Why hasn't there been a dedicated mission to an ice giant?

[Unstung]

Are you sure the solar panels were to power the orbiter? As I recall, some variants of the ice giant and Chiron missions propose using solar-electric propulsion as far as the asteroid belt. The SEP stage would then be jettisoned and the actual orbiter (or flyby probe) would use ASRGs for power and storable chemical propellants for propulsion after that.
As you suggest, solar power at Uranus doesn't make any sense. From what I've read powering Juno with solar is pushing the envelope, and they only opted for that to preserve Pu-238 stocks.

I can't find the original source I read it from, but here's a .pdf:
http://www.lpi.usra.edu/opag/march09/presentations/hofstadter.pdf
Scroll down a bit for an illustration. It's supposed to produce 100W.
And a longer version of the .pdf:
http://spacepolicyonline.com/pages/images/stories/PSDS Sat1 Hofstadter-Uranus and Sat Sci.pdf
Its specifications/objectives are quite different than the one proposed for NASA, which I believe is more like Cassini-Huygens while this has a highly elliptical orbit like Juno. The solar panels on that type of mission are purported to enable a 100kg greater payload.

Why not use a cheaper alternative like this if the NASA proposal is too expensive? A mission is only possible every 15 years.

 

[Tychonaut]

I can't find the original source I read it from, but here's a .pdf:
http://www.lpi.usra.edu/opag/march09/presentations/hofstadter.pdf
Scroll down a bit for an illustration. It's supposed to produce 100W.
And a longer version of the .pdf:
http://spacepolicyonline.com/pages/images/stories/PSDS Sat1 Hofstadter-Uranus and Sat Sci.pdf

Interesting. Thanks, I hadn't seen anything on that proposal. I guess if New Horizons can do good science on 200W as a flyby mission, a well-designed orbiter could also do quite a bit with 100W.
:tiphat:

 

[RisingFury]

Where are you pulling this figure of 30 minutes from? This blog posting shows the detailed breakdown of what each instrument is doing at which target for the Pluto encounter and it shows the 12 hours around closest approach with a flurry of activity.

New Horizons will fly within the orbital distance of Charon, about 10 000 km away from Pluto. If you do the math, you find that it'll spend about 30 minutes within that sweet spot of twice the orbital radius of Charon.

Of course, the experiments will be running for hours ahead and hours after, but given the small size of this system and high relative velocity, it's very short window for science. Probes flying close to Jupiter get about a 24 hour sweet spot and days worth of science.

 

[Unstung]

It's been a while, but there's been a recent article on this subject about a cheaper alternative mission to Uranus:
http://www.planetary.org/blogs/guest-blogs/van-kane/20130708-uranus-or-bust.html
It still appears that scientists want such a mission to fly soon. Relatively little is known about the ice giants. An option to complete science on a budget is to pursue a descoped New Frontiers class orbiter, but such a mission would have to compete with many other proposals. A Discovery class mission would fly by Uranus but it would obviously have to expand on Voyager 2's discoveries. It's a shame that a Uranus New Frontiers class orbiter may have to challenge a multiple flyby mission of Io - a truly awesome world.

There are a few presentations linked to in the article:
https://solarsystem.nasa.gov/docs/02_Missions to Uranus_N. Andre.pdf
http://www.lpi.usra.edu/opag/iceGiant/04_CastilloTurtle_UranianSatellites.pdf

 

[jedidia]

Though with Dawn proving how well ion propulsion works, you would think we would be putting that drive system in probes left and right.

We do, actually. Well, into satelites, to be precise. They're quite common.

The problem, again, is power. I am pretty impressed that dawn has enough power by its solar cells so far out, and maybe we can push those boundaries some more, but really, you cannot fight the inverse square law for too long...

 

[Graham2001]

There was this proposal:

The present OSS mission continues a long and bright tradition by associating the communities of fundamental physics and planetary sciences in a single mission with ambitious goals in both domains. OSS is an M-class mission to explore the Neptune system almost half a century after flyby of the Voyager 2 spacecraft.

...

The probe will embark instruments allowing precise tracking of the probe during cruise. It allows to perform the best controlled experiment for testing, in deep space, the General Relativity, on which is based all the models of Solar system formation.

...

The design of the probe is mainly constrained by the deep space gravity test in order to minimise the perturbation of the accelerometer measurement.

http://arxiv.org/abs/1106.0132

But because the Neptune (& possible Saturn) flybys were secondary to proving the Pioneer Anomaly real... :tumbleweed:

 

[N_Molson]

Cassini-Huygens was more than a success (and still is), it gave (and still gives) awesome informations on Saturn, Titan and a pack of other moons.

It would be interesting to reproduce that on Uranus and Neptune. Very little is known about their moons and rings systems, because they are farther away. But that distance also makes them more difficult to reach, too. But that's definitively feasible with current off-the-shelf technology, IMHO. Uranus and Neptune are even smaller gravity wells than Saturn, so orbital insertion is easier.

Redundant systems, though they add mass to the probe, can ensure reliability. After all, Cassini and the 2 Voyagers are in very good condition given their age. New Horizons is flying steady. Pioneer 11 lasted until 1995, Pioneer 10 until 2003, a very good performance. So it seems that class of very deep space probes have an excellent reliability record.

