Since the triumph of the moon landings more than four decades ago, presidents have made grand pronouncements about the next adventure for NASA astronauts, one that would culminate with humans on Mars. But each time, those ambitions faded, unfulfilled.

Now a panel of experts is warning that the same fate may be in store for President Obama’s declaration that NASA astronauts would reach Mars in the 2030s.

In a 285-page review of the human spaceflight program, the experts, convened by the National Research Council at the request of Congress, found that NASA had not detailed a viable strategy for getting there and that its budgets were too small to succeed.

“There is not a believable plan for getting there in a finite period of time,” said Jonathan I. Lunine, a Cornell astronomer and a co-chairman of the committee.