Venus 73 - A lost opportunity? Options

[gndonald]

I've been browsing through the NTRS and I've stumbled upon a series of reports dealing with an alternative mission plan for the 72/73 Venus launch window (The one used for Mariner 10).

The plan was this:

A modified version of the Mariner 9 probe carrying a 400lb balloon supported atmospheric probe would be launched atop an Atlas-Centaur or Titan IIIc. Upon arrival at Venus the atmosphere probe would be released and the remainder of the craft would either go into orbit or flyby Venus. The RTG powered balloon probe would operate for several months in the upper levels of the Venusian atmosphere.

While I know that Mariner 10 was a great success (albeit with a lot of luck) and the first pictures of Mercury were well worth the effort, I cannot shake the feeling that this plan was a lost opportunity. While the Venusian atmosphere has been studied from orbit and by short term drop probes, only the Soviet Vega probes attempted to carry out 'long term' studies from within the upper atmosphere, though this was limited by the flyby nature of the mission and the fact that the balloon probes were battery powered.

In fact I feel that a good case could be made for such a mission to be attempted in the future, the science payoff would be well worth it.

See:

http://ntrs.nasa.gov/archive/nasa/casi.ntr..._1969006258.pdf

for the 23mb summary report.

This post has been edited by gndonald: Aug 15 2007, 12:53 AM

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[qraal]

Hi

That mission does sound cool. Almost as tragic an opportunity lost as the Grand Tour missions proposed around the same time which would've included drop-probes as well as orbiters for all the Gas Giants.

I often wistfully wonder if Voyager I would've lasted out to Pluto - that was one option at the time.

Adam

gndonald wrote...

I've been browsing through the NTRS and I've stumbled upon a series of reports dealing with an alternative mission plan for the 72/73 Venus launch window (The one used for Mariner 10).

The plan was this:

A modified version of the Mariner 9 probe carrying a 400lb balloon supported atmospheric probe would be launched atop an Atlas-Centaur or Titan IIIc. Upon arrival at Venus the atmosphere probe would be released and the remainder of the craft would either go into orbit or flyby Venus. The RTG powered balloon probe would operate for several months in the upper levels of the Venusian atmosphere.

[JRehling]

I often wistfully wonder if Voyager I would've lasted out to Pluto - that was one option at the time.

Off topic, here, but that would have been at the expense of the close flyby of Titan by Voyager. It's safe to say that everything Voyager learned about would have been made up by now, but a consideration would have been if Cassini-Huygens would have been as well designed as it was without those observations. It's difficult to say -- remote observations verified a lot of Voyager's conclusions, but it's still harder to say if those observations would have been made without Voyager's data.

[nprev]

Just out of curiosity, JR, do you know what would have been V1's C/A distance to Titan if the Pluto option had been selected? I'm sure that the deep atmosphere would still have been optically evident (albeit probably no occultation opportunites).

Reason I ask is that I mourn for the original Grand Tour... Happy for NH, of course, but that sure would have been cool.

[ugordan]

If I'm not mistaken, Voyager 1 Titan flyby was of big significance because it allowed radio science occultation of the atmosphere, allowing pressure and temperature profiles to be deduced and even the actual physical radius of Titan's surface. Those results would be unavailable if you just saw Titan as an orange ball in the distance and it's questionable how well we would have been able to characterize its atmosphere based only on groundbased stellar occultations. As JRehling pointed out, it was likely this flyby that enabled Huygens to be so successful.

The flyby was very significant in that regard, something that tends to be shadowed by the cameras' inability to pierce through the haze, often leading to assumptions the flyby was a waste.

[edstrick]

"I often wistfully wonder if Voyager I would've lasted out to Pluto - that was one option at the time."

The titan occultation was absolutely critical to understanding the moon at a basic physical level. We literally did not know where the surface was below the top of the orange fog, and Voyager's other instruments would not have really solved the problem.. They'd have had a fair idea from the infrared spectroscopy, but not a solid, accurate atmosphere temperature/pressure profile and surface radius.

Huygens would have been nearly impossible without that data.

BUT WAIT, THERE'S MORE!!!!!

I believe Voyager 1 did a perfect radio-occultation profile of the entire extant of the rings, not possible from a grand-tour in-the-ecliptic trajectory like Voyager 2 took. They used doppler-delay analysis of the data much like synthetic aperture imagery analysis and resolved the ring structure at tens of meters resolution in low opacity zones, as well as measured the particle size distribution by scatter and different opacity at the 2 wavelengths. Ring studies are as "pre voyager' and "post voyager" due to the occultation data as much as Titan studues are.

[nprev]

<sigh>...thanks, you guys. I knew full well that there were unassailable reasons, just wanted to hear them. Don't tell Alan I said this, but a good look at Titan at that time obviously outweighed the possible gains from a Pluto flyby. IIRC, estimates of surface pressure pre-Voyager ranged from 20mb to 1.5 bar...and many people were excited then at the possibility that Titan might have a slightly denser atmosphere than Mars! (Significantly denser than Earth's appeared to be a long shot that few seemed publicly willing to advocate...)

[brellis]

I believe Voyager 1 did a perfect radio-occultation profile of the entire extant of the rings, not possible from a grand-tour in-the-ecliptic trajectory like Voyager 2 took. They used doppler-delay analysis of the data much like synthetic aperture imagery analysis and resolved the ring structure at tens of meters resolution in low opacity zones, as well as measured the particle size distribution by scatter and different opacity at the 2 wavelengths. Ring studies are as "pre voyager' and "post voyager" due to the occultation data as much as Titan studues are.

