Dr John Gribben on BBC Radio 4's Today programme (2'48")

The mile-wide object has the potential to create worldwide devastation if it hits the planet during its closest approach in October 2028.

Concerned scientists are tracking the rock, known as 1997 XF11, to project its course.

The Visiting Fellow of Astronomy at Sussex University, England, Dr John Gribben, said: "There is about a one in 1,000 chance that it will hit the Earth. Those are very short odds.

"It is big enough to cause immense devastation. You are not talking about wiping out a city, you are talking about wiping out a continent."

Asteroid specialist Jack Hills says the speeding space rock poses a real danger to Earth.

Most dangerous so far

Mr Hills, a scientist at America's Los Alamos National Laboratory, said: "This is the first really big one to pass this close.

"This is the most dangerous one we've found so far. An object this big hitting the Earth has the potential of killing many, many people."

The best estimate so far is that 1997 XF11 will track inside the orbit of the Moon and pass within 26,000 miles of Earth.

But the estimate has a margin of error of more than 180,000 miles making a collision with Earth possible.

A meteorite found on Earth

Asteroid 1997 XF11 was discovered on December 6 last year by the University of Arizona Spacewatch programme and was added to a list of 108 asteroids considered to be "potentially hazardous objects".

Mr Hills said an asteroid the size of 1997 XF11 colliding with the Earth at more than 17,000 miles an hour would explode with an energy of about 320,000 megatons of dynamite - the equivalent of almost two million Hiroshima-sized atomic bombs.

Widespread devastation

Depending on the collision site, it could cause mountainous tidal waves or a 20-mile-wide crater, throwing so much dust into the air that the Sun would be blotted out for weeks, Mr Hills said.

He added: "If one like this hit in the Atlantic Ocean, all of the coastal cities would be scoured by the tsunami. Where cities stood, there would be only mudflats."

Asteroids are routinely observed and plotted by astronomers around the world because of their potential for causing devastation on Earth.

Keeping watch

Revised dates and distances from Earth will be generated as astronomers plot its course over the next few years.

A notice filed by the International Astronomical Union said the asteroid will move out of view to all but the largest telescopes over the next few months.

It will become more visible once again in 2000. And two years later, it is predicted to pass within about six million miles of Earth on Hallowe'en Eve.

Mr Hills said the asteroid is lost from view when it passes behind the Sun, but that it will emerge into telescope range about every two years.

The IAU notice added: "The chance of an actual collision is small, but one is not entirely out of the question."

Other seemingly improbably happenings

Experts say that when an asteroid passes close to the earth in 2028 there is a 1,000 to one chance it might hit us.

Bookmakers William Hill currently quote the likelihood of the following events at 1000 to one:

• Oasis emigrating to Australia

• Paul Gascoigne to star in a second The Full Monty film

• Prince Charles, Prince Edward and Prince Andrew to stage a triple wedding in 1998

• Former Baywatch star Pamela Anderson to marry Michael Jackson

• England to beat Scotland in a penalty shoot-out in the World Cup final or vice versa

• The Millennium Dome to be opened by the Teletubbies

• All five Spice Girls to marry professional footballers this year

• Income tax to be abolished by the Millennium

• Coronation Street butcher Fred Elliot to be killed off by mad cow disease

• Any British girl to win the Wimbledon singles final this year.