http://www.spaceviews.com/2000/04/17a.html
The Future of Mars Exploration
Published: 2000 April 17
5:40 am ET (0940 UT)
by Bruce Moomaw
Special to SpaceViews
A revised Mars exploration plan may not
be released by NASA until this fall as
scientists and agency officials debate the
best ways to return useful data about the
Red Planet affordably, a key official said at
an astrobiology conference last month.
Dan McCleese, chief scientist of JPL's Mars Exploration
Directorate, told attendees of the first Astrobiology Science
Conference at NASA's Ames Research Center earlier this month
that a revised Mars exploration plan, which NASA said last
month would be released this summer, may not be ready until
the fall as the space agency works to completely overhaul the
program.
McCleese indicated that a key aspect of the original Mars
exploration program -- a sample-return mission -- might be
pushed back well into the next decade, as NASA realizes it
significantly underestimated the complexity and cost of such a
mission.
The Problems with Sample Return
When it was initiated after the 1993 failure of Mars Observer,
the Mars Surveyor program had three overall goals of equal
importance: understanding Mars' potential for past or present
life, its climate history, and the "natural resources" of the planet
(which included both the resources that might support future
manned expeditions, and -- rather awkwardly -- Mars' overall
geological history). Indeed, both the 1998 spacecraft were
designed to focus on the climate question.
But in 1996, following the revelation of possible microfossils
in Mars meteorite ALH84001, the program's central goal was
changed by NASA Administrator Dan Goldin to focus on the
hunt for present or (more likely) past life on Mars. Thus sample
return, which had been a longer-range goal, was suddenly
emphasized for the near future, without adequate consideration
of just how expensive it might turn out to be.
However, NASA's belief that a sample-return mission could
be conducted for as little as $400 million was so overly
optimistic that the Mars exploration program would have to
have been overhauled even if Mars Climate Orbiter (MCO) and
Mars Polar Lander (MPL) had not failed. In an earlier interview,
McCleese said that the cost estimates for such a mission had
soared to over $1 billion; at this conference he told another
reporter that he now regards a sample return mission as a
"Cassini-level program" -- and the Cassini Saturn probe cost
over $3 billion and took a decade of planning.
The current sample-return plan calls for a rover to collect
samples up to a kilometer (0.6 miles) from a lander. The
samples would then be launched into orbit with a small
solid-propellant rocket, where they would be picked up by a
French orbiter that would return the samples to Earth. However,
McCleese said at the conference that the plan is now open to
complete review, and other methods, such as directly launching
the samples from the Martian surface to Earth, might be used
instead.
But in any case, such an effort will require a lot of time and
money -- and that raises another question: given NASA's limited
Mars funding, should it immediately focus on developing a
sample-return mission, or should it instead devote the next
several Mars launch opportunities to using smaller orbiters and
landers to carefully survey Mars to locate the best possible
landing sites for sample-return missions? After all, sample
return flights will always be expensive and relatively few in
number, and each one will return at most a few kilograms of
samples, which means that we may have to put a great deal of
thought into where to land them to maximize the chance that
this small returned sample will contain evidence of ancient
microfossils. According to McCleese, this question is currently
bitterly dividing NASA's Mars science advisors -- and he rather
plaintively asked the astrobiologists assembled at the
conference for their input on the question.
Shifting Scientific Priorities
Now the question of whether life ever existed on Mars will
still be the guiding question in designing the Mars program, but
there has been a return to a multi-pronged investigative
approach to answer that question. As McCleese says, answering
that question one way or the other convincingly "will take
decades, require also understanding the climatological history
of Mars, and probably require deep drilling into its surface" --
several meters deep in the short run, and maybe eventually
kilometers deep, into or below the thick buried "cryospheric"
layer of permafrost that most scientists believe underlies Mars'
surface.
In addition, data returned from the Mars Global Surveyor
(MGS) spacecraft have left still dangling the central question of
whether Mars had large amounts of liquid water flowing across
its surface early in its history or whether it was always so cold
as to be frozen at the surface, with its "valley networks" forged
instead by slower trickles of liquid water (perhaps geothermally
heated) a short distance underneath that frozen surface.
MGS' thermal infrared spectrometer has also so far failed to
discover the expected large deposits of carbonate minerals that
should exist if (as believed) Mars' ancient dense carbon dioxide
atmosphere disappeared because it reacted with that liquid
water and the surface rocks to form carbonates.
