So NASA has the choice of 1) launching a lighter/smaller probe with less scientific capabilities and endurance, 2) spend $$$ and time developing a much more powerful launch vehicle, or 3) have a few years of patience waiting for JUICE to get to Jupiter on a slower trajectory.

Jupiter has already been visited by multiple probes so sending a small and not very capable probe would not gain us much scientifically. (But this option was taken with the New Horizons probe to Pluto some years ago.) (2) Wouldn't actually be faster and much much much more expensive (see how long the SLS rocket is taking to get done. But mr. Musk is taking this route too with SpaceX's Starship, so this will be an option in the future if all goes well). So that leaves option 3, which NASA has taken. They could probably have opted for an existing more powerful and expensive launcher to shave some years off the transit, but they preferred to wait a bit longer and not spend that money.

[1]: http://www.astronautix.com/a/ariane5eca.html [2]: http://www.astronautix.com/t/titaniiie.html

> So that leaves option 3, which NASA has taken.

As far as I'm aware, JUICE is an ESA probe. NASA is providing one of the instruments, but I believe that's it.

The length of time to get to Jupiter is pretty much dictated by orbital mechanics. As well as your ability to slow down and actually get into orbit instead of just doing a flyby.

So the longer journey is likely a combination of the launch vehicle, the cruise stage vehicle and how much fuel they want to dedicate into getting into orbit vs the mission since the probe is expected to visit multiple moons it’s likely that they prioritize the fuel for in mission orbit corrections so they are taking a longer and slower approach.

Also there are years when it’s retrograde and there are years when it’s prograde so there is that too.

With Voyager it also wasn’t just Jupiter it was an alignment of all the planets which is far rarer.

And again voyager didn’t entered orbit it had a very brief flyby of Jupiter.

Cassini also took a decade to reach Saturn because it needed to enter orbit around it and its moons.

"In the 1960s, mission designers at JPL noted that the next occurrence of a once-every-175-year alignment of the outer planets would happen in the late 1970s. A spacecraft could take advantage of this opportunity to fly by Jupiter and use its gravity to bend its trajectory to visit Saturn, and repeat the process to also visit Uranus, Neptune, and Pluto."

https://www.nasa.gov/feature/45-years-ago-voyager-1-begins-i...

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.

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...

[1] https://theconversation.com/the-much-anticipated-juice-missi...

[2] https://www.lpi.usra.edu/opag/mar2012/presentations/Friday/5...

[3] https://sci.esa.int/web/juice/-/49837-juice-assessment-study...

- "The mass injected into the Earth escape trajectory would be 4800 kg (without launch adapter), with a hyperbolic escape velocity of 3.15 km/s, which increases to 9.5 km/s after the last Earth swing-by"

The Voyagers were launched directly to >10 km/s — comparable to the speed JUICE will reach on its third flyby of Earth, in 2029.

https://sci-hub.ru/10.1016/j.actaastro.2019.06.013 ("C₃ of... 105.5 km²/s² and 102.4 km²/s² for Voyagers 1 and 2, respectively [75]")

The mass of JUICE is probably a big parameter here.

In particular https://trajbrowser.arc.nasa.gov/traj_browser.php?NEAs=on&NE...

This EVEEJ route doesn’t appear to be in the trajectory list, but it’s interesting to explore and see the various windows.

[1] - "JUICE interplanetary operations design drivers and challenges" - https://arc.aiaa.org/doi/pdf/10.2514/6.2018-2493

This is also an interesting Thesis on optimization required for the flybys due to the uncertainty of the Moons positions on arrival :

"Preliminary Navigation Analysis for the Flyby Tour of ESA’s JUICE Mission" - An investigation on the trajectory correction maneuvers design

https://repository.tudelft.nl/islandora/object/uuid:455ec2e9...

NASA’s Europa Clipper, due to launch in Oct 2024 on a Falcon 9, will take 5.5 years to get to Jupiter. The Congress directed NASA to investigate the option to launch it on an SLS, should one be available (it won’t, and it would be ridiculously expensive) – it would have been able to send the probe on a direct ~1.5 year Hohmann transfer to Jupiter without gravity assists, but no other existing launch vehicle could do that.

Also, you may end up bumping into payload limits of the launcher itself, so you can get there, but you'll need to take the scenic route with fewer delta-v tolls.