Scientifically, it was a very close decision -- as even Steven Squyres, the head of the Athena scientific team, admits -- but certainly this mission is more likely to interest the general public.

However, that same public popularity has apparently led to a sudden startling twist in the mission selection process that began with NASA's abrupt postponement of the press conference at which the selection was to be announced. The reason for this delay was due to the fact that NASA is suddenly considering a totally unexpected plan to launch a second, duplicate rover to another landing site also in 2003.

This idea obviously has scientific merits -- but it also has flaws. For one thing, there's the danger of having not one, but two, more Mars missions fail in 2003.

During the Sixties and Seventies, the U.S. usually launched duplicate spacecraft on its planetary missions -- and three separate times, the first probe failed but its duplicate saved the day. (The Soviets carried this philosophy even farther, sometimes launching three probes at a time -- and, once, four!)

But in those days, NASA had much more money with which to thoroughly test deep space probes before launch.

Today, in the era of tighter space budgets and NASA's declared philosophy of "better-faster-cheaper", the Agency tends to skimp on advance tests, and as a result has suffered several failures because of its resultant failure to detect a flaw in the craft's design -- a flaw that would also have ruined any duplicate craft launched at the same time.

For instance, the software flaw that probably caused the crash of the Mars Polar Lander wasn't discovered until months after the failure -- with a duplicate lander launched at the same time also facing a non-upright landing.

The same thing might very well have happened to a duplicate of the Mars Observer, which was destroyed a few days before arriving in Mars orbit in 1993, probably due to a very subtle and hard-to-detect flaw in the design of its propellant valves.

Losing one more Mars probe in 2003 would be bad enough for NASA; losing two of them could be possibly fatal for its public support.

And there's another problem. At the same time that NASA announced it was seriously considering adding a duplicate rover, it was forced to confirm rumors that it is suddenly considering cancelling or delaying the "Pluto Express" probe it had planned to launch in late 2004 to fly by the last unexplored planet, due to "expense problems".

And while it refuses to go into much detail on the latter, it's very hard to avoid concluding that the likely cost of a duplicate rover -- $175 to $200 million -- is probably the factor responsible, (along with a bottomless trough in Earth orbit).

But the Pluto mission does not tolerate delays well. If it's delayed beyond 2004, it is much harder to make a gravity-assist flyby of Jupiter to catapult the craft out to Pluto. (It might still be possible, but the flight would be prolonged by several more years.)

There are other possible flight techniques that might also be used -- such as having the Pluto probe make three successive gravity-assist flybys of Venus instead to finally fling it out to Pluto, or equipping it with a duplicate of the solar-powered ion drive system on the Deep Space 1 probe (which could give it an initial push out to Pluto if combined with a single Venus flyby). But these also involve a considerably longer and/or more expensive mission.

Moreover, Pluto is slowly moving farther away from the Sun on its eccentric orbit -- and its extremely thin but scientifically interesting atmosphere (one of the major scientific subjects of the mission) is about to start freezing onto the planet's surface.

The original 2004 probe is supposed to reach Pluto in 2012 -- and any probe that gets there after about 2015 may not find much atmosphere left to study.