The joint Mars exploration envisioned by the US and Europe is set for an overhaul, following an announcement by the Americans that their part of the budget is critically short of funds.
Nasa and Esa had agreed to send two rovers to the Red Planet in 2018.
In Europe's case, this vehicle is already designed and about to be built.
But a new report from the US National Research Council says the probable $3.5bn (ã2.2bn) cost of the American side of the mission is $1bn too high.
The planetary decadal survey - which is only an advisory document at this stage - recommends the effort be scaled back or postponed indefinitely.
As an example scenario of how the mission could be modified - or de-scoped - to fit within the new suggested budget, the report considers the situation in which the European rover is simply left behind on Earth.
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Such a one-sided outcome from a revision process is not thought to be likely, but it gives a sense of the difficulties Nasa and Esa now face in developing a joint initiative to land and rove on Mars later this decade.
We're quite confident that a really good mission can be done for $2.5bn, said the survey's chairman Professor Steve Squyres, but we leave it to those two agencies to work out exactly the details of what that would look like.
Critically, we feel that the de-scopes have to be shared equitably between Nasa and Esa because it's so important to preserve the partnership with Esa. We can't force all the bad news on to Esa; it's got to be a fair split.
