Nasa's UARS satellite fell far from major landmass

The "productive scientific life" of UARS ended in 2005 when it ran out of fuel

Nasa's UARS spacecraft fell to Earth north-east of the Samoan islands.

Orbital tracking experts have now established that the defunct satellite entered the atmosphere over the Pacific Ocean at 14.1 degrees South latitude and 170.2 degrees West longitude.

Any debris that survived Saturday's fiery descent would have plunged into open water, the US space agency says.

The exact time the six-tonne craft engaged the atmosphere is now given as 0401 GMT.

The Upper Atmosphere Research Satellite (UARS) was the largest American space agency research platform to return uncontrolled from orbit in about 30 years.

Its fall from the sky generated huge interest worldwide at the weekend, with the possibility that its destruction would produce a spectacular fireball in the sky for anyone close enough to see it.

The return was monitored by the Joint Space Operations Center (JSPOC) at Vandenberg Air Force Base in California.

On Saturday, JSPOC's best estimate for the timing of the re-entry was 0416 GMT. Post-fall analysis has now brought that forward by 15 minutes, meaning the event occurred much further from the west coast of North America than was originally thought.

Modelling work had indicated perhaps 500kg of mangled metal could have survived to the surface, spread over a path some 800km long. If the latest analysis is correct, it seems certain all of that debris would have gone into the ocean.

"We have a high accuracy assessment based on a large number of data points, and we're very, very confident in our latest assessment," said Nick Johnson, Nasa's chief scientist for orbital debris at the Johnson Space Center in Houston. "Because the re-entry of the UARS satellite took place over the mid-Pacific Ocean, it's unlikely that anyone actually observed the re-entry."

UARS was deployed in 1991 from the space shuttle Discovery on a mission to study the Earth's upper atmosphere.

It contributed important new understanding on subjects such as the chemistry of the protective ozone layer and the cooling effect volcanoes can exert on the global climate.

Tracking stations will typically witness the uncontrolled return of at least one piece of space debris every day; and on average, one intact defunct spacecraft or old rocket body will come back into the atmosphere every week.

Something the size of UARS is seen perhaps once a year. Much larger objects such as space station cargo ships return from orbit several times a year, but they are equipped with thrusters capable of guiding their dive into a remote part of the Southern Ocean.

UARS is supposed to be the last big Nasa satellite to return uncontrolled. Future re-entries are expected to be better targeted.

The German space agency (DLR) has an X-ray satellite, Rosat, coming back uncontrolled to Earth later this year which, although more compact than UARS, could produce a greater amount of surface debris.

The latest studies indicate perhaps 30 individual items weighing a total of 1.6 tonnes could make it through the atmosphere to hit the ocean or land. The largest single fragment is expected to be the telescope's mirror, which is very heat resistant.

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