LE BOURGET, France — The United States and Europe are moving closer toward a full-scale collaboration in Mars exploration but have agreed to go their separate ways, for now, in exploring the mysteries of dark energy, according to U.S. and European officials.

The science directors of NASA and the European Space Agency (ESA) are scheduled to meet in
the week of June 29 to craft an agreement calling for NASA to launch
‘s billion-dollar ExoMarslander and rover in early 2016 aboard an Atlas 5 rocket that will also carry NASA’s Mars Science Orbiter mission.

In a June 17 e-mail response to Space News inquiries, NASA spokesman Dwayne Brown said a prospective agreement would require that ExoMars lose enough weight to fit aboard the Atlas vehicle with the NASA orbiter.

“It will not be a simple task for NASA’s orbiter to carry the large lander ESA has designed,” Brown said. “Detailed studies and analyses are required. Also considered will be … the difficulty of the 2016 opportunity for landing due to orbital mechanics and dust storms.”

An Atlas launch would help solve a longstanding ExoMars financing problem at ESA. The mission’s current budget of 850 million euros ($1.192 billion) is insufficient to finance the experiment payload and a launch aboard a European Ariane 5 rocket. ESA has said its current ExoMars budget is around 200 million euros short of what’s needed to pursue the mission without outside help.

ESA has been negotiating with
for a Proton rocket launch, and for the commercial purchase, in
, of nuclear heaters to provide electrical power and keep the ExoMars rover instruments warm on the martian surface.

Under the agreement now taking shape with NASA, ESA would use U.S.-built nuclear heaters, known as radioisotope thermoelectric generators.

ESA Director-General Jean-Jacques Dordain, in a June 15 press briefing at the Paris Air Show here, said ExoMars will be unable to carry a key science package – the Humboldt suite of instruments to study martian geophysics – because of financial and weight-limit requirements. He voiced support for a long-term Mars exploration collaboration with NASA, starting with ExoMars.

ESA Science Director David Southwood said he hopes an initial letter of intent can be signed with NASA on an ExoMars collaboration during his June 29-30 meeting with Edward J. Weiler, NASA’s associate administrator for science.

NASA’s Brown said the meeting might produce the outline of such a letter, but that the two agencies “have not yet discussed an official letter of intent since the accommodation studies and independent reviews are not finished, and a final commitment on a 2016 partnership has not been made.” Brown confirmed NASA’s interest in making ExoMars “the first installment of a larger joint Mars exploration program between both agencies – a joint Mars Architecture looking at missions in 2016, 2018, leading to a joint Mars Sample Return mission sometime in the decade of the 2020s. These plans are predicated on a successful 2016 architecture.”

“This is a courtship,” Southwood said. “What we hope to accomplish is something that would be terrific for Europe, and for the United States. For a future mission like Mars Sample Return – even the
United States
cannot do that alone.”

But if NASA and ESA are moving closer on Mars exploration, the two agencies will be pursuing separate missions to investigate dark-energy sources. ESA has included the Euclid dark-energy mission among candidate missions to be evaluated late this year for future funding. Southwood said he has concluded that
cannot be combined with the Joint Dark Energy Mission (JDEM) planned by NASA and the U.S. Department of Energy.

The main problem, he said, is the calendar. Southwood said structuring a trans-Atlantic cooperative mission on dark energy in time to meet ESA’s competition deadline has proved impossible given the personnel changes in the
administration since January and the more-complex program management owing to the Department of Energy’s co-management of the program.

Jon Morse, NASA’s astrophysics director, agreed that integrating ESA as a full partner in the mission would be a challenge that would require high-level approval in
and at ESA and might threaten
‘s place in the ongoing competition among future ESA programs.

“They did not want to jeopardize
‘s standing in the competition” by starting a long process of ESA-NASA-Department of Energy negotiations, Morse said in a June 17 interview. “The discussions with ESA have been deferred until the next steps [in ESA’s future mission-selection process], but this does not preclude a future collaboration.”

Morse said NASA’s JDEM agreement with the Department of Energy, spelled out in a November 2008 memorandum of understanding, has not changed with the arrival of the administration of U.S. President Barack Obama. The memorandum identifies NASA as overall mission manager, and says: “In consultation with DOE, NASA will investigate the possibility for international in-kind contributions. NASA will be the principal point of contact in negotiation and conclusion of international agreements related to JDEM.”