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

Bowing to pressure from both the scientific community and the general public, NASA gave new life Wednesday to prospects for a Pluto mission, saying it would solicit proposals for a revised mission to the outermost planet in our solar system.

NASA officials said during a Wednesday teleconference that they would soon accept proposals from both within and outside of the space agency for alternatives to a Pluto mission proposal NASA stopped work on in September.

A draft "Announcement of Opportunity" (AO) outlining NASA's requirements for a Pluto mission will be released on December 26 for comment, with a final version of the AO to be issued next month. Scientists, engineers, and others will then have until March 19 to submit their proposals to the space agency for peer review. At least two of the submitted proposals will be selected for additional study, with the final decision on what mission proposal -- if any -- to be made in August.

The decision marks the first time the space agency has opened up for competition a mission to the outer solar system: NASA has developed past missions entirely in-house. "In the past decade a number of organizations outside NASA have gained the expertise to successfully fly deep space missions, and in the past few months we have heard the calls from many in the scientific community in favor of open competition in our outer planet program," explained Ed Weiler, NASA associate administrator for space science. "We hope that opening these missions to competition will greatly benefit science and space exploration."

NASA had been working on its own Pluto mission project, the Pluto/Kuiper Express (PKE), at JPL. However, on September 12 the agency issued a "stop work" order halting progress on PKE, citing escalating costs that made it difficult for the agency to afford to work on both PKE and a sibling mission, Europa Orbiter.

"Basically, I directed the project to go full speed on Europa to give us the earliest possible launch of that mission, and spend the remaining money -- and there is significant money remaining -- on reformatting, reformulating the program," Weiler told a Congressional subcommittee on the day the stop work order was issued.

Although Weiler insisted at the time that the intent of the stop work order was only to defer the mission, many scientists and space activists saw the order as tantamount to cancellation. They argued that delaying PKE much beyond its scheduled 2004 launch date would prevent it from using a gravity assist swingby of Jupiter, increasing the flight time to Pluto.

Scientists are also eager to get to Pluto as soon as possible in order to observe the planet's tenuous atmosphere. As Pluto moves away from the Sun in its elliptical orbit -- the planet passed perihelion, the closest point in its orbit to the Sun, in 1989 -- the planet grows colder. Scientists believe the thin atmosphere, with a surface pressure just a few millionths that of the Earth's, may freeze out by about 2020. If the atmosphere freezes out before a spacecraft can study it, scientists would have to wait more than 200 years until Pluto returned close enough to the Sun for the atmosphere to reappear.

Because of the potential to miss out on a once-in-a-lifetime mission, planetary scientists, as well as members of the general public, lobbied NASA to resume work on PKE. Ten days after the stop-work order was issued, the American Astronomical Society's Division for Planetary Sciences (DPS), the world's largest organization of planetary scientists and astronomers, publicly called on NASA to reconsider its decision. Expressing "serious concerns" about the decision to stop work on the project, the DPS called upon NASA and Congress "to find a way to fund this important mission, but not at the expense of other equally important planetary missions."

This is the clearest view available of the distant Pluto and its moon, Charon, as revealed by NASA's Hubble Space Telescope in 1994 when the planet was 2.6 billion miles from Earth, or nearly 30 times the separation between Earth and the sun. Photo: NASA/ESA

In November a letter to NASA from Michael Drake, a University of Arizona scientist and chair of an advisory committee studying NASA's space science programs, said that both PKE and Europa Orbiter should be the agency's highest priority outer solar system missions. "This is a mission to the frontier of the solar system," Drake wrote of PKE, "an appealing aspect to both scientists and the public."

The Planetary Society also called on its members to contact NASA and the Congress to urge them to find a way to resume the mission. More than 10,000 members sent in postcards to Congress to express their support for the mission. Besides the Planetary Society effort, Ted Nichols, a Pennsylvania high school student, established a web site at www.plutomission.com to rally support for the mission. More than 4,000 people signed an online petition at the site, attracting the attention of the international press as well as NASA itself, who invited Nichols to Washington to meet with agency officials.

Those scientists and activists who had lobbied in support of a Pluto mission considered Wednesday's announcement a victory. "We are gratified that NASA is trying to restore the Pluto mission to its launch schedule," Lou Friedman, executive director of the Planetary Society, said in a statement. "Public interest in exploring the only unvisited planet of our solar system is high, much higher than NASA may have realized. So, too, is the scientific interest -- Pluto and the Kuiper Belt hold key clues to the origin of the solar system."

"I think NASA was floored by the response from all quarters," said Alan Stern, director of the Department of Space Studies of the Southwest Research Institute and a longtime proponent of a Pluto mission. "They did the right thing, too."

The belief that there is strong public support for a Pluto mission was backed up on Wednesday by a new poll. The poll, conducted by Roper Starch Worldwide for Sky & Telescope magazine, found that 58% of Americans surveyed supported a Pluto mission, while 64% supported the Europa Orbiter mission. "Our poll shows that Americans want NASA to carry out both programs," said J. Kelly Beatty, Sky & Telescope's executive editor. "People have an insatiable appetite for interplanetary exploration, and these two missions would be dramatic voyages of discovery."

The challenge that now faces supporters of Pluto exploration is designing a spacecraft that can study the planet while meeting NASA's cost and schedule requirements. While the combined cost of both the Pluto and Europa missions had ballooned to twice the original estimate of $700 million, NASA is putting a cap of $500 million for the new Pluto mission proposals. While the agency doesn't have a required launch date for the mission, officials said Wednesday they would like to have the spacecraft reach Pluto by 2015.

"It'll be tough, as this is a long mission far away" Stern said of the work needed to fit the mission under the cost cap, "but it might just be doable."

For now, though, mission advocates are enjoying their revival of a mission to the only planet not yet explored by spacecraft. "NASA now understands the worth and public interest of exploring Pluto, and has given it a higher priority," said Nichols, who plans to retool his plutomission.com web site from "Save the Mission" to a "We Support the Mission" theme. "I don't think we could have been given a better Christmas present."