The Voyager ring data helped the Cassini team pick a route for that initial dash through the ring plane behind Saturn at arrival. Other than the launch itself, that must have been the nail-biting moment of the entire voyage of Cassini.

[tedstryk]

nprev, to put it another way, the spacecraft that should have gone to Pluto is not the one that flew by Titan, it is the one hanging from the ceiling in the Air and Space Museum.

[monitorlizard]

I hope it isn't too confusing if I mention the first Voyager program, of the 1960's. As it was originally envisioned, Voyager was a program to send spacecraft to both Mars and Venus. gndonald, the drawing you posted of the Venus spacecraft reminds me a lot of the proposed Voyager with atmospheric probe, one of which many scientists wanted to send to Venus. Then there was the Voyager with atmospheric probe for Mars, first targetted for 1969, then 1971. The U.S. then seems to have gotten very shy about sending non-landing probes to Venus and Mars in the later 1960's. I guess NASA figured there wouldn't be much public interest in probes that only studied a planetary atmosphere, and didn't send back pictures from the surface.

I agree a Venus balloon would have been very cool in 1973. If it can be done within a Discovery budget, it should be done. Maybe it would qualify for one of those enhanced missions (with Sterling/Rankine RTG's) that Alan Stern has proposed.

[JRehling]

IIRC, estimates of surface pressure pre-Voyager ranged from 20mb to 1.5 bar...and many people were excited then at the possibility that Titan might have a slightly denser atmosphere than Mars! (Significantly denser than Earth's appeared to be a long shot that few seemed publicly willing to advocate...)

Actually, one of the models (David Hunten, c. 1978) was for a pressure of 20 BARS and a greenhouse effect pumping temperatures up incredibly (almost Marslike).

One of the things that most wowwed me when the Huygens data came back was how Voyager had absolutely nailed both the pressure and the temperature. I knew of no precedent for a ground-truthing matching the remote sensing data so squarely.

The sad thing about missing the Voyager flyby of Pluto is that it would have imaged one hemisphere (including what is now the winter pole), enabling NH to image the other one. When NH has completed its flyby, there'll be quite a bit of the surface imaged only at a distance, and that area will not see a closeup for a very long time.

[edstrick]

"..how Voyager had absolutely nailed both the pressure and the temperature."

That's the power of Radio Occultation.
It takes 2-frequency occultation to separate ionosphere refraction from neutral atmosphere effects, but once you have that, you have only two main uncertainties. The first is the atmosphere temperature at the top of the occultation profile. (It's unconstrained, and the range of assumed values biases temps below that "top" of atmosphere before the curves converge.) The second is the mean molecular weight of the atmosphere. That puts a constant bias on the temperature at any altitude (I think).

In the case of Titan, the first uncertainty converges to the correct value way way above the surface. On Mars, it barely has time to converge before the occultation hits the surface. The second uncertainty puts a bias in the absolute value of the surface temp. If the titanian atmosphere had a lot of Argon, the molecular weight would have been higher, the temp would have been different. As it turned out, there was effectively no argon and the small amount of methane put only a tiny bias on the profile.

The other mismatch would have been due to weather or seasonal climate changes. Pretty small for such a dense atmosphere. Yep. Voyager NAILED it.

What's more interesting is that the Voyager occultation profile was a VERY good match to modeled pre-Voyager profiles! They just didn't know where the surface was under the profiles. They'd agreed that the high "temperatures" observed in some infrared bands were from dust-heated upper atmosphere, but long wave thermal IR data just couldn't show whether there was zero, small (what really exists), or a larger greenhouse effect raise in surface temperatures. The surface could have been at a few tenths of an atmosphere pressure, near the minimum in the temperature profile, or at tens of atmospheres pressure. The sharp cutoff occultation observed by Voyager pinned it precisely and proved that there was a solid or liquid but very well defined surface, and not a super-refractive non-cutoff like Venus occultations (which can't penetrate to the surface).

[tasp]

While Voyager inflight upgrades made possible an exquisite Triton flyby, knowing to a level of certainty at the time of the mission development that such upgrades could be pulled off wasn't feasible.

As launched, Voyager would not have done a very good Pluto flyby, but the robust and revamped Voyager II that arrived at Triton in 1989 certainly could have. Just not sure if that would have been understood in the mid 70s.

[elakdawalla]

I often wistfully wonder if Voyager I would've lasted out to Pluto - that was one option at the time.

This conversation seemed familiar -- with good reason, as it turned out:
http://www.unmannedspaceflight.com/index.php?showtopic=1297

--Emily

[NMRguy]

This conversation seemed familiar -- with good reason, as it turned out:

Ahhh. A nice trip down memory lane.

The sad thing about missing the Voyager flyby of Pluto is that it would have imaged one hemisphere (including what is now the winter pole), enabling NH to image the other one. When NH has completed its flyby, there'll be quite a bit of the surface imaged only at a distance, and that area will not see a closeup for a very long time.

This is really getting off topic, but I honestly don't believe that NH, PKE, or any of the other incarnations would have been on the table if Voyager had made the Pluto flyby. Although NH proved to be a budget outer planet mission, its cost could cover at least 1.5 Discovery missions. Support probably just wouldn't be there to make the LONG trip out to the edge. And I can't say that I'm unhappy with the results. Voyager was an astounding success on its own, and through hard work from lots of people New Horizons made it off the ground, completed a very nice Jupiter flyby, and is en route to Pluto/Charon with modern instruments that are optimized for the darkness that is beyond Neptune. I'm not sure Voyager can even compete with that.