Mars Express however, does have the disadvantage that - at least for the first two years - it will be left in an elongated orbit rather than being aerobraked into a low circular orbit, limiting its high-resolution mapping of Mars -- but the latitude of its orbit's periapsis (low point) will be constantly shifted, alleviating that problem. It may also be aerobraked afterward.

Mars Express will also carry the "MARSIS" long-wavelength radar sounder to try to look for layers of water ice or liquid water as much as 2-3 km below Mars' surface -- a goal which Briggs and McKay regard as extremely important, and which they would have assigned to a second American Mars orbiter only two years after their first one.

It's clear, though, that Mars Express -- despite being European -- is now a crucial central element of the U.S. Mars program. If it fails, the US will need to quickly launch something similar. Meanwhile, Briggs and McKay continue to favor a follow-up Mars radar orbiter, as they have doubts about the effectiveness of the MARSIS instrument and think that a more powerful and deeper-probing subsurface radar sounder will be needed in any case.

Such an orbiter could, of course, also further extend the kinds of mapping carried out by the earlier orbiters - including higher-resolution Infa-red mineral mapping, which a recent study group identified as very important.

Briggs and McKay's follow-up Mars orbiter is one of their leading candidates for the 2005 launch opportunity -- but not, perhaps, their favorite.

Their favored mission is for a full-fledged 2005 Mars soft-lander to test many crucial new exploration technologies -- some of which would have been tested on the now cancelled 2001 Mars Surveyor Lander.

The 2001 Lander was supposed to use a new system to actively modify its entry path down through the atmosphere, thus increasing its landing accuracy from the 20-30 km of current Mars landers to land within only 10 km of its target point -- and probably within only a few kilometers.

The same system can be used to allow future Mars orbiters to brake themselves into orbit around Mars by skimming though its upper atmosphere on arrival.

This "aerocapture" technique would mean that they will no longer have to carry large amounts of braking fuel, vastly lowering their weight and cost. This test won't be carried out on the 2003 rovers due to cost -- a move which some regard as a mistake.

The cancelled 2003 sample-return lander was supposed to use a scanning laser altimeter during the last 1 to 1.5 km of its descent to construct contour maps of its landing area, which it could then use to steer itself away from dangerously rough terrain and steep slopes.

Even given JPL's new and more rugged design for future soft landers -- in which the entire bottom of the lander (to which the landing engines are attached) is crushable -- such a hazard-avoidance system is vital.

JPL's next step would have been to further increase the targeting accuracy of landers -- including a system in which the craft would compare its constructed contour maps of its landing area with pre-stored maps constructed from the ground to try and precisely locate itself and steer itself toward its target point, a system which might enable it to come down within only 100-200 meters of its target.

The 2001 Lander would also have carried "MIP", the first package of engineering experiments to determine the feasibility of In-Situ Propellant Production -- in which an unmanned or manned lander designed to blast off later from Mars' surface can manufacture much or all of its needed propellants by chemically processing Mars' CO2 air, thus greatly lowering its launch weight and cost.

The MIP tests would have also included an attempt to manufacture a small amount of liquid oxygen -- as well as tests to find ways of preventing the accumulation of airborne Mars dust on solar panels or removing it later.

Solar panels on Mars landers are covered by dust at an effective rate of apx 1% every three days -- a fact which will limit the lifetime of the 2003 rovers to only three months or so, greatly limiting their range and science return.

NASA's failure to at least try testing a lightweight panel-cleaning system on the rovers may be another mistake -- if it worked even partially, it would greatly increase their scientific return for a very low cost.

And "PROMISE", a more sophisticated package of ISPP tests on the cancelled 2003 sample-return lander, would have manufactured enough CO2 and methane from Mars' air to fuel a small rocket thruster on the lander's side -- as well as trying to make carbon monoxide (a possible alternative fuel) and nitrogen (which, along with manufactured oxygen and water, could give a manned mission most of its life-support supplies).

Finally the 2001 Lander would also have carried two experiments to test the dangers of the Martian environment for manned crews. "MARIE", a radiation detector, is now scheduled to be carried on the 2001 Orbiter and the 2003 rovers -- but "MECA", a set of tests to check Mars' soil for hazards both to men and machines, hasn't found a new ride yet.

MECA would have chemically analyzed the soil for poisons -- including trace metals and fine quartz dust that could cause silicosis -- and it would have used microscopes and other sensors to determine whether the dust could gum up moving parts, and whether static charges built up by dust storms could damage electronics.

The cancelled 2003 lander would have carried follow-up tests (including a study of Martian dust devils). While the tests relating to manned Mars exploration maybe a little premature; the tests relating to the functioning of machinery are not -- and MECA's sensors would provide much science data on Mars' soil as a fringe benefit.