"While many known asteroids have passed by closer to Earth than Florence. They were estimated to be smaller," said Paul Chodas, Manager of NASA's Centre for Near-Earth Object Studies (CNEOS) at the agency's Jet Propulsion Laboratory in Pasadena, California.
The 2017 encounter that will occur around 5.35 pm on Friday (India time) is the closest by this asteroid since 1890 and the closest it will ever be until after 2500.
Florence will brighten to ninth magnitude in late August and early September, when it will be visible in small telescopes for several nights as it moves through the constellations Piscis Austrinus, Capricornus, Aquarius and Delphinus, NASA said.
This relatively close encounter provides an opportunity for scientists to study this asteroid up close.
Florence is expected to be an excellent target for ground-based radar observations.
The resulting radar images will show the real size of Florence and also could reveal surface details as small as about 10 metres.