The giant Toutatis asteroid passes by the Earth on Wednesday at distance of less than 1.6 million km.
This is roughly four times the distance from the Earth to the Moon and closer than this 4.6km-wide rock has come to us since at least the 12th century.
The timing of closest approach is at 1335 GMT. Toutatis is one of the best studied asteroids in the Solar System.
The plane of its orbit is closer to the plane of the Earth's orbit than any known Earth-orbit-crossing asteroid.
But astronomers stress that it poses no danger, even though officially they class it as a Potentially Hazardous Asteroid.
"This is the closest Toutatis will come for another 500 years, and its orbit is very well known," Dr Don Yeomans, of the US space agency's Jet Propulsion Laboratory (JPL) and manager of its Near-Earth Objects Program Office, said.
"What this fly-by provides is an opportunity to study one of our closest Solar System neighbours."
The object - its full name is 4179 Toutatis - was discovered in 1989 by French scientists and named after a Celtic god.
Its dimensions are 4.6 by 2.4 by 1.9km. It is considered one of the strangest bodies in the Solar System because of its peculiar rotation and odd shape, which resembles two chunks of rock connected by a narrow neck-like structure.
The rocky body's strange traits are believed to be the result of a history of violent collisions.
Future missions
Even though much is already known about Toutatis, Robin Scagell from the UK's Society of Popular Astronomy says many observations will be made during the flyby.
"Radar is a very important tool and this will be used to investigate precisely what Toutatis is made of," he told the BBC.
"It's a lumpy thing and it seems to be comprised of perhaps different chunks of rock that are hung together by a bit of gravity.
"It is really important to know this sort of thing because if one of these asteroids ever has our name on it and comes towards us, we need to know whether they are just big mountains which can be pushed to one side.
If you try to take the Armageddon-Bruce Willis approach and blast it to smithereens and it's composed of lots of chunks of rock, that's not going to work very well."
The European Space Agency is looking at an asteroid mission concept - called Don Quijote - which would try to understand how an asteroid on a collision course with Earth might be destroyed or deflected.
This mission would involve two spacecraft - Sancho and Hidalgo - launched on different trajectories to the same asteroid. Sancho would arrive first and orbit from a safe distance.
It would then measure the effects as Hidalgo is sent crashing into the asteroid's surface.
Nasa has a similar mission - Deep Impact - which will blow a hole in the comet Tempel 1 and measure the effects.
http://news.bbc.co.uk/1/hi/sci/tech/3700754.stm
Terrifying
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