This decision has been combined with NASA's decision to compensate for the funding cut by simply canceling the 2004 Pluto Express mission in order to try to maintain the Europa Orbiter for a 2006 launch.

This requires the Pluto probe being launched in 2009-10 at the earliest, which makes it highly unlikely that it will reach Pluto until about 2020 -- and also requires that unless the Pluto probe is launched before the next opportunity for a Jupiter gravity-assist flyby opens up in 2014, the probe must reach Pluto with the assistance of an ion engine or solar sail, which will sharply increase the mission's overall cost.

But the delay in the probe's arrival at Pluto produces another problem -- as Pluto moves farther away from its perihelion, its thin atmosphere (one of the mission's major scientific goals) is starting to freeze out on its surface. By contrast, the Europa Orbiter can tolerate a launch delay of any length with no loss in scientific return or increase in cost.

NASA, however, currently seems determined to launch the Europa probe first, on two grounds. One is the intense interest from the general public (and perhaps of NASA Administrator Goldin himself) in astrobiology, to which Europa is directly relevant.

The other is its argument that Pluto and the Kuiper Belt are not really all that high priority scientifically -- and that Pluto's atmosphere is even less so. Two guest speakers at the SSES meeting tried to counter the latter argument by going into detail as to Pluto's scientific importance.

The first, Dr. Jonathan Lunine, described the scientific importance of the Kuiper Belt itself -- whose existence has only been confirmed over the past decade, despite the fact that it apparently contains as many as 100,000 significant-sized icy objects of which Pluto is only the largest known member.

The Kuiper Belt, the closer (and much smaller) rocky Asteroid Belt, and the comets scattered into various highly eccentric orbits (or into the incredibly distant "Oort Cloud" of comets orbiting trillions of kilometers from the Sun, which we now think may also contain larger objects), seem to be the only remnants of the original cloud of small "planetesimals" which later crashed together to form the planets.

As such, Lunine explained, it's become increasingly clear in the past few years that studying the Belt is vital to understand the processes by which the Solar System as a whole was formed.

One major question is the way in which most of the original icy planetesimals in the "disk" out of which the Solar System coalesced collided to form the four giant planets.

To what extent did Jupiter, Saturn, Uranus and Neptune form in their current orbits, and to what extent did they tug gravitationally at each other and at the remaining planetesimals to shift their orbits inward or outward? At what rate did they grow, and by what process? (There are at least two major competing theories about the latter.)

To what extent did they "clear out" the remaining planetesimals, by flinging them either into the inner Solar System (where they may have provided the inner rocky planets -- including Earth -- with much of their supply of water) or away from the Sun completely into interstellar space (or into the incredibly distant "Oort Cloud" of comets orbiting up to trillions of kilometers from the Sun, which we now strongly suspect contains some larger objects as well)?

Another question concerns the Kuiper objects' chemical compositions. To what degree do they chemically resemble the contents of genuinely interstellar clouds of gas and dust, and to what extent may they have been chemically modified by the physical processes that made that original cloud around the Sun coalesce into comets and planetesimals?

There is some tentative evidence that they may vary from each other in surface composition to a surprising degree; why? Pluto, Charon and other Kuiper Belt debris are thought to be rich in organic compounds. In fact, they probably comprise, in total mass, the biggest supply of moderately complex organic compounds anywhere in the Solar System, which may mean incoming comets provided Earth not only with its water supply but a crucial supply of organic compounds out of which life was able to evolve. The question is, how complex are these organic compounds, and just what are they?

Lunine said that the Pluto-Kuiper Express should thus have the importance of its optional extension to fly by one or two smaller Kuiper objects (or "KBOs") after Pluto reemphasized. But he also said that an examination of Pluto and its moon Charon themselves, the largest known Kuiper objects, is vital for these goals.

For one thing, there is some evidence that Pluto's surface has undergone geological processes over time which may have erased many of the olderst craters on its surface -- which means that by comparing its crater population with that of Charon and of smaller KBOs, we may gain some idea of the way in which the collision rates of objects in the Belt has changed over the eons, and thus of how rapidly the original smaller objects in the Belt coalesced or were absorbed into the giant planets (or flung into interstellar space by them).

For another thing, there is some tentative evidence that Pluto is actually an "embryonic" giant planet -- which was just starting to undergo the process of "runaway accumulation", in which a growing planet's gravity pulls more and more objects into itself, at the time that the gravitational tuggings of the already-existing giant planets finished clearing most of the original Kuiper objects out of the Belt and thus aborted Pluto's development.

To confirm this, we need to know more about Pluto's precise density -- and thus the size of its probable rocky core -- as compared to the densities of more typical Kuiper objects, since the heat from the greater number of objects crashing into Pluto probably drove off much of its lighter icy materials.

However, it's risky to use this argument to justify launching a Pluto probe in 2004, since a later Pluto mission -- even if the atmosphere has frozen by its arrival -- could still make crater counts and surface composition measurements, and allow study of most of the geological processes on Pluto (not to mention Charon and the smaller Kuiper objects, which have no atmosphere).