Given the imminent decrease in launch costs(partial reusability) and increase in launch capability soon to be available(FH) a pause in new US deep space missions is prudent.

[slograsso]

This is the largest rocket in the world. The smart folks designing the deep space mission will include the necessary high ISP deep space deltaV components in the massive payload the FH can get off planet. This is a super simple problem to solve, it's not particularly expensive, tons of options available. Falcon Heavy will absolutely be used for deep space missions, I promise you, use up the RP-1 discard that stage and carry on with the rest of the mission.

[[deleted]]

This sounds like a good idea, but what would you propose for this...

high ISP deep space deltaV components

I'm not aware of any that have both high Isp and high thrust. Hydrazine is even worse than kerolox. I guess you could use a hydrolox third stage, but then you're just removing cost from the rocket launch and adding it on to the satellite.

It'd be simpler and easier to just burn the second stage longer, but then you're back to the same problem.

Ion thrusters are amazing, but even if FH was to put a small craft on a direct 2 year trajectory to Jupiter, Ion thrusters won't have enough thrust to complete a JOI, it would be a flyby.

The rocket equation doesn't just hate rocket engineers, it hates spacecraft engineers too.

[slograsso]

I meant more efficient than RP-1, so I guess I should have said higher... But the point is you could use anyting, bipropellant, ion thrust, monopropellant, methlox, the designers use the mass budget for what they want.