Friday, December 19, 2008

Tidbits from AGU

I'm flying home (to a major snowstorm) after attending the Fall American Geophysical Union (AGU) conference, which is one of the largest scientific gatherings for planetary science. For the science revelations, please look at Emily's blog at the Planetary Society. Here, I'll report on a number of tidbits related to future planetary missions. In future entries, I'll discuss the Venus Flagship proposal and the VALOR Venus balloon discovery proposal.

As I waited at the airport, I missed the kickoff of the next planetary Decadal Survey. Modeled on the similar surveys done by the astronomical community, these surveys look at the state of the science, options for future missions, and prioritize NASA's exploration efforts for approximately the next decade. (In a future blog entry, I'll provide a report card on the last Decadal Survey completed in in 2003.) A key goal of this survey, I'm told, is that it will make a strong effort to get much more accurate estimate of costs for its key recommendations. In the last astronomical survey, for example, the cost of the James Webb Space Telescope was severely underestimated. As a result, a number of missions of lower priority have had to be deferred to cover the cost overrun. A similar example in the planetary program might be the Mars Science Lab (MSL), but as explained in a previous entry, the Decadal Survey was recommending a modest technology development mission while the Mars community was recommending a major science mission. (The difference between the two at the time was around $1B.) I am told that the goal is to complete the next Decadal Survey in approximately six months.

I had a long chat with one of the Titan-Saturn Flagship proposal engineers. He explained the major reason that the proposed mission will image Titan at 50 m compared to the state of the art cameras at Mars that image that planet at sub meter resolutions. The Titan atmosphere extends so far into space that the orbiter has to orbit 1500 km above th surface. Compare that to the 250 distance of the Mars Reconnaissance Orbiter. I suspect that additional issues – such as imaging at longer wavelengths (which reduces resolution) and the ubiquitous haze – are contributing issues. (It’s also important to remember that the instruments listed in the proposal are placeholders. The actual instruments to be flown would be selected through a competition. Generally the selected instruments are more capable than the placeholders.)

The mission designer also explained why the orbiter delivers the balloon and lander into the Titan atmosphere months before Titan orbit insertion. This isn't an issue with the lander which will have a surface lifetime of hours. It does create relay issues (but not insurmountable ones) for the balloon. Apparently the weight of the two in situ craft is enough that the mission designers do not want to use to extra fuel that would be caused by carrying them longer.

He also explained why the option of simply launching the balloon and lander separately to arrive after the orbiter is in place at Titan isn't the plan of record. It is cheaper to have a single launch. More importantly, the balloon has its own plutonium power source. ESA's rules apparently prevent it from launching craft with radioactive power sources. NASA doesn't want the cost of a second launch on its books. In theory, ESA could contribute hardware to the orbiter equivalent to NASA's cost of a second launch. However, the ground rules of the proposal stated that NASA would pay for the orbiter and ESA the in situ probes. If Titan is selected as the destination of the Flagship mission, this rule can be re-examined.

And speaking of re-examining the redefinition of the Titan mission, Thomas Spilker dropped some hints. Spilker is one of the mission design wizards at JPL who examines options for future missions. (He's published papers, for example, on options for Neptune orbiters even though those are decades away.) He gave an overview of the challenges of designing a mission that includes both Enceladus and Titan as options. He didn't say too much that wasn't already in the mission proposal. He did say that the current mission configuration is subject to significant change should it be chosen.

I talked with another mission designer who thinks there may be options to first orbit Titan and then move to orbit Enceladus using relatively little fuel. He hasn't published his work, so I won't say more.

I stopped by the NASA booth at the conference. After trying to figure out how to “borrow” the detailed models of the Cassini and Phoenix spacecraft, I talked with the gentlemen at the radioisotope power table. This is the group that provides the radioisotope thermal generators (RTGs) for outer planet missions (including the MMRTG that will power the Mars Science Lab and the ASRG that reduces the use of plutonium dramatically). Right now, the generators are pretty big and sized to deliver hundreds of watts. They were polling attendees for mission ideas for power sources in the 10 to 50 watt range. Right now, mission designers have a choice of batteries, solar power, or large RTGs. If you want to do a mission with small probes where the probe has to operate for longer than batteries will suffice and solar power is difficult to impossible, then you are out of luck. If these small plutonium power generators were developed, they likely could enable a range of missions, such as network stations on Mars or Titan, long lived comet landers, long lived Venus balloons, small balloons carrying only cameras for Titan (think of four balloons instead of one imaging the surface), etc. These are only my ideas – the scientific community would come up with far more and more clever uses.

I'll write a separate blog entry or more likely a series to discuss the Venus Flagship proposal. One of the goals would be to fly a synthetic aperture radar that could image the surface at less than 10 m (6 m was mentioned). Only a portion of the planet could be imaged. However, I once read that each doubling of resolution increased the scientific content of an image by 10X. If I remember correctly, Magellan mapped Venus at a resolution of a hundred to hundreds of meters. This mission would do for our understanding of Venus what the high resolution cameras of the last decade did for our understanding of Mars. And those cameras imaged only a few percent at highest resolution.

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