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Posted by: Tom Tamlyn Jan 18 2006, 06:06 PM

During the press conference recorded on January 15 (rebroadcasted on NASA TV this morning), several of the New Horizons instruments have been described as the most capable ever carried for a planetary reconnaissance. For "reconnaissance" I guess we should understand "fly-by," i.e., the instruments are more powerful than those carried by the Pioneers & Voyagers.

It would be interesting to learn how they compare with the corresponding instruments on Cassini & Galileo, with their much higher payloads.

TTT

Edit: I just heard Craig Covault from Aviation Week catch this issue, but he didn't pursue the comparison beyond Voyager & Pioneer.

Posted by: ljk4-1 Jan 20 2006, 10:21 PM

A What If group of questions:

Suppose that in 1980, NASA/JPL, realizing that they could not penetrate the clouds of Titan or even hope to see any surface through gaps in the clouds (and even if they could have, no doubt it would have left everyone seriously confused), had redirected Voyager 1 to flyby Pluto around 1990.

In addition to the fact that we would have long known about a few more moons of Pluto besides Charon, what do you think would have been changed/modified/ replaced on New Horizons had the Voyager 1 flyby taken place? Would Voyager 1 been up to the task at all? I remember its images were not quite as sharp as Voyager 2's.

Would NH even exist if Voyager 1 had explored Pluto?

Do you think Pluto will be all that different from Triton - besides the fact that it won't have a big blue world in the background?

Posted by: mcaplinger Jan 20 2006, 11:09 PM

Clearly the NH payload is much more capable than Voyager's. That said, I don't think NH would have flown if Voyager 1 had visited Pluto, assuming that it had worked as well as Voyager 2 did at Neptune. We would have 1 km resolution visible color images of the illuminated faces of Pluto/Charon and probably some occultation data (I'm not sure that IRIS could have gotten anything useful.) That would have made spending the NH budget on " just another flyby" a pretty hard sell.

Posted by: JRehling Jan 20 2006, 11:42 PM

First of all, it's hard to say what we wouldn't have known about Titan. Perhaps Cassini/Huygens would have been designed in a well less adapted to the quirks of Titan. Some Voyager discoveries were duplicated from the ground, but who knows if people would have been clever enough to conduct the right investigations without the Voyager 1 benchmark.

It's very unlikely that Pluto would have earned a followup if Voyager 1 had flown by -- but that depends upon what Voyager would have found! If Pluto has rainbow-colored lakes and the message "HELLO" on one face and a duplicate of the Rolling Stones logo on the other, then yes, we would have flown a followup. If it looks like Oberon, we probably wouldn't have.

I don't expect anything so theatrical. The biggest differences vs. Triton should stem from:

1) Triton had and has huge tidal interactions from Neptune. Pluto probably faced something much milder in braking into synchrony with Charon. Pluto may show as little thermal evolution as Callisto (which tidally-braked into synchrony with Jupiter, but shows no surface scars from that).

2) Triton is always the same distance from the Sun, while Pluto has a wildly elliptical orbit introducing long seasons. Triton does have axial-tilt seasons, but the global solar heating budget is about constant.

3) Pluto probably was thumped royally in a collision that produced Charon, although that may have been so early in its formation as to show little present evidence. Triton may be different from Pluto in not having been thumped similarly.

4) Something else. Even when worlds seem to be similar in bulk composition and solar distance, they often end up quite different. The saturnian satellites show that fact off over and over. We'll have to wait and see.

Priors aside, we know that Pluto has some very dark areas on its surface while Triton does not. Something explains that. Maybe it's because seasons on Pluto lead to a significant freeze/thaw of the atmosphere, and the dark patches are now (still) thawed.

Posted by: edstrick Jan 21 2006, 09:25 AM

Voyager 1 would not have been retargeted for Pluto even if they hadn't "done" Titan, because a very high mission requirement for the mission was radio occultation studies of the rings, which (unless my memory has failed) was done by Voyager 1 and could not be done by 2 because of it's trajectory.

A Voyager would have done about as well at Pluto as they did at Triton which was amazingly well, considering the light levels. But the spectrometers did not do nearly as well as the cameras, especially the infrared one. The low temperatures shifted the peak emission to longer wavelengths, and required much longer integration times to get any data.

If a Voyager had done Pluto, unless Pluto's hiding a "spectacular" we can't guess now, we'd be sending the first Neptune orbiter out, probably, though maybe not for a few more years.

Also.. remember.. While Voyager 2 did the "grand tour", it was not designed for it. The originally planned mission was -- without a formal project name, it never got that far -- TOPS The Outer Planets Spacecraft. Designed with more redundancy, and instruments designed to work better beyond Saturn, there were ideally going to be 3, with the third one going on to Pluto. It was way $$$, and got descoped down to Mariner Jupiter Saturn, later renamed Voyager. I don't recall what they were going to launch it on, as Voyagers required the Titan 3, which was our largest launch vehicle by the time they flew, and I assume TOPS was going to be heavier.

Posted by: Rob Pinnegar Feb 4 2006, 12:59 AM

Just on a note unrelated to the rest of this thread (except for the title):

I've just done some amateurish calculations regarding the current size of Pluto and Charon as currently seen from New Horizons. Pluto subtends an arc of about 0.1", which is about one-tenth of a pixel diameter. Obviously we won't be getting multiple pixels on Pluto for a very long time.

However, the Pluto/Charon distance currently isn't that much less than an arc-second, so assuming Pluto and Charon can even be picked up at this stage of the game, *in theory* they ought to occupy adjacent pixels. If there are any tests of the high-resolution camera planned for the near future, it might be neat to do the test when Charon is at its maximum apparent separation from Pluto, to see if they can be picked up, as a faint pixel right next door to a fainter pixel.

Of course, since Pluto would only be filling up one-hundredth of the pixel's area, and Charon less than that, Pluto would currently appear to effectively be a 19th magnitude object. Problem, that.