Europe's flagship robotic rover mission to Mars now looks certain to leave Earth in 2018, two years later than recently proposed, the BBC understands.

BBC了解到，欧盟的火星探测旗舰级机器人2018年将会离开月球，这比最近一次发布的时间晚了两年。

Heavy rovers like Curiosity can be landed with a skycrane

The ExoMars vehicle is intended to search the Red Planet for signs of past or present life.

The delay is the third for the mission originally planned to launch in 2011.

While the switch will disappoint many people, officials say the change will open up a greatly expanded programme of exploration at the Red Planet.

The European Space Agency (Esa) will now join forces at Mars with the US space agency (Nasa). The two organisations believe they can achieve far more by combining their expertise¹（专家的意见，专门技术） and budgets.

The basis for this approach was agreed at bilateral²（双边的） discussions in Plymouth, UK, last month.

Since then, scientists and engineers on both sides of the Atlantic have been working up the basic architecture for a series of missions in 2016, 2018 and 2020 (launch opportunities to Mars come up roughly every two years).

Mass issues

The plan, or baseline, for this programme - including its implications（蕴涵式） for ExoMars - is now starting to emerge.

It would see the agencies launch a European orbiter to the Red Planet in 2016. Its main aim would be to track down the sources of methane³（甲烷） recently detected at Mars. The presence of methane is intriguing⁴（吸引人的，有趣的） because its likely origin is either present-day life or geological activity.

Confirmation⁵ of either would be a major discovery.

The American Atlas⁶ rocket used for this mission would also have capacity to carry sufficient mass to put some sort of static lander on the surface. The European orbiter would act as its data relay to Earth.

The 2018 launch opportunity would be taken by ExoMars, again launching on a US Atlas rocket. This mission window is actually one of the most favourable⁷ in terms of planetary alignment⁸（调整） for many years, and that makes it possible to send a very heavy surface mission.

The proposal on the table currently is that ExoMars should be joined by a slightly smaller rover（漂流，流浪） in the class of the US Spirit and Opportunity vehicles that are on the surface today.

ExoMars and its smaller cousin could be targeted at the Methane sources identified by the 2016 orbiter.

The 2020 launch opportunity would probably then be taken by a network of instrumented static landers.

Technological⁹ goals

Both Esa and Nasa will have tight finances going forward and will have to constrain¹⁰ their ambitions accordingly.

European ministers pledged sufficient monies（货币，钱） at their major triennial（每三年一次的事物） gathering¹¹ last year to take the budget for ExoMars to 850m euros. Esa officials believe the proposals they are formulating¹² with Nasa can broadly match the cost requirements and the technological goals of both parties.

For Europe, the primary goals are to land, to rove and to drill on Mars. However, under the plan outlined above, these objectives could not all be achieved during the ExoMars opportunity.

In 2018, it is likely the entry, descent and landing (EDL) of Europe's rover would be handled by the Americans, using the "skycrane" system they have designed for their big 2013 rover known as Curiosity.

If Europe really does want to do EDL, the option is open for it to take responsibility for the 2016 surface package of instruments.

Esa's director-general, Jean-Jacques Dordain, has promised to report to his member states in the autumn with firm proposals for a re-scoped Mars exploration programme.