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

NASA announced Wednesday that it had chosen two proposals for a mission to Pluto for additional study despite the fact that no funds for such a mission exist in the agency's budget for next year.

The announcement of the selection of Pluto Kuiper Belt (PKB) spacecraft proposals is the latest chapter in the saga of a mission concept that has faced cancellation more than once in the last year, but has managed to survive in part because of intense scientific and popular interest in a mission to the outermost planet.

"The PKB mission represents a possible opportunity to visit the only planet not yet explored by spacecraft," said Colleen Hartman, Pluto program director at NASA Headquarters. "It's really an opportunity to, in a sense, look into a deep-freeze of history which could tell us how our Solar System evolved to what it is today, including the precursor ingredients of life."

One proposal, called the "Pluto and Outer Solar System Explorer" (POSSE), is led by Larry Esposito of the University of Colorado in Boulder, and includes participants from JPL and several companies and universities. The other, titled "New Horizons: Shedding Light on Frontier Worlds", is led by Alan Stern of the Southwest Research Institute, also located in Boulder. Like POSSE, the New Horizons team includes participants from universities, companies, JPL, and NASA's Goddard Space Flight Center. No additional details about the proposals were released, and the principal investigators were unavailable for comment.

The teams behind the two winning proposals will each receive $450,000 from NASA to perform a three-month detailed study. NASA will then evaluate the results of those studies and determine if either mission should be selected for any future development, with an eye towards a launch between 2004 and 2006 and a flyby of Pluto before 2020.

The catch is that, at the present time, there is no funding available to build or launch a Pluto mission. No funding for a Pluto mission is included in President Bush's budget proposal for fiscal year 2002. NASA, however, is continuing to study a Pluto mission at the request of Congress, in the event that funding is added when Congress works on the budget later this year.

NASA had been working on a Pluto mission concept known as Pluto Kuiper Express (PKE) for several years as part of a family of spacecraft that included Europa orbiter and solar probe missions. However, concerned about spiraling cost estimates for those missions, NASA associate administrator Ed Weiler issued a stop-work order for PKE in September 2000, choosing instead to focus on the Europa Orbiter mission.

Both the professional scientific and the space activist communities greeted that decision with disapproval, noting that it was critical to visit the planet now before it moves farther away from the Sun in its elliptical orbit. Any lengthy delay could prevent scientists from studying the planet's tenuous atmosphere, which may freeze out in the next 20 years. Moreover, the planet's highly-inclined rotational axis and the geometry of its orbit means that more of the planet will be in permanent shadow in the coming decades, hiding it from view.

In December NASA backtracked, saying that it would instead solicit proposals for alternative, less expensive Pluto missions. Proposals were due in by mid-March, at which time they were to be evaluated and at least two selected for additional study. NASA planned to make a final decision on what proposal, if any, would be chosen to fly in August.

However, the Bush Administration's blueprint for the 2002 budget, released in late February, specifically excluded any funding for a Pluto mission. At that time NASA attempted to stop the competition, but changed their minds when the staff of the Senate Appropriations Committee specifically requested NASA to continue in the event that funding for such a mission was restored in the budget.

Congress has yet to take up in detail the proposed fiscal year 2002 budget for NASA, and the prospects for restoring funding are unknown. Organizations such as The Planetary Society are lobbying Congress to fund a Pluto mission: the society recently created a "Kids 4 Pluto Now!" section on its web site that invites children to send messages to members of Congress in support of such a mission. Individual efforts, such as plutomission.com, created by a Pennsylvania high school student, have also helped rally support for a mission.