Miami - An asteroid the size of a mountain is about to shave by Earth, in a rare type of flyby that will not be seen for another decade, astronomers said on Monday.The asteroid, known as 2004 BL86, runs no risk of a colliding with Earth and will be about three times farther than the Moon when it passes.At the time of its closest approach on 26 January, the asteroid will be about 1.2m km from Earth.As asteroid events go, this one is special because the space rock is so much larger than most, measuring about a third of a mile across (0.5km).The majority of near-Earth objects are 15 to 30m in diameter."It's the largest known space rock predicted to come this close to us until 2027," said Sky and Telescope magazine.The next large asteroid that is known to be making a close approach to Earth will be asteroid 1999 AN10, flying past in 2027.In the meantime, astronomers are excited for the opportunity to catch a glimpse of 2004 BL86, a mysterious asteroid that won't pass by again for about 200 years."At present, we know almost nothing about the asteroid, so there are bound to be surprises," said radar astronomer Lance Benner of Nasa's Jet Propulsion Laboratory."When we get our radar data back the day after the flyby, we will have the first detailed images."Telescope and binoculars Nasa said the asteroid will not be visible to the naked eye, but may be seen with the help of small telescopes and strong binoculars.The asteroid's closest approach to Earth is expected around 11 am (1600 GMT), but it will be dim "because Earth will see only a portion of its illuminated side," Sky and Telescope said, which has published a star chart to show its path.As the hours pass, it will gain in brightness and the best viewing for those in North and South America, Europe and Africa will range from 20:00 Monday to 01:00 Tuesday (01:00 GMT-06:00 GMT Tuesday).During that time period, the asteroid will be heading northward through the constellation Cancer. The asteroid orbits the Sun every 1.84 years, and was discovered 11 years ago by the LINEAR telescope in New Mexico. By using observations via telescope and radar mapping by Nasa scientists at facilities in California and Puerto Rico, astronomers hope to discover how fast the asteroid spins and learn more about its size, shape and surface."Asteroids are something special. Not only did asteroids provide Earth with the building blocks of life and much of its water, but in the future, they will become valuable resources for mineral ores and other vital natural resources," said Don Yeomans, retired manager of Nasa's Near Earth Object Program Office at the Jet Propulsion Laboratory in Pasadena, California."They will also become the fuelling stops for humanity as we continue to explore our solar system. There is something about asteroids that makes me want to look up."