Entering the atmosphere lowerthan that would likely create so much friction that the craft would havebeen scorched and torn apart.

The intended altitude atclosest approach was about 90 miles (140 to 150 kilometers), Cook said.Even that altitude had been revised from the originally targeted altitude,of about 125 miles (200 kilometers) above the surface.

Although Cook would not speculateabout the cause of the problem, he said that targeting a spacecraft toreach another planet is a tremendous feat of engineering and precisiontracking.

"What happens with deep spacenavigation, is on your way to a planet, you're basically trying to figureout where you are relative to the Earth -- and what you really care aboutis where you are relative to this planet that you're going to," Cook said.

That position relative toa target planet can only be determined with precision near the very endof the voyage, he said. "So it's only over the course of the last six toeight hours that we saw what was about a 100-kilometer (60-mile) drop inthe point of closest approach, and that's part of the mystery -- we don'tunderstand why that occurred."

Cook said a systematic investigationwould be necessary before the cause of the problem could be determined.

The search for the spacecraftis ongoing, though. Controllers will continue to scan the region aroundMars for some signal that the orbiter has survived until they determinefor certain what became of the craft.

While the loss of the orbiterwould be serious, it wouldn't be devastating, said Carl Pilcher, sciencedirector for solar system exploration in NASA's office of space science."It would mean a science delay, not a science loss," he said.

One of the science experimentson the orbiter has already been lost once before. The Pressure ModulatorInfrared Radiometer is a copy of an instrument that flew aboard the MarsObserver, which dropped out of contact just days before it was to reachMars in 1993. The instrument was designed to measure temperature changesand particle distribution throughout the atmosphere.

Pilcher downplayed the potentialspacecraft failure, saying that occasional loss is the price of an aggressivespace mission schedule. He said the loss would not affect the overall Marsexploration program too much.

"The advantage of havinga number of missions launched continuously over a long period of time isyou actually can make this program robust by including redundant systems,"Pilcher said.

The cost of the Mars ClimateOrbiter is estimated at $125 million.

On the other hand, the potentialloss could have an impact on the Mars Polar Lander's mission. The climateorbiter was supposed to serve as a communications and data-relay stationfor the lander, which is scheduled to reach Mars on Dec. 3.

The lander is capable ofcommunicating directly with Earth, but it can send data much more efficientlythrough an orbiting relay station. Loss of the climate orbiter would notcompromise the lander's mission, said Mars Surveyor ' 98 mission scientistRichard Zurek. The mission includes both the climate orbiter and polarlander.

"The Mars Surveyor programis resilient to the loss of any one spacecraft," he said. "Certainly wewere looking for a type of climate and weather data for this particularspacecraft, and that would be a loss. However the Mars Polar Lander, forinstance, will be able to carry out its surface mission even if we don'testablish contact with the orbiter again."