The world's major space agencies, armed forces and security officials have come together to monitor the heavens for a bus-sized spacecraft that will fall to Earth this week.

In an event prompted by the rule that what goes up must come down, the defunct satellite will plummet through the atmosphere, burn and break apart, and scatter hunks of steel, aluminium and titanium over a distance of hundreds of miles.

Much of Nasa's nearly six-tonne Upper Atmosphere Research Satellite (UARS) will disintegrate as it hurtles through the atmosphere, but the space agency anticipates that 26 potentially hazardous parts, weighing a total of 532kg, could remain intact and impact on the surface. The debris will spread over an estimated 500 miles.

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 (90kph) and 240mph (385kph).

Radar stations around the world, including RAF Fylingdales in north Yorkshire, are tracking the object and expect it to re-enter the atmosphere between Thursday and Saturday, but there is little chance of predicting with any accuracy where the debris will fall.

An update from Nasa on Wednesday said the satellite was 120 miles above the Earth and due to impact on Friday US time. The agency will issue further updates 24 hours before re-entry, then at 12, six and two hours before re-entry.

The spacecraft's orbit puts a great swathe 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.

Noting that safety was its top priority, Nasa declared the odds of someone being struck by a falling part of the spacecraft at one in 3,200. There are no confirmed injuries from man-made space debris and no record of significant property damage from a falling satellite.

"Most of the Earth's surface is covered by water or is uninhabited, so nobody tends to even see this kind of debris when it does land," Hugh Lewis, a space debris expert at Southampton University, told the Guardian.

"Those pieces that do survive re-entry have slowed down a lot, but they are still travelling quite fast. Because of their size, they would do significant damage if they hit a structure or a person, but the chances of that happening are remote," he added.

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

An organisation of major space agencies known as the Inter-agency Space Debris Coordination Committee (IADC) takes a lead role in monitoring threats from falling space junk and 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.

"There is a limit to what you can do in response, because you cannot give categorical information on where something is going to land. It would be irresponsible to order an evacuation, because you would put more people at risk than would ever be in danger from falling space debris," said Richard Crowther, a space surveillance expert at the UK Space Agency. "Fortunately, we are a small target compared with other landmasses."

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 region at risk will cover several thousand kilometres.

Under an international treaty, governments are obliged to return any parts of the satellite that are found to the owner, in this case Nasa. The space agency urged anyone who suspected they had 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.

Wherever the spacecraft lands, it will give the relevant authorities valuable experience ahead of a potentially more dangerous event in early November, when the German Rosat satellite re-enters at 28,000kph. The German space agency, DLR, said up to 30 pieces of the spacecraft might survive re-entry, with a combined mass of more than one-and-a-half tonnes.