The European Space Agency's selection of its next generation of space science probes will depend on upcoming decisions before science and budget panels in the United States and Japan.

Artist's concept of Solar Orbiter, a proposed mission to study the sun up-close. Credit: Max Planck Institute for Solar System Research

In a world of tightening government budgets, space agencies often rely on each other to fill in funding gaps on future missions. Europe is looking to the United States and Japan for cooperation, according to David Southwood, ESA's director of science and robotic exploration.

International partnerships are often required to make space missions a reality, but ESA is waiting on the outcome of a powerful board of U.S. scientists and Japan's sluggish budget process.

NASA and Japan are likewise standing by for an ESA decision on its next group of missions.

ESA has already selected finalists for the first launch slots of the agency's Cosmic Vision program, which is overseeing prospective space science missions between 2015 and 2025.

Capped at a total cost of 475 million euros, or $605 million, the medium-class candidates include Solar Orbiter, the Euclid dark energy mission, and the Plato telescope to search for extrasolar planets.

Three more ambitious candidate missions are under study. The finalists are an orbiter to Jupiter called Laplace, the International X-ray Observatory, and the LISA probe to study gravity waves. The L-class missions are supposed to cost less than 650 million euros, or $828 million.

A decision is due next year on two M-class missions launching in 2017 and 2018. One L-class mission will be approved for implementation for launch in 2020.

The Space Program Committee, made up of top European scientists, will make the final call on what missions will proceed toward full development and launch. The Space Science Advisory Committee and other working groups will recommend missions based on scientific and technical merit.

"Obviously, if a mission is clearly not feasible without a partner and there is no credible partner identified, this will impact the recommendation of the advisory structure," said Willy Benz, chairman of the Space Science Advisory Committee. "Hence, if a mission is not supported by an agency there is not much point to push it unless ESA could do it alone."

Benz is also director of the Physics Institute at the University of Bern in Switzerland.

"After all, a collaboration implies at least two partner willing to do work for the same goal," Benz told Spaceflight Now. "On the other hand, it is not to the advisory structure to negotiate which elements of a mission will be done in collaboration nor the extent of such a collaboration. Our main concern is scientific excellence and technological feasibility."

Political considerations are left up to ESA and the decision-making Science Program Committee.

Most of the proposed Cosmic Vision missions will need some level of international support, Southwood said.

It's difficult for any space agency to map out future science missions, especially when managers must consider the interests of other nations.

NASA chooses its top-priority missions from the results of the National Academy of Sciences decadal survey, a ranking of potential missions based on scientific, technical readiness and cost.

Artist's concept of the International X-ray Observatory, a cooperative project between NASA, ESA and JAXA. Credit: NASA

"We've got some major decisions to make about a year from now, and how we orchestrate the cooperation with the United States depends on how the decadal survey chooses priorities for NASA to shoot at," Southwood said.

A panel of U.S. researchers is currently reviewing 28 missions and will report their findings and recommendations in March 2011. The results will give NASA a blueprint for planetary exploration between 2013 and 2022.

"It doesn't mean we will follow the decadal survey," Southwood said. "It means we understand our American friends will have to."

The decadal survey should be released just in time for ESA's decision on the first three Cosmic Vision missions.

ESA is planning Solar Orbiter and the International X-ray Observatory assuming NASA participation. NASA's baseline for both missions also count on European involvement.

Europe's contribution to the tandem mission to Jupiter will include an orbiter of the planet's moon Ganymede. NASA is planning a probe to circle Jupiter's ice-covered moon Europa.

Southwood said the Euclid dark energy spacecraft would also likely warrant U.S. cooperation. Other ESA finalist missions could also be opened up to international partnerships.

The decadal survey results will also help decide whether NASA and ESA will first launch the Jupiter probes or a costly sample return mission to Mars. Both missions have fundamental links on both sides of the Atlantic Ocean.

Providing an example of the international worries facing modern space missions, Southwood said a powerful infrared telescope named SPICA was passed over during the selection process for finalist M-class missions last year.

ESA planned to contribute about one-third of SPICA's total cost, but the Japanese government was not ready to commit to the mission as Europe selected their Cosmic Vision candidates.

"We're trying to match our plans to those of the Japanese," Southwood said. "We would look kind of foolish all dressed up at the wedding when the bride-groom hasn't left his home."

SPICA would be an important tool for astronomers, providing imaging power comparable to Europe's flagship Herschel telescope now operating in space.

"There's a protocol here," Southwood said. "The Japanese have to lead. We have to be ready to say to the Japanese, yes, we'll go with you. But we've got to match our development and our decision-making process to when the Japanese are ready to make their decisions."

Artist's concept of SPICA. Credit: JAXA

ESA could still participate in the SPICA mission and will make a final decision by the end of 2011.

Another proposed ESA mission, named Marco Polo, was also turned down by Europe's space science advisory committee due to worries Japan could not meet its commitment to the project.

Marco Polo was supposed to build upon Japan's successful Hayabusa mission that returned to Earth last month, possibly carrying samples from the surface of an asteroid. The joint Marco Polo mission would have also returned rocks and dust from an asteroid.

"One thing that we're very interested in is the technology of picking up a sample and bringing it back to the surface of the Earth from a celestial body," Southwood said.

But Japan's follow-up missions to Hayabusa have been at the mercy of budget concerns.

"The Japanese element (of Marco Polo) wasn't there at the time of M-class downselection," Southwood said. "It was over-budget."

The Japanese Aerospace Exploration Agency last week announced it submitted a proposal to the country's Space Activities Commission for a less ambitious follow-on to Hayabusa costing about $310 million.

The commission and Japanese government will decide the fate of Hayabusa 2 when the JAXA budget is formulated later this year.

The Hayabusa 2 craft would blast off in 2014 or 2015 and visit a primitive asteroid, collect samples and return to Earth.

Benz the said the Cosmic Vision M-class proposal eliminated next year could be reconsidered in the next round of selections, which will soon begin for another M-class mission. ESA plans to release a call for proposals by the end of July for the third M-class project for launch in 2022.