Asteroid Florence is among the largest near-Earth asteroids that are several miles in size. Measurements from NASA's Spitzer Space Telescope and Near-Earth Object Wide-Field Infrared Survey Explorer (NEOWISE) mission indicate it is about 4.4 kilometres in size.
"While many known asteroids have passed by closer to Earth than Florence will on September 1, all of those were estimated to be smaller," said Mr Paul Chodas, manager of NASA's Center for Near-Earth Object Studies (CNEOS).
"Florence is the largest asteroid to pass by our planet this close since the NASA programme to detect and track near-Earth asteroids began," Mr Chodas 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, NASA said.
Asteroid Florence was discovered by Schelte Bus at Siding Spring Observatory in Australia in March 1981. It is named in honour of Florence Nightingale, the founder of modern nursing.
This year's encounter is the closest by the asteroid since 1890 and the closest it will ever be until after 2500, according to the US space agency.
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.
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