A concept of the European Space Agency's ExoMars Rover. Credit: European Space Agency/AOES Medialab artist's concept

PARIS — The European Space Agency (ESA) has issued stop-work orders on contracts for its billion-dollar ExoMars program pending negotiations with NASA on how U.S. and European Mars rovers planned for launch in 2018 might be merged into one, ESA Science Director David Southwood said April 20.

For the 18-nation ESA, the partnership with NASA on Mars missions in 2016 and 2018 is a single budget line item called ExoMars.

That is not the case at NASA, whose current budget uncertainties have caused the U.S. agency to cancel its plans for a rover to accompany ESA’s on the mission to be launched in 2018 but have not affected — at least, not yet — the 2016 mission. That launch will carry a European-built Mars orbiter and a European-provided package to demonstrate entry, descent and landing (EDL) technologies.

The 2016 and 2018 missions will both be placed into orbit by United Launch Alliance-built Atlas 5 rockets purchased by NASA. Neither launch has yet been called into question by NASA.

In an interview, Southwood said that ESA member governments approved the ExoMars mission as an indivisible package that included the 2016 orbiter and EDL package plus the 2018 rover.

With the ESA rover now likely to mutate into a larger vehicle with significant NASA involvement, the division of responsibility for the 2018 mission will be subjected to a major modification.

The NASA decision to cancel its own rover and seek to place some of that vehicle’s mission on the ESA rover — a decision announced to ESA the week of March 29 — has caused ESA to issue the stop-work orders.

Southwood said the agency hopes to come to terms with NASA on a work-share division for the rover — and notably an agreement avoiding U.S. technology-transfer obstacles under ITAR, the U.S. International Traffic in Arms Regulations, which can block trade in items including Mars parachutes — in time to present a fresh budget to ESA’s financial authorities in early May.

A late-May meeting of ExoMars managers from the governments financing the mission would then be asked to endorse the program changes, work-share modifications and the new budget, Southwood said.

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