PARIS—A Russian Proton heavy-lift rocket on March 14 successfully placed the Euro-Russian ExoMars 2016 mission into orbit, a launch intended as the first half of a program whose second half, scheduled for 2018, remains in doubt.

While the ExoMars 2016 entry, descent and landing technology demonstrator and trace-gas-detecting orbiter have value in their own right, ExoMars 2016 was conceived as a mission to include a second liftoff, in 2018, of a European-built Mars rover and a Russian-provided surface science platform.

Notable among the rover’s payload was a drill to dig 2 meters below the Mars surface, far enough below Mars’s radiation-scoured surface to where vestiges of life may be found.

The heads of the European and Russian space agencies, meeting at Russia’s Baikonur Cosmodrome spaceport in Kazakhstan for the launch, on March 13 made vague references to challenges and constraints that could delay ExoMars 2018 to 2020.

During the Baikonur briefing, European Space Agency (ESA) Director-General Johann-Dietrich Woerner elected to place the second ExoMars launch in 2020 during his slide presentation, saying he didn’t want to “disappoint people if, under certain circumstances, we have to move it.” For a 2018 launch, he said, the schedule is “very tight.”

Igor A. Komarov, head of Russia’s Roscosmos, said Roscosmos and ESA were still discussing ExoMars 2018 and had made no decision. “There are some constraints with the launch in 2018,” Komarov said.

Russia’s principal contribution to ExoMars 2018 is a Proton launch. But its role in the experiment package of the 2018 mission is larger than what it is providing for the 2016 mission.

ESA has long said it will need a final tranche of financing to complete the ExoMars 2018 mission. The agency has said that, to assure the ExoMars industrial consortium can complete the work in time, ESA would need financial commitments from its governments by June at the latest. ESA officials have estimated that the program would need up to 200 million euros ($218 million) in additional financing to complete the 2018 hardware production and finance operations.

ExoMars’ biggest contributor is Italy, followed by Britain – which is leading the rover’s design and manufacturing – and France. The heads of all three nations’ space agencies were in Baikonur alongside Woerner and Komarov.

Presumably the European and Russian representatives used the opportunity to deliver candid assessments of whether there was still time to make the 2018 deadline – and if so, how much money would be needed.

In a March 14 interview, Woerner said it’s not just a question of money. It’s also possible that issues related to the development of the 2018 mission’s payload may force a delay to 2020 even if the financial hurdles are cleared. Woerner said he would present ESA governments with cost scenarios for both a 2018 launch, and a delay to 2020, by this summer.

None of this is new for ExoMars, a program that has progressed by fits and starts since it was first approved by ESA governments. ESA’s former director-general, Jean-Jacques Dordain, made it is personal priority to keep ExoMars from collapsing.

Despite Dordain’s support, which included creative use of ESA’s financing mechanisms, several ESA governments were skeptical that the Thales Alenia Space-led industrial team could make the 2016 launch date.

This was especially true since the industrial consortium had to make do with a few months’ financing at a time, with no guarantee that a further tranche would be approved. ExoMars’s budget is 1.2 billion euros.

But after a suspected component defect that delayed the launch by two months, pushing it to the edge of its 2016 launch window, the mission’s preparation went smoothly to final assembly and prelaunch preparations at the Baikonur facility.

ExoMars 2016 is scheduled to reach Mars in October, initially entering into a highly elliptical orbit of 300 kilometers by 96,000 kilometers. The trace gas orbiter is the scheduled to circularize its orbit at 400 kilometers in altitude after a series of maneuvers ending in November 2017.

The 600-kilogram Schiaparelli entry, descent and landing demonstrator is scheduled to be jettisoned on October 2016 and to land on the Mars surface three days later. Operating on internal battery power, it is expected to take measurements for several days. Its main purpose is to demonstrate, for future planetary exploration missions, Europe’s entry-and-descent capabilities.

Peter B. de Selding was the Paris bureau chief for SpaceNews.