An asteroid bigger than an aircraft carrier will dart between the Earth and moon on Tuesday -- the closest encounter in 35 years.
But scientists insist it will not hit.
"We're extremely confident, 100pc confident, that this is not a threat," said the manager of Nasa's Near Earth Object Programme, Don Yeomans.
The asteroid named 2005 YU55 is being watched by ground antennas as it approaches from the direction of the sun. The last time it came within so-called shouting distance was 200 years ago.
The closest approach will occur on Tuesday when the asteroid passes within 202,000 miles of Earth.
That is closer than the roughly 240,000 miles between the Earth and the moon.
The moon will be just under 150,000 miles from the asteroid at the time of closest approach.
Both the Earth and moon are safe -- "this time", said Jay Melosh, professor of Earth and atmospheric sciences at Purdue University.
If 2005 YU55 were to strike Earth, it would blast out a crater four miles across and 1,700 feet deep, according to Melosh's calculations -- think a magnitude-7 earthquake and 70ft tsunami waves.
Scientists have been tracking the slowly spinning object since its discovery in 2005, and are positive it won't do any damage. "We know the orbit of this object very well," Mr Yeomans said.
The asteroid stretches a quarter-mile across. Smaller objects come close all the time, Mr Yeomans noted, but nothing this big will have ventured so close since 1976.
Radar observations from California and Puerto Rico will help scientists ascertain whether the asteroid is pockmarked with craters and holds any water-bearing minerals or even frozen water. Amateur astronomers would need a six-inch-or-bigger telescope -- and know exactly where to look -- to spot it.