In its MRO-class "Reduced" form, the Plan G mission would include a copy of MRO's HiRISE camera and of the SAR shallow-subsurface imaging radar mentioned previously.

Their purpose, as on the other possible missions, would be to continue studying the surface features of Mars in detail (including sedimentary layers and water-carved channels, with the SAR examining those features shallowly buried beneath Mars' wind-blown soil); examine the small changes that keep occurring on the surface (new eruptions of liquid water out of the gullies; new tiny meteor impact craters; dune movements); and continue appraising possible future landing sites for interest and safety. (It would also carry an IR climate sounder for continuing modest monitoring of Mars' weather.)

In the "Core" version of Plan G preferred by the scientists, the IR sounder would again be replaced by a full-fledged, high-resolution "SWIR-TIR" near-infrared and thermal-infrared mapping spectrometer, which could provide the most detailed orbital maps yet of Mars' minerals.

Moreover, all Plan G versions would be put into an orbit tilted only five degrees from Mars' poles, allowing its instruments to make the same kinds of studies of the polar Layered Terrains, and of changes over time in the current polar caps, that the Plan P polar mission would make -- except that Plan G couldn't observe the central regions of the caps.

Finally, the still more souped-up "Augmented" version of this mission would add the same Fourier solar-occultation IR spectrometer carried on the "Plan A" atmospheric version of MSO, to examine Mars' atmosphere for very small traces of methane and other interesting gases and to try to narrow down the location of their surface sources -- although the Augmented Plan G mission would lack the millimeter-wave spectrometer for the latter purpose, and its near-polar orbit would reduce the number of solar occultations that the Fourier spectrometer could observe at Mars' equatorial latitudes, reducing both its sensitivity and its ability to narrow down gas-source locations.

However, the Core and Augmented versions of Plan G would also have another major addition: a 200-kg Martian hard lander that would be carried piggyback on the main spacecraft, which would eject the lander just before braking into Mars orbit (as with the unsuccessful little Beagle 2 lander that rode on Europe's Mars Express). An alternate Augmented version of the "Plan A" atmosphere-focused version of MSO might also carry such a lander, instead of the SAR radar mapper.

Mars scientists, for decades, have wanted to fly a "network mission" that would scatter four or more tiny landers widely across Mars, carrying seismometers and weather sensors to give us a detailed look at the planet's internal structure and geological activity, as well as a better look at its surface weather.

But this mission, like Cinderella, has been repeatedly slighted and pushed into the future by the overriding need to look for evidence of life on the planet (while a Network Mission could focus on the nonliving scientific aspects of Mars). France did some serious work on developing a "Netlander" mission to do this before 2010, but finally dumped it due to unacceptable cost.

NASA now hopes to fly the "Mars Long-Lived Network Mission" to do this by 2020. But it turns out that even a single such little lander could not only serve as a prototype test of the later network landers' design (as the 1997 "Mars Pathfinder" lander was actually supposed to do, before NASA quietly dumped its successor missions) -- but could also, by itself, do some useful new science.

In particular, a single Mars seismometer could not only determine just how geologically active Mars still is (thus allowing better design of later seismometers); it could do a surprisingly good job of pinning down the general fuzzy location of Marsquakes and meteorite impacts, directly estimating the direction from which the shock waves had come, and estimating the distance of the source using the different speed with which various kinds of shock waves travel through the rock.

Thus it could also pick out good landing areas for the later Network Landers. And it could also make a preliminary appraisal of the thickness of Mars' crustal layer.

(NASA, remarkably, also thinks that if MSO carries a SAR radar system it could actually use it to do some similar work locating particular active parts of the planet -- by imaging the same area on the planet at different times and comparing the images interferometrically to detect changes in ground height as small as a few centimeters, such as could be produced by Marsquakes and buried volcanic activity.)

The small MSO lander could also carry a set of weather sensors like the ones planned for the 2009 Mars Surface Lab rover -- measuring not only pressure, winds and humidity but also atmospheric static electricity, and using small infrared sensors to profile air temperatures up to several kilometers above the surface -- and a tracking beacon by which Earth could make the best measurements yet of the very slow wobble of Mars' tilted spin axis, thus telling us more about the size and consistency of its core.

It could even carry a "heat-flow probe" to try and measure the small amount of excess heat streaming out of Mars' interior from the radioisotope traces in its rocks.

Such a probe would be a string of tiny temperature sensors, strung out through Mars' subsurface soil to a depth of two meters or more by a tiny cylindrical "mole" that would bury itself in the soil (a similar mole was planned for Beagle 2).

A single heat-flow probe's measurements would be seriously interfered with by Mars' daily and seasonl changes in surface temperature; but, again, it could provide data to better design such sensors for the Network Mission.

The value of a single small US geophysical lander could also be raised by the fact that the European Space Agency has just decided to enlarge its planned 2013 "ExoMars" mission, an attempt to carry out the same sophisticated surface studies as America's 2009 MSL, but with a smaller and cheaper rover (whose success is open to question).

And one of its new features would be a separate long-lived stationary package of sensors left behind at the landing site by the rover, and almost identical to those on the little MSO lander -- meaning that the two devices could work together to greatly improve their seismic and weather studies.

But there's a problem: JPL's current estimate of the cost of such a small piggyback lander is about $200 million -- and that's for a lander that could only carry 5 kg of instruments, half of what the MSO Science Analysis Group would like. Thus it could probably be included on this mission only if it was the prototype for a later network of Mars hard-landers -- and if Washington was willing to give NASA that big lump of additional technological development money so early in the Mars program.

(The fate of the 1997 Mars Pathfinder lander -- which was originally supposed to be the prototype for a network of similar small Mars landers before NASA's budget problems left it stranded as a stand-alone mission -- must be kept in mind.)

Indeed, cost is a general problem for the Mars Science Orbiter (a very old and familiar song where NASA is concerned). The MEPAG team made it clear that they want to at least fly the "Core" version of any of the MSO concepts, NASA's current guideline is that MSO must not cost more than the current Mars Recon Orbiter -- which would probably mean that it could only carry the barebones "Reduced" science version of any of the three mission concepts, which the group described as "much less desirable, as they required significant compromises with regard to measurement goals or supported too few cross-disciplinary elements."

Indeed, even the Reduced version of the "Plan G" mission -- which carries both a Hi-RISE quality camera and the SAR system -- would require a more complex and costlier spacecraft.

So, the decision as to just what payload MSO will finally carry still depends not only on the ongoing science discoveries being made at Mars by our current robots, but on just how much money NASA's Mars Exploration Program will have to play with.

It's quite possible that the final payload could be some hybrid mixture of the different science-instrument combinations listed above. At any rate, the decision must be made soon -- the official "Science Definition Team" to decide MSO's final recommended set of science measurements will deliver its decision in 3 to 4 months.

After that, NASA's Mars program planners must move on to their next major decision: determining which of four or five overall mission types will be flown in the 2016 and 2018 Mars launch windows -- including the next really major Mars lander mission after the Mars Surface Laboratory rover. I will discuss that particular quandary for NASA's Mars planners in a later article.

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