Updated: 01/09/17 : 11:42:10Printable Version
The largest asteroid in more than a century is expected to pass by Earth at a relatively close distance of 4.4m miles (7m km), Nasa says.
Florence measures 2.7 miles (4.4km) in size and will not pose a threat to Earth for centuries to come.
While other asteroids have passed closer to Earth, they were all estimated to be smaller.
Asteroids retain clues to the events that brought the Sun and the planets into being.
At its closest point, Florence - which was discovered in 1981 - will be at about 18 times the average distance between the Earth and the Moon.
"Florence is the largest asteroid to pass by our planet this close since the [American space agency] Nasa program to detect and track near-Earth asteroids began [over a century ago]," Paul Chodas, manager of Nasa's Center for Near-Earth Object Studies, said in a statement.
The 2017 encounter is the closest by this asteroid since 1890 and the closest it will ever be until after 2500, the US space agency added.
Scientists plan to study the asteroid up close, using ground-based radar imaging in California and Puerto Rico.
An object this size would have global effects were it to hit the Earth. Scientists believe they have now identified more than 90% of the monster rocks flying on space.