The rumours about MSL, mentioned a while back, seem to be getting stronger and stronger. Since the most recent MEPAG meeting a number of people have started more or less assuming that MSL is going to slip back at least one launch opportunity, to 2011; there's also talk of it being doubled up into an MSL A and an MSL B, like MER. It's not been announced yet and may not be a done deal, but the delay, at least, seems very much to be the way that the wind's blowing. It's possible that a formal decision is going to wait until NASA's new administrator, Mike Griffin, weighs in.
An ancillary rumour suggests that if this happens there will be serious consideration given to moving the next Scout opportunity for a small mission up from the 2011 launch opportunity to the 2009 opportunity. Four years is a very short time in which to go from a sheet of paper to a spacecraft, and a truncated selection process might be put in place, in which only the scouts that were shortlisted last time need apply. Those three were: Scim, a mission to pull dust from the upper atmosphere; Ares, an aircraft that would do magnetometry, inter alia; and Marvel, an orbiter with which to measure atmospheric trace gases, including methane, based on the bus for Mars Odyssey. If it were done that way, then the case for Marvel would be extremely strong: the science basis is all the stronger now we have reports of methane, and the technical risk has to be low compared to the other two, since it's a fairly straightforward orbiter, content to look at the atmosphere from above rather than rip through its edges with ramscoops flaring, or extend its wings and try and fly through the stuff. If you wanted to be really practical about the matter, you could save time by just selecting Marvel straight out of the gate -- but I'm not sure that that's actually legal for a scout mission.
Marvel would get really accurate data on methane levels, it would measure any seasonal variations, and it would pick up a bunch of other trace gases if they're there. It would also have a good chance of measuring the carbon-12/carbon-13 ratio in the methane, a measurement which, if it is different from the isotope ratio in the carbon dioxide, would offer strong evidence for a biological source. (The idea is that the methane-making organisms would feed on a well-mixed global reservoir of carbon dioxide, but would discriminate between isotopes as they did so, just as methane makers and photosynthesisers do on the earth; most non-biological processes don’t make such discriminations.) But in an ideal world -- say, for the sake of argument, a world in which the aim was not to fly an already accredited Scout design, but to fly the best methane mission you could -- you'd want to at least consider an alternative mission that looks for the same data a different way.
Marvel would be in an orbit that kept it close to the terminator; its spectrometer, instead of looking straight down, as most spacecraft instruments do, would look at the horizon -- the planet's limb -- in order to see the sun through the atmosphere. This means it gets a strong signal, because it's got a very bright light source shining through a lot of gas. An alternative, which Mike Mumma at Goddard has thought about, is to put a telescope in a halo orbit around the Martian L1 point, which is on a line between Mars and the sun. At L1 a telescope could spend all its time looking at the whole sunlit face of Mars -- and paying attention to any region that was of particular interest.
The challenge of an L1 mission is that the signal is a lot weaker. Your light source is the surface of Mars, which is a lot less bright than the sun. And the light passes through much less of the atmosphere. Light coming to Marvel on a tangent to the planet's surface goes through something like 40 times as much of the atmosphere as light reflected from the surface to the L1 point, and thus the spectral features due to methane, or anything else, will be considerably stronger.
The advantage of L1 is that you could look at specific places -- at resolutions down to a few tens of kilometres -- and look at them at any time of day that you chose. Marvel has to look only at sunsets or dawns, and it can't choose to focus down on a particular region (though it will see all the regions of the planet on a fairly regular basis). If methane levels are strongly variable in space and time, that would seem to be an advantage for the L1 mission. The lower signal at L1 should be something that can be compensated for in principle simply by taking data for longer; in principle, an L1 mission could stare at a particular spot all day.
I really don't know which mission might be better. If I knew for sure that Marvel's signal to noise ratio made it significantly more capable in terms of isotope analysis, I think I'd go with that. There again, the idea of being able to look down wherever and whenever you want is certainly appealing, especially if it were to turn out that there are point sources and complex surface chemistry. But developing a mission from scratch to fly in 2009 would be very, very hard, and I think that the greater amount of design work already done on Marvel would probably trump all other factors. So if there is to be a methane mission at all, it may well be a case of love the one you're with - not too hard, since Marvel is pretty damn loveable.
