This is the mission that will follow this year's Phoenix lander to study mars' northern permafrost; the very ambitious 2009 Mars Surface Laboratory (MSL) rover to drive dozens of kilometers across the surface, studying Mars' mineralogy in detail and looking for trace organics that might be fossil evidence of ancient life; and the small 2011 Mars Scout mission selected from proposals made by competing teams.

Beyond that point, the form of the Mars program has been a good deal fuzzier. The current 2013 mission was added after NASA cancelled the 2009 "Mars Telecom Orbiter" that it had planned to co-build with Italy. MTO would have been a Mars comsat, relaying back to Earth very large amounts of data efficiently from MSL and from future rovers and landers.

Its orbit would have been high and circular, allowing it to stay in line of sight with any individual lander for hours at a time -- but this would also have reduced its own scientific studies of Mars to a token trickle.

After Italy pulled out of the project, NASA decided that it wasn't worthwhile for the US to fly MTO on its own because it was overspecialized -- it actually had much more capability to relay back data from Mars landers than we will need for the next one or two decades. But some kind of Mars orbiter with data-relay capabilities must be launched by 2016 at the latest.

Currently, the big Mars Reconnaissance Orbiter that we installed at Mars in 2006 -- in addition to itself returning incredibly detailed and sweeping surface photos and huge amounts of data from other instruments -- is serving as the relay satellite allowing our two surprisingly durable 2003 Mars Exploration Rovers to return most of their data to Earth, with assistance from the smaller 2001 Mars Odyssey orbiter that continues to work well after five years in orbit.

Both craft are in low, circular polar orbits 250 to 400 km up, which allow them to swoop over the site of any Mars lander twice a day, maintaining line-of-sight radio contact with it for seven to ten minutes each time, during which time they record most of the daily science data collected by each rover in addition to relaying Earth's latest set of marching orders for the rovers' actions the next day.

While it would be nice to have a high-altitude Mars comsat capable of maintaining much more prolonged radio contact with MSL and our other future landers, NASA has decided that it isn't cost-effective to fly one for that purpose -- indeed, our Mars landers for at least the next 10 or 20 years can probably get along with a single MRO-type data relay satellites, rather than the two we have working right now.

But we do need at least one such orbiter, since sending large amounts of data directly back to Earth from landers would require that they land a heavy radio system on Mars, which is much more difficult than putting one into Mars orbit. And MRO is officially designed to work in Mars orbit only until the end of 2010 -- while Mars Odyssey is already 20 months past its own design lifetime. With any luck, MRO (like most US spacecraft) will actually work long past its official design lifetime -- but it would be very dangerous to count on that.

So, after the cancellation of the 2009 Telecom Orbiter, NASA decided that the 2013 Mars mission should be another big low-altitude polar orbiter that could do cost-effective double duty, making major additional science studies and serving as another longer-lived com relay that could work through at least 2024.

This mission was originally called the Mars Science and Telecom Orbiter -- but NASA got nervous about the possibility that the scientific part of this mission would be downplayed, and so has now renamed it simply the "Mars Science Orbiter" (MSO).

The obvious follow-up question is: what kind of scientific studies should it make? Mars is a spectacularly varied planet.

But the four orbiters that have been put there over the last decade have already carried a major collection of varied instruments to study the planet (including Europe's 2003 Mars Express orbiter, and the Mars Global Surveyor that finally succumbed to an erroneous ground command last year after surveying Mars for nine straight years and single-handedly revolutionizing our understanding of the planet). So, what new worthwhile observations can an expensive MSO craft make?

In early 2006, NASA assigned this problem to "MEPAG" (Mars Exploration Program Analysis Group), the agency's team of scientists and engineers who constantly monitor continuing developments to recommend adjustments in the future course of the program. MEPAG picked a sub team, which in May 2006 recommended that MSO should focus on studying Mars' atmosphere -- and especially two aspects of it: high-altitude aeronomy and trace gases.

