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Comment by exitb | Hacker News Reader

That mission features a complex trajectory that will use a lot of gravity assist to get to Jupiter. It will actually take eight years to get there, while Juno needed five and the Voyagers only two. Why do recent missions choose to do that? Is that a cost saving measure?

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The Voyagers took advantage of a special constellation that doesn't happen often:

"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...

That didn't affect the Earth-Jupiter leg, which was direct.

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

Efficiency [1] in the form of reduced delta-v requirements [2]. The “yellow book” assessment study report probably has even more details [3].

[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...

Here's a key figure [3]:

- "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 hyperbolic excess of >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]")

Voyager was also only weighing 773kg.

The mass of JUICE is probably a big parameter here.

[1]: "From Earth to Jupiter...JUICE trajectory is designed by ESOC Mission Analysis team. It is the result of an optimization, iterated with ESA project and Airbus teams, aiming at maximizing the dry mass at launch, keeping the total mass within the launcher capabilities and minimizing the transfer duration, while respecting other mission constraints, like total radiation dose, minimum Sun distance and maximum eclipse duration..."

[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...

JUICE has a launch mass of 5,963 kg while Juno had 3,625 kg and Voyager only 721.9 kg. I guess it may be a limit of the launch vehicle and the mass the spacecrafts have.

Indeed. The same rocket will give a light probe a much higher velocity than a large probe. The launch vehicle for JUICE is the Ariane 5 ECA which can launch 16 tonnes to low earth orbit[1], while the Titan IIIE that launched the Voyagers had a capability of 15.4 tonnes to a (lower) low earth orbit[2]. So the current launch vehicle is a bit more powerful, but not 8 times as much. There are some more powerful (and more expensive) rockets active at the time, but nothing 8 times as powerful as the Titan IIIE.

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 NASA has the choice of ...

> 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 Voyager missions were able to use a rare and convenient alignment of the planets that would slingshot them into deep space. Juno is orbiting Jupiter itself, which is not a hard thing to do in our solar system. JUICE is attempting flybys of several of Jupiter moons, to end up orbiting around Ganymede. This is a much more delicate orbital insertion and has much stricter requirements for the gravity assists on its way there.

https://trajbrowser.arc.nasa.gov/traj_browser.php

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.

The Voyagers, besides their much lower mass, were flyby missions. They could afford to be fast because they didn’t need to slow down for orbit insertion.

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.

It's always a tradeoff. For every kilogram of propellant you save on the trip, you get one extra kilogram for the payload.

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.

It's cost saving, but the cost saved is really weight (mass) of the fuel. Using the gravity assists means you can save on fuel and use that mass for the spacecraft and its instruments instead. JUICE masses 2400 kg, where Juno is 1600 and the Voyagers were 700.

The voyagers were launched within very specific window iirc which is why they managed to get to the outer planets so quickly. Also the voyagers only did a flyby so they could afford to go as fast as they want as they didn’t needed to enter orbit.

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.

Well, sure, the window was very specific to visit multiple planets, but Jupiter was the first stop. Similar Earth-Jupiter configuration happens every year.

The ISS made a "stop" in my general vicinity earlier today by your definition of "stop".

Sorry, English it not my first language, I meant it as a first destination, it obviously just flew by.

Jupiter makes an orbit around the sun once every 12 years there likely are years where it is closer to earth than others.

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.

I wonder what the predictions are like. When it finally gets to Jupiter, eight years from now, do we know now where the moons will be ? To what accuracy ?

I believe orbits are chaotic over a timespan of many millions of years (due to weak interactions with multiple other bodies), but accurately predictable over a human lifespan.

I just wonder whether they can say that, on date&time X1 (98% sure to be accurate within E1 hours) we know satellite Z will be X2 thousands of km away (98% sure to be accurate within E2 km).

Much higher confidence to not mess up those gravity assists? Control and sensors have come a long way since then

Cost saving as there is no prestige or gain in coming fast or first. Probes are also bigger and heavier.

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