An asteroid the size of a mountain is about to shave by Earth, in a rare type of fly-by that will not be seen for another decade, astronomers say.
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 January 26, the asteroid will be approximately 1.2 million kilometres from Earth," NASA's Jet Propulsion Laboratory said in a statement on Monday.
For this particular asteroid, it will mark the closest pass by Earth in around 200 years.
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But as asteroid events go, this one is special because the space rock is so much larger than most, measuring about a half of a kilometre while the majority of near-Earth objects are 15 to 30 metres 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 when it flies past in 2027.
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 11am (0300 AEDT Tuesday), but it will be dim "because Earth will see only a portion of its illuminated side," Sky and Telescope said.
