Over the last few years, as new scientific information on Mars has finally started coming in at a continuous landslide flow, the recommendations of these groups have changed somewhat -- although not, perhaps, as dramatically as one might think. A lot of their overall ideas have remained continuously relevant.

Once again, this is largely true of this new report. There's not all that much really new or surprising in its 108 text page (which are accompanied, by the way, with a spectacular album of recent photos that make it abundantly clear just how frequently localized flows of water across Mars' surface have affected it in the past, and maybe even that they are still occasionally doing so.)

But there are a few new ideas in it -- based on the most recent developments in the Mars Exploration Program -- that are worthy of note. First, the new committee has reached the conclusion that "Identification of appropriate landing sites for detailed [biological] analysis -- whether in situ or by sample return [to Earth] -- can be done with the data sets now available, or imminently available from currently active missions."

That is, there is little need at this point for still more new Mars orbiters -- after the most recent 2005 Mars Reconnaissance Orbiter -- to continue inspecting the surface to pick out good landing sites for Mars landers. What we already have, or are about to get from our current three still-operating Mars orbiters, is quite adequate to do that properly.

(This does not mean, however, that additional mapping from orbit is totally useless. The big orbiter that NASA plans to launch in 2013 -- both for scientific studies, and to serve as a new relay comsat for Mars landers after MRO finally dies of old age -- was originally supposed to focus mainly on atmospheric studies; but since then NASA has decided that part of those studies will be done instead by the smaller 2011 Mars Scout orbiter, and so a group was hastily convened to recommend new scientific goals for the 2013 craft. The report from that group will be officially released in the next few weeks. Advance word indicates that they will recommend several possible alternative science payloads for it, all of which include a good many sophisticated surface mapping instruments.)

Second, the group issues what can only be described as a veiled caution about sending men to Mars too early: "The scientific study of Mars, including return to Earth of astrobiologically valuable samples that can be used to address the questions being asked today, can be done with robotic missions...It is critical that any astrobiological evidence that might be present on Mars not be compromised by robotic or human activities before definitive measurements or sample return occur."

This is pretty clearly a reference to a fact that is worrying many Martian researchers: any manned landing on Mars, by its very nature, cannot help but very seriously contaminate the same local area that it's trying to study with a large number of Earth germs and organic materials. You can't sterilize humans. For that reason by itself, it may be scientifically very unwise to actually land humans (as opposed to human-run sterilized robots and sample-return craft) on Mars' surface for a long time to come.

Most importantly, however, the committee proposes a completely new approach to the problem that has most vexed Mars researchers in the last decade: how do you return scientifically adequate samples of the Martian surface to Earth, on unmanned craft, for really detailed study?

This is universally regarded as absolutely necessary for any remotely adequate study of the evidence for present-day or fossil life on Mars, regardless of whether the much smaller and simpler in-situ instruments on earlier landers have provided tantalizing initial evidence or not. Earth-based analytical instruments are tremendously larger and more sensitive than any instruments carried to Mars could possibly be -- they frequently weigh tons, and can examine both chemical traces far too rare, and microscopic phenomena far too small, for in-situ to even begin to observe.

Moreover: "If properly stored and isolated from the terrestrial environment, returned samples can be examined by more sensitive analytical techniques and methodologies that are likely to be developed in the future. Returned samples also offer great flexibility; in any scientific investigation, it is advantageous to be able to alter the analytical strategy as new information emerges. This is particularly critical in astrobiological investigations, where the characteristics of extraterrestrial organisms or prebiotic chemistry cannot be confidently predicted."

This has already been done to a massive degree where the study of Mars meteorites is concerned. Many of the studies done on them -- including those which provided evidence of microscopic fossils in the famous "ALH84001" meteorite, which is now mostly regarded as chimerical but was still extremely important -- simply could not have been done until years after the meteorites were first collected, and this process will continue to advance.

Finally, "[S]ample return is particularly important for astrobiology investigations on Mars, since it is certain that any significant finding with potentially far-reaching implications will require corroboration by multiple replications of the same analyses (ideally in different laboratories and by different investigators), and by different types of analyses."

All this is especially crucial where the search for something as elusive as fossil evidence of ancient Martian life is concerned. One section of the report points out the continuing great difficulties scientists have even in deciding whether possible fossil evidence of extremely ancient life in Earth rocks is concerned -- all of the recently publicized evidence (chemical and microscopic) of fossil microbial life older than 2.7 billion years has come under serious fire.

And this is on our own planet, where we have a huge amount of material that we can sift for such evidence. While such really old fossil evidence may actually be preserved much more easily on Mars -- with its long-time extreme dryness, and its lack of continental drift and crustal recycling -- when you can only hope to return a few kilograms of samples from an entire planet over the next few decades, you are going to have to hope for a great deal of luck.

And unmanned Mars sample-return missions are so hard that there's no chance that more samples than that WILL be returned over the coming decades. NASA currently pegs the likely cost of each such mission at about $5 billion, and no one would be surprised to see that turn out to be an underestimate. "...[T]he history of NASA's space science activities clearly demonstrated that each major [space] scientific community gets only one multibillion dollar mission per decade. Thus the only realistic number of sample-return missions that can be contemplated within the predictable time horizon is: one."

So how do you make that one or so Martian sample-return mission per decade -- which will return only 1 or 2 kilograms of samples -- count for as much as possible? NASA's strategy up to now has been to do very thorough advance studies of Mars before flying the first such mission, in order to pick the best possible site on the planet for biological evidence.

