The object, now called 2002 EM7, was hard to spot because it was moving outward from the innermost point of its orbit, 87 million km from the Sun. When it passed closest to the Earth - just 1.5 times the distance to the Moon - it was too close to the Sun to be visible.

Asteroids approaching from this blind spot cannot be seen by astronomers. If a previously unknown object passed through this zone on a collision course with Earth, it would not be identified until it was too late for any intervention.

Astronomers have made numerous calls in recent years for more funds to catalogue near-Earth objects and refine their orbits. This would reduce the number of unknown objects that could catch us unaware, and give early warning of potential future collisions.

Bigger than Tunguska

An asteroid-hunting telescope operated by the MIT Lincoln Laboratory first recorded the new asteroid on March 12, as it moved away from the Earth and more of its bright side came into view.

Further observations allowed Timothy Spahr of the Harvard-Smithsonian Center for Astrophysics to calculate its orbit. He found that it has a 323-day orbit that takes it as far as 188 million km from the Sun.

Invisible to the unaided eye, 2002 EM7 is too small to be classed as a "potentially hazardous asteroid". But it is probably between 50 to 100 metres across, making it larger than the object that exploded in 1908 over the Tunguska region of Siberia, flattening trees over 2000 square kilometres.

The approach puts it among the 10 closest known approaches by minor planets, says Brian Marsden of Harvard-Smithsonian. More ominously, only one of the 10 closest objects was larger. This was 1996 JA1, which passed only slightly closer to the Earth on 19 May 1996.

The discovery adds to the list of asteroids which are long shots to hit the Earth in the next century. Preliminary calculations indicate 2002 EM7 has Several chances to hit the Earth in that period, with odds of one in six million to one in a billion, Marsden told New Scientist. Further observations are likely to lengthen the odds.