An asteroid that is 400m (1,300ft) wide will pass by the Earth on Tuesday, closer to it even than the Moon.
It poses no danger to the Earth and it will be invisible to the naked eye.
Asteroid 2005 YU55's closest approach, at a distance of 325,000km (202,000mi), will be at 23:28 GMT. It is the closest the asteroid has been in 200 years.
It is also the largest space rock fly-by the Earth has seen since 1976; the next visit by such a large asteroid will be in 2028.
The aircraft-carrier-sized asteroid is darkly coloured in visible wavelengths and nearly spherical, lazily spinning about once every 20 hours as it races through our neighbourhood of the Solar System.
It will trace a path across the whole sky through to Thursday.
"This is the closest approach by an asteroid that large that we've ever known about in advance," said Lance Benner of Nasa's Jet Propulsion Laboratory.
But he stressed that there was no chance that the pass would be anything other than a close encounter.
Instead, the pass gives astronomers a rare opportunity to study the asteroid in detail."2005 YU55 cannot hit Earth, at least over the interval that we can compute the motion reliably - which extends for several hundred years," he said.
In particular, two radio telescopes - the Goldstone Observatory in California, US and the Arecibo Observatory in Puerto Rico, US - will be tracking radio echoes off it in a bid to understand better what it is made of and how it is shaped.
The precise details of the asteroid's path will also help scientists to predict where it will go much farther into the future.
Amateur astronomers may catch a glimpse of it with telescopes of 15cm or larger, Nasa suggests.
The Earth has several regular visitors like 2005 YU55 - most famouslythe Apophis asteroid. Apophis has in the past been claimed as a possible future impactor when it returns to our neighbourhood in 2029 and again in 2036.
There is, according to the latest calculations, no danger from Apophis either. However, it will pass much closer to the Earth on 13 April 2029 - at a distance of just 29,500km (18,300mi).
