Actually, another question comes before that one: to what extent SHOULD the new Mars program be planned out in advance? Several Conference participants pointed out that too much advance planning too far into the future can lead to serious trouble itself, for two reasons.

The first is that -- when near-future missions depend on the success of a mission -- its failure can foul up the whole program's schedule for years in advance. We saw this happen when the failure of Mars Polar Lander forced the cancellation of the similar 2001 Mars Surveyor Lander, despite the fact that it had been almost completely built.

The second is that -- from the start of its exploration -- the planet Mars has thrown one major surprise after another at us. The flybys of the Sixties had convinced most scientists that Mars was a desolate world that had never had much air or any liquid water on its surface -- but Mariner 9's 1971 orbital survey revealed large networks of ancient valleys apparently carved by liquid water.

The 1976 Viking landers discovered what appear to be powerful oxidizing compounds in Mars' soil -- whose origin, and even identity, remain puzzling, and which thoroughly fouled up interpretation of the life-detection experiments which had been carefully planned out on that mission for years in advance.

And Mars Global Surveyor's ongoing detailed orbital survey of Mars has thrown a whole series of surprises at us -- most of which still remain puzzling. A (by no means complete) list includes the apparent near-absence of carbonates or other water-weathered surface minerals such as one would expect if early Mars did have liquid water on its surface; the fact that most areas of its surface that looked smooth in the Viking Orbiter photos have turned out to be covered with parallel grooves and furrows a few dozen meters wide, carved by some erosive process that we don't understand; and -- most recently -- the discovery of what looks like evidence of recent eruptions of liquid water from just below Mars' surface, where its current existence was thought to be impossible.

Any Mars exploration strategy must include the ability to adjust its form fairly quickly in response to such new revelations, which could turn it topsy-turvy. (That discovery of possible near-surface water eruptions -- announced only a few months ago -- is already having a major impact on NASA's future Mars plans.)

For these reasons, several Conference participants -- including David Paige, Candace Hansen and M.I. Richardson -- urged that the Mars program take on a form similar to the Discovery Program for general Solar System exploration, in which NASA periodically releases Announcements of Opportunity (setting only cost and time constraints), various separate scientific teams respond by proposing entire unified mission candidates, and NASA then selects the best of them.

This format would allow total flexibility from moment to moment in planning the Mars program. Quoting Richardson: "The best strategy of exploration is... to have no strategy except to be as well prepared as possible for the next step ahead and to be fully aware of the available options."

But there are some limits to this approach as well, for Mars. The current Discovery program is a "scattershot" affair in which probes are aimed at dozens of different potential scientific targets all over the Solar System, so that the order in which missions are launched has virtually no effect on later missions.

In the case of Mars, though, we do have an overall top program goal -- determining whether early Mars supported life, or at least had an environment that could have supported life -- which we would like to advance toward as rapidly as possible; and this does require some advance planning of the overall sequence of missions.

Also, there are a number of very important new technologies which are unquestionably crucial for Mars exploration, and which need to be developed and tested as quickly as possible. Beyond the long-range rovers now firmly set for 2003, these include:

1. The ability to actively avoid dangerously rough terrain during landing. The bouncing-airbag system used on Pathfinder (and scheduled for the 2003 rovers) is very useful, but it does have limits. It can't be used with really big and complex landers, such as those needed to launch samples back to earth; and even if an airbag lander comes down precisely at its landing point, it's likely to then bounce and roll up to a kilometer after landing, as Pathfinder did -- which will foul up its ability to land near small but interesting geological features. We need full-fledged soft landers of the Viking and Polar Lander sort -- but capable of surviving landing with high reliability.

   In another Conference paper, Tom Rivellini described the new soft-landing system JPL has developed after the Polar Lander failure, in which the entire broad bottom of the lander (carrying the landing engines) can tolerate being crushed on landing, and six long outriggers sticking out in every direction prevent a tipover.

   But we still need to give landers the ability to detect especially rough areas or steep slopes and steer away from them during the final rocket-propelled landing phase. S.W. Thurman described JPL's concept for a scanning laser altimeter that would start working at 1 to 1.5 km altitude, rapidly sketching contour maps of the surface below the lander so that it could detect and guide itself away from such hazards.

2. The ability to land with pinpoint accuracy. Current Mars landers can only come down within about 30 km of their target point, which is hopelessly inadequate to study individual geological formations on Mars, even if you carry a long-range rover. The cancelled 2001 Lander was supposed to test a guidance system that would actively control its entry path into the Martian atmosphere to ensure that it could come down within 10 km of its target point (and probably a lot closer) -- but that test will not be attempted on the 2003 rovers because of inadequate funds. (The same technique will allow "aerocapture" of future craft into orbit around Mars without large amounts of braking fuel.)

   Beyond that, NASA would like to quickly develop the ability to put down a lander within only 100 meters of its target point, using the same scanning laser system used for avoiding hazardous terrain to construct maps of the current landing site which the lander would then compare with stored onboard maps of the desired landing point during its early descent by parachute -- so that, after the chute was cut loose and the craft completed its descent on throttleable rockets, it could steer itself toward the correct landing point.

3. "In-Situ Propellant Production (ISPP)" -- the ability to convert Mars' CO2 atmosphere into at least some of the propellants that a ship needs to blast off from the Martian surface back into Mars orbit (or completely back to Earth). If this can be made to work, it will tremendously lower the weight, and thus the cost, of sample-return missions, and later of manned ships. But it will require an extensive program of testing on Mars' surface -- which was supposed to be started by the 2001 Lander and continued on the cancelled 2003 sample-return lander.

4. Tests of the possible harmful effects that Mars' extremely fine dust will have on machinery -- and especially of possible ways to prevent it from building up on solar panels as it settles out of the atmosphere. The latter will probably limit the lifetime of the 2003 rovers to only about three months -- but if panel-cleaning techniques can be developed, the lifetime and range of Mars exploration vehicles will be vastly increased. This, again, was supposed to be studied by the 2001 Lander.

5. Development of techniques for drilling deeply into Mars' surface -- but using only lightweight, relatively low-power systems -- to try to reach the zone of subsurface liquid water which may serve as a last stronghold for "extant" (still-living) Martian microbes. Until this year, it was assumed that such water exists only a kilometer or more deep, below Mars' thick "cryosphere" of permafrost, where the planet's remaining geothermal heat can still keep it above freezing.

   MGS' new discovery of what looks like recent eruptions of liquid water from strata only a few dozen meters deep (water which may conceivably be able to remain liquid at low temperatures because of a high salt content) indicates that reaching this goal may perhaps be much easier than we had thought -- but it will still be one of the most difficult technical tasks needed for Mars exploration, and one that will definitely have to be developed one step at a time over the next two decades.

One well-received attempt was made at the Conference to design a fairly detailed program for U.S. Mars exploration over the next 15 years or so, by the Ames Research Center's Geoffrey Briggs and Christopher McKay.

Their plan allows considerable elasticity in the details of mission order -- but it does emphasize the importance of carrying out both sweeping reconnaissance of Mars' surface to look for the best landing sites, and tests of the various technologies we will require before we start launching either sample-return missions or attempts to drill deep into Mars' surface. In Part 3 of this report, I'll describe their plan.

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