A giant asteroid the size of three football pitches will make the closest flyby of Earth in recorded history for an object of its size, scientists said.
It will pass between the Earth and the Moon and will even come closer than the orbit of many telecommunications satellites, although astronomers insisted that there is little chance of a collision with the massive rock.
Anxious Earthlings need not worry too much for another 24 years, however, because asteroid 2004 MN4 is not due to make its closest approach to Earth until approximately 10pm London time on Friday 13th April 2029.
The latest calculations of the rock's orbit suggest that it will come so close that it will probably be visible to the naked eye from Britain. It will shine in the sky as a dim, fast-moving star - the first asteroid in modern times to be clearly visible from Earth without the aid of a telescope or binoculars.
The asteroid was first discovered in June 2004 and calculations of its orbit made by astronomers last Christmas Eve suggested that there was a one in 60 chance of it colliding with the Earth. However, within a week this was revised down to virtually zero probability of a collision.
If it did collide it would cause an explosion equivalent to several hydrogen bombs being detonated simultaneously, turning vast areas of land into desert or generating a giant tsunami if it landed in the ocean.
The latest revisions of the calculations have refined the asteroid's orbital path to suggest that it will pass our planet by the relative whisper of 36,350 km - well within the orbit of geostationary satellites and about a tenth of the distance to the Moon.
This is by far the largest of the top ten closest asteroids recorded by astronomers. Only two others have come closer and both were much smaller objects - tens of yards wide instead of the 320 metres of asteroid 2004 MN4.
Professor Mark Bailey, director of the Armagh Observatory, said that there is little danger from the asteroid even though it will come close enough for its orbit to be directly affected by the Earth's gravity - causing the path of the space rock to swing away.
"I think everyone is saying that it's going to miss. It'll pass so close though that you'll be able to see it with a small telescope and even with the naked eye," Professor Bailey said.
"It's like being on a train station platform and watching an express train go by three feet away. You're close, but it's not dangerous," he said.
Large asteroids have frequently collided with the Earth in the past and some of the larger ones have caused massive devastation on a global scale. They can send huge plumes of dust and debris into the atmosphere, blocking out sunlight for several years and causing the environmental equivalent of a "nuclear winter".
Last autumn a much bigger asteroid called Toutatis, which is about 2.9 miles long and 1.5 miles wide, made its closest flyby to Earth but its distance was still four times greater than that separating the Moon and the Earth.
Unlike asteroid 2004 MN4 and despite its size, Toutatis was not visible to the naked eye.
Steve Chesley, of Nasa's Jet Propulsion Laboratory in Pasadena, California, said that asteroid 2004 MN4 is unusual because so much is known about its orbit before it makes its closest approach in 24 years time.
"All the others in the top ten were discovered during the close approach, whereas for 2004 MN4 the close approach is predicted well in advance," Dr Chesley said.
Scientists estimate that on average an asteroid of this size would be expected to pass this close to Earth about once in every 1,300 years.
Asteroid 2004 MN4 circles the Sun, but unlike most asteroids that reside in a belt between Mars and Jupiter, the 323-day orbit of the asteroid lies mostly within the orbit of the Earth, making further encounters likely.
"However, our current risk analysis for 2004 MN4 indicates that no subsequent Earth encounters for the 21st Century are of concern," said Dr Chesley and his colleagues at JPL.
- The IndependentBy Steve Connor