If I may use your site to toot my own horn, I finally have that very long article on the first meeting of NASA's Mars Strategic Roadmap Committee (in January) up at SpaceDaily: http://www.spacedaily.com/news/mars-future-05f.html . (The main holdup was that I had also made a deal with "Astronomy" magazine to publish a super-shortened version of the article on their website, and they didn't get around to putting their version up until Feb. 28.) Anyway, I think it provides a prety thorough overview of the way things were shaping up then, in the Committee's eyes. And since then they've held their second meeting -- which I wasn't able to attend, but whose presentation materials can be seen at http://www.hq.nasa.gov/office/apio/mars_materials2.htm . (This is a sub-branch of NASA's extremely useful overall site for the Strategic Roadmaps at http://www.nasa.gov/about/strategic_roadmaps.html . I DID manage to attend the first meeting of the Solar System Strategic Roadmap meeting in February and am now working on an article on that.)
Anyway, to sum up the relevant parts: the Committe, virtually from the start, was VERY strongly in favor of one or two additional MSLs, for the simple reason that the MSL is yet another Mars reconaissance mission -- its primary function is to locate traces of complex organics on Mars' surface, after which the still more complex followup missions (Sample Return, and/or the Astrobiology Field Lab rover) would be sent to the same location to determine whether those organics actually do constitute biological evidence. But if the first MSL fails to turn up evidence of such organics, we will be very much in the dark as to where to send those missions -- and, since the cost of a sample return mission in particular has now risen to $4 billion, we damn well better send them to the best possible locations on Mars. Thus the committee's very firm-looking determination to recommend one or two more MSLs as additional reconaissance missions, although it's uncertain whether all of them would be launched before any of the more complex followups or they would be interleaved with them. (If none of the MSLs turn up interesting organics, the next step will probably be to start launching stationary "Deep Drill" landers to dig down 10-20 meters and look for possible biological traces there.)
There was also an awful lot of unease on the part of the Committee about launching the first Mars Telesat orbiter just a month before the first MSL in 2009 -- if the MTO fails, the MSL's data return will be tremendously choked off. And there are still serious technical worries bout MSL -- in particular, whether the proposed "Skycrane" landing system will work, and whether they can cram all of the 10 selected experiments (or even all of the top 6) onto the rover. So I think it extremely likely that the first MSL will be delayed till 2011, although I have no idea whether they'll try launching two of them at the same time. "Space.com" also has a recent article on this at http://www.space.com/news/mars_overhaul_050311.html . (What would really be wise is to build the second MTO in advance, so that if the first one fails it can be launched in 2011 to cover the first MSL -- with MTO-2 otherwise being held in storage until it finally is needed.)
But this does make it likely that the next Scouts -- or at least one of the two Scouts currently set for 2011 -- will be bumped up to 2009 to replace the MSL then. And the Committee has also started wrangling about whether to fiddle with the Scout program, for several reasons. For one thing, JPL's Mars program director Firouz Naderi told them flatly that the current $400 million Scout cost cap is not adequate to provide any more small Mars landers -- Phoenix was a fluke, since the spacecraft was already built. For another, there's a debate under way as to whether all the Scouts should be chosen completely independently of the goals of the main Mars program (as with the Discovery missions), or whether NASA should set possible scientific goals in advance for at least some of them (as with the New Frontiers missions). Phoenix ended up having its selection virtually ordered by NASA headquarters, precisely because its inspection of the near-surface polar ice layer did mesh so well with the main program -- and the same thing seems likely to be true of a methane-mapper mission; I'd be surprised not to see one fly as the second Scout.
Also, there's the entirely new additional line of "Mars Testbed" missions to make the studies and tests needed as preparation for eventual manned Mars expeditions -- added as part of the Bush Initiative -- with the first one currently set for 2011 along with the next two Scouts, and future ones going up every 2 to 4 years. These are fairly expensive -- the first one is likely to run about $600 million by itself -- and the more of their experiments can be carried instead as piggyback payloads on other Mars spacecraft, the better. The follow-up MSLs might carry some of them, but one of the presentations at the second Committee meeting makes it clear that consideration will also be given to trying to combine Mars Scout and Mars Testbed missions where possible.
Posted by: Bruce Moomaw | March 14, 2005 at 09:16 AM
Ben detto Anonimo.Su forza,fai unp sforzo,registrati,mettiti na foto come fcicao io,scrivi in chiaro.Mica siam un grupop di Al-Qaeda qui.Anzi,siamo tutti pronti per partire su Marte,io di sicuro.Se mi chiamassero mollo tutto e ci vado,marte,Io,Europa,Enceladus,Un Asteroide,un pezzo di sasso.Tutto,pur di metter il naso fuori dalla Terra.A gratis pure ci vado e in biciletta:DMa tanto,a 46 anni col tubo che potrei andarci nello spazio,non ti deve nemmeno venire la goccia al naso.
Posted by: Bayu | August 04, 2012 at 01:18 PM