RUSSIA'S damaged Mars probe, carrying four canisters of radioactive plutonium, fell to Earth on Sunday over the South Pacific, west of Chile, US and Australian officials said.
The Chilean Navy had received no official word of the crash, but recovery of any debris from the spacecraft would be difficult if not impossible, a Chilean naval officer said.
"This area has one of the deepest ocean floors in the world. Any kind of recovery, if it were possible, would be very, very tough," the officer said. The area where the spacecraft fell is up to 6,000 metres deep.
Initially the probe looked likely to hit Earth over Australia, causing Australian defence officials to jump into frenzied preparations for a possible emergency.
US Space Command spokesman, Mr Dave Knox, said his agency tracked objects in space, not in the atmosphere, and therefore it was impossible to know whether the six ton craft exploded upon re entry and where exactly the debris landed.
The craft most probably burned up upon entering the Earth's atmosphere, although NASA officials said two landing vehicles weighing 50 kilos each were built to survive a landing on Mars, and probably crashed into the ocean as debris.
The vehicles each carried two tiny plutonium batteries about the size of 35 mm film canisters containing some 200 grams of the radioactive material. Russian officials said they were designed to survive the intense heat of reentry and the impact of a crash.
US officials said they were confident the batteries had survived intact and were now at the bottom of the ocean.
The probe was carrying SLED II, an instrument intended to further study interplanetary charged particles. Designed by a team led by Prof Susan McKenna Lawlor, and working for Space Technology (Ireland) Ltd, Maynooth, it was a successor to the first Irish SLED instrument, which flew to Mars in 1989.
The spacecraft lifted off from Kazakhstan's Baikonur space centre on Saturday, but failed to break out of the Earth's orbit after a booster rocket misfired.
Before the probe landed, Mr Robert Bell, a National Security Council official, had outlined what would happen if the plutonium batteries did not survive.
One possibility was that the canisters would break up. If this happened at high altitude, Mr Bell said the small radioactive cloud that would result would dissipate harmlessly, but dispersal at a lower altitude or breakage upon impact could be lethal to humans.
Plutonium 238 causes lung or bone cancer if inhaled.