WASHINGTON - NASAs plan for exploring the outer planets is being tugged in two directions, with one group eager to make
Pluto the top priority and a second group just as strongly in favor of first launching a mission to Jupiters icy moon Europa. Scientists and space activists are pressuring NASA not to forfeit the chance to send a probe to Pluto in 2004, even if that means postponing the launch of the Europa mission for several years. Launching in 2004, they insist, is the best chance to avoid a two-decade wait for data about the most distant planet in the solar system.
"Europa doesnt lose much of anything by waiting," said Alan Stern, director of the department of space studies at the Southwest Research Institute, San Antonio, Texas. "But Pluto loses dramatically."
The other side, however, which counts among its supporters the senior NASA officials creating a new roadmap for the Outer Planets program, says it makes more sense to first tackle the Europa mission.
"They are both high priorities, but from the beginning of the Outer Planets program, it has been clear that the Europa Orbiter has been the linchpin mission," said Doug Stetson, program manager for solar system exploration at the Jet Propulsion Laboratory, Pasadena (JPL), California.
NASAs plan for exploring the outer planets had called for launching the Europa Orbiter in 2003 followed by the Pluto-Kuiper Express (PKE) in 2004. But that changed in September when
NASA directed JPL to stop work on the Pluto mission, citing concern about the technical challenges and rising cost of the project. NASA ordered JPL to concentrate instead on a plan for getting the Europa Orbiters costs under control while delaying the mission as little as possible. Arrival at Pluto, the only planet in the solar system not visited by spacecraft, would be delayed until no later than 2020, NASA officials said.
NASA's Pluto-Kuiper Express would be the first mission to visit the solar system's outermost planet.
NASA is not expected to decide before January which of the two missions will be the first to fly. However, the agency officials busy charting a new course for the troubled Outer Planets program have been laboring under the assumption that the Europa Orbiter -- not the Pluto-Kuiper Express -- will be the first of the two spacecraft to leave the launch pad.
Stetson said his team will present NASA Headquarters this month with several options for launching the Europa Orbiter as early as 2007 without busting the agencys budget of about $1.2 billion over five years for the Outer Planets program.
In every option developed by JPL, Stetson said, the Europa Orbiter would be launched first, followed several years later by a redesigned Pluto mission.
The primary objective of the Europa Orbiter mission is to verify that a liquid ocean lies beneath the moons icy crust. Some planetary scientists and astrobiologists contend that if life is to be found beyond Earth, Europa is a very good place to start the search. The possibility of such a scientifically tantalizing new frontier for future spacecraft missions helped NASA sell the Outer Planets program to the White House and Congress, several scientists said.
But other scientists argue that does not mean Pluto cannot be launched first. Michael Drake, a NASA science advisor and director of the department of planetary sciences at the University of Arizona in Tucson, said good science dictates that the Pluto Kuiper Express mission be the first Outer Planets mission off the launch pad.
"We need to return the Europa data first," Drake said. "But what we dont necessarily need to do is launch the Europa Orbiter first."
The flight to Europa would take about three years. The Pluto trip is expected to take at least seven years, and possibly much longer if the spacecraft is not launched until the end of the decade.
Drake said NASA should launch the Pluto probe in late 2004 to take advantage of a planetary alignment that would provide a maximum Jupiter gravity assist that would significantly reduce the travel time to Pluto. A 2004 launch would also give the mission
the best shot at viewing Plutos little-understood atmosphere before it freezes and collapses as the planet swings further into space in its 248-year orbit around the Sun. Unlike the other eight planets in the
solar system which have circular orbits, Pluto has a highly elliptical orbit that at apogee vastly increases its distance from the Sun and Earth. If JPL does not come up with a way to fit both missions into NASAs budget without pushing the Pluto-Kuiper Express out to the end of the decade, then NASA ought to consider opening the Pluto mission up to competition, Drake said.
"If JPL cannot do that after it scrubs its numbers, we believe it is important to open it up to serious competition to see if somebody could possibly find a way of implementing [Pluto-Kuiper Express] and Europa more effectively," Drake said.
JPLs Stetson is skeptical that reopening the competition would cure what ails both missions. "We dont think there is a significant cost reduction to be achieved through competition. Weve squeezed out as much as we can," he said.
Drake, who serves as chairman of NASAs Space Science Advisory Committees Solar System Exploration Subcommittee, made his case for moving the Pluto mission to the head of the line in a Nov. 27 letter to NASAs senior space science officials.
"There are parts of the letter that are unarguable," said Jay Bergstrahl, director of solar system exploration at NASA Headquarters here. "The part that there is no scientific reason for putting Europa ahead of [Pluto-Kuiper Express] is true."
But, Bergstrahl said, there are reasons for postponing the Pluto mission until later in the decade, including a concern that NASA may not be able to qualify an expendable rocket by 2004 to carry a nuclear-powered spacecraft like the Pluto-Kuiper Express.
Still, Bergstrahl said Drakes proposal is one of the options that will be considered when an executive committee briefs NASAs space science chief, Ed Weiler, on ideas for restructuring the Outer Planets program.
Colleen Hartman, deputy director of research in NASAs Office of Space Science and chairwoman of the Outer Planets Re-planning Committee, said nothing has been specifically ruled out. "What we are trying to do on the executive committee is pry the door open and make sure it stays open to any good ideas right to the end," she said.
Hartmans committee has not assigned anyone to analyze the cost and other programmatic implications of Drakes "Pluto first" proposal. "Right now we are concentrating on what it will take to do the Europa Orbiter mission," she said, adding that NASA Headquarters, JPL and the other centers have plenty of information on what the Pluto mission would cost.
"Its pretty clear at this point that we cannot afford to do [Pluto-Kuiper Express] now," she said. "The Europa Orbiter is first." Weiler is expected to decide on a new course that will put the Outer Planets program back on track in January.
"I get the impression that weve made a lot of headway with NASA and that the public interest [in Pluto] has really caught them a little bit by surprise," said Lou Friedman, director of the Pasadena, California-based Planetary Society, which mounted a campaign this fall to save PKE. "I get the feeling they are legitimately trying to be fair and consider all options."
Some proponents of the "Pluto first" approach said the future of the Outer Planets program hangs in the balance.
"NASA has to take a step back and look at what sequence of launches yields data returns from Europa and Pluto with the shortest possible interval between them," said Jonathan Lunine, a University of Arizona planetary science professor who chaired the PKE science definition team for NASA in 1995. "An Outer Planets program with a nine-to-10-year gap before the next mission returns data is not really a program."
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