PARIS -- The presence of water in the upper-stage engine has been identified as the probable cause of an Ariane 5 heavy-lift rocket failure in July, investigators have concluded.

The failure caused the loss of one satellite, degraded the operational life of a second and forced a shutdown of Ariane 5 operations.

The exact cause of the failure had engineers stumped for months. But recent tests of the engine in Lampoldshausen, Germany, showed that it exhibits the same defects when small amounts of water are added as it did in the July failure, said Jean-Yves Le Gall, chief operating officer of Arianespace, the European consortium that operates the vehicle.

"The characteristics of the engine ignition when water is introduced are very, very similar to what we saw during the July launch," Le Gall said Jan. 8.

He said the apparent discovery of such a simple cause for the failure has bolstered Arianespace's confidence that it can return the rocket to flight in late February.

The Ariane 5's upper-stage engine is routinely filled with water during hydraulic tests before it is delivered for final integration into the vehicle.