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Onwards to Uranus and Neptune!
Planet X
post Jan 14 2008, 03:08 PM
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QUOTE (JRehling @ Jan 13 2008, 04:40 PM) *
One tradeoff with orbiter missions is how long we can tolerate the cruise phase to be. With a flyby, it's not such an issue, but with an orbiter, the faster you get there, the more fuel (or heatshield) that you need to slam on the brakes at the end of the cruise. A minimum-energy trajectory to Neptune is prohibitive unless we expect the people running the mission at the onset to be retired (or dead) by the time the science mission begins. The more we shave off of that cruise, the more mass problems intrude. That's one big factor in Uranus's favor -- it's 11 AU closer.


Personally, I see Uranus as the better target. Though Neptune may have more to offer, it's just too distant for an orbiter mission, barring some miracle advancement in technology. For instance, the Hohmann transfer time to Neptune is 40 years, while the time to Uranus is 16 years. I see no problem with designing a probe to last a couple decades (16 year trip time + 4 year length of primary mission) for a Uranus mission. Besides, there's always the possibility of a mission profile similar to that of Cassini, involving Venus, Earth, and Jupiter that could shorten the trip time somewhat. Later!

J P
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vjkane
post Jan 14 2008, 04:35 PM
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Neptune has been ranked as a higher priority as ranked by the scientific community for a follow on mission because of the interest in Triton.

I looked at Titon's orbit relative to Neptune in the mid-2020's on the Solar System Simulator. As viewed from the sun (which I presume is the direction of arrival for a flyby mission), Triton's orbit remains well away from Neptune for this time frame. However, Triton's distance from Neptune is less than the distance of Io from Jupiter. A mission that targets a Triton close encounter still comes reasonably close to Neptune, and probably close enough to act as a relay for a Neptune probe. (However, the ammonia in Neptune's atmosphere will weaken the probe's signal; I don't know how this would effect this scenario.) In fact, a more distant flyby of Neptune than was done by Voyager might be required to give adequate viewing time for the probe relay.


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Greg Hullender
post Jan 14 2008, 05:48 PM
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I get 16 years for the Hohmann to Uranus, but just 30 2/3 to Neptune -- not 40. Calculating as follows:
1) Since the Hohmann is at 1 AU at perihelion and at the target planet at aphelion, the SMA of the Hohmann is (1+a)/2, where a is the SMA of the planet. 10.1147 AU for Uranus and 15.5518 for Neptune.
2) Kepler's third law says period is SMA to the 3/2 power, but we only want HALF the period (since we're done when we reach the target). That gives me 16.08422 years to Uranus and 30.6650 to Neptune.

As a cross check, using these numbers I calculate an 84-year period for Uranus and 165 years for Neptune, so I do think I have it right this time. :-)

Even so, 30 years is a long time to wait. So's 16. Add to that the fact that currently we can't even swing the delta-V for a Hohmann to JUPITER for a large probe -- much less one to Uranus or Neptune. That's why Cassini and Galileo had to make all those swings by Venus and Earth first.

Aerobraking, ion propulsion, and cheaper boosters all seem needed to get a reasonable-sized package to either destination in a reasonable time. On the other hand, all three look like they might be just over the horizon.

--Greg
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Del Palmer
post Jan 14 2008, 07:12 PM
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QUOTE (Greg Hullender @ Jan 14 2008, 05:48 PM) *
Add to that the fact that currently we can't even swing the delta-V for a Hohmann to JUPITER for a large probe -- much less one to Uranus or Neptune. That's why Cassini and Galileo had to make all those swings by Venus and Earth first.


Galileo could have reached Jupiter directly, had it been allowed to use its original liquid-hydrogen-fueled Centaur IUS instead of that pansy solid-fuel IUS...


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"I got a call from NASA Headquarters wanting a color picture of Venus. I said, “What color would you like it?” - Laurance R. Doyle, former JPL image processing guy
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Jyril
post Jan 14 2008, 10:40 PM
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But it would have missed Ida and Gaspra, and the cool photos of Earth and Venus.

On the other hand, the main antenna might have worked, and the probe would have followed the SL9 impacts from orbit...


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ugordan
post Jan 14 2008, 10:54 PM
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QUOTE (Jyril @ Jan 14 2008, 11:40 PM) *
and the probe would have followed the SL9 impacts from orbit...

It would have been on the wrong side of the planet at the time.


