By Jonathan Amos
Science reporter, BBC News
MSL would be expected to work on the surface for at least a year
Nasa is pushing ahead with plans to launch its next Mars mission in 2009, but acknowledges that extra funds are required to make it happen.
The Mars Science Laboratory (MSL) will be the biggest planetary rover yet; it will be the size of a Mini Cooper.
Engineers are grappling with a number of technical challenges, such as the complexity of the motors that will drive the vehicle across the surface.
The budget has already grown to $1.9bn from the original cost of $1.6bn.
Nasa will not currently say precisely how much extra cash is needed.
It is looking for support in Congress but is also assessing other missions to see if there is money that can be reallocated to MSL.
It needs the additional funding to make up delays in MSL's assembly schedule. Key bottlenecks centre on actuators - the motors that drive and turn the rover's wheels, and operate its robotic arm.
"Because of the mass of MSL and its size, those are reasonably complex motors and they're difficult to produce," explained Doug McCuistion, director of the Mars Exploration Programme at Nasa Headquarters.
"The lack of those deliveries puts schedule pressure - which in turn puts budget pressure - on the system."
The tools needed to do experiments on the surface of Mars, such as a drill, also require actuators, and these are delayed, too.
MSL is a "smart" rover that would be dropped on to the surface of the Red Planet by a rocket-powered "skycrane". At almost three metres in length and weighing 850kg, MSL is too big to land using the bouncing bags employed on a number of recent Mars missions.
The vehicle would be equipped with a small nuclear power pack to work its systems for at least one Martian year. MSL will do biology experiments as well extend the geology work currently being conducted by the Mars Exploration Rovers on the planet today.
The Mars Science Laboratory is designed to pave the way for a future mission that would return rocks to Earth.
Red Planet missions launch when Earth and Mars are favourably aligned. The best opportunities arise roughly every two years.
If MSL misses its 2009 slot, it faces a lengthy delay that is potentially much much more expensive than the additional staffing and resources needed to keep the project on its current schedule.
"This is a really important scientific mission," said Doug McCuistion.
MSL will be considerably bigger than any previous rover
"This is truly the push into the next decade for the Mars programme and for the discovery of the potential for life on other planets. And it's an extremely critical mission to further the science goals of the agency."
Dr Ed Weiler, the associate administrator for Nasa's science mission directorate, added: "We've poured over a billion and a half dollars into this.
"The science is critical, it's a flagship mission in the Mars programme; and as long as we think we have a good technical chance to make it, we're going to do what we have to do."
The current launch window for MSL runs from 15 September to 4 October, 2009. This would see the spacecraft arrive at Mars sometime between 10 July and 14 September, 2010.