"While many known asteroids have passed by closer to Earth than Florence will on September 1, all of those were estimated to be smaller," Paul Chodas, a NASA scientist at the space agency's Jet Propulsion Laboratory in Pasadena, California, said in a statement. "Florence is the largest asteroid to pass by our planet this close since the NASA program to detect and track near-Earth asteroids began."
Florence is about 2.7 miles (4.4 kilometers) in size, according to measurements from NASA's Spitzer Space Telescope. And given the asteroid's proximity to Earth, you'll be able to see it with a small telescope as it brightens to its ninth magnitude in late August and early September. Sky and Telescope has also provided a decent blueprint for where to look for Florence on the first couple evenings of September.