If I'm reading the specs correctly the Titan IIIE was a significantly more powerful vehicle than the Ariane 5 while launching a much smaller payload. JUICE also has to be going slow enough to enter orbit without carrying an insane amount of fuel for braking while Voyager just did a flyby.

- "Titan IIIE was a significantly more powerful vehicle than the Ariane 5 while launching a much smaller payload"

Yes, with a fourth stage. (I don't think it's particularly that modern rockets can't integrate such a solid-rocket boost stage, but rather there's no payload that needs this).

https://en.wikipedia.org/wiki/Star_(rocket_stage)

- "JUICE also has to be going slow enough to enter orbit without carrying an insane amount of fuel for braking while Voyager just did a flyby"

The flyby speeds are similar; but JUICE takes a time to get to that speed.

It's not really that difficult to capture into Jupiter. Parent's link [3] says it's only 900 m/s for JUICE. Part of the reason is the extremely deep gravity well; and another is that the moons are large enough to give you effective gravity assists, within the orbit of Jupiter.

>The flyby speeds are similar; but JUICE takes a time to get to that speed.

Ahhh, that makes sense. Essentially Voyager did do a direct flight while JUICE isn't, instead building speed by doing slingshots off Earth and Venus.

"First, it will do a fly-by of Earth and the Moon, then slingshot around Venus in 2025 before swinging past Earth again in 2029."

https://phys.org/news/2023-04-europe-juice-mission-jupiter-i...

Yes the problem is not only getting to Jupiter, it is stopping once you get there