PARIS — The 20-nation European Space Agency (ESA), which has been struggling with its governments for more than five years to finance the two-launch ExoMars mission to Mars, on June 17 will sign a final contract for the full 2016 mission and a part of its 2018 companion.

On June 18-19, Europe’s Science Program Committee, which has been solicited to make an exceptional contribution to ExoMars despite the mission’s being conceived and managed outside Europe’s space science program, is expected to approve an investment of 50 million euros ($65 million) in ExoMars, European government and industry officials said.

The June 17 contract will be signed during the Paris Air Show by ESA and ExoMars’ prime contractor, Thales Alenia Space of France and Italy. The contract’s nominal value is some 650 million euros, but ESA has been parceling out contract increments, often at three-month intervals, to Thales Alenia Space during the past three years so as not to lose the 2016 launch date.

Winning its member governments’ occasionally grudging approval every few months, ESA has already spent nearly 500 million euros on ExoMars, giving the program the heft, if not the formal status, of an approved ESA mission.

ESA Science Director Alvaro Gimenez said the agency is persuaded that Thales Alenia Space and its industrial team has made good on its promise to deliver the hardware needed for the January 2016 launch in time for its integration and launch aboard a Russian Proton rocket.

In a June 4 interview, Gimenez said he would not ask the Science Program Committee to approve contributing to ExoMars as a “Mission of Opportunity” were he not certain that the 2016 launch date, which has always been the most critical factor in the mission, would be respected.

“These guys have done an amazing job under difficult circumstances,” Gimenez said of the ExoMars industrial team and Thales Alenia Space’s ability to maintain forward motion on the program despite the stop-and-go funding.

“That is all over — finally,” Gimenez said of the three-month funding increments. “This three-month funding is really not the way to manage a program, but the project is advancing very well.”

Vincenzo Giorgio, director of exploration and science at Thales Alenia Space Italy, said the drip-fed funding schedule led to several “near-death” experiences for ExoMars, especially as Thales Alenia Space fought to keep its many subcontractors from abandoning the program for lack of payment.

“This was no easy thing to manage, but it was especially difficult for the smaller members of the contracting team,” Giorgio said in a June 6 interview. “We tried to organize their work as efficiently as possible, and of course they wanted to stay with a program as big as ExoMars. But it’s clear that this kind of funding scheme should not be repeated. There have been negative effects and it has cost everyone involved.”

One of the problems ExoMars had with several ESA delegations is that some thought Thales Alenia Space in any event could not make the 2016 deadline. Without the 2016 mission, which will place a telecommunications relay and an atmospheric-gas analyzer into Mars orbit, plus an ESA entry, descent and landing module, the 2018 rover mission would have problems sending data back to Earth.

It is for this reason that ExoMars has always been viewed at ESA as a single mission.

Russia’s arrival as a full partner in ExoMars preserved the mission after NASA’s partial withdrawal, but it also added some 200 million euros in total costs to ESA because the European agency has a larger role in the 2018 mission than it had when NASA was providing the two launches.

ESA has said total ExoMars mission costs are expected to be 1.2 billion euros. It has raised, so far, about 850 million euros from its delegations. Led by its director-general, Jean-Jacques Dordain, who has taken on ExoMars as a cause, the agency has found pockets of funding from a variety of sources but is still missing at least 150 million euros to complete its share of the project.

Gimenez agreed that this was the case, but said the remaining funds will not be needed until 2014. For now, he said, the full 2016 package is funded, with portions of 2018 also paid for. The contract to be signed June 17 notably continues work on the European rover and a Europe-provided carrier that will house both the rover and a Russian-provided descent module in 2018.

Dordain has been working with Russia’s Roscosmos space agency to see if Russia could partner with ESA in the exploration of Jupiter, a partnership that would include a Russian Proton launch of ESA’s Juice science mission in 2022.

If an accord can be reached, ESA will return to the Science Program Committee in 2014 to ask that monies the committee would otherwise set aside for Juice’s launch on a European Ariane 5 rocket be spent on ExoMars.

ExoMars is not the only Mars mission he will discuss with the Science Program Committee. The agency’s 10-year-old Mars Express orbiter, designed to operate for only two years, remains in good health and Gimenez said he will ask for a further two years of operations, to 2016.

The cost to ESA is about 2 million euros per year. ESA still has many plans for Mars Express, including an October 2014 encounter with Comet C/2013 Siding Spring in addition to helping with landing site selection for the ExoMars drill-equipped rover.

Peter B. de Selding was the Paris Bureau Chief for SpaceNews.