Wednesday, July 2, 2008

Where do you want to go today?

To the moons of Saturn, personally. More specifically, to Titan.

Why? Because it is exotic and beautiful and offers just the right amount of challenge. It has the potential for really good science about interesting things like where did life come from anyway? And hey, you've got to love the idea of ballooning around an alien moon. I clearly do, much to my distraction from real political issues!

Its a bandwagon that's easy to get on, and the two major space agencies (NASA and ESA -- I don't count China because they are merely showboating and NASDA and the individual European nations couldn't afford something like this and Russia is, well, Russia) have both proposed multiple missions to the Saturn system as a follow on to the still-active Cassini.

Almost all of them involve some degree of romantic technology: solar electric propulsion, aerocapture, ballooning, multiple-satellite tours. All these missions are compelling and I find myself falling into that perpetual habit of comparison shopping. Hm, here I am at Target, in the Space Missions aisle, trying to pick which Saturn/Titan mission I want to buy...

Pick it up, carry it to the counter, swipe your Visa card for the 3.2 billion dollars (imagine the Alaska Airlines miles I'd get with that!).

And so now, in order to assist all of you in selecting the Saturn/Titan exploration program that is best for you and your family, I present the following overview of the different products on the shelf.

TiPEx -- the Titan Prebiotic Explorer, is one of the oldest and most exotically romantic missions. Using solar electric propulsion and a quick dive inside Venus' orbit to pick up velocity for the long run out to Saturn, it also employed aerocapture into the thick Titan atmosphere for arrival. Both of these count as "sexy" in the minds of deep space exploration dreamers. They smack of science fiction and are technologies that the established space agencies have traditionally shied away from. For, in some ways, good reason. Aerocapture is, in particular, tricky: flying down a narrow corridor through the atmosphere of a distant and largely unknown world. Spooky stuff, the kind of stuff that causes probes to pass into radio shadow and never emerge.

That said, the TiPEx study also involved an unusual approach to the missions core payload, a helium/hot air balloon hybrid. TiPEx viewed the orbiter as secondary, carrying only a cloud penetrating radar, minimalist camera, and some radio science experiments. Otherwise, it was primarily a data relay platform for the big balloon with its gas chromatograph/mass spectrometer, radar, sounder, cameras and NIR spectrometer, and atmospheric science instruments. The drawback of a balloon -- the lack of ground truth -- was overcome with a sampling harpoon. While unable to actively hover, the balloon could never the less fire one of a dozen sample collection probes while drifting a few tens of meters over an interesting area of the surface. Tricky flying, for a robotic probe operating too far away for real-time human intervention.

On the whole, while daring, TiPEx was probably a little too adventuresome for the powers that be. After the failures of Mars Orbiter and Mars '94 and the near fiasco of the Huygens probe, space agencies around the world seem to have decided to pull back a little -- recognizing that sometimes discretion is the better part of valor and that an assured science return is worth greater expense or greater cost. And so, while exciting, this attempt to do a lot with a small but exotic mission seems destined to remain a paper study.

Never the less, the ideas of SEP propulsion and aerocapture remained strong. The next several studies, also performed by NASA's Jet Propulsion Laboratory, continued to use elements of this combined approach. A low cost "Billion Dollar Box" study did little but prove that valid Saturnian science could not be achieved for less than $1.3 billion -- and truly effective science would require at least $2.2 billion. It skipped the SEP exotica for chemical mid-course guidance but went for the aerocapture approach.

Finally a very well developed "Flagship Class" mission study put together a little bit of everything. Other than eschewing SEP boost, it went for all the rest that Titan dreamers hoped for: aerocapture to Titan orbit and not just a balloon but also a lander to obtain ground truth samples. This proposal reads as a real Cadillac effort, ambitious proposal and limited by launch vehicle constraints (Atlas V 551) and little else. The instrumentation package is well described and detailed, the hardware reasonably well defined and conservatively specified.

Meanwhile, on the other side of the pond, the often more ambitious (and innocent) planners of ESA were penning their own Saturn probe. TANDEM -- apparently an acronym standing for, in some language, Titan and Enceladus Mission, was a true blue sky effort including a Titan balloon, a Titan ground truth lander, and possibly multiple microprobes for icy Enceladus. Even more than JPL's Flagship, this seems a proposal in the "let's see what we could do..." mode of thinking. And, as with most of such proposals, it didn't last long when exposed to the corrosive light of day.

It did, however, open eyes to a new idea -- abandoning the almost obligatory aerocapture in favor of the seemingly retrograde use of chemical propulsion for capture at the destination. This wasn't out of engineering conservatism, but stemmed from the realization that a multi-stage capture, first into Saturn orbit and only later around Titan, offered significant science benefits. Such a "tour" would only be possible with chemical propulsion but, ironically, also enabled the use of this more conservative approach by a ballet of delicate flybys of various Saturnian moons to shift the spacecraft's orbit until a final, comparatively small burn for Titan orbit entry.

Picture 2.jpgNow sketched out as a cooperative project -- and bearing the thoroughly unattractive and clearly designed-by-committee name of Titan/Saturn System Mission -- TSSM would involve a NASA provided orbiter and a European provided lander and balloon.

That's the version I've put in my shopping cart. Not just because it is the latest, but because of the spectacular ability to generate a LOT of science. Not just one moon, but during the tour the fascinating planet of Saturn itself, the beautiful ring system, the icy and dynamic Enceladus, and the various other smaller moons.

I just hope that Bank of America will up my credit limit, because the bill's going to be a big one.

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