Dark spot on Uranus, Another giant planet gets the chicken pox Options

[JRehling]

The ideal time to arrive at Uranus would be near equinox. One is happening about now, and it's obviously too late for that. The next one is in 42 years. Put some pennies in the piggybank for a 2035ish launch.

A next-best arrival time might be when the anti-Voyager 2 hemispheres are in the light (to "finish" the coverage)... the next such solstice is in 20 some years (with lots of leeway for off-goal-but-OK illumination conditions). That's not going to happen (with NH2 being a dead possibility), and the next one after that is in 66 or so years.

For the satellites, obviously nothing short of close-up observations will do much justice to them. Uranus itself, though, is a decent target for Hubble and its successors.

This spot is a cyclone or anticyclone -- whichever it is would determine something about why the spot is dark, but that would seem to be a mystery for now. It has to be one of the two

[TritonAntares]

Hi again!

The ideal time to arrive at Uranus would be near equinox. One is happening about now, and it's obviously too late for that.
The next one is in 42 years. Put some pennies in the piggybank for a 2035ish launch.

A next-best arrival time might be when the anti-Voyager 2 hemispheres are in the light (to "finish" the coverage)...
the next such solstice is in 20 some years (with lots of leeway for off-goal-but-OK illumination conditions).
That's not going to happen (with NH2 being a dead possibility), and the next one after that is in 66 or so years. ....

That'll be probably the reason why we won't see a launch of a Uranus-orbiter-mission in say the next 40 to 45 years.
New inventions in propulsion systems will hopefully offer us a variety of capabilities to send and maintain an orbiter at Uranus then.
Maybe there could be 5 landers for the large moons on a platform or 2 corresponding missions -
one with such landers and another with a long surviving (21 years) orbiter.

Neptune and of course Triton for me seem to be the more rational targets.
There's no illumination problem like in the Uranian system - I'm still aware of Triton's 'chaotic' retrograde orbit!
But Neptun could be reached roughly every 12 years by a Jupiter swing-by with a launch window of 1-2 years.

Mission goals are clearly fixed - observation of Neptune itself and full cartography/remote sensing of Triton over years -
maybe for 1-3 decades, depending on the RTGs.
'Neptuneshine' should help on imaging Triton's dark hemisphere.
A Triton lander should be on board if manageable at this cold temperatures!
The rest few percent of the mission should aim at 340 km small Nereide (maybe like Phoebe at the begin of the mission)
and the other small satellites, most interesting here is 440×416×404 km large Proteus.

Bye.

[edstrick]

A Cassini class Uranus orbiter, even if it arrived at the solstice, would be able to image about 3/4 of each moon, using Uranus-shine on the solstice-night "quartisphere" that faces the planet.

I'm almost inclined to think it's simply not worth the effort to go back to Uranus, except on a probe-carrying "new frontiers" class mission, unless we fly a nuclear electric vehicle that can rendezvous and polar orbit each of the 5 major moons. Lidar from polar orbit would be able to image the entire surfaces as was done for Mars with Global Surveyor's lidar. A Multi-beam lidar and a radar scatterometer would be able to do a bang-up job on the moons, even the in-dark areas. We don't have to composition image the entire surface, though it would be nice.

[JRehling]

I'm almost inclined to think it's simply not worth the effort to go back to Uranus, except on a probe-carrying "new frontiers" class mission

Sooner or later, Uranus will be lined up for an opportunistic flyby mission en route to some KBO or other. There seemed to be a possible Jupiter-Uranus-Sedna trajectory according to my imprecise eyeballing of things. At some point, I think, we'll want to launch a fast flyby mission of some outer KBO and the added science of a Uranus flyby could be the tiebreaker in choosing which KBO is the target. I think the Uranus flyby could also accelerate the craft a bit more than the Jupiter flyby alone, as an added bonus.

[Rob Pinnegar]

A next-best arrival time might be when the anti-Voyager 2 hemispheres are in the light (to "finish" the coverage)...

Wouldn't the solstice be the worst time?

Away from the solstices, each moon's rotation allows us to see more than half of its surface. At a solstice, we get the minimum -- half, and always the same half. (For the purpose of this argument I'm simplifying things by assuming Uranus' inclination is 90 degrees to a first approximation.) There's probably a 1/2(1+cos 2 theta) term in the math here, but I won't dwell on that.

Basically the worst time to launch a Uranus orbiter would be the date that gets the orbiter to Uranus a year or two before a solstice (i.e. right about now). Assuming a Cassini-like tour, after four years, you'd have seen the same parts of the moons over and over again (plus whatever could be gotten from Uranus-shine, as has been pointed out above). At least if you arrived right at the solstice, after four years you'd be able to see down to about 20 degrees latitude in the unseen hemisphere by the end. That's a lot of territory.

Even halfway between a solstice and an equinox, you can still see about 85% of each moon's surface. That's not too bad at all, and it's not even a particularly good scenario.

[ngunn]

I've already advocated (some will say ad nauseam) a modest but very long-lived Titan orbiter to monitor seasonal and other secular changes on that world over decadal timescales. It strikes me that the Uranian system is another place where a similar approach might be appropriate. I'm thinking of a fully automatic imaging system that just goes on faithfully recording data for a century or so and which can be interrogated from time to time by more powerful fly-by craft on their way to the KB or beyond - a sort of long term weather-watch for the planet with the most extreme seasons. Of course this doesn't fit in with our current concept of a 'space mission' started and finished (usually) by the same team of scientists. It also has no category in space agency budget allocation, and no viable power source to date. All the same perhaps it's worth thinking about how it could be done.

