By Jonathan Amos
BBC News Online science staff, in Farnborough
The UK faces some tough choices if it wants to commit itself to a major programme to explore the Red Planet.
Aurora would put a rover on the Martian surface
It must decide in the next few months whether to spend about £25m a year, to get leading positions on missions that would land on Mars and return rocks.
But it is likely this investment could only be made if British participation is cut back in other areas, such as efforts to build giant telescopes.
Funding officials say some researchers are bound to be left disappointed.
"Money for scientific research is a scarce resource and no matter what you do, you are going to end up in difficult positions making priorities," said Professor Ian Halliday, the chief executive of the Particle Physics and Astronomy Research Council (PParc).
He was speaking at a Space Day event at the Farnborough International 2004 air show, where UK scientists were putting the case for Britain's full participation in Aurora, a European programme to send probes to Mars and beyond.
They feel the UK is in a prime position to lead several Aurora missions because of the expertise it acquired on the ill-fated Beagle lander.
Indeed, they think it would be a waste of the £45m spent on Beagle if that knowledge was not now taken forward on to other missions.
The European Space Agency (Esa) has told Britain that if it really wants a front seat on Aurora it must commit by 30 September about four or five million pounds and then a further £25m year after year from 2006 onwards.
Europe plans a huge telescope with a mirror 100m across
Despite the substantially increased monies going into science now under the government's new 10-year strategy, Professor Halliday said PParc was unlikely to be able to satisfy all of the demands being made of the funding agency.
British particle physicists would like to play an important role in the next generation linear accelerator to study fundamentals of matter.
Astronomers also want key influence in another international project to build a 100m-wide telescope that could resolve Earth-like planets around distant stars.
"There is real competition," Professor Halliday said. "The possibility of doing all three at a very high level, I don't think is high.
"In there, there is an either or [decision] and that pressure will go down into smaller projects."
But planetary scientists are fired up by the immense public interest generated by Beagle and the US rovers, and are lobbying hard for a positive Aurora commitment.
It could mean Britain's "subscriptions" through the Esa rising from £40m a year to £65m.
Dr Sarah Dunkin, from the Rutherford Appleton Laboratory, said the increase was significant but the payback to the British economy would be immense.
"We have a lack of cool role models to encourage young people into science and technology areas," she argued.
"By participating in a programme like this, it would be an enormous inspiration to our young. If the UK does not participate, a lot of our brilliant scientists and engineers will walk to another country that is taking part.
"To join Aurora at a leading level would cost 50 pence per head per year - the price of a king-sized Mars bar.
"To put that in some kind of context, we already spend £40 per head per year on grooming products - skin moisturizes and make-up products."
The UK is likely to ask Esa for more time beyond 30 September to make a decision on Aurora.
The implications of recent government spending announcements will take a while to work through, PParc says.
"We have put a lot of money into science because we believe this to be very important," Science Minister Lord Sainsbury told BBC News Online.
"What we're now in the process of doing is dividing that money up between the different parts of the scientific research system.
"We think Aurora is an important programme and that is one of the factors that will be taken into account when we divide up the budget."