An asteroid that measures nearly 2.7km (1.7 miles) across is set to fly past the Earth.
The space rock, which is called 1998 QE2, is so large that it is orbited by its own moon.
It will make its closest approach to our planet at 20:59 GMT (21:59 BST), but scientists say there is no chance that it will hit.
Instead it will keep a safe distance - at closest, about 5.8 million km (3.6 million mi).
That is about 200 times more distant than the asteroid "near-miss" that occurred in February - but Friday's passing space rock is more than 50,000 times larger.
Prof Alan Fitzsimmons, an astronomer at Queen's University Belfast, said: "It's a big one. And there are very few of these objects known - there are probably only about 600 or so of this size or larger in near-Earth space.
"And importantly, if something this size did hit us one day in the future, it is extremely likely it would cause global environmental devastation, so it is important to try and understand these objects."
Dark visitorThis fly-by will give astronomers the chance to study the rocky mass in detail.
Using radar telescopes, they will record a series of high-resolution images.
They want to find out what it is made of, and exactly where in the Solar System it came from.
Prof Fitzsimmons said: "We already know from the radar measurements, coupled with its brightness, that it appears to be a relatively dark asteroid - that it's come from the outer part of the asteroid belt."
Early analysis has already revealed that the asteroid has its own moon: it is being orbited by another smaller piece of rock that is about 600m (2000ft) across.
About 15% of asteroids that are large are "binary" systems like this.
This celestial event will not be visible to the naked eye, but space enthusiasts with even a modest telescope might be able to witness the pass.
After this, asteroid 1998 QE2 will hurtle back out into deep space; Friday's visit will be its closest approach for at least two centuries.
Researchers are becoming increasingly interested in potential hazards in space.
So far they have counted more than 9,000 near-Earth asteroids, and they spot another 800 new space rocks on average each year.
Source: http://www.bbc.co.uk/news/science-environment-22736709#sa-ns_mchannel=rss&ns_source=PublicRSS20-sa
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