Aeronomy involves the study -- mostly with in-situ instruments -- of the structure and processes in Mars' thin upper atmosphere and ionosphere.

It would involve particular focus on one question: how fast are solar ultraviolet light and the solar wind blowing past Mars' upper atmosphere causing the planet's remaining atmosphere to escape into space, given that Mars has both weak gravity and no magnetic field to shield its upper air from the solar wind? We know that Mars -- during its first billion years or so -- had a dense carbon dioxide atmosphere that would have made it much more hospitable.

There are several processes that could account for the loss of that dense air blanket -- and one of them is the very slow but steady eons-long leakage process that I've just mentioned.

Moreover, as the same processes remove the traces of water vapor in the air, Mars' surface ice sublimated into more atmospheric vapor to replace it -- so these same processes may have removed a significant amount of Mars' original water supply. If we know how efficient these processes are, we'll have a better idea of just how long it took for Mars to become as hostile a place as it now is.

MSO's other goal would have been the detection and mapping of trace gases in Mars' air, which can be scientifically important even when they exist in parts as small as one per trillion. There is already tantalizing evidence from Earth-based instruments and the Mars Express that a tiny amount of methane exists in Mars' air.

This is probably only a few parts per billion, but it is still extremely important, because the processes in Mars' atmosphere should destroy any methane very quickly -- for even this tiny amount to exist, some surface source must be replenishing it. This methane source could be biological: Martian microbes still surviving under the planet's surface and manufacturing methane as many Earth germs do.

Or it could simply be a nonliving geological source: chemical reactions occurring in Mars' subsurface rock due to the planet's shrinking but non-extinct geothermal activity.

Even in the latter case, pegging the cause of the methane -- and the specific places where it's being vented into the air -- would be extremely important, since such geothermal warm spots could well serve as isolated subsurface "oases" where liquid water and useful volcanic chemicals could support continuing Martian microbes even if they don't produce enough methane themselves to be directly detectable.

And there are many other scientifically important trace gases which, if they exist at all, could also serve as tracers of continuing biological and/or geothermal activity on Mars.

So the 2006 study group recommended that MSO first enter an elliptical orbit around Mars to study its aeronomy in detail -- reaching from only 150 km above the surface, all the way up to several thousand km from Mars to study the wake that the planet produces in the solar wind as that wind blows past it with some small new burden of escaped Martian air molecules.

MSO would then change to a circular 400-km altitude polar orbit (similar to the other US Mars orbiters) to use remote instruments to search for those minute trace gases and map their geographical distribution; and then, after several years, it would somewhat de-emphasize science and focus more on its second duty as a Martian comsat for the remainder of its decade-or-longer operating life.

But this January, the applecart got upset again. NASA picked its two competing finalist proposals for the 2011 Mars Scout mission.

The final choice won't be made until next January -- but both possible missions are small orbiters that would carry out exactly the same aeronomy studies that were supposed to make up half of MSO's science (although they apparently won't study trace gases).

So MEPAG hastily assigned a new team to come up with a new complete set of science goals for MSO, and that revised report was just released last month. This time, the team didn't settle firmly on a single scientific mission for MSO -- instead, they picked three alternate missions that would focus on different types of science studies, with NASA choosing one of them in the fairly near future largely on the basis of the science continuing to come in from our currently-functioning Mars spacecraft.

On top of that, the team also described "augmented" versions of each candidate mission, which could be flown if the Mars program's funding allows it -- and which would allow MSO to combine, to some extent, the studies of two of those "core" mission proposals.

As you might expect, the trace-gas studies may still comprise a lot of MSO's important science -- but a large number of new candidate instruments to study Mars' surface (or even to land a small piggyback craft there) are also listed, and in my next chapter I'll describe them.

Bruce Moomaw is our first "Space Blogger" at www.spaceblogger.com Feel free to create an account on SpaceBlogger and discuss this issue and more with Bruce and friends.

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