 

[Alfastar]

The only real good information we got from Uranus and Neptune are from Voyage 2, and that's a long time ago. Meanwhile, there is almost every year a new Mars mission from NASA. Why not sending a probe to the outer gas giants, we known almost everything now from Mars, but we known only a few things about Uranus and Neptune.

 

[N_Molson]

we known almost everything now from Mars

Oh no. Oh no no no no. 100 times no. We're hardly beginning to scratch its surface, literally. :lol:

 

[boogabooga]

Oh no. Oh no no no no. 100 times no. We're hardly beginning to scratch its surface, literally. :lol:

At this point, mars science can wait for the manned mission.

 

[Thunder Chicken]

Getting an orbiter to a ice giant would require a lot of mission resources.

• RTG or some other source of power other than solar (added mass).
• Propellant to enable capture of science probe with RTGs (added mass)
• Rocket big enough to get the above masses to a useful sling off Jupiter/Saturn.

The Voyagers needed a Titan III/Centaur rocket with ~1.3 million lbf thrust at launch, and it wasn't hauling any fuel for deceleration and capture. And I don't think there is near enough known about the atmospheres of the ice giants to attempt aerobraking (could accidentally become pagobraking if their understanding is off).

 

[Unstung]

Excuse the "Uranus" puns

Getting an orbiter to a ice giant would require a lot of mission resources.

• RTG or some other source of power other than solar (added mass).
• Propellant to enable capture of science probe with RTGs (added mass)
• Rocket big enough to get the above masses to a useful sling off Jupiter/Saturn.

The Voyagers needed a Titan III/Centaur rocket with ~1.3 million lbf thrust at launch, and it wasn't hauling any fuel for deceleration and capture. And I don't think there is near enough known about the atmospheres of the ice giants to attempt aerobraking (could accidentally become pagobraking if their understanding is off).

Galileo, Cassini, and Juno didn't/won't use aerobraking along with possibly any future mission. It may be too risky at a gas giant. The scientific community still wants to send something to Uranus, but the budget cuts are what's preventing a mission. An orbiter for Uranus is the third highest priority flagship mission in the Planetary Science Decadal Survey, after a Mars caching and Europa mission. Scientists are now studying cheaper Discovery and New Frontiers options to explore Uranus.

The cost of such a mission depends more on its scope than its destination. Any Uranus orbiter that has a chance of being approved must be cheaper than Cassini, for example. Being further away from the sun doesn't mean anything that is sent to Uranus will require a significantly larger booster and delta-v than any previous outer planet orbiter. The transit times will be longer, but to shorten it does require more fuel. Like Galileo, Cassini, and Juno, one would expect any outer solar system orbiter to conserve propellant by using the inner planets for gravity assists rather than launching directly to Jupiter.

Power is of course necessary for any spacecraft, and whatever power source will weigh it down, whether RTGs, solar panels, or batteries. Given that Uranus is so far away from the sun, the solar panels would have to be very massive. NASA is restarting production of plutonium, so RTGs are probably the better choice.

 

[NOMAD]

Have you checked out this page? While most of those mission concepts will never fly, at least not this decade, they make for interesting reading.

Achieving some of these concerpts in Orbiter would be interesting

 

[N_Molson]

Fire up your favorite 3D modelling software and your C++ compiler, guys !

 

[Loru]

 

[Thunder Chicken]

Galileo, Cassini, and Juno didn't/won't use aerobraking along with possibly any future mission.

Jupiter and Saturn are much deeper gravity wells, much less dV is needed to be captured. And they are not as high in the solar orbit as the ice giants. For an ice giant mission, you need to send the probe out farther from the sun and expend more dV to drop it into orbit than for a Jupiter/Saturn mission.

 

[Unstung]

Jupiter and Saturn are much deeper gravity wells, much less dV is needed to be captured. And they are not as high in the solar orbit as the ice giants. For an ice giant mission, you need to send the probe out farther from the sun and expend more dV to drop it into orbit than for a Jupiter/Saturn mission.

A probe could arrive at its destination more slowly, taking a longer time and requiring less fuel. The solar system escape velocity beyond Jupiter and Saturn is much lower enabling a lower velocity during transit. The change in velocity required also depends on the type of orbit a spacecraft is being placed into. For a specific example, Juno will reach a velocity of over 70 km/s before injecting itself into Jovian orbit. Jupiter is much more massive than Saturn, yet Juno will require more delta-V (1072 m/s) to get into its planned orbit than Cassini (633 m/s). Of course, Cassini changed its orbit many times over its mission, but it used Titan for gravity assists.

As I've mentioned, a mission to Uranus can fit under both the Discovery and New Frontiers program. The delta-v budget and power supply does not prevent cheaper missions and does not cause objectives to be scaled back more so than any other outer solar system mission. Although entering the atmosphere of Uranus and not orbiting the planet, but still going to a destination twice as far as Saturn, "...the requirements and complexity of such a mission would seem to be similar to that of a Saturn atmospheric probe mission". One competitor of a potential New Frontiers class Uranus mission, the Io Volanco Observer, will require 200 kilograms just for radiation shielding.