reactive, high-cadence series of small missions when it takes a decade to reach a target and return data. Since they are infrequent, missions to the outer planets have tended to be very expensive in order to accommodate as much science in one shot as possible. There are also additional complexities for operating in deep space, particularly if the spacecraft depend on Plutonium-238 for energy.

Some of these other constraints are changing. Juno is the first spacecraft to operate at Jupiter using solar panels. It will be followed up by the Europa Clipper and ESA's JUICE mission—both with solar panels instead of plutonium. In the previous low-cost Discovery competition, multiple missions were proposed to the Jupiter and Saturn systems using solar panels within a cost-cap of less than $500 million. So we know that there are feasible proposals already out there with proven technology that avoids the cost and complexities associated with radioactive power sources.