McCleese said that this could mean either that
wind-deposited materials, which have turned out to dominate
virtually all parts of Mars, are covering up the clues of its
ancient surface, or that our current theories about Mars' history
are significantly wrong. He suspects the latter. In either case, he
says that we're left with a major question: if surveys by Mars
orbiters continue to fail to reveal convincing evidence of
water-deposited minerals or sedimentary rocks on Mars'
surface, do we assume that they really don't exist -- or do we
send some landers down for a more thorough search?
The Shape of Mars Missions to Come
So now what? The 2001 Orbiter will definitely go up on
schedule in April 2001; it's a near-twin of Mars Climate Orbiter,
but all indications are that MCO would have worked well if its
controllers hadn't flown it into the planet due to an undersized
navigational staff. In addition, the 2001 Orbiter carries two
instruments which are very important in selecting promising
future landing sites to look for evidence of life.
The 2001 Lander, though, is a different matter. A whole series
of serious flaws was found in its design that could have ruined
it, besides the touchdown-contact switch problem that was the
most likely cause for MPL's failure. Proposed changes to the
lander, including an enlarged communications system to allow
engineering telemetry during the landing itself, and also a
direct backup communications link to Earth after the landing,
will add about 30 kg (66 lbs.) to the spacecraft's mass and push
back its launch until at least 2003.
Moreover, changes to its landing engine and a desire to add
both a computerized landing-obstacle avoidance system and
stronger (and heavier) landing gear to better deal with the
rugged Martian terrain may also be in the works. The lander is
already largely built; its avionics and scientific instruments will
probably be used at some point -- but it is by no means certain
that it will ever fly. That decision must be made within a month
or so.
Whether it flies or not, what happens to the future Mars
program? At his talk, McCleese briefly presented a whole table
of proposals for future Mars spacecraft in 2003 and beyond. He
also quoted the Polar Lander failure review board as taking one
look at this list and telling JPL, "No wonder you're having
failures -- you're spreading your design efforts much too thin.
You must focus firmly on one specific goal of Mars exploration
-- presumably the search for life -- and on designing a limited
variety of spacecraft to achieve it." If NASA does decide to
reconnoiter a whole set of possible landing sites before
dispatching sample-return spacecraft to a few of them, what's
the best way to do it?
McCleese described one idea that has been mentioned at
several recent conferences: the "Mars Scout". This proposal,
currently under study at JPL, would involve launching 5 to 10
small, cheap landers to various potential sample-return sites
(and maybe other interesting areas) over several Mars launch
opportunities, perhaps starting as early as 2003. They would be
similar to the Mars Pathfinder -- hard landers that would come
down fast, use inflatable airbag cocoons to cushion the landing
shock, and then prop themselves upright by unfolding side
petals with solar cells -- but they would be much smaller,
weighing perhaps only 30 kg or so, like the "Beagle 2" lander
that Europe plans to launch in 2003.
The Scouts would carry limited science payloads: two
cameras to photograph the area close-up both during descent
and after landing to check its safety and scientific interest for
more expensive landers, infrared spectrometers to map the local
minerals and check for signs that they were water-deposited,
and perhaps (like Beagle 2) a few other instruments to further
check the area's suitability for the return of samples containing
possible fossils -- and they would work for only a few weeks.
These might be small enough that they could be carried on the
100-kg (220-lb.) "Mars Micromission" spacecraft that the U.S.
plans to launch to Mars starting in 2003, as piggyback payloads
on Europe's Ariane 5 launches of large satellites.
There are already numerous proposals to have micromission
spacecraft carry various science payloads both into Mars orbit
and on tiny surface landers. One idea drawing great interest is
the "Mars Micromission Aerobot", in which a little helium
balloon would drift for weeks or months at about 4 km altitude,
transmitting thousands of pictures and carrying other
instruments such as IR mineral spectrometers, soil-water
detectors and magnetometers.
If NASA decides to start focusing immediately on
developing a sample return spacecraft, though, there will
probably not be enough money to fly many (if any) of these
little reconnaissance craft as well -- and if they are flown, we
probably won't see a sample-return mission launched until
early in the next decade. The agency's Mars funds, after all, are
sharply limited -- and the feeling now is that the remaining
money had better be very wisely spent.
At any rate, sometime later this year NASA should decide
how to proceed -- and with luck, we'll get some information on
this earlier, starting with the fate of the 2001 Lander.
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