What's the difference between inches, pounds and miles on the one hand, and centimeters, grams and kilometers on the other?

The answer could have saved the $125 million Mars Polar Orbiter from destruction last week, if watchdog computers at NASA's Jet Propulsion Laboratory in Pasadena had only known about the discrepancy.

In an embarrassingly simple mistake, NASA officials said yesterday, the spacecraft burned up on impact with the thin Martian atmosphere because two navigation teams and their computers had confused English and metric units.

After flying more than 415 million miles over nine months, the spacecraft was about to enter its first orbit around the Red Planet last Thursday when it accidentally flew too low toward Mars, mission engineers said at the time.

But yesterday the engineers said they had found the cause: It turned out that Lockheed Martin engineers in Colorado, who built the spacecraft and are responsible for its health, transmitted the orbiter's final course and velocity to Pasadena using the English term of pounds per second of force.

But the spacecraft's navigation team at JPL Mission Control in Pasadena --like almost all space scientists and engineers -- always use the metric system in their work. Their computers used the metric term newtons, or grams per second of force, to send final course and velocity commands to the Mars-bound spacecraft.

The ship flew just a hair too close and disintegrated in the Martian atmosphere.

"We're talking about velocities in millimeters per second,'' said Tom Gavin, deputy director of the laboratory's space and Earth sciences programs. "We had multiple systems of checks and balances, but they just didn't catch it," he said.

Gavin said previous Mars missions always used metric measurements.

Lockheed officials are reviewing contracts to see whether the space agency specified the units of measurement, said Noel Hinners, vice president of flight systems for Lockheed, Martin Astronautics in Denver. "We should have had them in metric units," he said.

The bad numbers had been used since the spacecraft's launch last December, but the effect was so small it went unnoticed. The difference added up over the months as the spacecraft jouneyed toward Mars.

The loss is not expected to affect NASA's relationship with Lockheed, which has built several probes for the space agency, including the Magellan probe to Venus and the Mars Global Surveyor orbiter.

"This country has not gone 100 percent metric," said Chris Jones, program manager for JPL's Mars Surveyor Program. "(Companies) continue to use the English system of units, and that something we have dealt with effectively on other programs."

Concerns over another spacecraft, the Mars Polar Lander, due to touch down near the planet's south pole December 3, are now on everybody's mind.

To Gavin, however, the problem that killed the Orbiter has already been solved:

"We didn't know what caused the navigation error at first," he said, "but now that we know it, the problem's easy to find and fix."

Gavin said review teams are checking the source of the error and others are hard at work to make sure the same error doesn't happen again.

Gavin said mission directors for the Polar Lander have recruited "the brightest groups of interplanetary people from industry and everywhere else" to make sure everyone speaks the same language and uses the same arithmetic terms when the lander finally reaches Mars.

The orbiter was designed to be the world's first complete weather satellite around another planet. Its mission was to collect data about the Martian atmosphere, seek evidence of water vapor, determine daily and seasonal weather patterns and find frost deposits in the south polar regions of Mars.

Unlike the mission of Mars Pathfinder in 1997, the Polar Lander this time will not deploy a roving vehicle on the surface, but instead will use a long flexible arm aboard the craft to scoop up samples of Martian soil for analysis by instruments inside the craft. And just a few minutes before it reaches the surface, the Lander will fire two "microprobes" deep into the Martian ground to see if water exists there.

The Associated Press contributed to this report.