By Jonathan Amos
Science reporter, BBC News, Le Bourget
Teams right across Europe are involved in the ExoMars project
The European Space Agency says its flagship Mars mission will lose a major instrument package to contain costs.
The Exomars venture will launch a rover to the Red Planet in 2016, to search for signs of past or present life.
It was hoped a static science payload called Humboldt could also be put on the surface to study the weather and, for example, listen for "Marsquakes".
But agency officials announced at the Paris air show that financial constraints now made this impossible.
Esa director-general Jean-Jacques Dordain said de-scoping ExoMars would also give extra margin to engineers who were concerned that the rover's design was pushing the limit of the maximum possible mass for the mission.
Mr Dordain said the loss of Humboldt was inevitable given the promise he had made to European governments in November last year to keep the cost of the project as close as possible to 850m euros.
He told the BBC he had given the Esa ExoMars development team three objectives: "To stay within the calendar; to try and stay within the money we have collected in November; and to keep the technology which I wish to demonstrate on Mars, which is landing, because we have never landed on Mars; moving on the surface; and drilling, because nobody has done that."
He said it was also likely now that the US would play a significant role in the endeavour, further limiting the financial burden on European taxpayers.
The American space agency (Nasa) has its own money worries and is keen to share the cost of Mars exploration with Europe.
Nasa will sign a "letter of intent" to this effect at a bilateral meeting in Plymouth, UK, on 30 June.
This would mean all future Red Planet missions being badged Nasa/Esa projects.
On ExoMars, the US is now set to provide the launcher - an Atlas rocket. It will also probably build the carrier spacecraft that delivers the rover to Red Planet; and the orbiter which will circle above Mars and relay its data back to Earth. It is possible, though, the US could source many elements from European industry.
The US offer represents a considerable investment, but the quid pro quo is that European money will then be put into future US-led missions.
"If Nasa is ready to contribute as much to ExoMars, it is a clear demonstration that ExoMars is important," said Mr Dordain.
Humboldt's omission from ExoMars will be a bitter blow to its scientists.
It was intended to study the surface environment and the geophysics of the deep interior.
Its sensors were being designed to undertake - among other things - meteorological investigations and an assessment of the radiation conditions on Mars.
Seismometers would have revealed remarkable new insights into the nature of Mars' geological structure.
Esa's science director, David Southwood, told the BBC that the scientific imperative for Humboldt was clear.
He said the science it would deliver was a necessary first step towards the agency's eventual goal of sending a mission to the planet that could fetch rocks back to Earth for detailed study.
"I'm absolutely confident we will see the elements of the Humboldt payload eventually deployed on Mars, but probably in a more dedicated circumstance," explained Professor Southwood.
"For instance, instead of having one stationary station, is it not better if you are looking at an entire planet to have multiple stations?"
Changes are necessary also to Esa's Bepi-Colombo mission to Mercury, which, like ExoMars, is due to leave Earth next decade.
Engineers are grappling with various technical problems as they try to design a spacecraft capable of withstanding the high heat and radiation experienced near the innermost planet.
The mission is going to be much heavier than planned, which will require a bigger, more expensive rocket.
"We are working with the industrial team to make a revised proposal to the member states, and especially to commit on the 'cost at completion'; because, as you know, the technical challenge we met with that mission [meant] we had to move from a Soyuz-class mission to an Ariane-class mission," Mr Dordain said.
The cost at completion - the full and final cost of the mission - is currently looking to be 904m euros, at 2007 prices.
The director general also announced that the first Soyuz rocket launch from Europe's spaceport in French Guiana would now take place in the "first few weeks" of 2010. The timeline had slipped because of delays in the construction of the new launch facilities in Guiana.
In addition, there is a delay of "several months" in the schedule leading to the maiden flight of Europe's small Vega rocket. The testing of all the launcher's new systems is taking longer than planned.
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