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Comment by krylon | Hacker News Reader

I'm still a little disappointed they chose to send a probe to Jupiter instead of Uranus or Neptune.

I am certain the Jovian system is madly interesting and there's lots and lots of fascinating things to be discovered. But on the other hand, Galileo has already spent several years exploring it, while we still know comparatively little about Uranus and Neptune.

Having said that, I am, of course, excited to see what new and interesting things Juno will allow us to find out. In a way, there are no boring places in our solar system.

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Something like Juno could only work as far as Jupiter, maybe Saturn, but at some point the size of the solar panels grows too unwieldy. Jupiter gets 1/25 the sunlight of Earth, Saturn is 1/100, Uranus is 1/400 and Neptune 1/900.

Material for RTGs running out is a thing unfortunately

According to Wikipedia, the United States restarted production of Plutonium-238 at circa 1.5 kg a year in 2013 after a c. 25-year hiatus.

Good

the Cassini one used 7kg https://en.wikipedia.org/wiki/GPHS-RTG so I guess it will be some time before new ones are made

They've been exploring smaller designs with better efficiency. Not just because of the shortage, but also because evergy gram counts in a spacecraft...

Well maybe every kilogram, but you got my point. Any savings in mass count, and if you get to cut 1% from the spacecrafts final mass, it can mean a lot in terms of speed and timings for the mission.

Wikipedia link: https://en.wikipedia.org/wiki/Radioisotope_thermoelectric_ge...

The thermophotovoltaic cells look lik a very interesting approach, don't know why they haven't been used yet. With them, Cassini could've used 2 RTGs instead of 3, resulting in a mass saving of 57.5 kg (2% lower dry mass from one less RTG) and some extra power. I do take into account that this technology ws probably not developed enough when Cassini launched.

I did not know Juno used solar panels.

I know that a probe to explore the ice giants would require an RTG (or some other energy source other than solar panels).

Still, I think it would be very interesting to study the ice giants and the respective moons and ring systems more closely.

It's the first probe going out that far that uses panels, all the previous ones use RTGs. When it was built (launched in 2011) there was a severe Pu-238 shortage; it's still in short supply, but back in production (as of 2013) and so some future long range RTG powered missions might be back on the table.

Oh well, I guess Uranus will still be around in few years. ;-)

But is Uranus or Neptune in a position that can be reached with gravity assists?

And even if it is, orbital insertion requires a lot of energy, so a flyby is probably the only possible option.

Mmmh. I can still hope, can't I?

I mean, really, assuming that all of the practical problems of building a probe and getting it to Uranus or Neptune were solved, wouldn't you just love to send it there and explore the $expletive out of them?

Well you can either keep up (false?) hopes or do the maths and see where the planets are located (pretty easy) and are there any potential gravity assists (not as easy, but there are software packages that help).

If you just want to eyeball the situation, take a look at Celestia.

I think orbital insertion is out of question for the outer planets using conventional space craft propulsion because the difference between high radial velocity required to reach the planet and the low tangential velocity of the planet itself is greater than the spacecraft's delta-v budget.

But there won't be another Voyager mission in our lifetimes, a suitable arrangement of planets happens only once in 400 years or so.

"The most favorable launch window for such a probe would be in August 2018, with arrival at Uranus in September 2030".

https://en.wikipedia.org/wiki/Exploration_of_Uranus

Juno is only possible because of new improvements in solar panels. And even so, it's a relatively low powered space probe at 400 watts. A lot of power is required for radio communication and heating.

Going any further with solar power is not feasible with the technology available today.

That's not how mission planning works. They don't start with something roughly like Juno and decide which planet to send it to. Rather, the planet and rough scientific mission is picked first, then the probe is designed around that.

I guess you could just mean "There is an RTG shortage, so we can't send anything past Jupiter", but I don't think this is actually how the decision was made.

The orbits of the planets are not apropriate yet for an Uranus/Neptune orbiter. That mission has been in the works for years, at a slow pace, but now slowly speeding up. During the 2020's the planing should go full speed for a late 2020 launch, and a mid 2030's arrival.

[Fix]: I was look for the links to post about the Neptune mission, and found this PDF where they had propsed a mission to launch january this year that would arrive late 2029. Obviously, it didn't get throught.

http://www.lpi.usra.edu/opag/jun_05_meeting/presentations/NE...

Now I'm wondering where did I read those other dates... still, with current technology wer'e talking about some 12 to 15 years of transit, with gravitational assist on Jupiter (almost 20 without it).

[Further edit]: It was the Uranus orbiter that had those dates. Now it's being marketed as a New Horizons sibling (probably to jump on the public acceptance of NH? and it's technological merits too -- little weight, simpler design, etc).

https://en.wikipedia.org/wiki/Uranus_orbiter_and_probe

Cassini also spent several years exploring Saturn system, but discovery of geysers on Enceladus and rivers/lakes on Titan only makes them more interesting.

Gas giants by itself are less interesting. Sure they will photograph those enormous clouds up close and measure their oxygen/hydrogen ratio with more precision, and maybe that's exciting for some scientists.

But you look at Europa and Enceladus and can't help but imagine rovers driving there one day, and can you imagine some craft floating down the river on Titan shooting 360 video?

I get this impression that everywhere we look, things out a lot more interesting than we had expected.

And I am certain that Juno will discover lots of unexpected weirdly interesting things, just as Galileo did before.

But Uranus and Neptune - and their moons and ring systems - are still fairly blank spots compared to Jupiter and Saturn. I am sure there's lots of interesting stuff waiting to be discovered.

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