The deputy director of Nasa's office of space sciences, Ed Weiler, described the presumed loss of the Polar Lander craft as "a crushing blow".
"We are not going to sit back and blandly go forward," he told reporters at Pasadena, California. The next Mars launch, planned for 2001, was now in doubt. "I am not convinced we will go forward with 2001; I have no confidence that it will be a successful mission."
The craft and orbiter for that mission are almost completed and Mr Weiler said they would not be assessed to see whether they could be used in 2003. "If we are not happy with 2001," he said, "we should not launch."
He said that a decision would be made in a matter of weeks about whether the 2001 mission would proceed as planned. The postponement of that launch could have a knock-on effect on the launch after that, planned for 2008, which is intended to retrieve samples from the planet and return them to Earth.
The failure of the Polar Lander project, which cost $200m ($125m), was established beyond doubt shortly after midday local time yesterday when the craft failed to make contact with the orbiting satellite, Mars Global Surveyor. This was the seventh attempt to make contact with the Polar Lander and was described by the scientists as their "last ace". There was no response.
The Nasa team will now conduct a post-mortem on the Polar Lander failure and the Mars programme as a whole. The loss of the Polar Lander is the second big failure in almost as many months.
The Mars Climate Orbiter vanished in September after it burnt up in the planet's atmosphere because one team of scientists had been working in metric measures and another in imperial, and a conversion had not been made.
The failure of these missions has already prompted much soul-searching in US scientific circles, where questions are being raised about the wisdom of the "faster, cheaper, better" policy instituted by the director of Nasa, Daniel Goldin.