Nasa satellite could hit North America

A redundant satellite falling back to Earth is now unlikely to enter the atmosphere early on Saturday UK time because its rate of descent is slowing, according to Nasa.

It is also now possible that debris from the bus-sized upper atmosphere research satellite (UARS)

could land in North America, the agency said, although the likelihood was still low.

More than half a tonne of material is expected to survive the former spacecraft's re-entry. Nasa had previously suggested the UARS would not arrive in its home continent. The final destinations of the debris remains uncertain, the space agency said in its latest update.

"Re-entry is expected late Friday 23 September, or early Saturday 24 September, eastern daylight time [five hours behind the UK] … The satellite's orientation or configuration apparently has changed, and that is now slowing its descent. There is a low probability any debris that survives re-entry will land in the United States, but the possibility cannot be discounted because of this changing rate of descent.

"It is still too early to predict the time and location of re-entry with any certainty, but predictions will become more refined in the next 12 – 18 hours."

Nasa anticipates that 26 potentially hazardous parts, weighing a total of 532kg, could remain intact and hit the Earth. The debris will spread along an estimated 500-mile corridor of the Earth's surface. Among the parts expected to survive the fiery re-entry are four titanium fuel tanks, four steel flywheel rims and an aluminium structure that alone weighs 158kg. Depending on their size and shape, the components will strike at speeds of between 55mph (90km/h) and 240mph (385km/h).

UK householders will be encouraged to know that most household insurance policies are likely to protect against any damage caused by the flying debris in what the Association of British Insurers called "a very unlikely but possible event". These cover against incidents "involving aircraft or flying objects including articles dropped from them", said a spokesman, with the "most obvious and tragic example" of payouts being from the Lockerbie bombing of Pan Am 103, in 1988.

Radar stations around the world, including RAF Fylingdales in North Yorkshire, are tracking the object, but there is little chance of predicting with any accuracy where the debris will fall. The spacecraft's orbit puts a great swath of the planet in its path between the latitudes of 57 degrees north and south. Mainland Britain lies between 50 and 60 degrees north. The satellite spends more time at higher latitudes, so there is a slightly higher risk in those regions.

Most likely by far is that the remains of the satellite will drop into the ocean, or be strewn across one of the planet's most desolate regions, such as Siberia, the Australian outback or the Canadian tundra. Nasa put the odds of anyone being struck by a falling part of the spacecraft at one in 3,200. The individual risk to a particular person is much less – one in 3,200 multiplied by the billions that live under the satellite's flight path. "The odds of you as an individual being hit by this are around one in 20 trillion," Dr Robert Massey of the Royal Astronomical Society told the BBC.

There are no confirmed injuries from man-made space debris and no record of significant property damage from a falling satellite. An organisation of major space agencies known as the Inter-Agency Space Debris Co-ordination Committee (IADC) is running back-to-back simulations to work out when, and roughly where, the spacecraft's remains will impact.

If the IADC or the Ministry of Defence, via RAF Fylingdales, found that the UK was at risk, they would inform the Cabinet Office civil contingencies committee, which is responsible for alerting the emergency services.

The UK Space Agency said in a statement: "The government continues to monitor the situation, share information nationally and at the local level. [The] Public will be aware through any press interest, but the risk to the UK is considered to be low and we will continue to monitor if that changes at any time.

"Due to uncertainties in predicting the rarefied atmosphere at these very high altitudes, the accuracy of re-entry prediction is of the order of 10% of the remaining lifetime, so even on the last orbit revolution (90 minutes), there is a nine-minute prediction uncertainty. If an object was about to fall on the UK we could only respond as we would in any other 'no notice event' such as a plane crash, at which time tried and tested procedures would be undertaken by the emergency response services."

When Nasa's Skylab fell to Earth in 1979, the space agency put the risk of human injury at 1 in 152, because the odds of the defunct space station striking a city were much higher. The partially controlled Skylab missed its expected impact site in South Africa and crash-landed in Australia.

Predicting where the debris will land is difficult for two main reasons. Unpredictable rises in the sun's activity warm the atmosphere and make it expand, which causes the spacecraft to experience more drag and re-enter more quickly. Another problem comes from uncertainties in the tracking of how the spacecraft disintegrates, which means that even just a few hours before impact, the corridor of the Earth's surface at risk will be several thousand long.

UN agreements oblige governments to return any parts of a satellite that are found to the owner, in this case Nasa, which will have to bear the costs of recovery. They also say a launching state shall be absolutely liable to pay compensation for damage on the Earth's surface or to aircraft.

Nasa urges anyone who suspects they have found debris from the spacecraft not to touch it and inform the local police. The satellite was launched in 1991 aboard the space shuttle Discovery and decommissioned in 2005.

Nasa says more than 22,000 objects larger than 10cm (4in) are currently tracked by the US Space Surveillance Network. Only about 1,000 of these represent operational spacecraft; the rest are orbital debris. Most orbital debris is within 1,250 miles (2,000km) of Earth's surface, says the agency, with the greatest concentrations found 500-530 miles (800-850km) up. During the past 50 years an average of one piece of debris fell back to Earth each day.