I had two reasons. First, the Pathfinder system is a good deal heavier, due to its big and rather thick crash airbags -- so that it has to be launched on a full-scale Delta 2 booster, rather than the smaller and cheaper "Medlite" Delta used by the Polar Lander (and its cancelled 2001 twin).

But NASA has decided to absorb this cost increase, since its Mars program has now been stretched out over a much longer schedule. Both of the possible 2003 missions (and the 2001 Mars Surveyor Orbiter) are all scheduled for full-scale Delta 2s.

Second, when the U.S. finally does launch an unmanned Mars sample-return vehicle, it will certainly have to be a full-fledged soft lander, equipped with an automatic obstacle-avoidance system (using scanning laser radar and/or a descent TV camera) to minimize landing risks.

But such a system will probably have to be tested under Martian conditions first, using a smaller and cheaper soft lander, before we commit something as expensive as a Mars sample-return mission to it. I remain convinced that this will be done at some point.

In case the Mars Mobile Lander isn't possible, though, NASA is also studying an alternative 2003 mission -- another Mars Surveyor orbiter spacecraft with a new set of instruments.

This spacecraft would probably be designed along the lines of the Mars Global Surveyor that is currently mapping Mars' surface with great success, although it might also be made lighter weight by incorporating some elements of the smaller Mars Climate Orbiter spacecraft (which would have worked very well if its ground controllers hadn't flown it into the planet, and which will have a near-duplicate launched to Mars in 2001).

The instruments it would carry (although there's some uncertainty) might include the following:

1. Another flight of the heavy "PMIRR" multichannel IR sounder carried on the Climate Orbiter (or a lighter instrument with the same capabilities), to map Mars' weather patterns -- air temperatures, dust clouds and ice clouds -- with unprecedented detail.
2. Another very high-resolution telescopic camera which, like the one on the current MGS, could photograph small patches of the Martian surface with a resolution of only about 2 meters.
3. A copy of the pair of tiny wider-angle cameras (only 0.6 kg apiece) on the Climate Orbiter.
4. A "VIMS" spectrometer to make very detailed maps of Mars' surface, using high-frequency infrared sunlight near the border of visible red light and reflected off the surface (thus mapping a different set of minerals than the longer-wavelength IR radiation emitted by Mars' surface because of its warmth and currently being mapped by MGS).
5. A UV spectrometer to analyze gases in the atmosphere and ionosphere.
6. Perhaps -- if weight allows -- another laser altimeter and magnetometer like those on MGS.