NASA officials today cast blame for the loss of the Mars Climate Orbiter on a misunderstanding over which units--English or metric--were being used to fine-tune the spacecraft's trajectory.

On a mission to study martian weather, the $87 million orbiter was lost on 23 September when it presumably burned up in the martian atmosphere (ScienceNOW, 23 September). NASA implied initially that the probe had made too low an approach to Mars because the navigation team at the Jet Propulsion Laboratory (JPL) in Pasadena, California, had erred somehow. That came as a big surprise, considering the team's solid-gold reputation. "We just knew there had to be more to the story than that," says a scientist familiar with the JPL operation.

Now it appears JPL was not at fault. The screw-up seems to have occurred between the JPL navigation team for the orbiter--which from tracking data determined the spacecraft's position and how its trajectory should be changed--and the spacecraft team at Lockheed Martin Astronautics in Denver, which told the probe how to fire its thrusters to get on target. The Lockheed Martin team was using English units for the forces acting on the spacecraft, according to a JPL worker, a practice still common in some engineering circles such as defense contracting. The JPL group had assumed the force data were in metric units, the convention agreed to at the outset, says the JPL worker.

NASA officials were quick today to indict the system, not any individual, for the spacecraft's loss. "The problem here was not the error," says Edward Weiler, NASA's associate administrator for space science in Washington, D.C. "It was the failure of NASA's systems engineering and the checks and balances in our processes to detect the error," he says, adding that "the lessons from [forthcoming] reviews will be applied across the board." With luck, any insights will come in time for Mars Polar Lander, scheduled to arrive at Mars on 3 December.