[JRehling]

Wouldn't the solstice be the worst time?

Away from the solstices, each moon's rotation allows us to see more than half of its surface. At a solstice, we get the minimum -- half, and always the same half. (For the purpose of this argument I'm simplifying things by assuming Uranus' inclination is 90 degrees to a first approximation.)

I should have been clearer.

A solstice (the correct, ie, anti-Voyager 2 solstice) would be the best time for a flyby: Voyager 2 saw one hemisphere of each moon. A flyby at the other solstice would see the other hemisphere. An equinox flyby would only show us half of each unseen half -- at closest approach, anyway.

A little before equinox would be best for an orbiter.

[alan]

Would it be possible to do a mission at Uranus similar to Cassini or Galileo? I believe the moons are too small, less than 3% of the mass of Titan, to be used to significantly alter a spacecraft's orbit.

[Guest_Myran_*]

alan wrote: Would it be possible to do a mission at Uranus similar to Cassini or Galileo? I believe the moons are too small, less than 3% of the mass of Titan, to be used to significantly alter a spacecraft's orbit.

With smaller moons it might still be possible if the flyby of each sattellite were correspondedly closer, in this case extremely close, but it certainly would be a cosmic billiard of a kind that havnt been attempted before. So I share your pessimism alan for that solution, less risky to simply let the orbiter carry more fuel.

[nprev]

Oh yeah, no question about that: we'd want to design in as much flexibility as possible if for no other reason than that reflying the mission would not be a realistic option!

One big concern, though, would be availability of favorable intrauranian system trajectories to eject & safely deposit (& communicate with!) a lander. Seems like the constraints would be very significant, unless a HUGE amount of fuel is available to cope with late-breaking discoveries.

[EDIT] Here's a thought: Would an ion propulsion system like Deep Space 1 or a xenon/Hall Effect thruster system (both strictly intended for use in Uranian orbit) be desirable? Although such systems surely would increase the time required to do maneuvers, they conserve reaction mass quite well...

[edstrick]

I've thought that one way to do certain types of missions, like a Uranus/KB fkyby or a Contour type mission is to build your entire spacecraft around a HiRise type camera and have all major instruments with pickoffs at the focal plane, like Hubble. You might have one "wide" angle, maybe 5 deg or something camera for close encounter observations when the motion blur is too great and you briefly get a range of phase and viewing angles you can't get from approach of flyout trajectories.

What would HiRise have gotten in terms of resolution at Uranus, if Voyager had had such a camera?

[ugordan]

If I have my calculations right, here is a comparison of pixel sizes of Miranda the two cameras would have had on Jan 24, 1986 00:00 UTC:

ISS NA --- HiRISE

HiRISE has an IFOV about 9 times narrower than ISS NA. What I did was use the IFOVs of both cameras and multiplied it with 800 to get the window the Solar System Simulator uses. In case of ISS NA, the width of the window would be the same as the camera FOV, whereas in case of HiRISE it would comprise only a tiny fragment of the instrument FOV. Coupled with the fact CCDs are used nowadays, which have a far superior S/N ratio compared to vidicons, a HiRISE-type camera would bring a more than decent resolution increase.

Here's a not very illustrative closest approach comparison (around 30 000 km, assuming the ephemeris is correct):
ISS NA --- HiRISE

The HiRISE swath width would probably cover the majority of the visible disc at C/A, eliminating the need for a dedicated wide-angle camera. This, of course, is due to the fact it's a push-broom camera instead of a framing one and has a very large CCD array. I'm not sure how useful it would be in case you're not in a circular orbit around the target so complex slews would need to be done.

[edstrick]

A HiRise type detector system would probably be a bad idea that far out at those low light levels. You probably want a framing camera with a mechanical shutter and filter wheel. Something like a small version of one of Hubble's cameras, in effect.

But the basic point is that you can get detailed global mapping and real mineralogical mapping during late far-encounter observations where a Voyager or a Stardust class camera system could only get gross morphology. And get that good quality data on ALL of the main moons and inner moons, unlike the "just barely see general geology" quality of imaging of Oberon and Umbriel from the best possible flyby trajectory they could get.

[tasp]

Would it be possible to do a mission at Uranus similar to Cassini or Galileo? I believe the moons are too small, less than 3% of the mass of Titan, to be used to significantly alter a spacecraft's orbit.

Alex Blackwell a while back posted an abstract from a report that shows a Cassini or Galileo style orbital tour of Uranus is possible. The Uranus system is almost a perfectly scaled down version of the Jupiter system.

4 of the moons are sufficiently massive (Ariel, Umbriel, Titania, and Oberon) to alter a spacecraft trajectory sufficiently to encounter a different moon on the next orbit. Some of the flyby altitudes are under 50 kilometers!

IIRC, about the only major hassle was that the plane of the initial orbit for the craft won't be correct for the majority of the time for Uranus. The inclination of the orbiter can be changed, but it takes time.

The out of plane portion of the mission would be advantageous for studies of the Uranian magnetsphere, however.

[ugordan]

Some of the flyby altitudes are under 50 kilometers!

That looks like a nontrivial issue given how far Uranus is and navigation uncertainties involved. While navigators these days are clearly able to fly a space probe through a keyhole, Uranus is quite far away and <50 km flybys are a bit on the low side. I still get the chills remembering Cassini will zoom some 30 km (?) above Enceladus.
I don't have access to the full paper to find out what the predicted 3-sigma delivery errors would be. I guess something on the order of 10 km wouldn't be unreasonable.