Dr. Ed Weiler, NASA's associate administrator for space science, said that for programmatic and technical reasons it is not feasible to implement Pluto-Kuiper Express as currently conceived. Escalating cost was among the reasons he cited in congressional testimony.

Doug Stetson, manager of JPL's Solar System Exploration Office, said the earliest launch date now under consideration would be either 2009 or 2010. Such a mission would not include a gravity assist at Jupiter, as did the 2004 launch plan.

A launch at the end of the decade might include an advanced propulsion system-most likely either solar electric propulsion or solar sails-with a planned arrival at Pluto no later than 2020.

Another Pluto option under consideration, Stetson said, is the next available gravity assist at Jupiter, which would call for a launch in 2014 or 2015 with a projected eight-year flight.

Pluto, the farthest planet from the sun and the only planet never explored by a NASA spacecraft, has an orbit of almost 250 years around the sun. It came closest to the sun in 1989.

Stetson said some scientific models indicate that the planet's atmosphere will condense, freeze and collapse onto the surface of the planet, "but there's not a clear understanding of when that would happen."

Even if the atmosphere freezes, the mission would still be a scientifically compelling for studies of the planet and Kuiper Belt objects related to the origin of the solar system.

A strategic planning process for the next Pluto mission will involve the findings of NASA's Solar System Exploration Subcommittee, which will be assessing options and recommend to NASA Headquarters within the next two years, Stetson said.

In that regard, Dr. Earle Huckins, deputy associate administrator for NASA's Office of Space Science, led a team meeting at JPL Sept. 22 to begin replanning the outer solar system program. Dr. Jay Bergstralh, the acting solar system exploration director, has been leading a team looking at science objectives for outer solar system exploration.

"A better understanding for when we can launch a Pluto mission will be forthcoming in the context of the overall outer planets program," Stetson said.

"NASA has asked us to give them a plan that would allow a Europa orbiter launch in 2006 plus a plan for the technology developments that would allow arrival of a Pluto spacecraft no later than 2020.

In the meantime, we're developing technology for other missions we're considering for the program, including a comet nucleus sample return, a Europa lander and an atmospheric mobility mission at Saturn's moon Titan."

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