An artist's concept of the Pluto-Kuiper Express spacecraft. Photo: NASA/JPL

Planetary scientists and space activists seeking support for a mission to Pluto won a major victory late last week when a U.S. Senate committee approved a NASA budget that includes some funding for such a mission.

The Senate Appropriations Committee approved a fiscal year 2002 budget for NASA on Thursday that includes $25 million for a Pluto mission by taking an equal amount away from a program to develop advanced propulsion technologies for a future Pluto mission.

"The Committee has deferred, without prejudice, the inclusion of full funding for the PKE," the committee noted in a report accompanying the budget bill. "It has however, included $25,000,000 for it by eliminating the proposed $25,000,000 for the Œquick sprint to Pluto' propulsion initiative contained in the core research and technology line for solar system exploration."

The committee, in effect, overturned a decision by NASA and the Bush Administration earlier this year to cancel the mission. At the time, NASA said it would instead seek funding for a program to develop advanced propulsion technologies, such as ion drives, that would enable a "quick sprint" to Pluto before 2020.

The decision to fund propulsion work rather that a spacecraft mission has been sharply criticized by scientists who support a Pluto mission. "The idea that a new propulsion system might be invented and implemented in the short term which would enable a mission to Pluto for a lower cost than using current effective technology has no basis in the history of space exploration," the Division of Planetary Sciences (DPS) of the American Astronomical Society claimed in a statement released Monday. "If anything, reliance on such a system will drive up mission costs to a point of impracticality."

The $25 million transferred from the propulsion program to the mission is only a small fraction of the overall cost of the mission, currently estimated by NASA at $500 million. Scientists involved with the development of two competing mission proposals say they're not sure yet whether the $25 million will be enough to sustain work on the mission for the full year.

There is the possibility, though, that more money will be found for the mission. The version of the NASA budget bill in the House of Representatives has a number of differences compared to the Senate version, including no funding for a Pluto mission. Once the full House and Senate pass their respective versions of the budget, a joint conference committee will meet to iron out the differences. At that time, the Senate Appropriations Committee report stated, "the Committee expects to address the issue of full funding for PKE [Pluto-Kuiper Express]."

Those involved with potential Pluto missions are hopeful that the Senate's action will keep a Pluto mission alive. "I'm optimistic," said Alan Stern, a longtime advocate of a Pluto mission and principal investigator of one of two Pluto mission concepts NASA selected for further study last month. "The Senate's support seems very strong."

Scientists and other Pluto mission supporters have been fighting hard for a mission since last September, when NASA issued a stop-work order on the Pluto-Kuiper Express mission under development at JPL. In December, NASA partially reversed its decision, saying it would accept proposals from outside the agency for a new Pluto mission that would cost no more than $500 million and arrive at Pluto no later than 2020.

Even while scientists were working on those proposals, though, NASA backtracked again, deciding in February fund propulsion technology for a future "sprint" mission to Pluto rather than a successor to PKE. At the request of the Senate, though, NASA continued the competition for mission proposals, and in June selected two proposals for three months of additional study.

Scientists have made a Pluto mission a priority because of the rare opportunity to study the planet and its thin atmosphere. "Pluto is moving away from the Sun, and its tenuous yet complex atmosphere may freeze out before we have a chance to study it in situ," the DPS explained in its statement Monday. "The Sun is moving towards higher latitudes on both Pluto and its moon, plunging more and more of their northern hemispheres into darkness, decreasing the amount of surface visible to a passing spacecraft."

"We are losing an opportunity which will not recur for 200 years," the DPS noted. "Time is of the essence."

Those involved with the Pluto mission proposals are confident their spacecraft can meet the scientific goals of such a mission given the stringent funding and schedule constraints. "We are really becoming convinced this is doable on schedule and on cost," said Stern.

There are limits, though, that even the staunchest supporters of a Pluto mission will not exceed in their efforts to fund the mission. "If Congress is unable to augment the NASA budget request by the amount needed to ensure the timely launch of PKB [Pluto-Kuiper Belt mission]," the DPS stated, "we request that no direction regarding PKB be made: the lost opportunity, while painful, does not justify the sacrifice of other NASA space science programs."