NASA and the European Space Agency have agreed on a strategic partnership for future robotic missions to explore Mars, officials announced last week.

Ed Weiler, the associate administrator for NASA's science mission directorate, unveiled the plan during a Thursday news conference announcing a two-year delay of the agency's next Mars rover.

An artist's concept of Europe's proposed ExoMars rover. Credit: ESA

The deal was finalized last Wednesday during a meeting between Weiler and David Southwood, director of ESA's science programs.

Officials started discussing the alliance in July, when the two leaders participated in an annual meeting between NASA and ESA.

The agencies have already worked together on previous Mars missions, including radars on ESA's Mars Express probe and NASA's Mars Reconnaissance Orbiter. Mars Express was also used as a communications relay station for NASA's Phoenix lander.

The partnership will help the agencies share mounting costs and responsibilities for future Mars missions.

"These missions get more and more expensive," Weiler said. "The easy stuff has been done."

The next two Mars rovers, NASA's Mars Science Laboratory and ESA's ExoMars mission, are multi-billion dollar flagship missions.

"It will be a long term cooperation where NASA could contribute to an ESA-led mission, like ExoMars, and ESA could contribute to NASA-led missions," said Jean-Jacques Dordain, ESA director general.

The partnership will help lay the groundwork for the Mars Sample Return mission, which is likely to cost between $6 billion and $8 billion, according to Weiler.

"The long term goal is MSR but there are steps on the way and we intend to take them together," Southwood said. "Mars is a diverse place. We need a lot of exploration, surveying and analysis before we commit to bring samples back."

Those steps include missions like MSL and ExoMars, in addition to other probes still awaiting selection for launch late in the next decade.

ExoMars depends on international participation outside Europe because ESA's member states have not committed to providing more than 1 billion euros, or about $1.3 billion, for the mission, Dordain said.

The total cost of ExoMars is projected to be more than 1.2 billion euros, or more than $1.5 billion, according to ESA officials.

That extra money will have to come from contributions from international partners, such as NASA or Russia.

"(If) I cannot succeed in getting all this cooperation, then I shall have to see how we can adjust the mission to stay within the 1 billion euro maximum that the member states have given me," Dordain said.

ExoMars is scheduled to launch in January 2016. European officials postponed the probe's launch from 2013 earlier this year.

Dordain said he is waiting until late next year to request a firm funding commitment from ESA member states. The probe's design and international agreements should be in place by then, he said.

The delay of MSL's launch will allow scientists to focus on developing the joint Mars architecture, including deciding details on NASA's involvement in ExoMars.

"We don't have to rush to come up with some idea for 2016," Weiler said. "We could probably do a heck of a lot better mission if we did it together than if we continued to compete with each other."

Officials expect the sample return mission to occur after 2020, considering the current budget and economic conditions.

"We will never, ever do a sample return mission unless we work together," Weiler said. "We both have the same goals scientifically. We want to get our science communities together and start laying out an architecture."

A mission to retrieve samples from Mars has long been on the wish list of U.S. and European scientists. But those plans have been repeatedly delayed due to technical and funding hurdles.

"For the sample-return mission, we've got to go to Mars, we've got to land, we've got to have a rocket that takes off, we've got to rendezvous with an orbiter, it's got to get the samples, then it's got to get to the Earth and it's got to land safely. Other than that, it's a piece of cake," Weiler said.

The partnership will allow the agencies to "pool" their resources to help tackle the immense challenges involved with such a mission, Southwood said.

Southwood said the agreement will also give scientists more opportunities to fly their experiments to the Red Planet.