For Pluto, discovered in 1930, the visit would be the first by a spacecraft. The most distant planet, Pluto is also the smallest just 1,400 miles (2,300 kilometers) in diameter.
But cost overruns on a wide array of NASA missions, including a doubling in cost of one unnamed project, are forcing the American space agency to consider paring down the fleet of spacecraft it intends to launch in the near term. Pluto-Kuiper Express is target number one.
In the wake of the loss of two NASA spacecraft at Mars last year, the agency has pulled back on what levels of risk it considers acceptable in piecing together its missions. And with lowered risk levels, costs have increased.
"Recent revisions of a lot of budgets of projects have come in at considerably higher numbers and that has caused a lot of concern," NASA spokesman Don Savage said. "There are now discussions about what can be done about it."
If the situation holds, NASA Associate Administrator for Space Science Ed Weiler said last week, according to one source, a major mission will face the chopping block.
"There was an awful lot of red," said the source, who sat in on a review of missions. "Weiler's got his hands full."
Absent the budgetary crunch, the Pluto mission, along with the Europa Orbiter and Solar Probe, already face their own particular problems, further complicating the issue.
The three separate missions are jointly managed by NASAs Jet Propulsion Laboratory (JPL) as the Outer Planets/Solar Probe Project. Although the three have different final targets, all share some hardware and will involve trajectories that take them looping around Jupiter.
And all three also share two distinct problems: They don't have launch vehicles and are lacking an electrical power source.
"Thats a real problem for the two heading permanently for the outer solar system," Drake said.
NASA officials want to see an Evolved Expendable Launch Vehicle (EELV) fly five to six times before the space agency attempts launching one of these missions atop the next-generation rocket. That seems unlikely to happen before 2004.
"You dont want to squander the publics money on something thats going to explode," Drake said.
And the Department of Energy now reports that work on a power source replacement for the traditional plutonium 238 radioisotope thermoelectric generators (RTGs) called the Advanced Radiological Power Source (ARPS) has not been productive, yielding lower than predicted output.
If a Europa Orbiter were to fly, some thought is now being given to using a more expensive Titan 4 launcher and using leftover Cassini-era RTGs. If the plutonium were used for Europa Orbiter, that means little, if any, would remain for the Pluto-Kuiper Express, effectively spelling its doom. (Were the mission simply delayed, it would be for a full 12 years, until it could use Jupiter again to gain a much-needed gravity boost to Pluto.)
That prospect has prompted the Pasadena, California-based Planetary Society to start a grass-roots campaign to save the Pluto mission. The group called the threatened cancellation a "political budgetary decision."
"For four decades we have sent missions of exploration into space, from heat-seared Mercury to the blue wonder of Neptune. What will it say of our generation and our lack of wonder and curiosity if we stop now, right before exploring Pluto, the last outpost planet of our solar system?" said Louis Friedman, the societys executive director.
Harsh as it may be, the cancellation of a single project is still preferable to losing an entire class of missions, said David Smith, study director for the Space Studies Board's Committee on Planetary and Lunar Exploration.
NASA has established thematic classes, or lines, of missions, like the Mars Surveyor program, that keeps the agency from having to go back each year to Congress looking for money. Congress funds these lines with NASA deciding on how best to spend the monies.
"If there's any danger in loosing a line, then canceling a spacecraft is certainly the lesser of the two evils," Smith told SPACE.com.
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