The trouble with this is -- as the new report says -- "[O]nce this approach has been accepted, the only viable sample is one for which there already is compelling evidence for life, or at least a high probability of there having been life. This would seem to require obtaining a sample from a place at which detailed in-situ astrobiological analysis has already been done" -- perhaps by the "Astrobiology Field Lab" rover that NASA hopes to launch around 2016 as a more sensitive and sophisticated follow-up to the already very complex and expensive "Mars Surface Laboratory" rover scheduled for 2009.

"NASA's 1995 report [on this subject] advocated sample return only after Mars had been thoroughly studied at global and regional scales, and following detailed local investigations that would ensure that any returned samples would have astrobiological relevance. NASA has implemented those recommendations in its exploration planning, with the unfortunate result that Mars sample return has been repeatedly pushed to (or beyond) the end of any planning cycle."

Is there a better way to plan sample return? The new report says there is: a technique that could allow the first Mars sample return mission to be flown fairly soon, at fairly low cost, while still returning a set of small samples from a very wide portion of the Martian surface.

"Utilize ALL the highly mobile, well-instrumented rovers" -- and possibly stationary landers, if they use drills to dig up samples from a moderate distance below the Martian surface -- "to cache collections of interesting samples for possible future sample-return missions.

A subsequent sample-return mission could then land near a sample cache (possibly guided by a beacon on the cache) and retrieve the cache" -- using a relatively simple, short-range rover without too much in the way of science instrumentation of its own -- "eliminating the need for analytical instruments [on the sample-return rover itself] to assess the nature of the samples...Such a strategy would increase the likelihood that astrobiologically interesting samples could be obtained, and it obviously would decrease the cost of a sample-return mission {or multiple missions) significantly."

Doing this would allow the very first Martian sample-return mission to be flown in the reasonably near future, and for minimum cost, while still maximizing the chances that it would return scientifically valuable samples. Before it was launched, NASA would be free to select its best target from a whole menu of several caches of samples that had been collected by previous landers (at different sites on Mars) because the scientific teams running those earlier missions had concluded from those landers' own scientific studies that they might well be of biological interest.

And, if those earlier sample-caching landers had been rovers, then each one's collection of cached samples would have been selected during a drive of as much as a dozen kilometers or more over the Martian surface, with the rover having spent months (or even years) carefully collecting samples from the most interesting individual rock formations that it came upon.

It is, unfortunately, too late at this point to build such a sample-caching kit into the big 2009 MSL rover -- but the new report urges that all US (and European) Mars rovers and deep-drilling missions include one, which would be made as simple, lightweight and low-cost as possible.

As the report points out, this also means that the lander that eventually does actually pick up the stored sample cache from the particular chosen rover will have to be able to make a precision landing within a kilometer or so of that old rover, but this kind of landing ability will be necessary for future Mars science missions anyway.

And -- as the report points out -- not only will this strategy allow us to maximize the scientific cost-effectiveness of the first Martian sample-return mission; it will allow us to ease into that expensive mission more gradually and smoothly. "Sample return should be seen as a program that NASA and the Mars science community have already embarked upon, rather than as a single, highly complex, costly and risky mission that is to occur at some future time... Programmatically, sample return should be phased over multiple launch opportunities.

A first phase could involve caching samples on Mars; a second phase, putting the samples into orbit" (around Mars, by having the sample-return lander retrieve the cache and load it into a small canister which would then be launched into a moderately low orbit) -- "and a third phase, returning the samples to Earth" (by launching a Mars orbiter that would automatically rendezvous and dock with the tiny orbiting sample canister, and then load it into a capsule that the orbiter would rocket out of Mars orbit and back to Earth.

"This approach would also allow some independent science investigations at each phase that would continue to engage the science community and the public; and it would increase resilience of the program in the face of the failure or delay of individual spacecraft missions." That is, the sample-return lander and its short-range rover for the second phase would carry some new instruments of their own to continue the in-situ study of the Martian surface in addition to their main task of retrieving the sample cache.

And obviously this kind of plan maximizes the flexibility of the sample-return mission -- not only in its ability to select good samples for return, but in its time schedule, and its ability to adjust itself to deal with the failure of a single one of the phases. If the first sample-return lander fails, you can send another to the same cache a few years later.

If the sample-retrieval orbiter fails before rendezvousing with the orbiting sample canister, that canister -- if it's in a reasonably high orbit and carries a long-lived solar-powered radio beacon, or even mirrored laser reflectors -- can wait a few more years for a repeat attempt to be made.

And the new technologies necessary for a sample-return -- precision landing; rendezvous of one rover with another on the Martian surface; automatic rendezvous and docking in Mars orbit; adequate sterilization of the sample-collection gear; construction of a facility on Earth to safely store the returned samples -- could all be done in a more stretched-out, flexible way at the same time that that part of the US Mars exploration program which does not involve returning samples to Earth was moving forward.

This new strategy for a "decentralized" first Mars sample return mission that's made as flexible as possible -- both in its variety of possible samples to be chosen for the return, and in its scheduling -- may, at this point, be the only realistic hope we have of ever actually getting this very big and very expensive mission started. Without this change in strategy, one can easily see us kicking that first sample-return mission further into the future indefinitely, just as NASA has already delayed it for two decades.

But a US Mars program that "multitasks" -- making the preparations for the first sample-return missions (and later ones) gradually, at the very same time that it is carrying on other major on-site scientific studies of that very complex planet -- could be a different matter.

Sensibly incorporating decentralization, flexibility and parallel multitasking into the Mars Exploration Program seems to be the wave of the future, as a necessity if we are to carry out the huge task of exploring that very complex world in a practical way.

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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