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Jyril
post Jan 14 2008, 11:04 PM
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QUOTE (ugordan @ Jan 15 2008, 12:54 AM) *
It would have been on the wrong side of the planet at the time.


How do you know that? Wouldn't the Jupiter tour have been different, if it had traveled directly to the planet? It would have been there already for years.

And even if wouldn't have seen the impacts, it would have had much better view of the impact effects than any observatory at Earth.


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ugordan
post Jan 14 2008, 11:06 PM
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QUOTE (Jyril @ Jan 15 2008, 12:04 AM) *
How do you know that?

Murphy's law.


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tedstryk
post Jan 15 2008, 12:54 AM
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Yes, but during the post-Challenger delay and replanning, they also discovered that the rocket motors were defective. Galileo would have gone the way of CONTOUR had it launched in 1986 or 87. In other words, things could have been a whole lot better, but they could have been a thousand times worse.


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tasp
post Jan 15 2008, 06:08 AM
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IIRC, the Galileo Jupiter trajectory provided by the Centaur stage would have included a nice flyby of Amphitrite, an otherwise obscure, largish main belt asteroid. Had that occured, it would have been the largest asteroid flown by till Dawn reaches Vesta in several years.
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Toma B
post Jan 15 2008, 09:02 AM
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QUOTE (Jyril @ Jan 15 2008, 01:40 AM) *
On the other hand, the main antenna might have worked,...

It would have worked.
If I remember correctly it was plan to open HGA while still near shuttle so that if any problem occurred it could be fixed right away... <spacewalk>
Also that heat shield for HGA would not be there.....
I am not sure about this just remembering..... sad.gif


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Greg Hullender
post Jan 15 2008, 02:32 PM
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Uh, I hope they didn't plan to open the main antenna before firing the Centaur upper stage. :-)

Anyway, I've heard the switch away from the Centaur blamed for the antenna problems. (e.g. Carting the probe around for so long led the lubrication to dry out so the pins didn't release.) But I've heard other explanations too.

Biggest problem with the upper stage (based on reading "Taming Liquid Hydrogen: The Centaur Upper Stage Rocket 1958–2002") was that the shuttle couldn't actually lift the Centaur-G-Prime because the shuttle never achieved its originally planned lift capability. They were talking about throttling up to 109% instead of 104% for the Galileo and Ulysses launches. The astronaut crews were already calling Centaur "the Death Star" before Challenger exploded.

After Challenger, safety changes made the shuttle heavier, and any changes to the Centaur would have made it heavier too. If it was marginal before that, it was hopeless afterwards.

Anyway, the real problem wasn't failing to use the Centaur upper stage; it was using the Shuttle in the first place. Galileo should have been a Titan-Centaur launch, and I don't think there's much dispute over that now. Depending on the shuttle for launching unmanned probes turned out to be a huge mistake. It may have put us as much as 15 years behind where we'd otherwise have been.

But this is old news, long hashed over here. However, if you haven't read it, do have a look at "Taming Hydrogen." It's a great read.

--Greg
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Toma B
post Jan 16 2008, 07:49 AM
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QUOTE (Greg Hullender @ Jan 15 2008, 05:32 PM) *
Uh, I hope they didn't plan to open the main antenna before firing the Centaur upper stage.
--Greg

Why not?
Actually that's what I thought they were planing.....I really don't know... huh.gif


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Greg Hullender
post Jan 16 2008, 04:29 PM
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Actually, I think you're right. The only reason they folded the antenna up in the first place was to hide it behind a sun screen, and the only reason they had to do that was to protect it during the Venus flyby. Obviously they wouldn't have done a Venus flyby if they could have launched with Centaur.

Sorry about that. :-)

--Greg
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Mariner9
post Jan 16 2008, 09:51 PM
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Some years ago at a JPL open house I spoke with one of the engineers on Cassini. I asked about the reasoning behind using the Titan 4 vs. using the shuttle. It turns out it would have been cheaper to launch on the shuttle, but only by the way NASA does accounting. The price charged to the Cassini project would have been the processing fees and upper stage, and not the actual shuttle flight itself.

And it wasn't payload capacity or safety that were the main drivers either. It was schedule. The Cassini team figured they would rather rely on the Titan to get a launch accomplished during a planetary launch window. The shuttle, with it's frequent delays and down times was just too unreliable.

Not exactly on topic for this thread, but since everyone was discussing launch issues with the shuttle......
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